_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 1-1/2 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.