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effect of restricted suction upon pump amps (high speed multi-stage centrifugal pump)
5

effect of restricted suction upon pump amps (high speed multi-stage centrifugal pump)

effect of restricted suction upon pump amps (high speed multi-stage centrifugal pump)

(OP)
The pump in question is 10 stage horizontal 7300rpm centrifugal pump.

It is driven through gearbox by a 600hp motor.

For some context, the pump was operating in steady state at 70% motor load (approx 400hp), when suddenly the current began to increase, speed began to decrease, indicated flow went down.

We are investigating a variety of scenarios to explain that (and there's a lot more info not shared), but here for purposes of this thread I'm not concerned with any scenario other than sudden restriction of the suction path.

Would you expect that to cause the amps to increase or decrease? (my uninformed vote is decrease, similar to effect of throttling the dischrage path in radial flow pump, but I'm interested in firming that up or knocking it down).


=====================================
(2B)+(2B)' ?

RE: effect of restricted suction upon pump amps (high speed multi-stage centrifugal pump)

∆P across the pump at constant speed is fixed based on fluid density. Any reduction in inlet conditions are going to reduce density and mass flow rate which will reduce pump power therefore amps.

RE: effect of restricted suction upon pump amps (high speed multi-stage centrifugal pump)

Pumping power is calculated as the volume of the fluid per unit time (flow capacity) times the density of the fluid times the gravitational constant times the pumping head (vertical distance to be pumped). Anything that reduces volume pumped will result in lower amps.

RE: effect of restricted suction upon pump amps (high speed multi-stage centrifugal pump)

"Speed began to decrease"??

How? A fixed speed motor is exactly that - fixed within a few rpm and if you noticed that happening then there is something seriously amiss, but I guess you know that.

On the sudden suction loss - kind of depends what type of pump. Some pumps increase power as the flow goes down - mainly axial or mixed flow so you really need to look at or post the pump curve.

If it's a "standard" centrifugal then yes, less flow normally means less shaft power. You need to add in pump efficiency to bimrs list of things as it is shaft power you're interested in.

But sounds much more "mechanical" than loss of suction.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: effect of restricted suction upon pump amps (high speed multi-stage centrifugal pump)

(OP)
Yes, it was a significant reduction of RPM. We have prox probes and keyphasors on the high speed side which record speed. We can use the gear ratio to compute motor speed from that. The motor nameplate speed is 3551rpm (it's an old motor, these days most large motors have much lower slip). The inital speed was steady state at 3565rpm (71% load estmiate based on slip). Over the course of 2 minutes the speed dropped gradually to 3509rpm (185% load estimate based on slip) at which time the motor tripped. There are obviously a lot of scenarios we can look at including low/unbalanced voltage or motor problem or mechanical problems in the train or increased fluid demand, but in terms of the loss of suction scenario I take it the extreme change in RPM tends to confirm this is not a suction blockage problem?

I do recall for slow speed axial flow pumps, the bhp vs flow curve tends to have a negative slope, especially near shutoff. I realize this is the opposite of axial (pure radial) but I bring it up to mention fluid power is not the whole story, there are also potential losses in heat and possibly vibration. But I gather this type of behavior applies less to pure radial flow. I'm not sure why that is (why the shape of bhp vs flow tends to have negative slope on axial and positive slope on radial), maybe losses associated with internal recirculation play a bigger role in axial than radial? That's a little bit of a detour from original topic but just wondering if I have that right or off in left field.


=====================================
(2B)+(2B)' ?

RE: effect of restricted suction upon pump amps (high speed multi-stage centrifugal pump)

It doesn't look like it (being a suction issue) and yes, basically you're correct about the axial units- usually high flow low head type machines with something approaching propellers, not a 10 stage unit. 10 stages is actually quite a lot - there must be some serious output pressure to that?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: effect of restricted suction upon pump amps (high speed multi-stage centrifugal pump)

What power quality monitoring do you have? From your description either the load went up or the supply voltage went down. Do you have voltage and current measurements across all 3 phases?

RE: effect of restricted suction upon pump amps (high speed multi-stage centrifugal pump)

(OP)
You didn't read my red bolded words!!!

... haha, that's ok, the original question is answered so it's fair to move to the inevitable followup.

