Pump making more Differential Head than rated

[PKEngineer]

Could anyone explain possible reason(s) for having more differential head through centrifugal pump than rated (more than shut-in pressure)for liquid butane service pumping from suction sphere. Flow rate is within the operating range as measured?
I would assume suction and discharge pressure gauges are providing the right measurements.

Please ask me more specifics if more explanation required

Thanks/Regards

 

[JJPellin]

The additional speed is not enough to cause any problem with the pump as long as the pump is still being operated above the required minimum flow. The difference between 1775 rpm verses 1750 rpm is not enough to justify changing the motor. If you want to have a curve that correctly matches your running speed, you can generate a new curve using the actual speed and the affinity laws.

Johnny Pellin

 

[BigInch]

That's about a 3% head affinity law difference; only 7% more to find. It would be nice to have a temperature at the pump to get a double check on that 0.56 SG. Might be a percent or two, then the pump curve could be off a few percent too you know. Unless you have an in-place curve test, I would say 5% difference between actual and mfgr's curve is not IMpossible.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Artisi]

A 5% variance with an on-site testing considering all the variables against a carefully controlled shop test is more than reasonable.

 

[PKEngineer]

I believe we find the anomaly after finding the higher rpm which is giving us the numbers not matching with the curve. Also, as BigInch mentioned we probably have higher SG (which I believe it is)liquid at pump which is changing our output also. Plus some variance is always expected as Artisi mentioned. So looks like we are even!

Currently recirc. line at pump discharge is routed back to vessel so I believe have no control for the minimum flow. Do you guys agree it should be tied in to suction instead of vessel OR to look for something else as high temp. from discharge could boil off the butane at suction.

I TELL YOU GUYS: ALL OF YOU MAKE MY DAY OR EVEN WEEK

Thanks/Regards

 

[BigInch]

Recirc back to the tank would be the preferred solution if you have the pressure to do so. Heating more fluid means a temperature will take much longer to rise.

The recycle of 40% to the tank may be higher than necessary, but as long as its above minimum and temperatures are kept manageable, perhaps its not severe, although you wouldn't want to do that constantly. And it does mean some unnecessary running costs. If the other 60% was going to the trucks during that time, then there would be no spillback through the pump, you may be able to lower the head and cooling would not be hindered, so it would be better during low flowrate loading.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[PKEngineer]

Thanks BigInch