=73. Capacity of Receiving Well.=—The use of receiving wells is

restricted to small installations which require, in addition to the

standby unit, only one pump, the capacity of which is equal to the

maximum rate of sewage flow. When the receiving well has been pumped dry

the pump stops, allowing the well to fill again. Although the use of a

large receiving well, or an equalizing reservoir, for a large pumping

station would permit the operation of the pumps under more economical

conditions, the storage of sewage for the length of time required would

not be feasible. The sewage would probably become septic, creating odors

and corroding the pumps. The extra cost of the reservoir might not

compensate for the saving in the capacity and operation of the pumps.

The capacity of the receiving well should be so designed that the pump

when operating will be working at its maximum capacity, and the period

of rest during conditions of average rate of flow should be in the

neighborhood of 15 to 20 minutes. For example, assume an average rate of

flow of 2 cubic feet per second, with a maximum rate of double this

amount. The pump should have a capacity of 4 cubic feet per second, and

if the receiving well is to be filled in 15 minutes by the average rate

of sewage flow its capacity should be 15 × 5 × 60 × 7.5 or 14,000

gallons. Under these circumstances the pump will operate 15 minutes and

rest 15 minutes, during average conditions of flow.

=74. Types of Pumping Machinery.=—The two principal types of pumping

machines for lifting sewage are centrifugal pumps and reciprocating

pumps. A centrifugal pump is, in general, any pump which raises a liquid

by the centrifugal force created by a wheel, called the impeller,

revolving in a tight casing, as shown in Fig. 50. A reciprocating pump

is one in which there is a periodic reversal of motion of the parts of

the pump.

Centrifugal pumps are sometimes classified as volute pumps and turbine

pumps. A volute pump is a centrifugal pump with a spiral casing into

which the water is discharged from the impeller with the same velocity

at all points around the circumference, as shown in Fig. 51. A turbine

pump is a centrifugal pump in which the water is discharged from the

impeller through guide passages into a collecting chamber, in such a

manner as to prevent loss of energy in changing from kinetic head to

pressure head. A turbine pump is shown in section in Fig. 51.

Centrifugal pumps are sometimes classified as single stage and

multi-stage. A centrifugal pump from which the water is discharged at

the pressure created by a single impeller is called a single-stage pump.

If the water in the pump is discharged from one impeller into the

suction of another impeller the pump is known as a multi-stage pump. The

number of impellers operating at different pressures determines the

number of stages of the pump. A three-stage pump is shown in Fig. 52.

[Illustration:

FIG. 50.—Section through de Laval Single-Stage, Double Suction

Centrifugal Pump.

]

375 Lubricating ring.

380 Oil hole cap.

383 Oil drain tube.

404 Shaft sleeve lock nut.

440 Driving coupling.

441 Driven coupling.

443 Coupling check nut.

450 Coupling bolt.

451 Coupling bolt nut.

452 Coupling rubber.

453 Coupling rubber steel tube.

500 Pump case.

550 Bearing bracket cap.

551 Bearing.

552 Shaft.

553 Shaft sleeve, right hand thread.

PW Impeller.

554 Shaft sleeve, left hand thread.

555 Shaft stop collar, inner.

555–1 Shaft stop collar, outer.

556 Guide ring.

560 Packing gland.

563 Bearing.

567R Impeller protecting ring, right hand thread.

567L Impeller protecting ring, left hand thread.

583 Pump case protecting ring.

567 Labyrinth packing.

583 Labyrinth packing.

600 Pump case cover.

692 Impeller key.

815 Bearing bracket, outer.

815–1 Bearing bracket, inner.

[Illustration:

FIG. 51.—Types of Centrifugal Pumps.

]

[Illustration:

FIG. 52.—Section of a Multi-Stage Centrifugal Pump.

Courtesy DeLaval Steam Turbine Co.

]

Reciprocating pumps are generally driven by steam and are either

direct-acting, or of the crank-and-fly-wheel type. Power pumps are

reciprocating machines which may be driven by any form of motor, to

which they are connected by belt, chain or shaft. A Deming triplex

power pump is shown in Fig. 53. Power pumps can be used only where the

character of the sewage will not clog the valves nor corrode the pump.

A direct-acting steam pump is one in which the steam and water

cylinders are in the same straight line and the steam is used at full

boiler pressure throughout the full length of the stroke. The

crank-and-fly-wheel type of pumping engine permits the use of steam

expansively during a part of the stroke, the energy stored in the

flywheel carrying the machine through the remainder of the stroke.

Reciprocating pumps are sometimes classified as plunger pumps and

piston pumps. In the action of a plunger pump the water is expelled

from the water cylinder, by the action of a plunger which only partly

fills the water cylinder, as shown in Figs. 54 and 55. In a piston

pump the water is expelled from the water cylinder by the action of a

piston which completely fills the water cylinder, as shown in Fig. 63,

which illustrates a direct-acting piston pump.

[Illustration:

FIG. 53.—Triplex Power Pump.

Courtesy, The Deming Co.

]

Plungers are better than pistons for pumping sewage as the wear between

the pistons and the inside face of the cylinder soon reduces the

efficiency of the pump. Outside packed plungers are better than the

inside packed type because the packing can be taken up without stopping

the pump and the leakage from the pump is visible at all times. Outside

packed pumps are more expensive in first cost, but are easier to

maintain and have a longer life than piston pumps.

[Illustration:

FIG. 54.—Water End of Inside Center-Packed Plunger Pump.

]

In selecting a pump to perform certain work the size of the water

cylinder and the speed of the travel of the piston should be

investigated to insure proper capacity. The average linear travel of the

piston for slow speed pumps is estimated at about 100 feet per minute,

dependent on the length of stroke and the valve area. For short strokes

and small valve areas the speed may be as low as 40 feet per minute, and

for long stroke fire pumps with large valves the piston can be operated

at a speed of 200 feet per minute.[45] Vertical, triple-expansion,

crank-and-fly-wheel, outside packed plunger pumps with flap valves can

be operated at speeds of 200 feet per minute when lifting sewage, and

when equipped with mechanically operated valves and lifting water they

can be run at speeds of 400 to 500 feet per minute. The speed of travel

multiplied by the volume of piston or plunger displacement, with proper

allowance for slippage, will give the capacity of the pump. The slippage

allowance may be from 3 to 8 per cent for the best pumps, and for pumps

in poor conditions it may be a high as 30 to 40 per cent.

[Illustration:

FIG. 55—Water End of Outside Center-Packed Plunger Pump.

Courtesy Allis-Chalmers Co.

]

There are two types of ejector pumps used for lifting sewage. One of

these depends on the vacuum created by the velocity of a stream of water

or steam passing through a small nozzle. The operation of this pump is

described in Art. 139 and it is illustrated in Fig. 97. The other type

of ejector pump is known as the compressed-air ejector. It is operated

by means of compressed air which is turned into a receptacle containing

sewage. The details of this type are explained in Art. 83 and are

illustrated in Fig. 68.

=75. Sizes and Description of Pumps.=—The size of a centrifugal pump is

expressed as the diameter of the discharge pipe in inches. It has

nothing to do with the head for which the pump is suited. On the

assumption of a velocity of flow of 10 feet per second through the

discharge pipe the capacity of the pump can be approximated.

The size of a reciprocating pump involves the expression of the

diameters of the steam cylinders, the water cylinder, and the length of

the stroke in inches, in the order named, beginning with the steam

cylinder with the highest pressure. A complete description of a steam

pumping engine might be; compound, duplex, horizontal, condensing,

crank-and-fly-wheel, outside-center-packed, 12″ × 24″ × 18″ × 24″ pump.

The word compound means that there are a high-pressure and a

low-pressure steam cylinder; the word duplex means that there are two of

each of these cylinders; the word horizontal means that the axes of

these cylinders are in a horizontal plane; the word condensing means

that the steam is discharged from the low-pressure cylinders into a

condenser; the name crank-and-fly-wheel is self-explanatory; the name

outside-center-packed means that the water cylinder is divided into two

portions between which the plunger is exposed to the atmosphere, and

that the packing rings are on the outside of the two portions of the

cylinder as shown in Fig. 55; the figures shown mean that the

high-pressure steam cylinder is 12 inches in diameter, the low-pressure

24 inches in diameter, the water cylinder is 18 inches in diameter, and

the stroke of the pump is 24 inches.

=76. Definitions of Duty and Efficiency.=—The duty of a pump is the

number of foot-pounds of work done by the pump per million B.T.U., per

thousand pounds of steam, or per hundred pounds of coal, consumed in

performing the work. These units are only approximately equal as 100

pounds of coal or 1,000 pounds of steam do not always contain the same

number of B.T.U. and may only approximately equal 1,000,000 B.T.U.

Since 1,000,000 B.T.U. are equal to 778,000,000 foot-pounds of work, a

pump with a duty of 778,000,000 will have an efficiency of 100 per cent.

The efficiency of a pump is therefore its duty based on B.T.U. divided

by 778,000,000. The efficiencies or duties of various types of pumps are

given in Table 26.[46]

TABLE 26

APPROXIMATE DUTIES OF STEAM PUMPS

Small duplex, non-condensing 10,000,000

Large duplex, non-condensing 25,000,000

Small simple, flywheel, condensing 50,000,000

Large simple, flywheel, condensing 65,000,000

Small compound, flywheel, condensing 65,000,000

Large compound, flywheel, condensing 120,000,000

Small triple, flywheel, condensing 150,000,000

Large triple, flywheel, condensing 165,000,000

=77. Details of Centrifugal Pumps.=—A section of a centrifugal pump with

the names of the parts marked thereon is shown in Fig. 50. Among the

important parts which require the attention of the purchaser are: the

impeller (_PW_), the impeller packing rings (567 _R_ & _L_), the

bearings (551, 563), the thrust bearings (555–1), the shaft (552), and

the shaft coupling (440).

The impeller should be of bronze, gun metal, or other alloy, because

there is no rusting or roughening of the surface, and the efficiency

does not fall with age. Normal fresh sewage is not corrosive, but septic

sewage and sludge are usually so corrosive that iron parts cannot be

used with success in contact with them. The impeller should be machined

and polished to reduce the friction with the liquid. Impellers are made

either closed or open, i.e., either with or without plates on the sides

connecting the blades to avoid the friction of the liquid against the

side of the casing. The closed type of impeller is shown in Fig. 50.

Closed impellers are slightly more expensive, but generally give better

service and higher efficiencies than the open type. Single impeller

pumps should have an inlet on each side of the impeller to aid in

balancing the machine, unless the plane of the impeller is to be

horizontal when operating. Multi-impeller pumps usually have single

inlet openings for each impeller. Vibration in the pump is sometimes

caused by an unbalanced impeller. The moving parts may be balanced at

one speed and unbalanced at another. To determine if the moving parts

are balanced the pump should be run free at different speeds and the

amount of vibration observed. If the impeller is removed from the pump

its balance when at rest can be studied by resting it on horizontal

knife edges. If there is a tendency to rotate in any direction from any

position the impeller is not perfectly balanced.

Packing rings are used to prevent the escape of water from the discharge

chamber back into the suction chamber. These rings should be made of the

same material as the impeller. Labyrinth type rings, as shown in Fig.

50, are sometimes used as the long tortuous passages are efficient in

preventing leakage.

The bearings must be carefully made because of the high speed of the

pump. They are usually made of cast iron with babbitt lining. They

should be placed on the ends of the shaft on the outside of the pump

casing, as shown in Fig. 50, and should be split horizontally so as to

be easily renewed. Exterior bearings are oil lubricated by means of ring

or chain oilers with deep oil wells. Where interior bearings are

necessary, because of the length of the shaft, they should be made of

hard brass and should be water lubricated.

[Illustration:

FIG. 56.—Marine Type Thrust Bearing.

Courtesy, DeLaval Steam Turbine Co.

]

Thrust bearings or thrust balancing devices are used to take up the end

thrust which occurs in even the best designed pumps. To overcome this

pumps are designed with double suction, opposed impellers, or two pumps

with their impellers opposed may be placed on the same shaft. Due to

inequalities in wear, workmanship or other conditions, end thrust will

occur and must be cared for. Various types of thrust bearings are in

successful use, such as: the piston, ball, roller or marine types. The

marine type thrust bearing is shown in Fig. 56. The piston type of

hydraulic balancing device is shown in Fig. 57. In the figure _A_

represents the impeller, and _B_ a piston fixed to the shaft and

revolving with it. There is a passage for water through the openings

(1), (2), and (3) leading from the impeller chamber to the atmosphere or

to the suction of the pump. If the impeller tends to move to the right

opening (1) is closed resulting in pressure on the right of the impeller

forcing it to the left. If the impeller moves to the left (1) is opened

thus transmitting pressure to the piston _B_ forcing the impeller to the

right. The flange _C_ is not essential, but is advantageous in pumps

handling gritty matter. As the channel (2) wears larger the pressure

against the piston decreases allowing it to move to the left. This

partially closes (3) building up the pressure again.

[Illustration:

FIG. 57.—Piston Type of Thrust Balancing Device.

]

Flexible shaft couplings should be used if the shaft of the driving

motor and the pump are in the same line, as direct alignment is

difficult to obtain or to maintain. Where connected to steam turbines,

reduction gearing and rigid couplings are usually used on sewage pumps

to obtain slow speed and permit large passages. Flexible couplings are

of various types, one of which is shown in Fig. 50. A rigid coupling

would be formed by bolting the flanges firmly together. Shaft couplings

are usually not necessary where the pump is driven by belt connection to

the engine or motor, or where the pump and pulley rest on only two

bearings.

The stuffing box shown in Fig. 50 is packed loosely with two layers of

hemp between which is a lantern gland, in order to permit a small amount

of leakage. A drip box is placed below this gland to catch the leakage

and return it to the pump. The leakage is permitted as it aids in

lubrication and the tightening of the gland will cause binding of the

shaft. The gland on the suction side of the pump should be connected by

a small pipe to the discharge chamber in order to keep a constant supply

of water for lubrication and to prevent the entrance of air to the

suction end of the pump.

=78. Centrifugal Pump Characteristics.=—The capacity of a centrifugal

pump is fixed by the size and type of its impeller and by the speed of

revolution. Roughly, the capacity of a pump, for maximum efficiency,

varies directly as the speed of revolution, the discharge pressure

varies as the square of the speed, and the power varies as the cube of

the speed. These relations are found not to hold exactly in tests

because of internal hydraulic friction in the pump.

The characteristic curves for a centrifugal pump, or the so-called pump

characteristics, are represented graphically by the relation between

quantity and efficiency, quantity and power necessary to drive, and

quantity and head, all at the same speed. The quantities are plotted as

abscissas in every case. The curve whose ordinates are head and whose

abscissas are quantities is known as “the characteristic.” The curve

showing the relation between quantities and speeds is sometimes included

among the characteristics. Characteristics of pumps with different

styles of impellers are shown in Fig. 58. Fig. 59 shows the

characteristics of a pump run at different speeds, the efficiencies at

these speeds when pumping at different rates, and the maximum efficiency

at different speeds. It is to be noted that the information given in

this figure is more extensive than that in Fig. 58. The operating

conditions under any head, rate of discharge, and speed are given. The

curves of constant speed are parallel, and their distances apart vary as

the square of the speed. The line of maximum efficiency is approximately

a parabola.

