+-----+--+----+-----+------+------+------+------+-------+-------+-------+
|S E D|B | T | I D |Circumference| Transverse |Square |Length |Nominal|
|i x i|. | h | n i | | Area | Feet |in Feet|Weight |
|z t a|W | i | t a | |Square Inches| of | per |Pounds |
|e e m|. | c | e m +------+------+------+------+ Exter |Square | per |
| r e| | k | r e |Exter-|Inter-|Exter-|Inter-| -nal |Foot of| Foot |
| n t|G | n | n t | nal | nal | nal | nal |Surface| Exter | |
| a e|a | e | a e | | | | | per | -nal | |
| l r|u | s | l r | | | | |Foot of|Surface| |
| |g | s | | | | | |Length | | |
| |e | | | | | | | | | |
+-----+--+----+-----+------+------+------+------+-------+-------+-------+
|1-1/2|10|.134|1.232| 4.712| 3.870|1.7671|1.1921| .392 | 2.546 | 1.955 |
|1-1/2| 9|.148|1.204| 4.712| 3.782|1.7671|1.1385| .392 | 2.546 | 2.137 |
|1-1/2| 8|.165|1.170| 4.712| 3.676|1.7671|1.0751| .392 | 2.546 | 2.353 |
| 2 |10|.134|1.732| 6.283| 5.441|3.1416|2.3560| .523 | 1.909 | 2.670 |
| 2 | 9|.148|1.704| 6.283| 5.353|3.1416|2.2778| .523 | 1.909 | 2.927 |
| 2 | 8|.165|1.670| 6.283| 5.246|3.1416|2.1904| .523 | 1.909 | 3.234 |
|3-1/4|11|.120|3.010|10.210| 9.456|8.2958|7.1157| .850 | 1.175 | 4.011 |
|3-1/4|10|.134|2.982|10.210| 9.368|8.2958|6.9840| .850 | 1.175 | 4.459 |
|3-1/4| 9|.148|2.954|10.210| 9.280|8.2958|6.8535| .850 | 1.175 | 4.903 |
| 4 |10|.134|3.732|12.566|11.724|12.566|10.939| 1.047 | .954 | 5.532 |
| 4 | 9|.148|3.704|12.566|11.636|12.566|10.775| 1.047 | .954 | 6.000 |
| 4 | 8|.165|3.670|12.566|11.530|12.566|10.578| 1.047 | .954 | 6.758 |
+-----+--+----+-----+------+------+------+------+-------+-------+-------+
Dimensions are nominal and except where noted are in inches.
Pipe Material and Thickness--For saturated steam pressures not exceeding
160 pounds, all pipe over 14 inches should be 3/8 inch thick O. D. pipe.
All other pipe should be standard full weight, except high pressure
feed[77] and blow-off lines, which should be extra strong.
For pressures above 150 pounds up to 200 pounds with superheated steam,
all high pressure feed and blow-off lines, high pressure steam lines
having threaded flanges, and straight runs and bends of high pressure
steam lines 6 inches and under having Van Stone joints should be extra
strong. All piping 7 inches and over having Van Stone joints should be
full weight soft flanging pipe of special quality. Pipe 14 inches and
over should be 3/8 inch thick O. D. pipe. All pipes for these pressures
not specified above should be full weight pipe.
Flanges--For saturated steam, 160 pounds working pressure, all flanges
for wrought-iron pipe should be cast-iron threaded. All high pressure
threaded flanges should have the diameter thickness and drilling in
accordance with the "manufacturer's standard" for "extra heavy" flanges.
All low pressure flanges should have diameter, thickness and drilling in
accordance with "manufacturer's standard" for "standard flanges."
The flanges on high pressure lines should be counterbored to receive
pipe and prevent the threads from shouldering. The pipe should be
screwed through the flange at least 1/16 inch, placed in machine and
after facing off the end one smooth cut should be taken over the face of
the flange to make it square with the axis of the pipe.
[Illustration: 2000 Horse-power Installation of Babcock & Wilcox Boilers
and Superheaters, Equipped with Babcock & Wilcox Chain Grate Stokers at
the Kentucky Electric Co., Louisville, Ky.]
For pressures above 160 pounds, where superheated steam is used, all
high pressure steam lines 4 inches and over should have solid rolled
steel flanges and special upset lapped joints. In the manufacture of
such joints, the ends of the pipe are heated and upset against the face
of a holding mandrel conforming to the shape of the flange, the lapped
portion of the pipe being flattened out against the face of the mandrel,
the upsetting action maintaining the desired thickness of the lap. When
cool, both sides of the lap are faced to form a uniform thickness and an
even bearing against flange and gasket. The joint, therefore, is a
strictly metal to metal joint, the flanges merely holding the lapped
ends of the pipe against the gasket.
