ACTUAL TDH NOT MATCHING PUMP CURVE

[TechSupport1]

I have a centrifugal pump that should be working fine but isn't. According to the pump curve, this pump should be transporting the required flow, which it is. The TDH on the pump curve is about 300' but only about 1/2 of that is needed. I would expect the control valve downstream of the pump to take the pressure drop. Instead, the pump is just not putting up the discharge pressure. When the pump discharge is blocked in, the pump does shows the right discharge pressure (and the right TDH). The NPSHa is just above the NPSHr and the pump motor is large enough. The pump efficiency is poor at about 35%.

The pump has been repeatedly abused since it was installed.

What would be causing the discharge pressure to drop 50%, when suction pressure is constant and when the flow through the pump does not warrant it, according to the pump curve?

 

[PUMPDESIGNER]

It is a little hard because of terminology.
If by "blocked in" you mean the pump is operating against a closed valve, which I call "shut-Off" head, and you mean that the shut-off head matches what the pump curve shows.
Then,
Assuming that rpm is correct, you have thereby verified that the impeller diameter is correct. Good information.

Assuming impeller is not blocked/clogged in any way, I then quickly jump to look at either wear ring clerance on a closed impeller, vane clearance against the case on open impeller. It is surprising to some people how just a little extra clearance on the wear ring/vane clearance can dramatically affect head output.

If flow rate has been verified, and then you check that flow rate against the pump Head Capacity curve, you should be reading roughly what the pump curve states for head at that flow rate minus or plus intake head, as the case may be.

Now, that is all assuming that you are pumping water if the pump characteristic is based on water. If you are pumping a lighter liquid than water, and the pump characteristic is based on water, your pressure gauage will indeed read less than the rated pressure.

PUMPDESIGNER

 

[electricpete]

If TDH is correct at shutoff but lower than expected during operation, might suggest cavitation due to pressure drop somwhere in suction line (suction pressure gets lower as flow increases).

Other general thoughts
1 - Is your pump operating at rated speed?
2 - Do you trust your flow measurement instruments?
3 - Has fluid density been considered in flow measurement?

=====================================
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[rmw]

If you have a pump curve, showing the head curve, as well as the power required curve, and you can make some reasonable assumptions about your electrical supply, correct voltage, PF, etc, then calculate your pump horsepower. If the pump is doing what it is supposed to, it will require a certain amount of power to get the job done. I often check the motor to verify the pump. The motor is a dumb (sorry electrical guys) device, and is only going to give what is asked of it.

Pumpdesigner, if it were a wear ring clearance problem, wouldn't that affect the shut off head as well, preventing the pump from showing true shut off pressure while 'blocked in'?

Techsupport1, one more thing to check. You give npshr and npsha information, but what is the total positive suction head. Is your fluid saturated, or do you get a significant contribution to TDH from your suction pressure.

With your pump 'blocked in' you have no line losses on the suction side, and with the pump flowing, the line losses before the pump rob you of TDH. Things get worse if you have flashing or cavitation on the suction side.

rmw

rmw

 

[jet1749]

If TDH is correct at shut off, but drops off at points on r/hand side of the curve it would seem to indicate wear ring clearance too large. This could be confirmed by observing the increased power absorbed by the motor and subsequent drop in efficiency.
If as you say NPSHa is above NPSHr then there shouldn't be any suction problems (assuming here that you have an operational pressure guage at the suction of the pump, and are not just going on initial calculations made when specifying the pump)Cavitation however, should be making itself known if the suction conditions are badly wrong.
In any case if NPSHa is less than NPSHr, power absorbed by the motor would drop off. Again this can be confirmed by observation. If there is less pressure drop across the control valve then usual or expected, this would indicate that the valve is opening up to allow more flow.
The clue looks to be the comment that "the pump has been repeatedly abused since installation"

 

[smckennz]

Have you checked the rotation is correct?
If NPSHA is just above NPSHR then you will be cavitating.
If the pump has been abused then it is worth looking inside before doing too many more measurements. At 35% efficiency, volt/amp readings wont tell you a lot.
I suspect its stuffed.

Cheers

Steve

 

[Scipio]

What's your viscosity? Correct TDH at shutoff, lower-than-expected differential to the right, and lower-than-expected efficiencies sound like the difference between a water-based performance curve and one that needs to be corrected for high viscosity service.

