Pumps operating in end-of-curve 1

[EngMecBra]

Hi,

Here I have pumps selected for specified rated head and flow, but they are always operating in end-of-curve. There isn't any globe valve or control valve in discharge line to set specified rated flow.

For process, it's no problem because the service is water transfer from a basin to a tank.

But about pump and/or motor, can you see any problem for one of them?

Thanks

Rodrigo Alves

 

[LittleInch]

In a word Yes. Operating beyond the rated duty of the pump will cause the pump to operate in a zone of much higher NPSHR and you run the risk of severe and significant erosion of the impellor caused by cavitation of the fluid at the inlet. The pump itself will also be subject to increased vibration affecting the life of the seals and bearings.

The motor will also be affected as will your energy bill, by operating beyond the Best efficieny Point (BEP) and pumping at a much lower efficiency. It is likely that it is only because the motor is over sized for the rated duty, that it does not trip on excess amps. However temperature windings are probably at a higher temperature than they should be and hence may fail over time.

As you note, your system should include a retrofit of a control valve, either manual or actuated to keep the pump close to it's duty point for flow.

If you could post any more details such as the pump curve and where it operates plus some idea of size of pump and motor then that wpould be useful.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Pumpsonly]

If the pump is not cavitating and the motor is not overloaded. Just leave it.

 

[LittleInch]

Pumpsonly
Surely not. The OP is asking for advise and to operate any machinery significnatly beyond its duty point will greatly increase the chance of premature failure. Cavitation is sometimes difficult to detect due to the noise of the motor and other machinery and may only occur at certain times such as low water level (I assume it's water). The motor may well be OK if was a standard size and has spare capacity, but the OP has not given any details of the pump and motor or curves or system description. What he really needs by the sound of it is a re-sized pump to match his system curve (motor is probably Ok), but some sort of throttling device is probably easier to install.

Rodrigo - what is your comment.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[TenPenny]

When you operate to the right of BEP, the shaft stresses go up, so depending on the design of the shaft and bearings, you might have problems with overloading the bearings, or you may be fine. I've seen both.

 

[bimr]

You post leaves much to the imagination.

If your system is large, then you may have expensive failures and expensive costs to rectify. For a small system, the opposite.

How long has the system been in operation and why are you asking the question now?

Why was the system specified differently than it is operating?

Why would some recommend changes when the scope of the problem is not known?

 

[JJPellin]

We have some pumps that run very well at the end of curve. We have pumps where end of curve is within 5% of BEP. We have other pumps that would cavitate to destruction at EOC. We have vertical turbine pumps that would up-thrust and buckle the shaft. There is no way to answer this question with the information provided.

Johnny Pellin

 

[nabilbasha]

I understand it is just an open system where the pump used just to lift up water to storage tank. Most likely the pump head was over calculated hence the pump operate to the right end of the curve. Simple solution is to fit globe valve and lose it slightly to move the operating point to the left close to the BEP or alternatively trim the impeller to match the required ACTUAL head.

 

[EdStainless]

We have had similar situations and they ran fine until summer and then we got cavitation with warmer water.
We put an orifice plate in a flange down the discharge line. Not much restriction, just enough to move us back closer to BEP.

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Plymouth Tube

 

[LittleInch]

Cavitation and NPSHR both tend to climb rapidly as you go right on the curve so it wouldn't take much temperature difference or change in suction water level to experience cavitation.

Rodrigo - Do you have any feedback to offer us?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way