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short time (ordinarily, perhaps, 3 to 5 minutes), unless fires are first aroused to make fresh steam to replace that withdrawn.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.