Page 125 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

side of the pump is separated from the activity on the discharge side of
the pump. Gauge 1 continues to read 25 psi. Gauge 2 should also
record 25 psi. Gauge 3 should now be reading 15 psi, because this
gauge is 23.1 feet above gauges 1 and 2.
However, gauge 3 is recording 13 psi (it should be reading 15 psi) with
the system running. The APSr is 12 psi.

Gauges 1 and 2 should be reading the same pressure with the system running, as
gauge 1 was reading with the system off. If you're using precision digital pressure
instrumentation gauges, gauge 2 might possibly record a fraction psi less. This is
because gauge 2 is now recording minute losses between the tank and the gauge
including losses through the opening into the pipe and the losses through the gate
valve.
If there should be a divergence in the readings of the two gauges, something is out of
control. There might be an obstruction at the tank drain line or maybe the gate valve
is not totally open. Maybe the level has dropped in the tank. Maybe the vent valve on
the tank top is not open. Maybe the gauges need calibration. Send them to a
calibration shop a couple of times per year. But, isn't it interesting how much more
you know about your system after learning to interpret the pressure gauges. Who is
responsible for specifying, selling, and buying pumps without adequate
instrumentation?

Now we consider the pump. We've already discussed in this book that
the pump takes the energy that the suction gives it, the pump adds
more energy, jacking the energy up to discharge pressure. In this case
the pump is designed with a REP of 94 feet, which also is the TDH of
the system. The 94 feet indicate that the pump can generate about 40
psi at 300 gpm (94 + 2.31 = 40.6 psi if the liquid is water). This is
confirmed with a flow meter and a pump curve. The suction pressure is
13 psi. The discharge pressure gauge (4 gauge) should be reading 53
psi (40 + 13).

The pump's discharge pressure is a function of the suction pressure. Regrettably, most
pumps in the world don't have a gauge reading suction pressure. In our example here,
if our pump is generating less than 40 psi, the pump is operating to the right of its
BEP, and is losing efficiency. Was the pump assembled correctly? Was it repaired
correctly, with all parts machined to their correct tolerances? Is the motor's velocity
correct? Is there a flow meter installed? The pump is always on its curve. If this pump
were generating more than 40 or 41 psi, it would be operating to the left of its BEP.
Verify the other factors.