 

[PUMPDESIGNER]

rmw is correct about power curve to flow rate. However, if specific speed is Ns=2500-8000, then current may not be useful because current does not change much. You have a lot of head at 300 feet indicating you may have a low specific speed impeller if there is only one stage.

rmw - Wear ring affect on shut-off head is variable, you are correct that it may have an effect. On flat curve pumps however there may be very little difference, and on high specific speed pumps the effects at shut off are small also unless the wear ring leakage pushes the flow rate beyond a critical point where the curve starts to drop, but still at shut-off the affect may be so small that shut-off head appears correct even with excessive clearance.

That is one problem with these forums, makes you sharp because the people posting often do not have much information about the pumps, what kind they are, stages, specific speeds, diffuser, scroll, double volute, double suction, etc. Makes it really hard and we end up speculating quite a lot.

PUMPDESIGNER

 

[rmw]

Pumpdesigner,

Thanks for the info.

I agree with the part about groping around in the dark, but often many posters do not know that much about a pump, in order to give us what we need to analyze situations.

rmw

 

[ccfowler]

TechSupport1,

The first thing that I noticed was your comment:

"The pump has been repeatedly abused since it was installed."

What manner of abuse(s) have been visited upon this beloved piece of machinery? Abrasive slurries, large damaging solids, broken/bent vanes, repairs that would be better characterized as vandalism, ...?

How well is the axial alignment being maintained? If the axial position is not well controlled, this could account for damage and excess leakage at the wear rings.

Another thought involves the accuracy of the pump curves that you have. Were the curves based on actual test performance of this pump, generic curves for the pump model, or "advertizing" curves for a series of pump models? The combination of an abused (or worn) pump and uncertainty of the true applicability of the presumed pump curves can readily account for a significant portion of the apparent performance discrepancy.

How does the actual shaft speed compare to the shaft speed that applies to the pump curves that you are using? Apply the affinity laws to see the effects of the speed differences on expected performance values.

 

[Kawartha]

TechSupport1,

I don't think that you have described your system in detail. What you are observing makes me think that you may have air (or some type of compressible gas) which is entrained in the liquid entering the pump. When flow is reduced to shut-off, possibly the air is escaping prior to the pump suction. However, if you have entrained air in significant volume (>6%) - this could result in major decrease in performance. Entrained air will cause the performance curve to drop below the standard curve at an increasing rate with increasing flow rate.

Entrained air does not result in cavitation, but would probably cause some noise.

You should be able to make a judgement on this possibility by checking the HP drawn at higher flow rates. Actual HP will be well below expected pump curve HP.

Richard

 

[friartuck]

What are the inlet conditions like on the pump? I work with fans and if the inlet conditions are poor, i.e. if there is a sharp bend on the inlet to the fan, you can lose 30% or so of the duty and the fan curve reduces dramatically. This might be a contributing factor. And since when you do a closed head test the pump duty is ok, then this also matches the scenario since the bad inlet conditions at zero flow will not affact the closed head operation.

Drapes

 

[toothless]

Like someone said, check the rotation. We had a similar problem and were astonished at how well the pump performed when running backwards.

 

[vanstoja]

Following friartuck's theme, centrifugal pump inlet flow conditions are the first thing I would suspect when first-time or all-time operations show TDH to be less than or greater than the manufacturer's curve when flowrate is properly measured. If the pump has inlet counterswirl (opposite to impeller rotation), then TDH will get progressively higher than the pump curve head as flowrate is increased. If the inlet flow is coswirling then the head will progressively drop below the curve head as flowrate is increased. Because rotational flow effects are so low at near zero flow, the measured head at shutoff may show no difference from the pump curve. It is not obvious from your description whether or not your pump ever produced the manufacturer's head-flow performance and has since degraded by wear ring leakage, incresed cavitation, etc. To check for inlet swirl effects you need to measure pumping power since the driver has to supply more than normal power to deliver higher head with counterswirl or lesser power for the lower head with coswirl. Multiple pipe bends of particular orientations upstream of the pump can leave appreciable swirl at the pump inlet even when the distance to the last bend meets the 7-diameters criterion advertised in the pump literature. Multiple pipebend complexes like three back-to-back 90 degree elbows need something like 188 pipe diameters downstream to dissipate swirl if the bends are out-of-plane and the the 3rd bend has right spatial orientation. We ran tests of such a configuratuion and found we could turn coswirl into counterswirl by changing the spatial orientation of the last elbow by 180 degrees. Incidentally a pump efficiency of 35% is not necessarily bad if the specific speed of the pump is below 1000 US units. You have to have specific speeds in the 2000-4000 range and relatively high flowrates to get pump efficiencies of 80% and higher.