======================+===============+================
| [B] Boiler |
Pump Sizes. | Power | Full Speed.
| Required |
------+-------+-------+------+--------+-------+--------
Steam.| Water.|Stroke.|Horse | Steam | Rev. |Piston
| | |Power.| Pres. | Per |Travel
| | | |at Pump,|Minute.|Feet Per
| | | | lbs. | |Minute.
------+-------+-------+------+--------+-------+--------
14 × 7 × 12 | 100 | 40 | 70 | 140
14 × 7-1/4 × 12 | | | |
----------------------+------+--------+-------+--------
16 × 9 × 12 | 115 | 45 | 70 | 140
----------------------+------+--------+-------+--------
18 × 20 × 12 | 150 | 45 | 70 | 140
18-1/2 × 10-1/4 × 12 | | | |
----------------------+------+--------+-------+--------
20 × 12 × 16 | 200 | 50 | 60 | 160
| | | |
----------------------+------+--------+-------+--------
[Footnote B:
This boiler power is required for continuous running at full speed
and pressure. It is, however, often best to put in a larger pump than
the existing boilers could drive at full capacity, as a _small boiler
will drive a 750-gallon pump at the 500-gallon speed with very nearly
as good economy as it can drive a 500-gallon pump at full speed_. The
pump then does not have to be changed when the plant is enlarged and
the boiler power increased.
A steam piston relatively larger than necessary is a source of
weakness. It takes more volume of steam, and gives more power with
which to burst the pipes if the throttle is opened wide suddenly
during excitement.
It has been common to make all fire-pumps with water plunger of
only one-fourth the area of steam piston, with the idea that pump
could thereby be more readily run at night, when steam was low. The
capacity in gallons is thus reduced 25 per cent. as compared with a 3
to 1 plunger on the same steam cylinders.]
_b._ The above sizes of steam and water cylinders and length of stroke
have given general satisfaction and will now be considered as standard.
Often, especially with large pumps, “4 to 1” construction is a
mistake, and gives no additional security, although the pump might
start and give a few puffs with 30 lbs. of steam on banked fires;
because, if any pump of whatever cylinder ratio draws 50 or 100
horse-power of steam from boilers with dead fires, it can run
effectively only a very short time (ordinarily, perhaps, 3 to 5
minutes), unless fires are first aroused to make fresh steam to
replace that withdrawn.
Steam pressures stated above must be maintained _at the pump_, to
give full speed and 100 lbs. water pressure. Pressure at boilers must
be a little more, to allow for loss of steam pressure between boiler
and pump. Pumps in poor order, or too tightly packed, will require
more steam.
The boiler horse-powers above are reckoned on the A. S. M. E. basis
of 34-1/2 lbs. of water evaporated from and at 212 degrees Fahrenheit
as the unit of boiler horse-power. From 12 to 15 square feet of
water-heating surface in the boiler is commonly assumed necessary for
the generation of one horse-power.
Smaller boilers than called for above, if favorably set, and having
excellent chimney draft, can sometimes be forced to nearly double
their nominal capacity for a short run, as for fire service.
_c._ _250 gallons per minute is the standard allowance for a good
1-1/8-inch (smooth nozzle) fire stream._
A so-called “Ring Nozzle” discharges only three-fourths as much water
as a smooth nozzle of the same bore, and is not recommended.
From fifteen to twenty automatic sprinklers may be reckoned as
discharging about the same quantity as a 1-1/8-inch hose stream under
the ordinary practical conditions as to pipes supplying sprinkler and
hose systems respectively.