[Illustration:

FIG. 58.—Characteristics of Centrifugal Pumps with Different Styles of

Impellers at Constant Speed.

]

A study of the characteristics of any particular pump should be made

with a view to its selection for the load and conditions under which it

is to be used. Among the important things to be considered in the

selection of a centrifugal pump for the expected conditions of load are:

the capacity required, the maximum and minimum total head to be pumped

against, the maximum variations in suction and discharge heads, and the

nature of the drive. For example, the pump, whose characteristics are

shown in Fig. 59, should be operated at about 800 revolutions per

minute. Under total heads between 40 and 50 feet, the discharge for the

best efficiency will vary between 600 and 670 gallons per minute.

[Illustration:

FIG. 59.—Efficiency and Characteristic Curves of a Centrifugal Pump at

Different Speeds.

]

[Illustration:

FIG. 60.—Efficiencies of Centrifugal Pumps.

]

The efficiencies of centrifugal pumps increase with their capacities as

is shown approximately in Fig. 60.

=79. Setting of Centrifugal Pumps.=—In setting a centrifugal pump, care

should be taken to provide a firm foundation to hold the shafts of the

pump and the electric motor or the reduction gearing in good alignment,

or to prevent the pump from being displaced by the pull of a belt. It is

desirable that the foundation be level. Centrifugal pumps should be set

submerged for small pumping stations automatically controlled. Sludge

pumps must be set submerged as otherwise they will not prime

successfully. Provision should be made by which the pump can be lifted

from the sewage, or sludge, for inspection and repair. In many cases the

pump can be made self-priming by setting it in a dry, water-tight vault

below the low level of sewage flow. Where possible it is desirable not

to set the pump submerged as it will receive better care when easily

accessible.

[Illustration:

FIG. 61.—Centrifugal Pump in Manhole at Duluth, Minn.

Eng. Contracting, Vol. 43, 1915, p. 310.

]

The suction pipe should be free from vertical bends where air might

collect and should be straight for at least 18 to 24 inches from the

pump casing. An elbow on the suction pipe, attached directly to the

casing of the pump gives a lower efficiency than a suction pipe with a

short straight run. Centrifugal pumps will operate with as high a

suction lift as reciprocating pumps, but at the start they must be

primed and some provision must be made for priming them. The suction

pipe should be equipped with foot valves to hold the priming, or some

method may be provided for exhausting the air from the suction pipe. The

foot valves should be so installed as to form no appreciable obstruction

to the flow of water. They should have an area of opening at least 50

per cent greater than the cross-section of the suction pipe. A strainer

on the suction pipe is undesirable as it becomes clogged and is usually

in an inaccessible position for cleaning. A screen should be placed at

the entrance to the suction well to prevent the entrance of objects that

are likely to clog the pump. A gate-valve and a check-valve should be

provided on the discharge pipe, the former to assist in controlling the

rate of discharge and the latter to prevent back flow into the pump when

it is not operating.

Centrifugal pumps are well adapted to service in either large or small

units. An installation in a manhole at Park Point, Duluth, is shown in

Fig. 61. This station is controlled by an automatic electric device

which is operated by a float in the suction pit. Such automatic control

is an added advantage of the use of electrically driven centrifugal

pumps. The Calumet Pumping Station in Chicago, shown in Fig. 49, has a

capacity of approximately 1,000 cubic feet per second. The simplicity of

the layout of this station is shown in Fig. 62.

[Illustration:

FIG. 62.—Interior Arrangement of the Calumet Sewage Pumping Station,

Chicago.

Eng. News-Record, Vol. 85, 1920, p. 872.

]

=80. Steam Pumps and Pumping Engines.=—The direct-acting steam pump, one

type of which is shown in Fig. 63, is adapted to pumping sewage the

character of which will not corrode or clog the valves. In this form of

pump it is necessary to utilize the steam at full pressure throughout

the entire length of the stroke, which results in high steam

consumption. A flywheel permits the use of steam expansively during a

part of the stroke, thus increasing the economy of operation. Other

devices used for the same purpose are known as compensators. They are

not in general use.

Steam engines are classified in many different ways, for example;

according to the type of valve gear, as, plain slide valve, Corliss,

Lentz, etc.; or according to the number of steam expansions, as, simple,

compound, triple-expansion, etc.; or according to the efficiency of the

machine as low duty or high duty; or as

[Illustration:

FIG. 63.—Section of Duplex Piston Steam Pump.

Courtesy, The John H. McGowan Co.

]

STEAM END

2 Steam cylinder and housing combined.

8 Steam piston head.

9 Steam piston follower.

10 Steam piston inside ring.

11 Steam piston outside ring (2).

12 Steam cylinder head.

14 Steam chest.

16 Steam chest cover.

17 Steam slide valve.

18 Steam valve rod.

20 Steam valve rod, pin and nut.

22 Steam valve rod, collar and set screw.

23 Steam valve rod, stuffing box.

24 Steam valve rod, stuffing box, nut and gland.

38 Piston rod.

47 Piston rod stuffing box.

48 Piston rod, stuffing box, nut and gland.

49 Valve gear stand.

51 Long valve crank and shaft.

52 Short valve crank and shaft.

PUMP END

115 Pump body.

127 Brass liner.

129 Water piston head.

130 Water piston follower.

137 Cylinder head.

139 Valve plate.

140 Cap.

152 Suction flange.

161 Discharge flange.

162 Valve seat, suction or discharge.

163 Valve, suction or discharge.

164 Suction valve spring.

167 Discharge valve spring.

168 Valve plate, suction or discharge.

169 Valve stem, suction or discharge.

STEAM END

55 Crank pin.

56 Valve rod link.

61 Long rocker arm.

62 Short rocker arm.

63 Rocker arm wiper.

69 Cross head.

condensing or non-condensing, etc. Throttling engines or automatic

engines refer to the method of control of the steam by the governor. In

throttling engines the governor controls the amount of opening of the

throttle valve, in automatic engines the governor controls the position

of the cut-off.

The simple slide valve, low-duty, non-condensing, throttling engine, is

the lowest in first cost and the most expensive in the consumption of

fuel. The triple-expansion Corliss, or the non-releasing Corliss,

high-duty pumping engine is the most expensive in first cost but

consumes less steam for the power delivered than any other form of

reciprocating engine. For pumps of very small capacity the cost of fuel

is not so important an item as the first cost of the machine. For this

reason and because of the lower cost of attendance low-duty pumps are

more frequently found in small pumping stations.

[Illustration:

FIG. 64.—Diagram Showing Rates of Steam Consumption for Different Size

Units under Different Loads.

]

TABLE 27

WATER RATES OF PRIME MOVERS AT FULL AND PART LOADS

───────────────────────────────┬──────┬─────────────────────────┬──────

Type of Engine │ │ │Boiler

│Power,│ │Press.

│ K.W. │ Per Cent of Full Load │ Lbs.

───────────────────────────────┼──────┼────┬────┬────┬─────┬────┼──────

│ │ 25 │ 50 │ 75 │ 100 │125 │

───────────────────────────────┼──────┼────┼────┼────┼─────┼────┼──────

Single cylinder, high speed, │ │ │ │ │ │ │100 to

non-condensing │ 25│ 33│ 27│26.3│ 27.0│27.5│ 150

│ 250│ 42│37.5│ 35│ 34.0│34.0│

│ │ │ │ │ │ │

Automatic, flat four valve, │ │ │ │ │ │ │100 to

high speed │ 150│ │ 32│ 30│ 26.5│29.0│ 125

│ 250│ │ 33│ 31│ 28│30.0│

│ │ │ │ │ │ │

Tandem compound condensing, │ │ │ │ │ │ │100 to

high speed │ 125│ │ 23│ 19│ 17│ 18│ 150

│ │ │ 25│ 20│ 19.5│ 21│

│ │ │ │ │ │ │

Cross compound, condensing, │ │ │ │ │ │ │ 125

high speed │ │ 30│ 26│ 24│ 23│23.5│

│ │ │ │ │ │ │

Cross compound, non-condensing,│ │ │ │ │ │ │ 125

high speed │ │ 39│ 31│ 27│ 26│27.5│

│ │ │ │ │ │ │

Single cylinder Corliss, │ │ │ │ │ │ │ 100

condensing │ 120│23.7│20.4│ 19│ 18.5│19.0│

│ 500│26.3│22.8│21.3│ 20.8│21.3│ 125

│ │ │ │ │ │ │

Compound Corliss, condensing │ │16.5│ 14│12.5│ 12.1│12.5│ 100

│ │22.2│ 19│17.0│ 16.5│17.0│ 150

│ │ │ │ │ │ │

Single cylinder, rotary four │ │ │ │ │ │ │ 100

valve, non-condensing │ 75│26.2│22.3│21.3│ 21.6│22.8│

│ 400│35.0│27.2│26.4│ 26.0│26.8│ 180

│ │ │ │ │ │ │

Rotary four valve, tandem │ │ │ │ │ │ │ 100

compound non-condensing │ 125│32.0│22.0│ 20│18.25│18.5│

│ 600│40.0│28.3│23.2│ 22.5│22.7│ 150

│ │ │ │ │ │ │

Cross compound, non-condensing │ │ │ │ │ │ │ 100

rotary four valve │ 125│ 25│ 21│19.1│ 18.5│19.0│

│ 600│39.4│ 28│22.3│ 20.6│20.7│ 150

│ │ │ │ │ │ │

Single cylinder, poppett valve,│ │ │ │ │ │ │ 100

non-condensing │ 120│22.7│20.5│19.7│ 19.1│20.1│

│ 600│28.5│26.0│25.0│ 24.3│25.5│ 150

│ │ │ │ │ │ │

Single cylinder, poppett valve,│ │ │ │ │ │ │ 100

condensing │ 120│18.5│16.7│16.1│ 15.6│16.4│

│ 600│24.6│22.3│21.4│ 20.8│21.9│ 150

│ │ │ │ │ │ │

Compound condensing, poppett │ │ │ │ │ │ │ 100

valve │ 200│14.2│13.0│12.5│ 12.2│12.9│

│ 1200│18.4│16.9│16.3│ 15.9│16.8│ 150

│ │ │ │ │ │ │

Uniflow │ 125│14.6│13.7│13.4│ 13.4│13.3│ 150

│ 600│15.0│14.3│13.7│ 13.5│14.0│

│ │ │ │ │ │ │

Steam turbines, condensing, │ │ │ │ │ │ │ 125

Allis-Chalmers │ 300│ │ 24│ 17│ 160│16.5│

│ 2000│ │31.9│26.3│ 23.8│23.0│ 175

│ │ │ │ │ │ │

Steam turbines, condensing, │ │ │ │ │ │ │ 125

Westinghouse │ 300│ │13.7│12.8│ 12.2│12.6│

│ 2000│ │18.2│16.9│ 16.2│16.8│ 175

│ │ │ │ │ │ │

Steam turbines, high pressure, │ │ │ │ │ │ │

non-con., 12″ to 36″ wheel, │4 to 8│ │ │ │ │ │

1000 to 3600 R.P.M. │stages│ │ │ │ 28 5│ │

│ │ │ │ │116.5│ │

│ │ │ │ │ │ │

Ditto. Condensing, 26–inch │ │ │ │ │ 17 3│ │

│ │ │ │ │112.0│ │

│ │ │ │ │ │ │

───────────────────────────────┴──────┴────┴────┴────┴─────┴────┴──────

The steam consumption per indicated horse-power, better known as the

water rate of the engine, for various types of engines at full and at

part load is shown in Fig. 64. The steam consumption of other types at

full load is shown in Table 27. The indicated horse-power (I.H.P.) of a

steam engine is the product of the mean effective pressure (M.E.P.), the

area of the steam pistons, the length of the stroke, and the number of

strokes per unit of time. A common form of this expression is,

I.H.P = _PLAN_⁄33,000,

in which _P_ = the M.E.P. in pounds per square inch;

_L_ = the length of the stroke in inches;

_A_ = the sum of the areas of the pistons in square inches;

_N_ = the number of revolutions per minute.

The I.H.P. multiplied by the mechanical efficiency of the machine will

give the brake or water horse-power, that is, the horse-power delivered

by the machine. The product of the M.E.P., the sum of the areas of the

steam pistons and the mechanical efficiency of the machine, should equal

the product of the total head of water pumped against expressed in

pounds per square inch and the sum of the areas of the water pistons or

plungers. The M.E.P. is determined from indicator cards taken from the

steam cylinders during operation. These cards show the steam pressure on

the head and crank ends of each cylinder at all points during the

stroke.

=81. Steam Turbines.=—Among the advantages in the use of steam turbines

as compared with reciprocating steam engines for driving centrifugal

pumps are their simplicity of operation, the small floor space needed,

their freedom from vibration requiring a relatively light foundation,

and their ability to operate successfully and economically either

condensing or non-condensing under varying steam pressure. They can be

operated with steam at atmospheric or low pressure, thus taking the

exhaust from other engines. The greatest economy of operation for the

turbine alone will be obtained by operating with high pressure,

superheated steam and with a vacuum of 28 inches. In large units the

economy of operation of steam turbines is equal to that of the best type

of reciprocating engines. In order to develop the highest economy

turbines are operated at speeds from about 3,600 to 10,000 r.p.m. or

greater, the smaller turbines operating at the higher speeds. As these

speeds are usually too great for the operation of centrifugal pumps for

lifting sewage, reduction gears must be introduced between the turbine

and the pump. Although the best form of spiral-cut reduction gears may

obtain efficiencies of 95 to 98 per cent, or even higher, their use,

particularly in small units, is an undesirable feature of the steam

turbine for driving pumps.

The steam consumption of DeLaval turbines of different powers, and the

steam consumption of a 450 horse-power DeLaval turbine at different

loads are shown in Fig. 64. Some steam consumptions of other turbines

are recorded in Table 27. It is to be noted that the steam consumption

of the 450 horse-power turbine at part loads is not markedly greater

than that at full loads. This is an advantage of steam turbines as

compared with reciprocating engines. The steam consumption of any

turbine is dependent on the conditions of operation and is lower the

higher the vacuum into which the exhaust takes place.

[Illustration:

FIG. 65.—The DeLaval Trade Mark, Illustrating the Principle of the

DeLaval Steam Turbine.

Courtesy, DeLaval Steam Turbine Co.

]

There are two types of turbines in general use, the single stage or

impulse machines, and the compound or reaction type. The DeLaval is a

well-known make of the single stage or impulse type. The principle of

its operation is indicated in Fig. 65, which is the trade mark of the

DeLaval Steam Turbine Co. The energy of the steam is transmitted to the

wheel due to the high velocity of the steam impinging against the vanes.

