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Kevin Long
science forum beginner

Joined: 11 May 2005
Posts: 32

Posted: Fri Jun 02, 2006 2:06 am    Post subject: Centrifugal Pump Analysis

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.

Regards,

KL
p_bihari@yahoo.com
science forum beginner

Joined: 03 May 2006
Posts: 13

 Posted: Fri Jun 02, 2006 7:28 am    Post subject: Re: Centrifugal Pump Analysis 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

Posted: Fri Jun 02, 2006 12:23 pm    Post subject: Re: Centrifugal Pump Analysis

 Quote: 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 we try to inrease the flow (ie. opening the discharge valve) it does
trip.

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

Have been using the curves, but things don't seem to be adding up.

 Quote: 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.

Thank you for your offer, but to send you this information would violate
confidential company information.

Regards,

KL
Kelvin Hales
science forum beginner

Joined: 03 May 2005
Posts: 45

Posted: Fri Jun 02, 2006 2:10 pm    Post subject: Re: Centrifugal Pump Analysis

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

 Posted: Sat Jun 03, 2006 8:44 am    Post subject: Re: Centrifugal Pump Analysis 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

Posted: Sun Jun 04, 2006 11:31 am    Post subject: Re: Centrifugal Pump Analysis

KL <me@place.com> wrote in message news:F2Nfg.220461\$P01.90225@pd7tw3no...
 Quote: 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 Check the electrical side first, as it's relatively easy to exonerate with

confidence. Do an injection test on the current measurement and protection
relay. Also check the supply voltage AND voltage balance, if the latter
isn't within a percent or two, you can get proportionately high current
imbalance, overheating and premature tripping. If all that is OK and the
motor isn't *seriously* overheating or vibrating, then you can be pretty
confident that the electrical side is performing to spec.

You don't mention how large the shortfall is, if it's not fairly gross then
in my experience your measurement based approach may not tell you a lot.
Also, is this a new untried installation or one that has worked fine in the
past but recently degraded, if the latter you'd be better off taking a
really good look at what's changed.

Not saying that there may not be something amiss, but the majority of these
cases I've observed with new installations have been simple cases of
incorrect sizing.

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