Page 348 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 333

the motor. Reciprocating Pumps
I did say there’s a standard formula for pump Many boiler plant applications were predominantly
horsepower. There is, it’s called Hydraulic Horsepower, served by reciprocating piston pumps until the middle
th
is also called theoretical horsepower, and it can be cal- of the 20 century when multi-stage centrifugal pumps
culated by multiplying the flow in gallons per minute displaced them. For that matter most of the liquids in
by the head in feet and dividing by 3960. If the liquid the plant were moved by the standard duplex reciprocat-
isn’t water at 8.33 pounds per gallon, multiply by the ing pump (Figure 10-68) which was the mainstay of the
specific gravity of the liquid. Note that it’s theoretical power plant at the beginning of that century. The pump,
horsepower. Divide by the pump efficiency to get brake powered by steam from the boiler, was capable of pro-
horsepower, the amount the driver has to produce. If ducing very high pressures and, despite the reciprocating
you don’t know the efficiency use 33% (multiply the operation, produced a reasonably constant output.
theoretical horsepower by 3) to be safe. The pressure differential of the pumped liquid is
determined by the difference between the steam supply
and exhaust pressures and the ratio of
the cylinder areas. The maximum pres-
sure that could be produced, an impor-
tant consideration for selecting valves
and piping materials, is the area of the
face of the steam piston less the area of
the connecting rod times the difference
in steam supply and exhaust pressures
divided by the area of the fluid piston
less the area of the connecting rod (Fig-
ure 10-69).
There were, and still are, single
piston pumps consisting of one steam
cylinder and one fluid cylinder but it was
difficult to adjust them so they would
operate continuously, occasionally hang-
Figure 10-68. Duplex ing up at one end of the stroke or an-
reciprocating pump other. Most of those were larger pumps
used for fuel oil and ballast (water) trans-
fer aboard the ships. The duplex pump
practically eliminated problems with the
pumps hanging up because the stroking
of one piston tripped the valve to reverse the other. It’s
difficult to see in the photograph but the linkage attached
to one shaft operates the control valve for the other. A
significant problem with these pumps was the lubrication
which tended to get into the condensate and then into
the boiler. They also had a lot of sliding parts that would
wear and required constant maintenance. Internal or
external check valves also slammed open and shut with
eventual wear and breakage.
Another form of reciprocating pump that can still
be found, principally in boiler feed use is a three piston
eccentric cranked motor driven pump. The pistons are
solid so they only pumped in one direction. Each of the
three pistons operated off a different crank arm so the
output was a little more uniform. The balance of pistons
Figure 10-69. Areas of pistons for pump pressure and a heavy counterweight on the shaft helped reduce