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

 

[BigInch]

Assuming the power supply is right, I'd guess that the pump is hotter than the sphere and the butane's pressure has increased due to temperature, but w/o any data, its a WAG.

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/

 

[PKEngineer]

BigInch
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

 

[BigInch]

Pump Discharge Pressure at shutoff =
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/

 

[Compositepro]

The vapor pressure will not directly affect the head of the pump. Liquid density will, which changes with temperature and composition.

 

[BigInch]

vp tells us if its liquid or gas, so what it says about the process could affect the values we have to calculate and also tell us if there is a possibility that gas is being created, which if it is, will affect the TDH delivered.

**********************
"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]

BigInch
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

 

[BigInch]

OK let's wait for the right curve.
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/

 

[Artisi]

is the pump runing at 1750? - could be above this say 1780 rpm - only a small difference but it increases the head.

 

[PKEngineer]

Artisi
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

 

[PKEngineer]

Thanks BigInch
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

 

[impeller1]

Yes pressure at pump suction should always be greater than substance vapour pressure..otherwise the liquid would start to boil and gases would form which would cause cavitation.

 

[JJPellin]

You suggest that your pump is operating above the curve. This is impossible. The curve is defined as all of the points where the pump can operate. If you are plotting the performance above the curve, then you have the wrong curve or bad data. The data could be wrong in a number of ways. As already mentioned, there is some inaccuracy in measurements of pressures and flows. The product temperature or specific gravity could be different than you are using. Pumping out of a closed sphere in equilibrium creates another dynamic. As the fluid level drops, the vapor space is filled by vaporization of liquid product. This results in auto-refrigeration and the product temperature drops. Cooler product has a higher specific gravity than warmer product. So, your specific gravity could be changing over time as the pump runs.

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

 

[PKEngineer]

Johnny
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

 

[BigInch]

I still think we should wait for better data and a curve.

**********************
"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]

Please find the pump curve as we discussed.

Waiting for comments

Thanks/Regards

 

[BigInch]

That looks like it fits.
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/

 

[JJPellin]

One thing strikes me as unusual when I look at these curves. There are NPSH curves for a pump built with an inducer and one without. The difference in NPSH required is about 7 feet. This suggests that the inducer could be generating as much as 7 feet of head. But, there is only one curve for flow versus head. They do not allow for the head generated by the inducer when plotting the capacity curve. If you have an inducer on your pump, this could be contributing some of the extra head. I don't often work with pumps at speeds this low. But, on higher speed pumps, I know that the head generated by the inducer can make a significant contribution to the overall head produced by the pump.

Johnny Pellin

 

[PKEngineer]

I got 1775 rpm driver speed so running higher than supposed to be.

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

 

[PKEngineer]

JJPellin

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