We collected a fair amount of data on this event and conducted what we call a fault tree. The fault tree approach tries to brainstorm and methodically list all remotely-plausible explanations and then categorize their likelihood and develop plans to evaluate or test each one until it can be ruled out or confirmed. That is what leads me to ask questions like this.

Unfortunately all we are left with on this particular event seems quite unlikely and improbable. I don't think there is any easy way to explain it other than to sanitize our writeup (to remove identifying info) and post it here. Maybe I'll do that in a new post.

=====================================
(2B)+(2B)' ?

RE: effect of restricted suction upon pump amps (high speed multi-stage centrifugal pump)

All good. I made my first response with the red text in mind but then I started thinking about the problem and forgot about the red text for my second response.

RE: effect of restricted suction upon pump amps (high speed multi-stage centrifugal pump)

Maybe better if you could post the pump Q-h-speed curve, and a process sketch which shows the control scheme for this pump. Is this a regenerative type high speed centrifugal pump? - the performance curves are unlike those for standard centrifugals.

RE: effect of restricted suction upon pump amps (high speed multi-stage centrifugal pump)

(OP)
> Maybe better if you could post the pump Q-h-speed curve, and a process sketch which shows the control scheme for this pump

In terms of control scheme, it is a non-vfd induction motor and there are no automatic flow control valves in the system. There are manually operated throttle valves.

I don't have Q-h curve handy. Maybe later (it's a little outside of my personal focus). The part that seems most relevant to me is that we suspect BHP will increase with flow over the range of interest.

> Is this a regenerative type high speed centrifugal pump? - the performance curves are unlike those for standard centrifugals.

It is not a regenerative pump, it is a 7 stage centrifugal pump (I was incorrect to say 10 stages earlier).

=== ==== === ADDITIONAL INFO ==== ==== ====

As promised, here is additional info, starting with facts, then a table outlining the possibilities, and finally the actions to investigate further. I tried to remove identifying info.

For various reasons beyond the scope of this thread, we we don't want to do intrusive disassembly of the pump and we do not want to run the pump for troubleshooting purposes (we already did run motor and gearbox uncoupled from the pump). We do plan to send the motor out for off-site load test. Personally I think the problem will be found to be degradation of the end ring of the squirrel cage rotor in the motor. It would be surprising for degradation to proceed that quickly (the motor was running the same speed as all the others normally do just two minutes before the trip), but that's still where my bet is.

=====================================
(2B)+(2B)' ?

RE: effect of restricted suction upon pump amps (high speed multi-stage centrifugal pump)

Either that or some solid material got into the pump. Your fault tree examines a potential foreign material suction line blockage, not pump ingestion.

Still - take a shop vacuum and block the suction side. The RPM will rise and the output will drop, but so will the amperage as the motor is no longer accelerating as much material (less m * delta-V).

A shop rag from some previous service perhaps, It would be unlikely to cause much vibration as it is a braking element. The remaining fragments might have been released during the slowdown when the power was removed.

I saw a relatively expensive diesel engine wiped out because someone working on the radiator pushed a rag into the radiator hose to stop it from dripping. Little bits of it were distributed throughout the engine cooling jacket, also about 400 HP. I think that water pump had more clearance than the one you are using.

RE: effect of restricted suction upon pump amps (high speed multi-stage centrifugal pump)

A drop in pump shaft speed from 3565rpm to 3509rpm isnt much so that would explain why flow and press remain more or less constant during this event just prior to overcurrent trip.
Gearbox G (acceleration) increasing? - presume this means negative acceleration ?
Doesnt seem like there is a suction restriction. My guess is some problem with the electric motor, since the report indicates no problems with the speed increasing gearbox.

RE: effect of restricted suction upon pump amps (high speed multi-stage centrifugal pump)

(OP)
> Gearbox G (acceleration) increasing? - presume this means negative acceleration ?

You lost me on that one. I don't know what negative acceleratIon is. We have an acceleromter on the gearbox. It gives a bandlimited "overall" acceleration ("overall" for us means sqrt2*rms... don't blame me for the the terminology) as I recall up to 10khz. Our computer recording system "clamps" the value at 10g's for whatever reason... it probably went a lot higher.

=====================================
(2B)+(2B)' ?

RE: effect of restricted suction upon pump amps (high speed multi-stage centrifugal pump)

Sounds like an accelerometer, which is typically for vibration detection - high input or output shaft vibration at the gearbox?

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