In the compound or reaction type of machine the steam expands from one

stage to the next imparting its energy to the wheel by virtue of its

expansion in the passages of the turbine. For this reason the

single-stage or impulse type is operated at higher speeds than the

compound or reaction machines.

=82. Steam Boilers.=—Among the important points to be considered in the

selection of a steam boiler for a sewage pumping station are: the

necessary power; the quality of the feed water; the available floor

space; the steam pressure to be carried; and the quality and character

of the fuel. Tubular boilers of the type shown in Fig. 66, are lower in

first cost than other types of boilers. They are not ordinarily built in

units larger than 250 to 300 horse-power and where more power is desired

a number of units must be used. They are objectionable because of the

relatively large floor space required, and because of their relatively

poor economy of operation. The efficiencies of water-tube boilers of

different types are given in Table 28. Large power units of the

water-tube type, as shown in Fig. 67, although more expensive in first

cost, require less floor space. Almost any desired steam pressure can be

obtained from either type but water-tube boilers are more commonly used

for high pressures. The grate or stoker can be arranged to burn almost

any kind of fuel under either water-tube or fire-tube boilers. The use

of poor quality of water in water-tube boilers is undesirable as the

tubes are more likely to become clogged than the larger passages of the

fire-tube boilers. If necessary, a feed-water purification plant should

be installed, as it is usually cheaper to take the impurities out of the

water than to take the scale out of the boiler.

[Illustration:

FIG. 66.—Horizontal Fire-tube Boiler.

]

[Illustration:

FIG. 67.—Babcock and Wilcox Water-tube Boiler.

]

Not less than two boiler units should be used in any power station,

regardless of the demands for power, and if the feed water is bad, three

or even four units should be provided, as two units may be down at any

time. An appreciable factor of safety is provided by the ability of a

boiler to be operated at 30 to 50 per cent overload, if sufficient draft

is available, but with resulting reduction in the economy of operation.

The number of units provided should be such that the maximum load on the

pumping station can be carried with at least one in every 6 units or

less, out of service for repairs or other cause.

TABLE 28

EFFICIENCIES OF STEAM BOILERS

From Marks’ Mechanical Engineer’s Handbook

────────┬───────────┬──────────┬──────┬────────┬──────┬──────┬──────────

Type │Horse-power│ Furnace │ │ │ │Evap. │

│ │ │ │ │ │ from │

│ │ │ │ │ │and at│

│ │ │ │ │B.T.U.│ 212° │ Combined

│ │ │ Sq. │Per Cent│ per │ per │Efficiency

│ │ │ Ft. │of Rated│ Lb. │ Lb. │of Boiler

│ │ │Grate │Capacity│ Dry │ Dry │ and

│ │ │ Area │D’v’l’d │ Coal │ Coal │ Furnace

────────┼───────────┼──────────┼──────┼────────┼──────┼──────┼──────────

Babcock │ 300│Hand-fired│ │ │ │ │

& │ │ │ │ │ │ │

Wilcox│ │ │ 84│ 118.7│11,912│ 8.81│ 71.8

Babcock │ 640│Hand-fired│ │ │ │ │

& │ │ │ │ │ │ │

Wilcox│ │ │ 118│ 121.5│14,602│ 10.83│ 72.0

Stirling│ 1128│B. & W. │ │ │ │ │

│ │ chain │ │ │ │ │

│ │ grate │ 187│ 198.3│12,130│ 9.51│ 76.1

Rust │ 335│Hand-fired│ 68│ 210.5│13,202│ 9.42│ 68.9

Heine │ 400│Green │ │ │ │ │

│ │ chain │ │ │ │ │

│ │ grate │ 83.5│ 123.8│11,608│ 8.79│ 73.5

Maximum efficiency recorded │ │ │ │ │ 83

───────────────────────────────┴──────┴────────┴──────┴──────┴──────────

The steam delivered by a boiler is the basis of the measurement of its

capacity or power. A boiler horse-power is the delivery of 33,320 B.T.U.

per hour. It is approximately equal to the raising of 30 pounds of water

per hour from a temperature of 100° Fahrenheit, to steam at a pressure

of 70 pounds per square inch, or to 34 pounds of water per hour changed

to steam from and at 212° Fahrenheit, at atmospheric pressure. The

horse-power of a boiler is sometimes approximated by the area of its

grate or heating surface. Such a method of measuring has a low degree of

accuracy on account of the variations in the quality of the fuel, and

the rate of combustion. For example, the rate of combustion under a

locomotive boiler is high and there is less than ⅒th of a square foot of

grate area and about 4.5 square feet of heating surface per boiler

horse-power. The Scotch Marine type of boiler used on steam ships, has

slightly more grate area and slightly less heating surface than the

locomotive type of boiler, because the rate of combustion is lower.

Stationary water-tube boilers may have 2 to 3 times as much grate area

and heating surface per horse-power as is found in locomotive boilers.

If a poor type of fuel is to be used the area of the grate should be

increased about inversely as the heat content of the fuel. The

approximate heat content of various types of fuels is shown in Table 29.

TABLE 29

APPROXIMATE HEAT VALUE OF FUELS

─────────────────────────────────────┬────────────────┬────────────────

Fuel │ │Pounds of Water

│ │Evaporated from

│ │ and at 212° F.

│ │ All heat

│B.T.U. per Pound│ utilized

─────────────────────────────────────┼────────────────┼────────────────

Anthracite │ 13,500│ 14.0

Semi-bituminous, Pennsylvania │ 15,000│ 15.5

Semi-bituminous, best, West Virginia │ 15,000│ 15.8

Bituminous, best, Pennsylvania │ 14,450│ 15.0

Bituminous, poor, Illinois │ 10,500│ 10.9

Lignite, best, Utah │ 11,000│ 11.4

Lignite, poor, Oregon │ 8,500│ 8.8

Wood, best oak │ 9,300│ 9.6

Wood, poor ash │ 8,500│ 8.8

─────────────────────────────────────┴────────────────┴────────────────

=83. Air Ejectors.=—The Ansonia compressed-air sewage ejector is shown

in Fig. 68. In its operation, sewage enters the reservoir through the

inlet pipe at the right, the air displaced being expelled slowly through

the air valve marked B. The rising sewage lifts the float which actuates

the balanced piston valve in the pipe above the reservoir when the

reservoir fills. The lifting of the valve admits compressed air to the

reservoir. The air pressure closes valve A and the inlet valve at the

right, and ejects the sewage through the discharge pipe at the left. As

the float drops with the descending sewage it shuts off the air supply

and opens the air exhaust through the small pipe at the top center.

Sewage is prevented from flowing back into the reservoir by the check

valve in the discharge pipe. Other ejectors operating on a similar

principle are the Ellis, the Pacific, the Priestmann and the Shone.

=84. Electric Motors.=—The most common form of alternating current

electric motor used for driving sewage pumps where continuous operation

and steady loads are met is the squirrel-cage polyphase induction motor.

These motors operate at a nearly constant speed which should be selected

to develop the maximum efficiency of the pump and motor set. While Fig.

59 shows the best efficiency under varying heads to be obtained with

variable speed, the advantages of cost, attention, and availability make

the use of a constant speed motor common.[47] This type of motor is

undesirable where stopping and starting are frequent because it has a

relatively small starting torque and it requires a large starting

current. Such motors can be constructed in small sizes for high starting

torques by increasing the resistance of the rotor, but at the expense of

the efficiency of operation.

[Illustration:

FIG. 68.—Ansonia Compressed-Air Sewage Ejector.

]

Alternating current motors are more generally used than direct-current

motors because of the greater economy of transmission of alternating

current, but where direct current is available constant speed shunt

wound motors should be adopted.

In the selection of a motor to drive a centrifugal pump it is important

that the motor have not only the requisite power, but that its speed

will develop the maximum efficiency from the pump and motor combined. If

the pump and motor operate on the same shaft the speed of the two

machines must be the same. If the two are belt connected, the size of

the pulleys may be selected so as to give the required speed. If the

motor is to be connected to a power pump an adequate automatic pressure

relief valve should be provided on the discharge pipe from the pump, to

prevent the overloading of the motor or bursting of the pump in case of

a sudden stoppage in the pipe. The motor must be selected to suit the

conditions of voltage, cycle, and phase on the line. Transformers are

available to step the voltage up or down to practically any value.

Rotary converters are used to change direct to alternating current or

vice versa.

=85. Internal Combustion Engines.=—Internal combustion engines are used

for driving pumps. Units are available in size from fractions of 1

horse-power to 2,000 horse-power or more, although the use of the larger

sizes is exceptional. These engines are not commonly used for sewage

pumping but when used they are ordinarily belt connected to a

centrifugal pump, or to an electric generator which in turn drives

electric motors which operate centrifugal pumps. This type of engine is

more commonly adapted to small loads, although not entirely confined to

this field, as they serve admirably as emergency units to supplement an

electrically equipped pumping station. The fuel efficiency of internal

combustion engines is higher than for steam engines as is indicated in

Table 30, but the fuel is more expensive.

The four-cycle gas engine shown in Fig. 69 is the type most commonly

used. Its horse-power is the product of: the mean effective pressure,

the length of the stroke, the area of the piston, and the number of

explosions per second divided by 550. The M.E.P. is dependent on the

character of the fuel used and the compression of the gas before

ignition. Producer gas will furnish mean effective pressures between 60

and 70 pounds per square inch, natural gas and gasoline, 85 to 90 pounds

per square inch, and alcohol from 95 to 110 pounds per square inch.

TABLE 30

COMPARATIVE FUEL COSTS FOR PRIME MOVERS

───────────────────────────────────────┬───────────────┬───────────────

Type of Engine │ Quantity of │Cost of Fuel in

│ Fuel per H.P. │ Cents per

│ Hour │ Horse-power

│ │ Hour

───────────────────────────────────────┼───────────────┼───────────────

Reciprocating steam engines, simple, │ 21 to 8 lb. │ 4.2 to 1.6

non-condensing, 25 to 200 H.P. │ coal │

Triple condensing, 2000 to 10,000 │2.3 to 1.9 lb. │ 0.46 to 0.37

H.P. │ coal │

───────────────────────────────────────┼───────────────┼───────────────

Steam turbines, high pressure, │ │

non-condensing, │ │

200 to 500 K.W. │6.5 to 4.2 lb. │ 1.3 to 0.86

│ coal │

500 to 3000 K.W. │2.6 to 1.9 lb. │ 0.52 to 0.37

│ coal │

Condensing 5000 to 20,000 K.W. │1.8 to 1.43 lb.│ 0.36 to 0.28

│ coal │

───────────────────────────────────────┼───────────────┼───────────────

Gas engines │ │

Natural gas, 50 to 200 H.P. │ 19 to 11 cu. │

│ ft. │

Producer gas, 50 to 200 H.P. │ 2 to 1.5 cu. │

│ ft. │

Illuminating gas, 10 to 75 H.P. │ 26 to 19 cu. │ 2.1 to 1.5

│ ft. │

Gasoline, 10 to 75 H.P. │ 1.5 to 0.8 │ 5.6 to 3.0

│ pints │

───────────────────────────────────────┼───────────────┼───────────────

Oil engines, 100 to 500 H.P. │1.1 to 0.75 lb.│

│ oil │

───────────────────────────────────────┴───────────────┴───────────────

NOTE.—Coal assumed at $4.00 per ton, illuminating gas at 80 cents per

thousand cubic feet, and gasoline at 30 cents per gallon.

[Illustration:

FIG. 69.—Bessemer Oil Engine. Twin Cylinder, Valve Side.

]

The Diesel Engine is the most efficient of internal combustion engines.

The original aim of the inventor, Dr. Rudolph Diesel, was to avoid the

explosive effect of the ordinary internal combustion engine by injecting

a fuel into air so highly compressed that its heat would ignite the

fuel, causing slow combustion of the fuel thus utilizing its energy to a

greater extent. The fuel and air were to be so proportioned as to

require no cooling. Although the ideal condition has not been attained,

the heat efficiency of Diesel engines is high. They will consume from

0.3 to 0.5 of a pound of oil (containing 18,000 B.T.U. per pound) per

brake horse-power hour, giving an effective heat efficiency of 25 to 30

per cent. Although not now in extensive use in the United States it is

probable that this engine will be more generally adopted for conditions

suitable for internal combustion engines.

=86. Selection of Pumping Machinery.=—Centrifugal pumps are particularly

adapted to the lifting of sewage because of their large passages, and

their lack of valves. The low lifts, nearly constant head, and the

possibility of equalizing the load by means of reservoirs are

particularly suited to efficient operation of centrifugal pumps. They

require less floor space than reciprocating pumps of the same capacity,

and because of their freedom from vibration they do not demand so heavy

a foundation. The discharge from the pump is continuous thus relieving

the piping from vibration. In case of emergency the discharge valve can

be shut off without shutting down the pump, an important point in “fool

proof” operation.

Volute pumps are better adapted to pumping sewage as their passages are

more free and they are better suited to the low lifts met. Gritty and

solid matter will cause wear on the diffusion vanes of turbine pumps in

spite of the most careful design. Although turbine pumps can possibly be

built with higher efficiency than volute pumps, their efficiency at part

load falls rapidly and the fluctuations of sewage flow are sufficient to

affect the economy of operation. Turbine pumps are more expensive and

heavier than volute pumps on account of the increased size necessitated

by the diffusion vanes.

Multi-stage pumps are used for high lifts and are seldom if ever

required in sewage pumping. As ordinarily manufactured, each stage is

good for an additional 40 to 100 pounds pressure, but wide variations in

the limiting pressures between stages are to be found.

Reciprocating plunger pumps are sometimes used for sewage pumping where

the character of the sewage is such that the valves will not be clogged

nor parts of the pump corroded. These pumps are seldom used in small

installations or for low lifts. They are not adapted to automatic or

long distance control as are electrically driven centrifugal pumps. The

use of reciprocating pumps for sewage pumping is practically restricted

to very large pumping stations with capacities in the neighborhood of

50,000,000 gallons per day or more. Steam-driven pumps are the most

common of the reciprocating type, but power pumps are sometimes used in

special cases for small installations and may be driven by either a

steam or gas engine or an electric motor.

Compressed air ejectors, as described in Art. 83 are used for lifting

sewage and other drainage from the basement of buildings below the sewer

level.

Centrifugal pumps electrically driven are, as a rule, the most

satisfactory for sewage pumping. Electric drive lends itself to control

by automatic devices, which are particularly convenient in small pumping

stations. The control can be arranged so that the pump is operated only

at full load and high efficiency, and when not operating no power is

being consumed, as is not the case with a steam pump where steam

pressure must be maintained at all times. The electric driven pump is

thrown into operation by a float controlled switch which is closed when

the reservoir fills, and opens when the pump has emptied the reservoir.

The choice between steam and electric power for large pumping stations

is a matter of relative reliability and economy.

