Page 123 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

Consider all the mathematical gyrations required just to determine the
Hv and Hf. This is a lot of math for one pump. Imagine the work to
specify pumps for a paper mill or beer brewery or municipal water
system. Now you can see why governments and pharmaceutical
companies contract consulting engineering companies to do this work
and specify the pumps. Finally, we can calculate the TDH of the
sys tem :
TDH = HS + Hp + Hf + HV
= 80 ft + 0 ft + 12.595 ft + 1.06 ft
= 93.655 ft
This system requires a pump with a best efficiency point (BEP) of 94
feet at 300 gallons per minute. If this is a conventional industrial
centrifugal pump with a BEP of 94 feet, the shut-off head should be
approximately 110 feet. And if the motor is a standard NEMA four-
pole motor spinning at about 1800 rpm, the diameter of the impeller
should be approximately 10.5 inches. If this pump were bought off the
shelf from local distributor stock, it would probably be a 3 x 4 x 12
model end-suction centrifugal back pullout pump with the impeller
machined to about 10.5 inches before installing the pump into the
system. And that’s the way it is done.
If the system already exists and the equipment is running, we can
recover the Hf and Hv from gauges using the Bachus & Custodio
Method, and forget about all those calculations. See Figure 8-8
opposite, with the corresponding elevations and placement of pressure
gauges installed into the piping numbered 1 through 5.
In this system drawing, pressure gauges 1, 2, and 3 are in the suction
piping. Gauges 4 and 5 are in the discharge piping. With the system
and pump turned off, we would open the vent valves on both the
suction and discharge tanks, this assures that both sides of the system
are atmospheric and cancels the Hp. The discharge tank and all piping
should be full with water for the test, or if required, the pumped liquid.
Remember that gauge readings will be adjusted by the specific gravity.
Expel all air bubbles in the piping. Some pumps have a little petcock
valve to allow expelling any trapped air in the volute. On the pump,
conventional stuffing boxes can also trap air. This must be expelled too.
Vertical valve stems in the piping can trap air. Loosen the packing to
expel this trapped air. This is done so that there is a complete column of
liquid from the top to the bottom of the system. Air pockets and
bubbles might cause inaccurate pressure gauge readings. All valves in
the column (including the check valve) should be opened, except for
the gate valve between gauges 1 and 2. It should be closed to hold the
column of liquid and prevent draining the line.