A special grade of soft flanging pipe is selected to prevent breaking.
The bending action is a severe test of the pipe and if it withstands the
bending process and the pressure tests, the reliability of the joint is
assured. Such a joint is called a Van Stone joint, though many
modifications and improvements have been made since the joint was
originally introduced.
The diameter and thickness of such flanges should be special extra
heavy. Such flanges should be turned to diameter, their fronts faced and
the backs machined in lieu of spot facing.
In lines other than given for pressures over 150 pounds, all flanges for
wrought-iron pipe should be threaded. All threaded flanges for high
pressure superheated lines 3½ inches and under should be "semi-steel"
extra heavy. Flanges for other than steam lines should be manufacturer's
standard extra heavy.
Welded flanges are frequently used in place of those described with
satisfactory results.
Fittings--For saturated steam under pressures up to 160 pounds, all
fittings 3½ inches and under should be screwed. Fittings 4 inches and
over should have flanged ends. Fittings for this pressure should be of
cast iron and should have heavy leads and full taper threads. Flanged
fittings in high pressure lines should be extra heavy, and in low
pressure lines standard weight. Where possible in high pressure flanges
and fittings, bolt surfaces should be spot faced to provide suitable
bearing for bolt heads and nuts.
Fittings for superheated steam up to 70 degrees at pressures above 160
pounds are sometimes of cast iron.[78] For superheat above 70 degrees
such fittings should be "steel castings" and in general these fittings
are recommended for any degree of superheat. Fittings for other than
high pressure work may be of cast iron, except where superheated steam
is carried, where they should be of "wrought steel" or "hard metal".
Fittings 3½ inches and under should be screwed, 4 inches and over
flanged.
Flanges for pressures up to 160 pounds in pipes and fittings for low
pressure lines, and any fittings for high pressure lines should have
plain faces, smooth tool finish, scored with V-shaped grooves for rubber
gaskets. High pressure line flanges should have raised faces, projecting
the full available diameter inside the bolt holes. These faces should be
similarly scored.
All pipe ½ inch and under should have ground joint unions suitable for
the pressure required. Pipe ¾ inch and over should have cast-iron
flanged unions. Unions are to be preferred to wrought-iron couplings
wherever possible to facilitate dismantling.
Valves--For 150 pounds working pressure, saturated steam, all valves 2
inches and under may have screwed ends; 2½ inches and over should be
flanged. All high pressure steam valves 6 inches and over should have
suitable by-passes. All valves for use with superheated steam should be
of special construction. For pressures above 160 pounds, where the
superheat does not exceed 70 degrees, valve bodies, caps and yokes are
sometimes made of cast iron, though ordinarily semi-steel will give
better satisfaction. The spindles of such valves should be of bronze and
there should be special necks with condensing chambers to prevent the
superheated steam from blowing through the packing. For pressures over
160 pounds and degrees of superheat above 70, all valves 3 inches and
over should have valve bodies, caps and yokes of steel castings.
Spindles should be of some non-corrosive metal, such as "monel metal".
Seat rings should be removable of the same non-corrosive metal as should
the spindle seats and plug faces.
All salt water valves should have bronze spindles, sleeves and packing
seats.
The suggestions as to flanges for different classes of service made on
page 311 hold as well for valve flanges, except that such flanges are
not scored.
Automatic stop and check valves are coming into general use with boilers
and such use is compulsory under the boiler regulations of certain
communities. Where used, they should be preferably placed directly on
the boiler nozzle. Where two or more boilers are on one line, in
addition to the valve at the boiler, whether this be an automatic valve
or a gate valve, there should be an additional gate valve on each boiler
branch at the main steam header.
Relief valves should be furnished at the discharge side of each feed
pump and on the discharge side of each feed heater of the closed type.
Feed Lines--Feed lines should in all instances be made of extra strong
pipe due to the corrosive action of hot feed water. While it has been
suggested above that cast-iron threaded flanges should be used in such
lines, due to the sudden expansion of such pipe in certain instances
cast-iron threaded flanges crack before they become thoroughly heated
and expand, and for this reason cast-steel threaded flanges will give
more satisfactory results. In some instances, wrought-steel and Van
Stone joints have been used in feed lines and this undoubtedly is better
practice than the use of cast-steel threaded work, though the additional
cost is not warranted in all stations.
Feed valves should always be of the globe pattern. A gate valve cannot
be closely regulated and often clatters owing to the pulsations of the
feed pump.
Gaskets--For steam and water lines where the pressure does not exceed
160 pounds, wire insertion rubber gaskets 1/16 inch thick will be found
to give good service. For low pressure lines, canvas insertion black
rubber gaskets are ordinarily used. For oil lines special gaskets are
necessary.
