Centrifugal Pump Analysis

Kevin Long · science forum beginner Joined: 11 May 2005 Posts: 32

Hello,

I have a centrifugal pump that is not producing the flow that it is rated
for even when running the motor attached is at full load amps (The fluid
being pumped is water).

I know the following for the pump: flow, suction pressure, discharge
pressure, pump efficiency, inlet diameter, outlet diameter.

For the motor I have: voltage, current, power factor and efficiency. The
motor is a 3 phase AC (60 Hz).

It is suspected that the motor for this pump is "maxed out" meaning that if
we increase the suction pressure and reduce any downstream back-pressures,
we will not get any more flow thru the pump because the motor is maxed out.

After some thinking, I devised a plan. Using the pump process data stated
above, I would calculate a brake horse power (bhp). When you know the bhp,
you use the motor efficiency and calculate the electrical input in kW.

Next using the motor electrical data, I can calculate the electrical input
using the equation:
kW = (sqrt(3) x V x I x pf)/1000 (Voltage is in V, Current is in
Amps, pf is the power factor)

Now that I have the electrical kW using both methods, I would compare each
kW answer and determine if the motors are really maxed out.

I would appreciate some comments.

Regards,

KL

p_bihari@yahoo.com · science forum beginner Joined: 03 May 2006 Posts: 13

I doubt your "Maxed Out" is a true situation or not.
Because if u have margin in ur pump e.g. (reducing head developed), the
flow thru the pump will go up & if for that flow motor is not
sufficient it should immediately trip.

If this is not happening in ur case then first consult ur curves.

As u told that pump is not developing rated flow & u know all actual
parameters there are many other possibilities than doubting on motor.

If u have this design & actual data send it on excel sheet, Probably I
can tell you the cause, as I have done more than 20 Energy Audits in
utilities.

Kevin Long · science forum beginner Joined: 11 May 2005 Posts: 32

Thank you for your response.

Kelvin Hales · science forum beginner Joined: 03 May 2005 Posts: 45

I'm guessing either:

a) You are not interpreting using the pump curves correctly
b) The pump suction conditions are not as per the design; e.g. you have
a different density of fluid at the inlet; possibly due to aeration or
flashing.
c) The pump impeller is damaged/corroded/eroded.
d) The pump recycle/bypass valve is open.

Using conservation of energy: if you are putting in the full rated
power and not getting the equivalent increase in head & flow, then the
pump efficiency must have gone down significantly and the energy must
be going into heat.

Re-do the measurements and calculations.

Kelvin B. Hales
Kelvin Hales Associates Limited
Consulting Process Control Engineers
Web: www.khace.com

p_bihari@yahoo.com · science forum beginner Joined: 03 May 2006 Posts: 13

My purpose is simple. Not asking for Confidential info. If u CAN share
design flow, head, power & actual Flow Head & Power + Max Flow point on
the curve with power & head, THEN the purpose is solved.

U decide whether this info violates any law or not.

But it seems that U r making some mistake in judging
the basic reason of failure.

bruce varley · science forum beginner Joined: 31 May 2005 Posts: 38

KL <me@place.com> wrote in message news:F2Nfg.220461$P01.90225@pd7tw3no...

Google