The selection of the proper type of pump, whether reciprocating or

otherwise, requires some experience in the consideration of the factors

involved. Fig. 70 is of some assistance. In discussing this figure,

Chester states:

“Fig. 70 attempts to represent graphically, the writer’s ideas

under general conditions, of the machines that should be selected

for certain capacities for both principal engine and alternate and

the station duty they may be expected to produce, but you must

realize that this intends the principal engine doing at least 90

per cent of the work and that the head, the cost of coal, the load

factor, the cost of real estate ... the boiler pressure, and the

space available, and finally ... the funds available, are factors

which may shift both the horizontal and curved lines. In the field

of low service pumps of 10,000,000 capacity or over, the

centrifugal pump reigns supreme, and for constant low heads of

20,000,000 capacity or over the turbine driven centrifugal usurps

the field.”

A reciprocating pump of any type would have to be specially built for

pumping sewage not carefully screened or otherwise treated, as the

valves, ordinarily used in such pumps for lifting water, would clog. The

vertical triple-expansion pumping engine with special valves and for

large installations, and the centrifugal pump for large or small

installations are the only suitable types for pumping sewage. With steam

turbine or electric drive the centrifugal has the field to itself.

[Illustration:

FIG. 70.—Expectancy Curves for Pumping Engines Working against a

Pressure of 100 Pounds per Square Inch.

J. N. Chester, Journal Am. Water Works Ass’n, Vol. 3, 1916, p. 493.

]

=87. Costs of Pumping Machinery.=—The cost of pumping machinery can not

be stated accurately as the many factors involved vary with the

fluctuations in the prices of raw materials, transportation, labor, etc.

The actual purchase price of machinery can be found accurately only from

the seller. The costs given in this chapter are useful principally for

comparative purposes and for exercise in the making of estimates. The

costs of complete pumping stations are shown in Table 31.[48] These

figures represent costs in 1911.

TABLE 31

COSTS OF COMPLETE PUMPING STATIONS

These costs include the best type of triple-expansion engines,

high-pressure boilers, brick or inexpensive stone building with slate

roof, chimney and intake. Cost of land is not included.

─────────────────┬─────────────────┬─────────────────┬─────────────────

Discharge │ Horse-power per │ │

Pressure, Lbs. │ Million Gals. │Cost, Dollars per│Cost, Dollars per

per Sq. In. │ Pumped │ Horse-power │ Million Gallons

─────────────────┼─────────────────┼─────────────────┼─────────────────

30│ 12│ 562│ 6,750

40│ 16│ 438│ 7,000

50│ 20│ 362│ 7,250

60│ 24│ 312│ 7,500

70│ 28│ 277│ 7,750

80│ 32│ 250│ 8,000

90│ 36│ 229│ 8,250

100│ 40│ 213│ 8,500

110│ 44│ 200│ 8,750

120│ 48│ 187│ 9,000

130│ 52│ 192│ 10,000

│ │ │

─────────────────┴─────────────────┴─────────────────┴─────────────────

=88. Cost Comparisons of Different Designs.=—In the design of a pumping

station and its equipment the relative costs of different designs should

be compared, and the least expensive design selected, due consideration

being given to serviceability, reliability, and other factors without

definite financial value. In comparing the costs of different types of

machinery, all items in connection with the pumping station should be

considered. For example, the cost of an electrically driven centrifugal

pump and equipment may be less than the total cost of a steam driven

reciprocating pump and equipment because of the saving in the cost of

boilers, boiler house, etc., but a comparison of the capitalized cost of

the two might show in favor of the reciprocating steam pump because of

the lower cost of operation.

The total cost of a plant, or any portion thereof, may be considered as

made up of three parts: (1) The first cost, (2) operation and

maintenance and, (3) renewal. The total cost S can be expressed as

_S_ = _C_ + _O_⁄_r_ + _R_,

in which _C_ = the first cost;

_O_ = the annual expenditure for operation and maintenance;

_R_ = the amount set aside to cover renewal;

_r_ = the rate of interest.

_S_ is called the capitalized cost of a plant. The annual payment

necessary to perpetuate a plant is

_A_ = _Sr_ = _Cr_ + _O_ + _Rr_.

The value of _R_ is useful when expressed in terms of the life of the

plant or machine and the current rate of interest. It is sometimes

called the depreciation factor or capitalized depreciation. If it is

borne in mind that _R_ is the amount to be set aside at compound

interest for the life of the plant, at the end of which time the accrued

interest should be sufficient to renew the plant, it is evident that

_R_(1 + _R_)^n − _R_ = _C_

or _R_ = _C_⁄((1+_r_)^n − 1)

in which _n_ is the period of usefulness, or life of the plant,

expressed in years, no allowance being made for scrap value.

A comparison of the annual expense of three different plants is shown in

Table 32. It is evident from this comparison that the machinery with the

least first cost is not always the least expensive when all items are

considered.

A sinking fund is a sum of money to which additions are made annually

for the purpose of renewing a plant at the expiration of its period of

usefulness. The annual payment into the sinking fund is equivalent to

the term _Rr_ in the expression for annual cost, or in terms of _C_,

_r_, and _n_, the annual payment is

_Cr_⁄((1 + _r_)^n − 1).

It is the same as the capitalized depreciation multiplied by the rate of

interest. The expression _r_⁄((1 + _r_)^n − 1) is sometimes called the

rate of depreciation.

The present worth of a machine is the difference between its first cost

and the present value of the sinking fund. If _m_ represents the present

age of a plant in years, then the present worth is

_P_ = _C_(1 – ((1 + _r_)^n − 1)⁄((1 + _r_)^m − 1)).

TABLE 32

COMPARISON OF COSTS OF THREE DIFFERENT PUMPING STATIONS. NOMINAL

CAPACITY THIRTY MILLION GALLONS PER DAY RAISED THIRTY FEET

────────────────┬──────────────────────────────────

Equipment │ Plant A

────────────────┼──────────────────────────────────

│One Acre of Land. Brick Building,

│ Steel Trussed Roof, Slate

│ Covered. Cross Compound

│ Condensing Horizontal Pumping

│ Engine

────────────────┼───────┬──────────┬───────┬───────

│Annual │ Years of │Sinking│ Total

│Payment│Usefulness│ Fund │

│ on │ │Payment│

│ First │ │ │

│ Cost │ │ │

────────────────┼───────┼──────────┼───────┼───────

Land │ 100│ │ 0│ 100

Permanent │ 1188│ 50│ 1080│ 2,260

Structures[49]│ │ │ │

Pumps and │ 440│ 15│ 435│ 875

Machinery │ │ │ │

Boilers │ 280│ 10│ 446│ 726

Labor │ │ │ │ 14,000

Fuel │ │ │ │ 5,500

Repairs, etc. │ │ │ │ 480

────────────────┼───────┼──────────┼───────┼───────

Total │ │ │ │ 23,941

────────────────┴───────┴──────────┴───────┴───────

────────────────┬──────────────────────────────────

Equipment │ Plant B

────────────────┼──────────────────────────────────

│One Acre of Land. Brick Building.

│ Steel Trussed Roof, Slate

│ Covered. Compound Condensing Low

│ Duty Horizontal Pumping Engine

│

────────────────┼───────┬──────────┬───────┬───────

│Annual │ Years of │Sinking│ Total

│Payment│Usefulness│ Fund │

│ on │ │Payment│

│ First │ │ │

│ Cost │ │ │

────────────────┼───────┼──────────┼───────┼───────

Land │ 100│ │ 0│ 100

Permanent │ 1180│ 50│ 1080│ 2,260

Structures[49]│ │ │ │

Pumps and │ 390│ 15│ 395│ 785

Machinery │ │ │ │

Boilers │ 252│ 10│ 400│ 652

Labor │ │ │ │ 14,000

Fuel │ │ │ │ 7,200

Repairs, etc. │ │ │ │ 400

────────────────┼───────┼──────────┼───────┼───────

Total │ │ │ │ 25,497

────────────────┴───────┴──────────┴───────┴───────

────────────────┬──────────────────────────────────

Equipment │ Plant C

────────────────┼──────────────────────────────────

│One Acre of Land. Frame Building,

│ Shingle Roof. Compound Duplex

│ Non-Condensing Pumping Engine.

│

│

────────────────┼───────┬──────────┬───────┬───────

│Annual │ Years of │Sinking│ Total

│Payment│Usefulness│ Fund │

│ on │ │Payment│

│ First │ │ │

│ Cost │ │ │

────────────────┼───────┼──────────┼───────┼───────

Land │ 100│ │ 0│ 100

Permanent │ 810│ 50│ 775│ 1,585

Structures[49]│ │ │ │

Pumps and │ 360│ 15│ 352│ 712

Machinery │ │ │ │

Boilers │ 308│ 10│ 490│ 798

Labor │ │ │ │ 14,000

Fuel │ │ │ │ 8,200

Repairs, etc. │ │ │ │ 550

────────────────┼───────┼──────────┼───────┼───────

Total │ │ │ │ 25,945

────────────────┴───────┴──────────┴───────┴───────

Where straight-line depreciation is spoken of it is assumed that the

worth of a machine depreciates an equal part of its first cost each

year. For example, if the life of a plant is assumed to be 20 years,

straight-line depreciation will assume that the plant loses 1/20 of its

original value annually. The present worth of a plant under this

assumption would be the product of its first cost and the ratio between

its remaining life and its total life. This method of estimating

depreciation and worth is frequently used, particularly for short-lived

plants and for simplicity in bookkeeping, but it is less logical than

the method given above.

=89. Number and Capacity of Pumping Units.=—In order to select the

number and capacity of pumping units for the best economy, a comparison

of the costs of different combinations of units should be made and the

most economical combination determined by trial. The principles outlined

in the preceding articles should be observed in making these

comparisons. In a steam pumping station, when the number of units

operating is less than the average daily maximum for the period, steam

must nevertheless be kept on a sufficient number of boilers to operate

the maximum number of pumps. This, and corresponding standby losses must

not be overlooked, as they may show that a smaller number of larger

units is ultimately more economical.

TABLE 33

SUMMARY OF FLUCTUATIONS OF SEWAGE FLOW AT A PROPOSED PUMPING STATION

─────────────────┬─────────────────┬─────────────────┬─────────────────

Number of Days │Flow in Thousand │ │

Loads Occurred in│ Gallons per │ │

One Year │ Minute │ Lift in Feet │ Horse-power

─────────────────┼─────────────────┼─────────────────┼─────────────────

1│ 293│ 6.0│ 450

8│ 163│ 8.6│ 354

15│ 119│ 10.0│ 300

18│ 106│ 10.6│ 284

23│ 88│ 11.2│ 249

31│ 69│ 12.2│ 211

32│ 65│ 12.4│ 204

45│ 51│ 13.4│ 173

41│ 50│ 13.5│ 169

30│ 45│ 13.8│ 158

28│ 44│ 13.9│ 154

23│ 40│ 14.2│ 143

21│ 38│ 14.4│ 137

18│ 35│ 14.6│ 129

12│ 29│ 15.0│ 111

8│ 24│ 15.6│ 95

5│ 20│ 16.0│ 79

3│ 16│ 16.5│ 65

2│ 14│ 16.8│ 58

1│ 6.5│ 18.0│ 29

─────────────────┴─────────────────┴─────────────────┴─────────────────

Total horse-power days for one year, 102,000.

Average load in horse-power, 280.

TABLE 34

POSSIBLE COMBINATIONS OF FIVE PUMPING UNITS TO CARE FOR THE LOADS SHOWN

IN TABLE 33[50]

──────────────────────────────────┬───────────────

40 Horse-power │ Load

Type 1[51] │

────────┬──────┬───────────┬──────┼───────┬───────

Per Cent│Pounds│ Load in │Pounds│Number │ Total

of Rated│Steam │Horse-power│Steam,│of Days│ Load

Capacity│ per │ │Units │Load is│Carried

│ H.P. │ │10,000│Carried│ on

│ Hour │ │Pounds│in Year│ these

│ │ │ │ │Days in

│ │ │ │ │ H.P.

────────┼──────┼───────────┼──────┼───────┼───────

151│ 45│ 60.4│ 6.5│ 1│ 681

120│ 44│ 48│ 40.5│ 8│ 542

102│ 45│ 40.8│ 66.1│ 15│ 458

96│ 45│ 38.4│ 74.8│ 18│ 434

98│ 45│ 39.2│ 97.5│ 23│ 381

│ │ │ │ 31│ 322

│ │ │ │ 32│ 312

│ │ │ │ 45│ 264

│ │ │ │ 41│ 258

101│ 45│ 40.4│ 131│ 30│ 242

98│ 45│ 39.2│ 119│ 28│ 235

│ │ │ │ 23│ 218

│ │ │ │ 21│ 210

│ │ │ │ 18│ 198

│ │ │ │ 12│ 170

104│ 45│ 41.6│ 20.9│ 8│ 145

│ │ │ │ 5│ 121

│ │ │ │ 3│ 100

99│ 45│ 39.6│ 8.5│ 2│ 89

113│ 44│ 45.2│ 4.8│ 1│ 45

│ │ │ ————│ │

Sub-total │ 596.6│ │

Grand total in pounds, 65,700,000

──────────────────────────────────────────────────

──────────────────────────────────┬───────────────

50 Horse-power │ Load

Type 1[51] │

────────┬──────┬───────────┬──────┼───────┬───────

Per Cent│Pounds│ Load in │Pounds│Number │ Total

of Rated│Steam │Horse-power│Steam,│of Days│ Load

Capacity│ per │ │Units │Load is│Carried

│ H.P. │ │10,000│Carried│ on

│ Hour │ │Pounds│in Year│ these

│ │ │ │ │Days in

│ │ │ │ │ H.P.

────────┼──────┼───────────┼──────┼───────┼───────

151│ 45│ 75.5│ 8.2│ 1│ 681

120│ 44│ 60.0│ 50.7│ 8│ 542

102│ 45│ 51.0│ 82.7│ 15│ 458

90│ 45│ 48.0│ 93.5│ 18│ 434

98│ 45│ 49.0│ 122.0│ 23│ 381

104│ 45│ 52.0│ 174.5│ 31│ 322

101│ 45│ 50.5│ 174.8│ 32│ 312

│ │ │ │ 45│ 264

103│ 45│ 51.5│ 228│ 41│ 258

│ │ │ │ 30│ 242

│ │ │ │ 28│ 235

│ │ │ │ 23│ 218

│ │ │ │ 21│ 210

│ │ │ │ 18│ 198

│ │ │ │ 12│ 170

│ │ │ │ 8│ 145

109│ 44│ 54.5│ 28.8│ 5│ 121

│ │ │ │ 3│ 100

99│ 45│ 49.5│ 10.7│ 2│ 89

│ │ │ │ 1│ 45

│ │ │ ————│ │

│ 973.9│ │

──────────────────────────────────────────────────

──────────────────────────────────┬───────────────

60 Horse-power │ Load

Type 1[51] │

────────┬──────┬───────────┬──────┼───────┬───────

Per Cent│Pounds│ Load in │Pounds│Number │ Total

of Rated│Steam │Horse-power│Steam,│of Days│ Load

Capacity│ per │ │Units │Load is│Carried

│ H.P. │ │10,000│Carried│ on

│ Hour │ │Pounds│in Year│ these

│ │ │ │ │Days in

│ │ │ │ │ H.P.