For pressure above 160 pounds carrying superheated steam, corrugated
steel gaskets extending the full available diameter inside of the bolt
holes give good satisfaction. For high pressure water lines wire
inserted rubber gaskets are used, and for low pressure flanged joints
canvas inserted rubber gaskets.
Size of Steam Lines--The factors affecting the proper size of steam
lines are the radiation from such lines and the velocity of steam within
them. As the size of the steam line increases, there will be an increase
in the radiation.[79] As the size decreases, the steam velocity and the
pressure drop for a given quantity of steam naturally increases.
There is a marked tendency in modern practice toward higher steam
velocities, particularly in the case of superheated steam. It was
formerly considered good practice to limit this velocity to 6000 feet
per minute but this figure is to-day considered low.
In practice the limiting factor in the velocity advisable is the
allowable pressure drop. In the description of the action of the
throttling calorimeter, it has been demonstrated that there is no loss
accompanying a drop in pressure, the difference in energy between the
higher and lower pressures appearing as heat, which, in the case of
steam flowing through a pipe, may evaporate any condensation present or
may be radiated from the pipe. A decrease in pipe area decreases the
radiating surface of the pipe and thus the possible condensation. As the
heat liberated by the pressure drop is utilized in overcoming or
diminishing the tendency toward condensation and the heat loss through
radiation, the steam as it enters the prime mover will be drier or more
highly superheated where high steam velocities are used than where they
are lower, and if enough excess pressure is carried at the boilers to
maintain the desired pressure at the prime mover, the pressure drop
results in an actual saving rather than a loss. The whole is analogous
to standard practice in electrical distributing systems where generator
voltage is adjusted to suit the loss in the feeder lines.
In modern practice, with superheated steam, velocities of 15,000 feet
per minute are not unusual and this figure is very frequently exceeded.
Piping System Design--With the proper size of pipe to be used
determined, the most important factor is the provision for the removal
of water of condensation that will occur in any system. Such
condensation cannot be wholly overcome and if the water of condensation
is carried to the prime mover, difficulties will invariably result.
Water is practically incompressible and its effect when traveling at
high velocities differs little from that of a solid body of equal
weight, hence impact against elbows, valves or other obstructions, is
the equivalent of a heavy hammer blow that may result in the fracture of
the pipe. If there is not sufficient water in the system to produce this
result, it will certainly cause knocking and vibration in the pipe,
resulting eventually in leaky joints. Where the water reaches the prime
mover, its effect will vary from disagreeable knocking to disruption.
Too frequently when there are disastrous results from such a cause the
boilers are blamed for delivering wet steam when, as a matter of fact,
the evil is purely a result of poor piping design, the most common cause
of such an action being the pocketing of the water in certain parts of
the piping from whence it is carried along in slugs by the steam. The
action is particularly severe if steam is admitted to a cold pipe
containing water, as the water may then form a partial vacuum by
condensing the steam and be projected at a very high velocity through
the pipes producing a characteristic sharp metallic knock which often
causes bursting of the pipe or fittings. The amount of water present
through condensation may be appreciated when it is considered that
uncovered 6-inch pipe 150 feet long carrying 3600 pounds of high
pressure steam per hour will condense approximately 6 per cent of the
total steam carried through radiation. It follows that efficient means
of removing condensation water are absolutely imperative and the
following suggestions as to such means may be of service:
The pitch of all pipe should be in the direction of the flow of steam.
Wherever a rise is necessary, a drain should be installed. All main
headers and important branches should end in a drop leg and each such
drop leg and any low points in the system should be connected to the
drainage pump. A similar connection should be made to every fitting
where there is danger of a water pocket.
Branch lines should never be taken from the bottom of a main header but
where possible should be taken from the top. Each engine supply pipe
should have its own separator placed as near the throttle as possible.
Such separators should be drained to the drainage system.
Check valves are frequently placed in drain pipes to prevent steam from
entering any portion of the system that may be shut off.
Valves should be so located that they cannot form water pockets when
either open or closed. Globe valves will form a water pocket in the
piping to which they are connected unless set with the stem horizontal,
while gate valves may be set with the spindle vertical or at an angle.
Where valves are placed directly on the boiler nozzle, a drain should be
provided above them.
High pressure drains should be trapped to both feed heaters and waste
headers. Traps and meters should be provided with by-passes. Cylinder
drains, heater blow-offs and drains, boiler blow-offs and similar lines
should be led to waste. The ends of cylinder drains should not extend
below the surface of water, for on starting up or on closing the
throttle valve with the drains open, water may be drawn back into the
cylinders.