Pump making more Differential Head than rated
Pump making more Differential Head than rated
(OP)
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
I would assume suction and discharge pressure gauges are providing the right measurements.
Please ask me more specifics if more explanation required
Thanks/Regards





RE: Pump making more Differential Head than rated
Post the pump make, model and P,Q,H, & rpm rating, sphere temperature, the flowrate, the pump suction pressure and temperature, the discharge pressure and temperature and the pump curve and what you think the differential head is and maybe someone can make sense of it.
**********************
"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/
RE: Pump making more Differential Head than rated
I do appreciate your earliest response.
The cut sheet provided by manfr. shows 97.5 psia max pressure at shut off but the pump curve depicts 182' ft shut off head so does not match after conversion to same unit (as I understand). I don't have the pump curve at the moment otherwise I would have attached it. I can get it Monday. Other data is as:
Pump: Dickow Mag Drive Pump Model: PRM
Service: i-butane
sp. gravity: 0.56
Q: 455 gpm
TDH: 172'
RPM: 1750
Current conditions:
Flow rate: 405 gpm
Suction reading: 56 psi
Discharge reading: 102 psi
Sphere temp.: 78F
No temperature readings available for pump suction or discharge
One of my doubts about about this anomaly is not having the right curve. Kindly comment.
Regards,
Malik
RE: Pump making more Differential Head than rated
Assuming it is a differential shutoff head.
182ft*62.4pcf/(144in2/ft2) = 78.8 psi (cold water)
Assuming the suction pressure is 0 psig,
and the cut sheet's 97.5 is psiA, then we have to add atmospheric pressure 14.7 psiA
I get pump's discharge pressure at shutoff of
93.5 psiA (with 0 psiG) suction pressure. That's not 97.5, but not too far off either. And that's with zero psig suction pressure.
Isobutane's vapor pressure at 80F is 40 psig and you have 56, which I assume is psig, so the butane should be liquid, if you haven't raised the temperature from sphere to pump.
TDH of 172 ft is 74.5 psi water * 0.56 SG = 41.7 psi Butane
With a suction pressure of 56 psig, plus 41.7 psi TDH = 97.7 psig. Your discharge pressure is 102 psig. Only a difference of 5 psig, say 5% error. If you've raised the temperature by only a few degrees before or inside the pump and raised the vp, or have any error in the pressure gage readings, it could be less than 5% error. I'd say that's not too bad for the first try matching calculations to real world data.
**********************
"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/
RE: Pump making more Differential Head than rated
RE: Pump making more Differential Head than rated
**********************
"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/
RE: Pump making more Differential Head than rated
Sorry I may have confused you with the data. Let me try to explain:
Design data as per spec sheet
Q = 455 GPM
TDH = 172 ft
Suction pressure = 53.6 psia
Max pressure @ shut off = 97.5 psia
Calcs provide 78.8 psi shut off which will be:
78.8*2.31 = 182' and this agrees with the pump curve
Calcs with real world data:
Suction pressure = 56 psi
Discharge pressure = 102 psi
So, TDH = (102-56)*2.31/0.56
= 190'
As per pump curve shut off head is somewhere 182' and dilemma is we are making 400 gpm at 190' TDH.
Please correct me if my calcs are wrong as may be missing something or need to correct the concept.
Regards
RE: Pump making more Differential Head than rated
Try to get the suction pressure, discharge pressure and temperature when at shutoff condition too.
**********************
"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/
RE: Pump making more Differential Head than rated
RE: Pump making more Differential Head than rated
I don't have access at the moment for operating rpm of the pump/driver. Let you know about rpm and amperage with the pump curve by tomorrow.
But again, at or beyond the shut off head we shouldn't be making any positive flow as I understand.
Regards
RE: Pump making more Differential Head than rated
I will try to get the data at shut off as you mentioned but not sure whether operation will allow me to operate at this limit.
Do we need to have pressure at pump suction above than the vapor pressure all the time in order to avoid two phase/ cavitation. We have light hydrocarbon service to transfer.
Regards
RE: Pump making more Differential Head than rated
RE: Pump making more Differential Head than rated
The curve could be wrong in a number of ways. As already noted, the actual rpm could be higher than the curve is based on. The impeller diameter could be different. The impeller could have a different over-file or under-file than what was used for the curve.
As already suggested, if you can get performance data at a number of operating points, including shut-off, this may suggest which of these errors is affecting your result.
Johnny Pellin
RE: Pump making more Differential Head than rated
I agree that pump should ride on the pump curve if it is the right one. If it is not then could be any of the reasons as you mentioned or it could be my misinterpretaton. I will try to get all these things cleared plus I will attach pump curve tomorrow.
As we have multipoint temperature readings in the sphere and was verified before so I would rule out any cause or discrepency in the temperature readings and inferred SG values.
Regards
RE: Pump making more Differential Head than rated
**********************
"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/
RE: Pump making more Differential Head than rated
Waiting for comments
Thanks/Regards
RE: Pump making more Differential Head than rated
Is the 1750 rpm verified and we're sure about the pressures, right?
What's the pump discharge line tying into?
What's the pressure in the discharge line with the pump off?
**********************
"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/
RE: Pump making more Differential Head than rated
Johnny Pellin
RE: Pump making more Differential Head than rated
Pump discharge line is branched off about 150 ft away from pump to our truck loading rack and other to transport to railcars.
I could not check pump discharge pressure with pump off as operating at the moment and I can not ask to stop it.
I am sorry not mentioning before that we have recirculation line from dishrage to sphere which is recirculating all the time because of aparently wrong setpoint for the control valve on this line (as I just checked). I believe 30-40% of our flow is going back to the sphere. I don't understand yet why we have this valve there.
So looks like high rpm is the culprit and the root cause as I understand.If it is so what could be the consequences running at this speed. DO we need to change the motor?
Regards
RE: Pump making more Differential Head than rated
We don't have inducer installed. It is mentioned on the curve though as we were concerned about the NPSH in the design stage and was provided as an option.
Regards
RE: Pump making more Differential Head than rated
Johnny Pellin
RE: Pump making more Differential Head than rated
**********************
"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/
RE: Pump making more Differential Head than rated
RE: Pump making more Differential Head than rated
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
RE: Pump making more Differential Head than rated
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/
RE: Pump making more Differential Head than rated