────────┼──────┼───────────┼──────┼───────┼───────

151│ 45│ 90.6│ 9.8│ 1│ 681

120│ 44│ 72.0│ 60.8│ 8│ 542

102│ 45│ 61.2│ 99.2│ 15│ 458

96│ 45│ 57.6│ 112│ 18│ 434

│ │ │ │ 23│ 381

104│ 45│ 62.4│ 209.0│ 31│ 322

101│ 45│ 60.6│ 210│ 32│ 312

102│ 45│ 61.2│ 325│ 45│ 264

│ │ │ │ 41│ 258

│ │ │ │ 30│ 242

│ │ │ │ 28│ 235

│ │ │ │ 23│ 218

│ │ │ │ 21│ 210

│ │ │ │ 18│ 198

106│ 45│ 63.6│ 137│ 12│ 170

│ │ │ │ 8│ 145

109│ 44│ 65.4│ 34.5│ 5│ 121

│ │ │ │ 3│ 100

│ │ │ │ 2│ 89

│ │ │ │ 1│ 45

│ │ │ ————│ │

│1197.3│ │

──────────────────────────────────────────────────

──────────────────────────────────┬───────────────

100 Horse-power │ Load

Type 4[51] │

────────┬──────┬───────────┬──────┼───────┬───────

Per Cent│Pounds│ Load in │Pounds│Number │ Total

of Rated│Steam │Horse-power│Steam,│of Days│ Load

Capacity│ per │ │Units │Load is│Carried

│ H.P. │ │10,000│Carried│ on

│ Hour │ │Pounds│in Year│ these

│ │ │ │ │Days in

│ │ │ │ │ H.P.

────────┼──────┼───────────┼──────┼───────┼───────

151│ 28│ 151│ 10.2│ 1│ 681

120│ 25│ 120│ 57.5│ 8│ 542

102│ 25│ 102│ 62.5│ 15│ 458

96│ 25│ 96│ 103.8│ 18│ 434

98│ 25│ 98│ 135.1│ 23│ 381

│ │ │ │ 31│ 322

│ │ │ │ 32│ 312

│ │ │ │ 45│ 264

│ │ │ │ 41│ 258

│ │ │ │ 30│ 242

│ │ │ │ 28│ 235

│ │ │ │ 23│ 218

│ │ │ │ 21│ 210

│ │ │ │ 18│ 198

106│ 25│ 106│ 76.5│ 12│ 170

104│ 25│ 104│ 29.1│ 8│ 145

│ │ │ │ 5│ 121

100│ 25│ 100│ 32.4│ 3│ 100

│ │ │ │ 2│ 89

│ │ │ │ 1│ 45

│ │ │ ————│ │

│ 507.1│ │

──────────────────────────────────────────────────

──────────────────────────────────┬───────────────

200 Horse-power │ Load

Type 5[51] │

────────┬──────┬───────────┬──────┼───────┬───────

Per Cent│Pounds│ Load in │Pounds│Number │ Total

of Rated│Steam │Horse-power│Steam,│of Days│ Load

Capacity│ per │ │Units │Load is│Carried

│ H.P. │ │10,000│Carried│ on

│ Hour │ │Pounds│in Year│ these

│ │ │ │ │Days in

│ │ │ │ │ H.P.

────────┼──────┼───────────┼──────┼───────┼───────

151│ 23│ 302│ 16.7│ 1│ 681

120│ 20│ 240│ 92.0│ 8│ 542

102│ 20│ 204│ 147│ 15│ 458

96│ 20│ 192│ 166│ 18│ 434

98│ 20│ 196│ 216│ 23│ 381

104│ 20│ 208│ 309.5│ 31│ 322

101│ 20│ 202│ 310│ 32│ 312

102│ 20│ 204│ 481│ 45│ 264

103│ 20│ 206│ 405│ 41│ 258

101│ 20│ 202│ 291│ 30│ 242

98│ 20│ 196│ 264│ 28│ 235

109│ 20│ 218│ 241│ 23│ 218

105│ 20│ 210│ 212│ 21│ 210

99│ 20│ 198│ 171│ 18│ 198

│ │ │ │ 12│ 170

│ │ │ │ 8│ 145

│ │ │ │ 5│ 121

│ │ │ │ 3│ 100

│ │ │ │ 2│ 89

│ │ │ │ 1│ 45

│ │ │ ————│ │

│3322.2│ │

──────────────────────────────────────────────────

TABLE 35

FINANCIAL COMPARISON OF PUMPING EQUIPMENTS

The loads to be cared for are shown in Table 34. An emergency unit is

supplied to bring the overload capacity of the plant, less the largest

unit, equal to the maximum load on the plant. No unit will be

overloaded more than fifty per cent of its rated capacity.

───────────┬───────────┬───────────┬───────────┬───────────┬───────────

Number of │ │ │ │ │

Units │ │ │ │ │

Exclusive │ │ │ │ │

of │ │ │ │ │

Emergency │ │ │ │ │

Unit │ 5 │ 4 │ 3 │ 2 │ 1

───────────┼───────────┼───────────┼───────────┼───────────┼───────────

Capacity │ 40 h.p.,│ │ │ │

and Type of│ Type 1│ │ │ │

Units │ 50 h.p.,│ 50 h.p.,│ │ │

│ Type 1│ Type 1│ │ │

│ 60 h.p.,│ 100 h.p.,│ 50 h.p.,│ │

│ Type 1│ Type 4│ Type 1│ │

│ 100 h.p.,│ 125 h.p.,│ 150 h.p.,│ 200 h.p.,│

│ Type 4│ Type 4│ Type 5│ Type 5│

│ 200 h.p.,│ 175 h.p.,│ 250 h.p.,│ 250 h.p.,│ 450 h.p.,

│ Type 5│ Type 5│ Type 6│ Type 6│ Type 7

───────────┼───────────┼───────────┼───────────┼───────────┼───────────

Emergency │ │ │ │ │

Unit, │ │ │ │ │

Capacity │ 200 h.p.,│ 175 h.p.,│ 250 h.p.,│ 250 h.p.,│ 450 h.p.,

and Type │ Type 5│ Type 5│ Type 6│ Type 6│ Type 7

───────────┼───────────┼───────────┼───────────┼───────────┼───────────

Annual │ │ │ │ │

payments,│ │ │ │ │

Dollars │ │ │ │ │

First │ │ │ │ │

cost of│ │ │ │ │

pumps │ 1,560│ 1,660│ 1,480│ 1,440│ 1,500

Renewal │ │ │ │ │

of │ │ │ │ │

pumps │ 1,340│ 1,430│ 1,270│ 1,240│ 1,290

First │ │ │ │ │

cost, │ │ │ │ │

boilers│ 1,024│ 1,089│ 1,125│ 1,115│ 1,410

Renewal, │ │ │ │ │

boilers│ 800│ 935│ 966│ 958│ 1,210

Fuel │ 13,140│ 11,860│ 10,490│ 9,420│ 9,400

Repairs, │ │ │ │ │

oil, │ │ │ │ │

etc. │ 2,000│ 1,800│ 1,500│ 1,300│ 1,200

Labor │ 35,000│ 31,500│ 29,500│ 27,000│ 27,000

Emergency│ │ │ │ │

unit. │ │ │ │ │

First │ │ │ │ │

cost │ 640│ 560│ 800│ 800│ 1,500

Emergency│ │ │ │ │

unit. │ │ │ │ │

Renewal│ 550│ 480│ 690│ 690│ 1,290

───────────┼───────────┼───────────┼───────────┼───────────┼───────────

Total │ 56,134│ 51,314│ 47,821│ 43,963│ 45,800

───────────┴───────────┴───────────┴───────────┴───────────┴───────────

Type 1. Simple duplex, non-condensing, horizontal.

Type 4. Compound condensing low duty horizontal.

Type 5. Low duty, triple, condensing, horizontal.

Type 6. Cross compound, condensing, horizontal.

Type 7. High duty, triple, condensing, vertical.

For example, the sewage flow expected at a proposed pumping station is

shown in Table 33. The steps involved in the selection of the number and

capacity of pumping units to care for these quantities are as follows:

(1) Determine the rated capacity of the equipment to be provided. In

this case the capacity will be taken as 450 horse-power, which is the

maximum load to be placed on the pumps. (2) Select any number of units

of such different types and capacities as are available for comparison,

and arrange them in different combinations so that each unit will

operate as nearly as possible at its rated capacity. The work involved

in such a study for 5 units is shown in Table 34. The weight of steam

consumed per indicated horse-power hour corresponding to the per cent of

the rated capacity at which the unit is operating is read from Fig. 64

or other data. (3) Repeat this step for other numbers and types of

units. (4) Prepare a table showing the annual costs of combinations of

different numbers and types of units as shown for this example in Table

35. The figures in Table 35 show that the least expensive of the

combinations of the units studied is one 200 horse-power unit, and one

250 horse-power unit, with a 250 horse-power unit in reserve. It is to

be noted that a reserve unit has been provided in each combination, the

capacity of which is equal to that of the largest unit of the

combination.

CHAPTER VIII

MATERIALS FOR SEWERS

=90. Materials.=—The materials most commonly used for the manufacture of

sewer pipe are vitrified clay and concrete. Cast iron, steel, and wood

are also used, but only under special conditions. For pipes built in the

trench, concrete, concrete blocks, brick, and vitrified clay blocks are

used. Concrete is being used to-day more than bricks or blocks because

it is cheaper. A decade or more ago all large sewers were built of

bricks. Vitrified clay and concrete are used for manufactured pipe 42

inches and less in diameter. Concrete is used almost exclusively for

larger sizes of pipe, particularly for pipe constructed in place,

although a brick invert lining is advisable when high velocities of flow

are expected.

The character of the external load, the velocity of flow and the quality

of sewage are important factors in determining the material to be used

in the construction of sewers. Reinforced concrete should be used for

large sewers near the surface subjected to heavy moving loads. A high

velocity of flow with erosive suspended matter demand a brick wearing

surface on the invert. Many engineers consider concrete less suitable

than vitrified clay or brick for conveying septic sewage or acid

industrial wastes, as concrete deteriorates more rapidly under such

conditions. Concrete should be used on soft yielding foundations,

whereas a hard compact earth, which can be cut to the form of the sewer,

is suitable to the use of brick or concrete.

Cast-iron pipe with lead joints is used for sewers flowing under

pressure, or where movements of the soil are to be expected. If the

sewage is not flowing under pressure, cement joints are sometimes used

in the cast-iron pipe. Movements of the soil are to be expected on side

hills, under railroad tracks, etc. Steel pipe is used on long outfalls

or under other conditions where external loads are light and the cost is

less than for other materials. Because of the thin plates used and the

liability to corrosion steel is not frequently used. It should never be

deeply buried nor externally loaded because of its weakness in resisting

such forces. Like wood pipe, its lightness is favorable to use on

bridges, but the greater heat conductivity of steel than wood

necessitates protection against freezing in exposed positions. Wood is

preferable only where the economy of its use is pronounced and the pipe

is running full at all times. It is desirable that the wood pipe should

be always submerged as the life of alternately wet and dry wood is

short.

Corrugated galvanized iron and unglazed tile have been used for sewers,

but usually only in emergencies or as a makeshift. Corrugated iron is

not suitable on account of its roughness and liability to corrosion, and

unglazed tile because of its lack of strength.

[Illustration:

FIG. 71.—Diagrammatic Section through Clay-pipe Press.

]

=91. Vitrified Clay Pipe.=—In general the physical and chemical

qualities of clays before burning are not sufficient to cause their

condemnation or approval by the engineer, as their behavior in the

furnace is quite individual and depends greatly on the manner in which

they are fired. The engineer is interested in the result and writes his

specifications accordingly.

In the manufacture of clay pipe, the clay as excavated is taken to a

mill and ground while dry, to as fine a condition as possible. It is

then sent to storage bins from which it is taken for wet grinding and

tempering. In this process the clay is mixed with water to the proper

degree of plasticity. A variation of 1 to 1½ per cent in the moisture

content will mean failure. Too wet a mixture will not have sufficient

strength to maintain its shape in the kiln. Too dry a mixture will show

laminations as it is pressed through the discs.

A press used in the manufacture of clay pipe is shown in cross-section

in Fig. 71. With the piston heads in the steam and mud cylinders at

their extreme upward positions, the mud cylinder is filled with clay of

the proper consistency. Steam is then turned into the steam cylinder

under pressure and the clay is squeezed into the space between the inner

and outer shells of the die and mandrel to form the hub of the pipe. The

pressure on the clay may be from 250 to 600 pounds per square inch. When

clay appears at the holes, marked _hh_ at the bottom of the mud

cylinder, the bottom plate and the center portion of the die are removed

and the remainder or straight portion of the pipe is formed by squeezing

the clay between the mandrel and the outer wall of the die. A completely

formed pipe can be seen issuing from the press in Fig. 72. Any sized

pipe that is desired can be formed from the same press by changing the

size of the dies and mandrel.

[Illustration:

FIG. 72.—Clay-pipe Press.

Courtesy, Blackmer and Post Manufacturing Co.

]

Curved pipes are made in two ways—by bending directly as they issue from

the press, or by shaping by hand in plaster of paris molds. Junctions

are made by cutting the branch pipe to the shape of the outside of the

main pipe, fastening the branch in place with soft clay and then cutting

out the wall of the main pipe the size of the branch. Special fittings

are usually made by hand in plaster molds.

After being pressed into shape the pipes are taken to a steam-heated

drying room where a constant temperature is maintained in order to

prevent cracking of the pipes. They remain in the drying room from 3 to

10 days until dry, when they are taken to the kilns. If taken to the

kilns when moist blisters will be produced.

The dried pipes are piled carefully in the kiln so that heat and weight

may be as evenly distributed as possible, and the fire is then started

in the kiln. The process of burning can be roughly divided into five

stages:

1st. Water smoking, which lasts about 72 hours during which the

temperature is raised gradually to 350 degrees Fahrenheit.

2nd. Heating, during which the temperature is raised to 800 degrees

Fahrenheit in 24 hours.

3rd. Oxidation, during which the temperature is raised to 1,400 degrees

Fahrenheit in 84 hours.

4th. Vitrification, in which the temperature is raised to 2,100 degrees

Fahrenheit in 48 hours, and finally,

5th. Glazing, during which the temperature is unchanged but salt (NaCl)

is thrown in and allowed to burn.

Oxidation must be complete before vitrification is started as otherwise

blisters will be raised due to imprisoned carbon dioxide. The important

points in vitrification are to make the required temperature within a

reasonable time and to maintain a uniform distribution of heat

throughout the kiln. When vitrification is complete as shown by a glassy

fracture of a broken sample taken from the kiln, glazing is accomplished

by throwing a shovelful of salt on the hottest part of the fire. About

five to six applications of salt from two to three hours apart may be

needed. The kiln is then allowed to cool and the manufacture of the pipe

is complete. The completeness of vitrification is indicated by the

amount of water that the finished pipe will absorb. Completely vitrified

pipe will absorb no moisture. Soft-burned pipe may absorb as much as 15

per cent moisture.

