11 replies to this topic

[Axis and Matrix]

Good day. I have several questions regarding pump throttling.

1. What will happen to the performance curve and system curve if the discharge of pump is throttled? Is the increase of temperature of the fluid being pumped expected? What are the other effects of throttling?
2. Is it advisable to throttle the pump suction to avoid the undesirable effects of discharge throttling? What are the pros and cons for each?
3. Can throttling lead to motor high amperage reading?

I know there are many pump experts here that can help me.

Thanks and Best Regards.

[breizh]

Hi,
I would advise you to post sketch about your system as pump curve for us to support your query . I guess this post is the continuation of an other one.
How big is your pump vs what you want to achieve ?
Breizh

[DB Shah]

Pump will operate at lower flow and corresponding head given in char curve.

Pump opertaes at a point where sys curve and char curve meets.

If pump is operated above min flow (flow indiated by vendor in view of temp rise and thrust) no prob of temp rise.

A BIG NO FOR SUCTION THROTTLING

Throttling will lead to operate pump at lower flow and corresponding head. Check the curve it also indicates power v/s flow. This are the basics taught in fluid flow operation in collage. Try it yourself to chk the effect of lower flow on the power consumed.

[fallah]

Axis and Matrix,

1-Working point moves to the left side of the pump curve,
2-No, NPSHA decrease may lead to cavitation,
3-Normally no, but depends on pump type.

Fallah

[S.AHMAD]

1. The system performance is of the following form:
H = H₀ + KQ²
where K is a combination of loss coefficient due to piping and control valve (common throttling device) and others such as pi and pipe diameter.
K inversely proportional to valve opening. That is when we throttle the valve, let say 50% opening, K increases. Therefore, for the same flowrate Q, the new H is higher than the previous H. This means that we can plot H vs Q at various valve opening. As mentioned by Fallah, the curve moves to the left side.
2. Same as mentioned by DBShah and Fallah
3. Axial type of pump requires higher BHP at lower flowrate.

Edited by S.AHMAD, 14 February 2012 - 08:57 PM.

[breizh]

Regarding point 1 you need to have a minimum flow rate to keep the integrity of the pump .
This point should be clarified with the vendor . Overheating may lead to fire too .
Breizh

[Zubair Exclaim]

If its a centrifugal pump it will require less power on lower flow

[S.AHMAD]

Exclamation

See attached file. Axial flow centrifugal pump behaves differently from radial flow

Attached Files

•  PumpCharacteristicVSImpellerType.pdf   429.19KB   233 downloads

[agorag]

1. The performance curve remains unchanged. Throttling the discharge valve increases the system resistance and therefore causes the operating point to the left. As the curve moves to the left, head increases. P=rgQH Since Q reduces, P reduces (though H increases a bit). Since recirculation results, part of the energy input goes to heat the pumped liquid.

2. Never. NPSHa drops. Incident angle of velocity triangle drops, resulting in severe eddies at suction.

3. In axial flow pumps, Yes. Not in radial flow pumps. See # 1 above.

[processjr]

• Throttling should not be carried out in the suction line as it may lead to cavitation due to pressure drop

[meetyourmaker]

1- pump operating point shifts to the left and the result will be lower flow rate at higher head due to increase in K value of throttled valve. pump efficiency at the operating flowrate determines  the temperature increase of the fluid so if the pump has lower efficiency on its new operating point on the left then I guess the temperature would increase. this tempereature increase is however significant only when operating at very very low flow rates.

2. why on earth would you throttle the suction valve on a centrifugal pump? It can increase the pressure drop in the suction line and lead to cavitation. It will do nothing to mitigate the effects of discharge throttling.

3. Not in most of the pumps we have in our plants, but there are a few units with high specific speeds(axial flow impellers) that have power- flow curves that drop with increase in flow rates, the discharge valves of such pumps should not be throttled and these should not be started against a blocked discharge

[Amirfarhangy]

Throttling the discharge will decrease the amperage reading on your pump, as you will have less flow passing through your system.