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short time (ordinarily, perhaps, 3 to 5 minutes), unless fires are first aroused to make fresh steam to replace that withdrawn.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 341⁄2 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 11⁄8-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 11⁄8-inch hose stream under the ordinary practical conditions as to pipes supplying sprinkler and hose systems respectively.
4. Capacity.
a. Plunger diameter alone will not tell how many gallons per minute a pump can deliver, and it is not reasonable to continue the old time notion of estimating capacity on the basis of 100 feet per minute piston travel.
b. The capacity of a pump depends on the speed at which it can be run, and the speed depends largely on the arrangement of valves and passageways for water and steam.
c. It is all right to run fire-pumps at the highest speed that is possible without causing violent jar, or hammering within the cylinders. Considerations of wear do not affect the brief periods of fire services or test, hence these speeds are greater than those allowable for constant daily duty.
d. Careful experiments on a large number of pumps of various makes at full speed, show that in a new pump with clean valves, and an air-tight suction pipe, and less than 15 feet lift, the actual delivery is only from 11⁄2 to 5 per cent. less than plunger displacement. This slip will increase with wear, and for a good average pump in practical use, probably 10 per cent. is a fair allowance to cover slip, valve leakage, slight short-stroke, etc.
e. Largely from tests, but partly from “average judgment,” and recognizing that a long stroke pump can run at a higher rate of piston travel in lineal feet per minute than a short stroke pump, and that a small pump can make more strokes per minute than a very large one, the speeds given in the preceding table have been adopted as standards in fire service for direct acting (non-fly-wheel) steam pumps, which have the large steam and water passages herein specified.
f. Rated capacity is to be based on the speed in the preceding table, correcting the plunger displacement for one-half the rod area and deducting 10 per cent. for slip, short-stroke, etc.
Men sometimes ask why, (if they can run a pump smoothly so as to get a delivery of 1,000 gals. per minute,) we should not accept as “a thousand gallon pump,” irrespective of its suction valve area or its exhaust port area or the size of its cylinders.
To this we reply that when new and favorably set almost any pump built according to these specifications can run at a much greater delivery than here rated, but when lift is unusually high or suction pipe long, or when the pump takes its suction under a head, no pump can be run so fast as on, for instance, a 5-foot lift. A solid foundation is also a great and indispensable aid in running a pump fast.
Standard 500-gallon pumps have often delivered 800 gallons, and 1,500-gallon pumps have delivered 2,000 gallons; but some margin must be allowed for unfavorable conditions and for deterioration as a pump grows old, or for the absence of an expert to get its utmost duty.
5. Capacity Plate.
a. Every steam fire-pump must bear a conspicuous statement of its capacity securely attached to the inboard side of air chamber, thus:—
NATIONAL STANDARD FIRE PUMP
16 × 9 × 12
CAPACITY
750 GALLONS PER MINUTE, OR
3 GOOD 11⁄8-IN. SMOOTH NOZZLE
FIRE STREAMS
FULL SPEED
70 REVOLUTIONS PER MINUTE
NEVER LET STEAM GET BELOW
50 POUNDS, NIGHTS, SUNDAYS
OR AT ANY OTHER TIME
The name “Underwriter” has been largely used for a considerable time to designate the type of pump covered by the principal features of these specifications. While our preferences are against the use of this word as designating any piece of apparatus objections will not be raised at the present time to its being continued on name plates in place of the words “National Standard,” if manufacturers so desire.
b. This plate must have an area of not less than one square foot, and must be made of an alloy at least two-thirds aluminum and the remainder zinc. The letters must be at least one-half inch in height, plain and distinct, with their surfaces raised on a black background and buffed off to a dead smooth finish.
The name of pump manufacturer may also be placed on this plate, if desired.
c. A smaller plate of composition must be attached to steam chest bearing the size of pump, the shop number, and the name of shop in which the pump was built.
6. Strength of Parts.
a. The maker must warrant each pump built under these specifications to be at time of delivery, in all its parts, strong enough to admit of closing all valves on water outlet pipes while steam valve is wide open and steam pressure eighty pounds, and agree to so test it before shipment from his works.
b. The pump must be warranted so designed and with such arrangement of thickness of metal that it shall be safe to instantly turn a full head of steam on to a cold pump without cracking or breaking the same by unequal expansion.
7. Shop Inspection.
A systematic shop inspection must be given to each pump to ensure completed workmanship, and to prevent the use of defective parts, improper materials, or the careless leaving of foreign matter in any part of the cylinders or chests.
Several instances have occurred in which chisels, bolts, or core irons have been found in steam chests or steam cylinders. This has resulted in a serious crippling of the pump and in some cases requiring repairs to be made before the pump could be used for fire purposes.