Vitrified clay blocks are made of the same material and in the same

manner as vitrified clay pipe.

The following data on vitrified pipe have been abstracted from the

specifications for vitrified pipe adopted by the American Society for

Testing Materials.

Pipes shall be subject to rejection on account of the following:

(_a_) Variation in any dimension exceeding the permissible

variations given in Table 36.

(_b_) Fracture or cracks passing through the shell or hub, except

that a single crack at either end of a pipe not exceeding 2 inches

in length or a single fracture in the hub not exceeding 3 inches

in width nor 2 inches in length will not be deemed cause for

rejection unless these defects exist in more than 5 per cent of

the entire shipment or delivery.

(_c_) Blisters or where the glazing is broken or which exceed 3

inches in diameter, or which project more than ⅛ inch above the

surface.

(_d_) Laminations which indicate extended voids in the pipe

material.

(_e_) Fire cracks or hair cracks sufficient to impair the

strength, durability or serviceability of the pipe.

(_f_) Variations of more than ⅛ inch per linear foot in alignment

of a pipe intended to be straight.

(_g_) Glaze which does not fully cover and protect all parts of

the shell and ends except those exempted in Sect. 31. Also glaze

which is not equal to best salt glaze.

(_h_) Failure to give a clear ringing sound when placed on end and

dry tapped with a light hammer.

(_i_) Insecure attachment of branches or spurs.

_Workmanship and Finish_

(29) Pipes shall be substantially free from fractures, large or

deep cracks and blisters, laminations and surface roughness.

(31) The glaze shall consist of a continuous layer of bright or

semi-bright glass substantially free from coarse blisters and

pimples.... Not more than 10 per cent of the inner surface of any

pipe barrel shall be bare of glaze except the hub, where it may be

entirely absent. Glazing will not be required on the outer surface

of the barrel at the spigot end for a distance from the end equal

to ⅔ the specified depth of the socket for the corresponding size

of pipe. Where glazing is required there shall be absence of any

well defined network of crazing lines or hair cracks.

(32) The ends of the pipe shall be square with their longitudinal

axis.

(33) Special shapes shall have a plain spigot end and a hub end

corresponding in all respects with the dimensions specified for

pipes of the corresponding internal diameter.

TABLE 36

PROPERTIES OF CLAY SEWER PIPE

Abstracts from Tentative Specifications of the American Society for

Testing Materials

─────────┬─────────┬───────────┬───────┬────────┬──────┬──────┬─────────

Internal │ Minimum │ Maximum │Laying │Diameter│Depth │Taper │ Minimum

Diameter,│Crushing │Absorption,│length,│ of │ of │ of │Thickness

Inches │Strength,│ Per Cent │ Feet │ Inside │Socket│Socket│ of

│ Pounds │ │ │ of │Inches│ │ Barrel.

│ per │ │ │Socket, │ │ │ Inches

│ Linear │ │ │ Inches │ │ │

│ Foot. │ │ │ │ │ │

│See Note │ │ │ │ │ │

│ 2 │ │ │ │ │ │

─────────┼─────────┼───────────┼───────┼────────┼──────┼──────┼─────────

│ │ │ │ │ │ │

│ │ │ │ │ │ │

│ │ │ │ │ │ │

│ │ │ │ │ │ │

│ │ │ │ │ │ │

─────────┼─────────┼───────────┼───────┼────────┼──────┼──────┼─────────

│ │ │ │ │ │ │

│ │ │ │ │ │ │

─────────┼─────────┼───────────┼───────┼────────┼──────┼──────┼─────────

6 │ 1430 │ 5 │2, 2½, │ 8¼ │ 2 │1 : 20│ ⅝

│ │ │ 3 │ │ │ │

8 │ 1430 │ 5 │2, 2½, │ 10¾ │ 2¼ │1 : 20│ ¾

│ │ │ 3 │ │ │ │

10 │ 1570 │ 5 │2, 2½, │ 13 │ 2½ │1 : 20│ ⅞

│ │ │ 3 │ │ │ │

12 │ 1710 │ 5 │2, 2½, │ 15¼ │ 2½ │1 : 20│ 1

│ │ │ 3 │ │ │ │

15 │ 1960 │ 5 │2, 2½, │ 18¾ │ 2½ │1 : 20│ 1¼

│ │ │ 3 │ │ │ │

18 │ 2200 │ 5 │2, 2½, │ 22¼ │ 3 │1 : 20│ 1½

│ │ │ 3 │ │ │ │

21 │ 2590 │ 5 │2, 2½, │ 26 │ 3 │1 : 20│ 1¾

│ │ │ 3 │ │ │ │

24 │ 3070 │ 5 │2, 2½, │ 29½ │ 3 │1 : 20│ 2

│ │ │ 3 │ │ │ │

27 │ 3370 │ 5 │ 3 │ 33¼ │ 3½ │1 : 20│ 2¼

30 │ 3690 │ 5 │ 3 │ 37 │ 3½ │1 : 20│ 2½

33 │ 3930 │ 5 │ 3 │ 40¼ │ 4 │1 : 20│ 2⅝

36 │ 4400 │ 5 │ 3 │ 44 │ 4 │1 : 20│ 2¾

39 │ 4710 │ 5 │ 3 │ 47¼ │ 4 │1 : 20│ 2⅞

42 │ 5030 │ 5 │ 3 │ 51 │ 4 │1 : 20│ 3

─────────┴─────────┴───────────┴───────┴────────┴──────┴──────┴─────────

─────────┬────────────────────────────────────────────────┬────────

Internal │ Permissible Variations │ Number

Diameter,│ │ of

Inches │ │Scorings

│ │ on

│ │ Spigot

│ │ and

│ │Socket ⅛

│ │ Inch

│ │ Deep

─────────┼───────┬─────────────┬────────┬───────┬─────────┼────────

│Length,│ Internal │ Length │ Depth │Thickness│

│ Inches│ Diameter, │ of Two │ of │ of │

│ (-), │ Inches │Opposite│Socket,│ Barrel, │

│ per │ │ Sides, │Inches │ Inches │

│ Foot │ │ Inches │ (-) │ (-) │

─────────┼───────┼──────┬──────┼────────┼───────┼─────────┼────────

│ │Spigot│Socket│ │ │ │

│ │ (±) │ (±) │ │ │ │

─────────┼───────┼──────┼──────┼────────┼───────┼─────────┼────────

6 │ ¼ │ 3/16 │ ¼ │ ⅛ │ ¼ │ 1/16 │ 2

│ │ │ │ │ │ │

8 │ ¼ │ ¼ │ 5/16 │ ⅛ │ ¼ │ 1/16 │ 2

│ │ │ │ │ │ │

10 │ ¼ │ ¼ │ 5/16 │ ⅛ │ ¼ │ 1/16 │ 2

│ │ │ │ │ │ │

12 │ ¼ │ 5/16 │ ⅜ │ ⅛ │ ¼ │ 1/16 │ 2

│ │ │ │ │ │ │

15 │ ¼ │ 5/16 │ ⅜ │ ⅛ │ ¼ │ 3/32 │ 3

│ │ │ │ │ │ │

18 │ ¼ │ ⅜ │ 7/16 │ 3/16 │ ¼ │ 3/32 │ 3

│ │ │ │ │ │ │

21 │ ¼ │ 7/16 │ ½ │ 3/16 │ ¼ │ ⅛ │ 3

│ │ │ │ │ │ │

24 │ ⅜ │ ½ │ 9/16 │ ¼ │ ¼ │ ⅛ │ 4

│ │ │ │ │ │ │

27 │ ⅜ │ ⅝ │11/16 │ ¼ │ ¼ │ ⅛ │ 4

30 │ ⅜ │ ⅝ │11/16 │ ¼ │ ¼ │ ⅛ │ 4

33 │ ⅜ │ ¾ │13/16 │ ¼ │ ¼ │ 3/16 │ 5

36 │ ⅜ │ ¾ │13/16 │ ⅜ │ ¼ │ 3/16 │ 5

39 │ ⅜ │ ¾ │13/16 │ ⅜ │ ¼ │ 3/16 │ 5

42 │ ⅜ │ ¾ │13/16 │ ⅜ │ ¼ │ 3/16 │ 5

─────────┴───────┴──────┴──────┴────────┴───────┴─────────┴────────

NOTE 1. For methods of making tests see Proc. Am. Soc. for Testing

Materials.

NOTE 2. Concentrated load at end of vertical diameter.

(_a_) Slants shall have their spigot ends cut at an angle of

approximately 45 degrees with the longitudinal axis.

(_b_) Curves shall be at angles of 90, 45, 22½, and 11¼ degrees as

required. They shall conform substantially to the curvature

specified.

(_c_) ... All branches shall terminate in sockets.

[Illustration:

FIG. 73.—Standard Clay Pipe Specials.

Courtesy, Blackmer and Post Manufacturing Co.

]

In Fig. 73 are shown the various forms of vitrified pipe and specials

which are ordinarily available on the market.

The life of vitrified clay sewers and some observations on the results

of the inspection of the sewers in Manhattan are discussed in Chapter

XII. The strength of vitrified sewer pipes is shown in Table 37.

TABLE 37

STRENGTH OF SEWER PIPE

Strength in pounds per linear foot to carry loads from ditch filling

material such as ordinary sand and thoroughly wet clay, with the under

side of the pipe bedded 60° to 90° by ordinary good methods. From Proc.

Am. Society for Testing Materials, Vol. 20, 1920, page 604.

────────┬──────────────────────────────────────────────────────────────

Height │

of Fill │

Above │

Top of │

Pipe, │

Feet │ Breadth of the Ditch a Little Below the Top of the Pipe

────────┼───────────┬───────────┬───────────┬────────────┬─────────────

│ 1 Foot │ 2 Feet │ 3 Feet │ 4 Feet │ 5 Feet

────────┼───────────┴───────────┴───────────┴────────────┴─────────────

│ Ditch Filling Material

────────┼─────┬─────┬─────┬─────┬─────┬─────┬─────┬──────┬──────┬──────

│sand │clay │sand │clay │sand │clay │sand │ clay │ sand │ clay

────────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼──────┼──────┼──────

2│ 265│ 280│ 615│ 635│ 970│ 990│ 1330│ 1,350│ 1,690│ 1,710

4│ 400│ 450│ 1055│ 1125│ 1745│ 1825│ 2455│ 2,535│ 3,165│ 3,250

6│ 470│ 545│ 1370│ 1500│ 2370│ 2525│ 3405│ 3,575│ 4,460│ 4,740

8│ 505│ 605│ 1600│ 1790│ 2875│ 3115│ 4215│ 4,495│ 5,595│ 5,890

10│ 525│ 640│ 1765│ 2015│ 3275│ 3610│ 4900│ 5,295│ 6,590│ 7,020

12│ 535│ 660│ 1880│ 2185│ 3600│ 4030│ 5485│ 6,000│ 7,460│ 8,035

14│ 540│ 675│ 1965│ 2320│ 3855│ 4380│ 5975│ 6,620│ 8,225│ 8,950

16│ 545│ 680│ 2025│ 2425│ 4065│ 4675│ 6395│ 7,165│ 8,890│ 9,775

18│ 545│ 685│ 2070│ 2505│ 4230│ 4920│ 6750│ 7,630│ 9,480│10,520

20│ 545│ 690│ 2100│ 2565│ 4365│ 5130│ 7050│ 8,060│ 9,995│11,190

22│ 545│ 690│ 2125│ 2610│ 4470│ 5305│ 7305│ 8,425│10,445│11,795

24│ 545│ 690│ 2140│ 2645│ 4560│ 5445│ 7525│ 8,750│10,840│12,340

26│ 545│ 690│ 2150│ 2675│ 4630│ 5575│ 7705│ 9,035│11,185│12,830

28│ 545│ 690│ 2160│ 2695│ 4685│ 5680│ 7860│ 9,280│11,490│13,270

30│ 545│ 690│ 2165│ 2715│ 4725│ 5765│ 7990│ 9,500│11,755│13,670

Very │ │ │ │ │ │ │ │ │ │

great │ 545│ 690│ 2180│ 2770│ 4910│ 6230│ 8725│11,075│13,635│17,305

────────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴──────┴──────┴──────

=92. Cement and Concrete Pipe.=—Although there is no general recognition

of a difference between cement and concrete pipe, there is a tendency to

term manufactured pipe of small diameter cement pipe, and large pipes or

pipes constructed in place, concrete pipe. Cement, unlike clay, is used

in the manufacture of pipe in the field or by more or less unskilled

operators in “one man” plants. Great care should be used in the

selection of cement, aggregate, and reinforcement for precast cement

pipe since the shocks to which it is subjected in transit are more

liable to rupture it than the heavier but steadier loads imposed on it

in the trench.

The United States Government, various scientific and engineering

societies, and other interested organizations have collaborated in the

preparation of specifications for cement and cement tests. These

specifications can be found in Trans. Am. Soc. Civil Engineers, Vol. 82,

1918, p. 166, and in other publications.

The following abstracts have been taken from the proposed tentative

specifications for Concrete Aggregates, of the Am. Society for Testing

Materials, issued June 21, 1921:

1. Fine aggregate shall consist of sand, stone screenings, or

other inert materials with similar characteristics, or a

combination thereof, having clean, hard, strong, durable uncoated

grains, free from injurious amounts of dust, lumps, soft or flaky

particles, shale, alkali, organic matter, loam or other

deleterious substances.

2. Fine aggregates shall preferably be graded from fine to coarse,

with the coarser particles predominating, within the following

limits:

Passing No. 4 sieve 100 per cent

Passing No. 50 sieve, not more than 50 per cent

Weight removed by elutriation test, not more than 3 per cent

Sieves shall conform to the U. S. Bureau of Standards

specifications for sieves.

3. The fine aggregate shall be tested in combination with the

coarse aggregate and the cement with which it is to be used and in

the proportions, including water, in which they are to be used on

the work, in accordance with the requirements specified in Section

6....

7. Coarse aggregate shall consist of crushed stone, gravel or

other approved inert materials with similar characteristics, or a

combination thereof, having clean, hard, strong, durable, uncoated

pieces free from injurious amounts of soft, friable, thin,

elongated or laminated pieces, alkali, organic or other

deleterious matter.

* * * * *

The following Table indicates desirable gradings, in percentages,

for coarse aggregate for certain maximum sizes.

