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# Formula For Calculating The Electric Motor Power Rating

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9 replies to this topic

### #1 Edavan

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Posted 16 September 2009 - 08:01 AM

Please provide the formulas for calculating the electric power rating for positive displacement pump (progressive cavity, screw pumps, gear pump and reciprocating pumps). As currently im working in a project. Till now i worked only with centrifugal pumps.

So please let me know the procedure for calculating the electric motor power for the PD pumps.

### #2 ankur2061

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Posted 16 September 2009 - 08:39 AM

vinprab,

Why do you think that the motor power rating calculation or equation for PD pumps should be different from centrifugal pumps? How do you think it is different for PD pumps?

In my humble opinion there is no difference in the basic formula or equation except for hydraulic efficiencies (n) for PD pumps which are higher compared to centrifugal pumps when dealing with lower flow rates.

You would still need the same inputs:
Q: flow rate, m3/h
rho: density of liquid, kg/m3
n: efficiency

What you need to know is the hydraulic efficiency (n) for your case in order to calculate the brake kW for your PD pump.

Hope this helps.

Regards,
Ankur.

### #3 Art Montemayor

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Posted 16 September 2009 - 10:31 AM

vinprab:

Ankur is exactly correct in what he states and in what he recommends.

All pumps - regardless of type - move a liquid through a developed head. Do not rely simply on formulas; employ logic and reasoning and you can't go wrong. For example, I am presently working on a project where some high pressure PD pumps are to be bought and I needed to check the horsepower requirements as per API Standard 674 - and guess what??? API furnishes the WRONG equation. If you don't believe that is possible, review and study the attached workbook I used on this project.

As Ankur infers, it much more important to know the reasoning and algorithm behind the equation than the equation itself. If you know the basics, you can always derive the equation - if need be.

### #4 Edavan

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Posted 16 September 2009 - 12:37 PM

Dear Ankur and Art

I read in some article; while calculating power requirement for centrifugal pump, we consider the differential head. but for calculating the power for PD pumps, we need to consider the "Discharge relief valve pressure" instead of differentail head.

### #5 Homayun

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Posted 16 September 2009 - 03:21 PM

The reason for PD pump to use the Safety Valve discharge pressure setting is that in case there is a blocked outlet, you still want the motor to be strong enough to pop the SV and NOT werck the pump. That's why you need to knwo the maximum possible differential head.

regards

### #6 ankur2061

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Posted 17 September 2009 - 12:04 AM

Vinprab,

I disagree with the concept of a PD pump motor being rated for shut-off (blocked discharge). In fact the very same principle that applies for centrifugal pump shut-off is applicable for PD pump - i.e., it will draw less current and thus draw less power in a shut-off condition. That due to shut-off or blocked outlet the pressure will build-up very rapidly for a PD pump is perfectly acceptable and a relief valve to protect the pump and associated piping is an absolute must still does not imply the motor of a PD pump needs to be sized for shut-off head. In my opinion, motor is still to be sized for the maximum differential head generated for pumping the rated (constant) volumetric flow rate of the pump.

I would like to know Art Montemayor's response on the motor being rated based on discharge safety valve set pressure. To me it doesn't seem right.

Hope 'Art' would make some detailed & incisive comments.

Regards,
Ankur.

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Posted 21 September 2009 - 09:21 PM

Vinprab,

I disagree with the concept of a PD pump motor being rated for shut-off (blocked discharge). In fact the very same principle that applies for centrifugal pump shut-off is applicable for PD pump - i.e., it will draw less current and thus draw less power in a shut-off condition. That due to shut-off or blocked outlet the pressure will build-up very rapidly for a PD pump is perfectly acceptable and a relief valve to protect the pump and associated piping is an absolute must still does not imply the motor of a PD pump needs to be sized for shut-off head. In my opinion, motor is still to be sized for the maximum differential head generated for pumping the rated (constant) volumetric flow rate of the pump.

I would like to know Art Montemayor's response on the motor being rated based on discharge safety valve set pressure. To me it doesn't seem right.

Hope 'Art' would make some detailed & incisive comments.

Regards,
Ankur.

Dear Vinprab

I totally agree with Ankur. The safety valve must be installed at the discharge of a PD pump to protect the pump as well as the piping and fittings at the discharge line. The discharge pressure is determined by how much resistance to flow along the discharge line and the method of calculation is the same as that of centrifugal pump - ie line pressure drop.

### #8 bam88

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Posted 22 September 2009 - 02:17 AM

The reason for PD pump to use the Safety Valve discharge pressure setting is that in case there is a blocked outlet, you still want the motor to be strong enough to pop the SV and NOT werck the pump. That's why you need to knwo the maximum possible differential head.

regards

Dear,

I would think that PSVs are sized after the blocked in condition of the PD pump, and not the other way round as you mentioned.

I have very limited experience in this area, please correct me if I'm wrong.

Regards,
bam

### #9 Edavan

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Posted 22 September 2009 - 12:53 PM

Hii all

The below is my assumptions

In case of centrifugal pumps (discharge blocked condition): maximum head the Centrifual pumps can develop is shufoff head, which will be lesser than discharge pipe design pressure.

But in case of PD pumps (discharge blocked condition): Irrespective of differential head used in calculating the motor rating, PD pump can develop head till pipe or valve breaks in the discharge pipe. As the performance curve for PD pump is vertical line.

Hence after determining the discharge pressure. PSV should be set 10% above the discharge pressure.

Question on viscosity:

Up to what viscosity, centrifual pump and PD pump will have no effect on the pump performance. I mean, from which viscosity we need to do refer viscosity correction factor. I heared it as 40Cst, however, i would like to know the experts views

### #10 fallah

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Posted 23 September 2009 - 03:49 AM

Hence after determining the discharge pressure. PSV should be set 10% above the discharge pressure.

Minimum design pressure at PD pump discharge (as a tyupical):

*MOP+1 bar for MOP<10 barg
*MOP+10% for MOP>10 barg

Relevant PSV should be set at minimum design pressure