GRADINGS OF COARSE AGGREGATES

─────────┬─────────────────────────────────────────┬───────────────────

Maximum │ Circular Openings, Inches │ Passing Screen

Size of │ │ Having Circular

Aggregate│ │Openings ¼ Inch in

Inches │ │diameter, not more

│ │ than

─────────┼───┬───┬───┬─────┬─────┬─────┬─────┬─────┼───────────────────

│ 3 │2½ │ 2 │ 1½ │ 1¼ │ 1 │ ¾ │ ½ │

─────────┼───┼───┼───┼─────┼─────┼─────┼─────┼─────┼───────────────────

3 │100│ │ │40–75│ │ │ │ │ 15 per cent

2½ │ │100│ │ │40–75│ │ │ │ 15 per cent

2 │ │ │100│ │ │40–75│ │ │ 15 per cent

1½ │ │ │ │ 100 │ │ │40–75│ │ 15 per cent

1¼ │ │ │ │ │ 100 │ │ │35–70│ 15 per cent

1 │ │ │ │ │ │ 100 │ │40–75│ 15 per cent

¾ │ │ │ │ │ │ │ 100 │ │ 15 per cent

─────────┴───┴───┴───┴─────┴─────┴─────┴─────┴─────┴───────────────────

The manufacture of small size cement pipe requires relatively more skill

than equipment. As a result great care must be observed in the

inspection of cement pipe and in the enforcement of specifications. For

large size concrete pipe and reinforced concrete pipe the difficulty of

holding the pipe together during transportation and lowering into the

trench aid in insuring a good product.

Cement pipe is made by ramming a mixture of cement, sand, and water into

a cylindrical mold and allowing it to stand until set. The mold is then

removed and the pipe stands for a further period of time to become

cured. The selection and proportion of materials, the amount of water,

the method of ramming, the period of setting, the length of time of

curing, and the control of moisture and temperature during this period

are of great importance in the resulting product. E. S. Hanson[52]

states that the most conservative engineers recommend a mixture of one

sack of cement to 2½ cubic feet of aggregate measured as loosely thrown

into the measuring box. In making up the aggregate, clean gravel or

broken stone up to ¼ inch in size is used. The American Concrete

Institute recommends that 100 per cent pass a ½-inch screen, 70 per cent

a ¼-inch screen, 50 per cent a No. 10, 40 per cent a No. 20, 30 per cent

a No. 30, and 20 per cent a No. 40. The materials should be carefully

graded by experiment and not guessed at, as the behavior of all

aggregates is not the same. Too coarse an aggregate is difficult to

handle in manufacturing. It causes loss of pipe when the jacket or mold

is removed and results in rough pipe, stone pockets, and pin holes

through which water spurts when pressure tests are applied. Too fine an

aggregate causes loss of strength and with ordinary mixtures tends to

produce a pipe which will show seepage under internal pressure tests.

The amount of water in the mixture will vary, from 15 to 20 per cent.

The mixture should appear dry but should ball in the hand under some

pressure.

[Illustration:

FIG. 74.—Details of 24–Inch Concrete Pipe Form.

]

The mixture can be rammed into the molds by hand or machine. A

machine-made pipe is preferable as it produces a more even and stronger

product. There are two types of machines for this purpose. One type

consists of a number of tamping feet which deliver about 200 blows to

the minute with a pressure of about 800 pounds per square inch of area

exposed. In the other type a revolving core is drawn through the pipe,

packing and polishing the concrete as it is pulled through, with special

provision for packing the bell of the pipe. The tamping machines can

make 1,500 feet of small size pipe to 300 feet of 24–inch pipe in a day.

Machines of the second type can make 750 feet of 8–inch to 200 feet of

30–inch pipe in 30–inch lengths in 9 hours. The inside and outside forms

for a 24–inch pipe are shown in Fig. 74 as used with the tamping

machines. The forms are swabbed with oil before being filled in order to

facilitate their removal. In making a Y-branch or other special, a hole

is cut in the pipe or mold the size of the joining pipe which is then

set in place and the joint wiped smooth with cement.

TABLE 38

PROPERTIES OF CEMENT CONCRETE SEWER PIPE

1917 Specifications of American Society for Testing Materials, with

Subsequent Revisions

─────────┬───────┬────────┬───────┬───────┬──────┬─────────

│Laying │Diameter│Normal │ Depth │Taper │ Minimum

│Length,│ at │Annular│ of │ of │Thickness

│ Feet │ Inside │Space, │Socket,│Socket│ of

│ │ of │Inches │Inches │ │ Barrel,

│ │Socket, │ │ │ │ Inches

│ │ Inches │ │ │ │

Internal │ │ │ │ │ │

Diameter,│ │ │ │ │ │

Inches │ │ │ │ │ │

─────────┼───────┼────────┼───────┼───────┼──────┼─────────

│ │ │ │ │ │

│ │ │ │ │ │

│ │ │ │ │ │

│ │ │ │ │ │

│ │ │ │ │ │

─────────┼───────┼────────┼───────┼───────┼──────┼─────────

│ │ │ │ │ │

│ │ │ │ │ │

─────────┼───────┼────────┼───────┼───────┼──────┼─────────

│2, 2½, │ 8¼ │ ½ │ 2 │1 : 20│ ⅝

6 │ 3 │ │ │ │ │

│2, 2½, │ 11 │ ⅝ │ 2¼ │1 : 20│ ¾

8 │ 3 │ │ │ │ │

│2, 2½, │ 13¼ │ ⅝ │ 2½ │1 : 20│ ⅞

10 │ 3 │ │ │ │ │

│2, 2½, │ 15⅝ │ ⅝ │ 2½ │1 : 20│ 1

12 │ 3 │ │ │ │ │

│2, 2½, │ 19¼ │ ⅝ │ 2½ │1 : 20│ 1¼

15 │ 3 │ │ │ │ │

│2, 2½, │ 22¾ │ ⅝ │ 2¾ │1 : 20│ 1½

18 │ 3 │ │ │ │ │

│2, 2½, │ 26½ │ ¾ │ 2¾ │1 : 20│ 1¾

21 │ 3 │ │ │ │ │

│2, 2½, │ 30¼ │ ¾ │ 3 │1 : 20│ 2⅛

24 │ 3 │ │ │ │ │

27 │ 3 │ 34 │ ⅞ │ 3¼ │1 : 20│ 2¼

30 │ 3 │ 38 │ 1 │ 3½ │1 : 20│ 2½

33 │ 3 │ 41½ │ 1 │ 4 │1 : 20│ 2¾

36 │ 3 │ 45½ │ 1¼ │ 4 │1 : 20│ 3

39 │ 3 │ 49 │ 1¼ │ 4 │1 : 20│ 3¼

42 │ 3 │ 53 │ 1½ │ 4 │1 : 20│ 3½

─────────┴───────┴────────┴───────┴───────┴──────┴─────────

─────────┬──────────────────────────────────────┬─────────┬───────────

│ Limits of Permissible Variations │ Minimum │ Maximum

│ │Crushing │Absorption,

│ │Strength,│ Per Cent

│ │ Pounds │

│ │ per │

│ │ Linear │

Internal │ │ Foot at │

Diameter,│ │ End of │

Inches │ │Diameter │

─────────┼───────┬─────────────┬──────┬─────────┼─────────┼───────────

│Length,│ Internal │Depth │Thickness│ │

│ Inch │ Diameter, │of Hub│of Barrel│ │

│ per │ Inches │ (-) │ (-) │ │

│ Foot │ │Inches│ Inches │ │

│ (-) │ │ │ │ │

─────────┼───────┼──────┬──────┼──────┼─────────┼─────────┼───────────

│ │Spigot│Socket│ │ │ │

│ │ (±) │ (±) │ │ │ │

─────────┼───────┼──────┼──────┼──────┼─────────┼─────────┼───────────

│ ¼ │ 3/16 │ 3/16 │ ¼ │ 1/16 │ 1430 │ 8

6 │ │ │ │ │ │ │

│ ¼ │ ¼ │ ¼ │ ¼ │ 1/16 │ 1430 │ 8

8 │ │ │ │ │ │ │

│ ¼ │ ¼ │ ¼ │ ¼ │ 1/16 │ 1570 │ 8

10 │ │ │ │ │ │ │

│ ¼ │ ¼ │ ¼ │ ¼ │ 1/16 │ 1910 │ 8

12 │ │ │ │ │ │ │

│ ¼ │ ¼ │ ¼ │ ¼ │ 3/32 │ 1960 │ 8

15 │ │ │ │ │ │ │

│ ¼ │ ¼ │ ¼ │ ¼ │ 3/32 │ 2200 │ 8

18 │ │ │ │ │ │ │

│ ¼ │ 5/16 │ 5/16 │ ¼ │ ⅛ │ 2590 │ 8

21 │ │ │ │ │ │ │

│ ⅜ │ 5/16 │ 5/16 │ ¼ │ ⅛ │ 3070 │ 8

24 │ │ │ │ │ │ │

27 │ ⅜ │ 5/16 │ ⅜ │ ¼ │ ⅛ │ 3370 │ 8

30 │ ⅜ │ ⅜ │ ⅜ │ ¼ │ ⅛ │ 3690 │ 8

33 │ ⅜ │ ⅜ │ ⅜ │ ¼ │ 3/16 │ 3930 │ 8

36 │ ⅜ │ ½ │ ½ │ ¼ │ 3/16 │ 4400 │ 8

39 │ ⅜ │ ½ │ ½ │ ¼ │ 3/16 │ 4710 │ 8

42 │ ⅜ │ ½ │ ½ │ ¼ │ 3/16 │ 5030 │ 8

─────────┴───────┴──────┴──────┴──────┴─────────┴─────────┴───────────

After the removal of the mold the pipe may be cured by the water or the

steam process. Hanson states:

By the former the pipe are simply set on the floor of the plant

and as soon as they are sufficiently strong so that they can be

sprinkled with water without falling down; sprinkling is commenced

and continued at such intervals for 6 or 7 days that the pipe will

be moist at all times. This is a slower process than steam curing.

It is also less uniform and less subject to control than where the

product is cured by steam.

In the steam process the pipe is exposed to low-pressure steam with

plenty of moisture in a closed receptacle for 24 hours, or until

hardened. It has been found by tests that pipes sprinkled for 28 days

are as strong as steam-cured pipes.

The dimensions of cement concrete sewer pipe as recommended by the Am.

Society for Testing Materials are shown in Table 38.

The following has been abstracted from the description of the

manufacture of one form of concrete pipe by G. C. Bartram.[53] All pipe

are manufactured in 4–foot lengths near the site at which they are to be

installed because of their great weight, for example, 36–inch pipe

weighs one ton. The plant for the manufacture of the pipe consists of

cast-iron bottom and top rings for each size to be used on the job, and

inside and outside steel casings. There are three bases for each steel

casing as the pipes stand on the bases for 72 hours and the steel casing

remains on for only 24 hours after the concrete has been poured. The

pipes are then lifted off the bases and stored for aging. The pipes are

cast with the spigot end up.

The concrete is ordinarily mixed in the proportions of 1 : 2 : 4. The

materials are placed in the mixer in the following order: first, the

stone, then the sand, then the cement, and finally the water. Sufficient

water is added to make the concrete flow freely. In cold weather or for

a hurry-up job the molds are covered with canvas and are steamed for 2

or 3 hours immediately after the concrete is poured. The molds are then

removed but the pipe should be steamed before use. Otherwise they are

allowed to stand 72 hours, as explained above. In cold weather the steam

is used to prevent freezing and not to hasten the completion of the

pipe.

[Illustration:

FIG. 75.—Triangle Mesh Reinforced Concrete Pipe.

As made by the Am. Concrete Pipe and Pile Co., Chicago.

]

[Illustration:

FIG. 76.—Methods of Joining and Reinforcing Concrete Pipe.

]

One layer or ring of reinforcement is used for sizes from 24 to 48

inches and two layers or rings for larger pipe. A type of reinforcement

sometimes used is the American Steel and Wire Company’s Triangular Mesh,

an illustration of which is shown in Fig. 75. The wire mesh is cut to

fit and is placed in a slot in the cast-iron base. The slot is then

filled with sand so that the concrete cannot enter, thus leaving a

portion of the reinforcement exposed. The inside reinforcement extends

through and out of the spigot of the completed pipe. In the trench the

two reinforcements overlap in the key-shaped space left on the inside of

the pipe by the design of the bell and spigot. This space is shown in

Fig. 76 A. When the pipe is placed in the trench the key-shaped space is

plastered with mortar and a piece is knocked out of the bell to receive

the grout with which the joint is closed. A spring steel band is then

put on the outside of the joint and grout poured into the hole at the

top. The band is removed as soon as the joint materials have set.

The rules for the reinforcement of concrete pipe recommended in Volume

XV, 1919, of the Transactions of the Concrete Institute are as follows:

No reinforcement is approved for pipe between 30 and 60 inches in

diameter or in rock or hard soils. For pipe 36 inches in diameter

or less the minimum thickness of shell shall be 5 inches. For

60–inch pipe the minimum thickness shall be 7 inches with

intermediate sizes in proportion. Reinforcement for circular pipe

shall consist of one or two rings of circular wire fabric or rods

of the areas shown in Table 39. All sewers near the surface and

subject to vibration should be reinforced. For sewers 6 feet or

less in diameter the reinforcement should consist of at least ½ of

1 per cent of the area of the concrete. It should be placed near

the inside at the crown and near the outside at the haunches. If

large horizontal pressures are expected the pipe should be

reinforced for these reverse stresses, which involves placing the

reinforcement near the outside at the crown and near the inside at

the haunches. The minimum thickness of the walls of sewers greater

than 6 feet in diameter with flat bottom and arch, with or without

side walls, should be 8 inches.

TABLE 39

REINFORCEMENT FOR CIRCULAR CONCRETE SEWER PIPE

(See Vol. XV, Proceedings Am. Concrete Institute)

─────────────────┬─────────────────┬─────────────────┬─────────────────

Diameter in │Minimum Thickness│ Number of Rings │ Cross Sectional

Inches │ of Shell in │ │Area of Each Ring

│ Inches │ │

─────────────────┼─────────────────┼─────────────────┼─────────────────

24 │3 │1 │.058

27 │3 │1 │.068

30 │3½ │1 │.080

33 │4 │1 │.107

36 │4 │1 │.146

39 │4 │1 │.146

42 │4½ │1 │.153

48 │5 │2 │.107

54 │5½ │2 │.123

60 │6 │2 │.146

66 │6½ │2 │.168

72 │7 │2 │.180

84 │8 │2 │.208

96 │9 │2 │.245

─────────────────┴─────────────────┴─────────────────┴─────────────────

Three methods for the reinforcement of concrete sewers are shown in Fig.

76 B.

=93. Proportioning of Concrete.=—In the proportioning of concrete

questions of strength, of permeability, and of workability[54] may need

consideration. All of these qualities are affected by the amount of

cement, the nature and gradation and relative proportions of the fine

and the coarse aggregate, and the amount of mixing water used.

Other things being equal the strength varies with the amount of cement

put into the concrete. For the same amount of cement and the same

consistency of the mixture, the strength increases with increased

density of concrete (that is, with decreased voids), and the effort

should be made so to proportion the fine and coarse aggregates as to

produce the densest concrete (least voids) with the aggregates

available. For the same consistency, the strength then will vary with

the ratio of the amount of cement to the amount of the voids.

So far as the mixing water is concerned, the greatest strength in the

concrete will be attained at a rather dry mix; that which produces the

least volume of concrete. The addition of more water results in a

concrete of less strength; 40 per cent more water may give a concrete of

less than half the normal strength. The reduction in strength is then

very marked for the wetter mixes, and the water content used is a

feature of considerable importance in the design of concrete mixtures.

Permeability is affected by the same elements as strength, but the size

and discontinuity of the pores have a greater influence.

Workability is an important quality; in some respects it will have to be

obtained at the expense of strength. Increasing the amount of mixing

water increases the workability of the mixtures, with a resulting

decrease in strength which may have to be accepted or else overcome by

increasing the cement in the mix.

An excess of water is often used unnecessarily through ignorance of the

injurious results. A high proportion of coarse aggregate, up to a

certain limit, will give concrete of high strength, but the mixture will

be harsh-working and not easy to place. Lower proportions of coarse

aggregate will give greater workability and better uniformity of

product, the latter being an important matter. It is apparent that the

degree of workability of the mixture needed will depend upon the nature

of the construction—for a pavement where the concrete will receive

substantial tamping or working the water content may be much less than

that which may need to be used in placing concrete around reinforcement

in narrow members, or where little tamping or spading can be done. The

nature of the work will affect the standard of consistency to be

specified.

The proportioning of the concrete should then be dependent upon the

needs of the structure and the manner of placing the concrete. The

proportions selected should be carefully adhered to and especially

should care be taken to see that the right quantity of mixing water is

used.

The materials are commonly measured volumetrically (by bulk). Because of

the variations which are introduced by volumetric measurement of the

materials by the presence of varying degrees of moisture, measurements

by weight would be more accurate, but these would also be affected by

differences in the specific gravity of the materials. The methods of

measuring, the allowance for moisture, as well as the proportions of the

materials, should be specified.

The methods for proportioning concrete are:

(1) Arbitrarily selected proportions.

(2) Proportions based on minimum voids.

(3) Proportions based on trial mixtures.

(4) Proportions based on a sieve analysis curve.

(5) Proportions based on the surface area of the aggregates.

(6) Proportions based on the water-cement ratio and the fineness

modulus.

(7) Proportions based on mortar-voids and cement-voids ratio.

Arbitrarily selected proportions are in quite general use; they are

intended to apply to the materials most commonly used in the vicinity of

the work. The most common practice is to use twice as great a volume of

coarse aggregate as fine aggregate, as for instance 1 part cement, 2

parts fine aggregate, and 4 parts coarse aggregate. Decreasing the ratio

of coarse aggregate to fine aggregate may give a more easily worked mix

or require relatively less water for a given workability, and in some

cases it will be proper to increase this ratio and thus secure an

increase of strength. Judgment and experience with given materials may

warrant changes from a stated ratio. The proportions are now frequently

given as one part cement to a certain number of parts of the mixed

aggregate, leaving the proportions of the fine to coarse to be

determined otherwise, since small variations in the relation of these

will not greatly affect the strength. Proportions in common use are:[55]

Mortar for

Laying brick and stone masonry from 1 : 0 to 1 : 3

Filling joints in sewer pipe 1 : 0 to 1 : 2

Surfaces, floors, sidewalks, pavements 1 : 0 to 1 : 2

Waterproof linings 1 : 0 to 1 : 2

Cement, bricks, and blocks 1 : 2½ to 1 : 4

Concrete for

Gravity retaining walls, heavy

foundations, structures needing mass

more than strength from 1 : 3 : 6 to 1 : 4 : 8

Retaining walls, piers, sewers,

pavements, foundations, and work

requiring strength. (Compressive

strength in 28 days, 1,500 to 2,000

pounds per square inch) from 1 : 2 : 4 to 1 : 3 : 6

Floors, beams, pavements, reinforced

concrete, arch bridges, low-pressure

tanks. (Compressive strength in 28

days, 2,000 to 3,000 pounds per

square inch) from 1 : 1½ : 3 to 1 : 2½ : 4½

Reinforced concrete columns, conduit

pipe, impervious concrete.

(Compressive strength in 28 days,

3,000 to 4,000 pounds per square

inch) from 1 : 1 : 2 to 1 : 1½ : 3

The usual method of proportioning based on minimum voids is to assume

that the particles of fine aggregate should fill the voids in the coarse

aggregate and that the particles of the cement will fill the voids in

the fine aggregate. About 5 to 10 per cent additional fine aggregate is

generally added to push the particles of the coarse aggregate apart and

thus give a more easily worked concrete and one freer from void spaces.

This method is inaccurate, principally because of the effect of the

moisture on the volume of the voids, and because the effect on the

volume by the addition of water is unknown.

Trial mixtures may be made by carefully weighing each of the ingredients

and then combining them to give a workable concrete. Using a given

amount of cement, the proportion of ingredients, of the same total

weight, which will give the least volume and therefore the densest

concrete is adopted. When making the comparison the consistency of the

mixes must be maintained constant.

Proportioning may be based on an ideal sieve analysis curve of the mixed

cement and aggregates. The sieve analysis of the aggregates is made by

screening a predetermined weight of the sample through a series of 5 to

8 sieves graded in size from slightly below the size of the largest

particle to slightly above the smallest particle of the aggregate. The

analysis is then expressed in the form of a curve. The ideal curve,

according to Fuller,[56] is shown in Fig. 77.

[Illustration:

FIG. 77.—Gravel Analysis.

The dotted line indicates the ideal combination of the coarse and fine

portions. The heavy full line indicates the combination attained.

]

The method of proportioning concrete by surface areas is based on the

theory that the strength of a concrete depends on the amount of cement

used in proportion to the surface area of the aggregates.[57]

The proportioning of concrete on the basis of a water-cement ratio and a

fineness modulus was introduced by Prof. D. A. Abrams.[58] It is based

on the theory that with fixed conditions of aggregate, moisture, etc.,

the ratio of water to cement determines the strength of the concrete.

A method of proportioning concrete by determining experimentally the

voids in mortars made up with a given amount of sand and definite

proportions of cement, and then calculating the voids in the concrete

made up by adding a definite amount of coarse aggregate to the mixture,

has been developed.[59] The method is based on the theory that the

strength of the concrete is a known function of the ratio of the volume

of cement to the volume of the voids in the concrete. The effect of

varying the proportion of the ingredients, including an increase in the

amount of mixing water beyond that required to give the densest mixture,

may be found by the method, and a comparison may be made of results

obtainable with different classes of fine and coarse aggregates.

Arbitrarily selected proportions, proportions based on voids, and

proportions based on trial mixtures are usually satisfactory for small

jobs where the amount of materials involved is not large. Where the

saving in materials will permit, more accurate methods should be used.

The methods can be studied more fully by reference to the original

articles quoted in the footnotes, or to the following texts:

Materials of Construction, Johnson, 5th Edition, 1918.

Materials of Engineering, H. F. Moore, 2d Edition, 1920.

Masonry Construction, I. O. Baker, 10th Edition, 1912.

Concrete Engineer’s Handbook, Hool and Johnson, 1918.

Concrete, Plain and Reinforced, Taylor and Thompson, 1916.

=94. Waterproofing Concrete.=—The waterproofing of concrete is most

satisfactorily done by making dense mixtures. In practice such

substances as hydrated lime, clay, alum and soap, and proprietary

compounds such as Ceresit, Medusa, etc., are frequently mixed with the

concrete under the theory that these very fine substances will fill any

remaining voids and render the concrete impervious. The specifications

of the Joint Committee issued on June 4, 1921, are much briefer and

contain less detailed instruction than those issued earlier.[60] The

earlier instructions follow.

Many expedients have been resorted to for making concrete

impervious to water. Experience shows, however, that when mortar

or concrete is proportioned to obtain the greatest practicable

density and is mixed to the proper consistency, the resulting

mortar or concrete is impervious under moderate pressure.

On the other hand concrete of dry consistency is more or less

pervious to water, and, though compounds of various kinds have

been mixed with the concrete or applied as a wash to the surface,

in an effort to offset this defect, these expedients have

generally been disappointing, for the reason that many of these

compounds have at best but temporary value, and in time lose their

power of imparting impermeability to the concrete.

In the case of subways, long retaining walls, and reservoirs,

provided the concrete itself is impervious, cracks may be so

reduced, by horizontal and vertical reinforcement properly

proportioned and located, that they will be too minute to permit

leakage, or will be closed by infiltration of silt.

Asphaltic or coal tar preparations applied either as a mastic or

as a coating on felt cloth or fabric, are used for waterproofing,

and should be proof against injury by liquids or gases.

For retaining and similar walls in direct contact with the earth,

the application of one or two coatings of hot coal tar pitch,

following a painting with a thin wash of coal tar dissolved in

benzol, to the thoroughly dried surface of concrete is an

efficient method of preventing the penetration of moisture from

the earth.

Tar paper and asphaltic compounds are not often used in sewer work as

absolute imperviousness is seldom necessary.

=95. Mixing and Placing Concrete.=—Careful workmanship is desirable in

the mixing and placing of concrete in sewers since water-tight

construction is desired. Because of the difficulty of inspecting

concrete in wet, dark and crowded excavations, and the careless habits

of workmen experienced in concrete sewer construction, the highest class

of concrete work cannot be expected. The situation is met by designing

thick walls as shown in the sections illustrated in Fig. 22 and 23.

In the report of the Joint Committee on Concrete and Reinforced Concrete

in Transactions of the American Society of Civil Engineers for 1917, on

page 1101 the recommendation is made concerning the mixing and placing

of concrete as follows:[61]

The mixing of concrete should be thorough and should continue

until the mass is uniform in color and is homogeneous. As the

maximum density and greatest strength of a given mixture depends

largely on thorough and complete mixing, it is essential that this

part of the work should receive special attention and care.

Inasmuch as it is difficult to determine by visual inspection

whether the concrete is uniformly mixed, especially where

aggregates having the color of cement are used, it is essential

that the mixing should occupy a definite period of time. The

minimum time will depend on whether the mixing is done by machine

or hand.

(_a_) Measuring Ingredients: Methods of measurement of the various

ingredients should be used which will secure at all times separate

and uniform measurements of cement, fine aggregate, coarse

aggregate and water.

(_b_) Machine Mixing: The mixing should be done in a batch machine

mixer of a type which will insure the uniform distribution of the

materials throughout the mass, and should continue for the minimum

time of 1½ minutes after all the ingredients are assembled in the

mixer. For mixers of 2 or more cubic yards capacity, the minimum

time of mixing should be 2 minutes. Since the strength of the

concrete is dependent on thorough mixing, a longer time than this

minimum is preferable. It is desirable to have the mixer equipped

with an attachment for automatically locking the discharging

device so as to prevent the emptying of the mixer until all the

materials have been mixed together for the minimum time required

after they are assembled in the mixer. Means should be provided to

prevent aggregates being added after the mixing has commenced. The

mixer should also be equipped with water storage, and an automatic

measuring device which can be locked if desired. It is also

desirable to equip the mixer with a device recording the

revolutions of the drum. The number of revolutions should be so

regulated as to give at the periphery of the drum a uniform speed.

About 200 feet per minute seems to be the best speed in the

present state of the art.

(_c_) Hand Mixing: Hand mixing should be done on a water-tight

platform and especial precautions taken after the water has been

added, to turn all the ingredients together at least 6 times, and

until the mass is homogeneous in appearance and color.

(_d_) Consistency: The materials should be mixed wet enough to

produce a concrete of such a consistency as will flow sluggishly

into the forms and about the metal reinforcement when used, and

which at the same time can be conveyed from the mixer to the forms

without separation of the coarse aggregate from the mortar. The

quantity of water is of the greatest importance in securing

concrete of maximum strength and density; too much water is as

objectionable as too little.

(_e_) Retempering: The remixing of concrete and mortar that has

partly reset should not be permitted.

_Placing Concrete_

(_a_) Methods: Concrete after the completion of the mixing should

be conveyed rapidly to the place of final deposit; under no

circumstances should concrete be used that has partly set.

Concrete should be deposited in such a manner as will permit the

most thorough compacting such as can be obtained by working with a

straight shovel or slicing tool kept moving up and down until all

the ingredients are in their proper place. Special care should be

exercised to prevent the formation of laitance; where laitance has

formed it should be removed, since it lacks strength and prevents

a proper bond in the concrete.

Care should be taken that the forms are substantial and thoroughly

wetted (except in freezing weather) or oiled, and that the space

to be occupied by the concrete is free from all debris. When the

placing of concrete is suspended, all necessary grooves for

joining future work should be made before the concrete has set.

When work is resumed concrete previously placed should be

roughened, cleansed of foreign material and laitance, thoroughly

wetted and then slushed with a mortar consisting of one part

Portland cement and not more than 2 parts of fine aggregate.

The surfaces of concrete exposed to premature drying should be

kept covered and wet for at least 7 days.

Where concrete is conveyed by spouting, the plant should be of

such a size and design as to insure a practically continuous

stream in the spout. The angle of the spout with the horizontal

should be such as to allow the concrete to flow without separation

of the ingredients; in general an angle of about 27 degrees or 1

vertical to 2 horizontal is good practice. The spout should be

thoroughly flushed with water before and after each run. The

delivery from the spout should be as close as possible from the

point of deposit. Where the discharge must be intermittent, a

hopper should be provided at the bottom. Spouting through a

vertical pipe is satisfactory when the flow is continuous; when it

is checked and discontinuous it is highly objectionable unless the

flow is checked by baffle plates.

(_b_) Freezing Weather: Concrete should not be mixed or deposited

at a freezing temperature, unless special precautions are taken to

prevent the use of materials covered with ice crystals or

containing frost, and to prevent the concrete from freezing before

it has set and sufficiently hardened.

As the coarse aggregate forms the greater portion of the concrete,

it is particularly important that this material be warmed to well

above the freezing point.

The enclosing of a structure and the warming of a space inside the

enclosure is recommended, but the use of salt to lower the

freezing point is not recommended.

(_c_) Rubble Concrete: Where the concrete is to be deposited in

massive work, its value may be improved and its cost materially

reduced by the use of clean stones saturated with water,

thoroughly embedded in and completely surrounded by concrete.

(_d_) Under Water: In placing concrete under water, it is

essential to maintain still water at the place of deposit. With

careful inspection the use of tremies, properly designed and

operated, is a satisfactory method of placing concrete through

water. The concrete should be mixed very wet (more so than is

ordinarily permissible) so that it will flow readily through the

tremie and into place with practically a level surface.

The coarse aggregate should be smaller than ordinarily used and

never more than one inch in diameter. The use of gravel

facilitates the mixing and assists the flow. The mouth of the

tremie should be buried in the concrete so that it is at all times

entirely sealed and the surrounding water prevented from forcing

itself into the tremie. The concrete will then discharge without

coming in contact with the water. The tremie should be suspended

so that it can be lowered quickly when it is necessary either to

choke off or to prevent too rapid flow. The lateral flow

preferably should not be over 15 feet.