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Npshr For Double Head Pump (Chemical Injection)

chemical injection pump metering pump npshr double head pump

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#1 Sherif Morsi

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Posted 06 February 2017 - 12:15 PM

Hi,

 

I am currently verifying some tubing sizes for existing  chemical injection pumps. The pumps are diaphragm, double head with motor rpm of 1800.

 

When I am calculating the NPSHR and specifically the acceleration head loss, do I used the whole 1800 rpm in the equation or I should use a lower value? If a lower value is to be used, how do I calculate it?

 

ha: Acceleration head loss (ft) = Length x velocity x RPM x Constant / K x 32.2  

 

Regards,

Sherif



#2 rdcrags

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Posted 14 February 2017 - 10:47 PM

Others will correct me if I am mistaken, but for chemical injection pumps, the NPSHR is stated as "flooded." That means the head at pump suction need only to be greater than zero.



#3 Sherif Morsi

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Posted 15 February 2017 - 05:22 AM

Thanks for your reply

 

These pumps are already existing and running, we are only replacing some of the old tubing on the skid. However, the original pump datasheet states the NPSHR as 2.3 psia (3 kPaa) and my hydraulic calculation for the line show a suction pressure pressure at the pump nozzle of at least 85 kPaa.

 

You think this is acceptable?

 

Sherif



#4 farid.k

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Posted 16 February 2017 - 07:54 PM

Npsh R or npsh A?
For rpm value, for me is not motor rpm, it should be diaphragm rotation per minute. You should find it in the datasheet by vendor.

#5 Sherif Morsi

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Posted 17 February 2017 - 06:49 AM

Sorry NPSHA. 

 

It's a plunger pump. 3 head pump, one motor

 

Head 1: 90 SPM (stroke/minute)

Head 2: 102 SPM

Head 3: 66 SPM.

 

So do I use this spm and not the total 1800 rpm motor speed?



#6 farid.k

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Posted 17 February 2017 - 08:49 AM

Sorry NPSHA.

It's a plunger pump. 3 head pump, one motor

Head 1: 90 SPM (stroke/minute)
Head 2: 102 SPM
Head 3: 66 SPM.

So do I use this spm and not the total 1800 rpm motor speed?


Yeap. Based on my previous calc, i used that value

#7 Sherif Morsi

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Posted 17 February 2017 - 11:23 AM

So why does API 14 E specify the speed as RPM? I am just asking out of curiosity



#8 farid.k

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Posted 18 February 2017 - 09:15 PM

So why does API 14 E specify the speed as RPM? I am just asking out of curiosity

if u use that value, you will get unrealistic value for the acceleration head loss. suppose range is about 50-120 rpm (refer attached picture for the formula). for the motor RPM, depends on the frequency and pump stages, minimum of the RPM is 1500 rpm and the highest 3550 rpm which i don't think you should use that value.

Attached Files



#9 Sherif Morsi

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Posted 20 February 2017 - 06:47 AM

Thanks for your feedback Farid.

 

Which textbook are you using? Can you send me the reference?



#10 MTumack

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Posted 21 February 2017 - 05:11 PM

You ought to use the RPM that the pump is actually running at.

 

The motor may be spinning at 1800 RPM (likely not, I'd assume a synchronous or actual speed of ~1750 RPM) but it is quite rare that a motor directly drives a pump! Chances are there is some sort of Chain or belt driving the shaft that does the fluid pumping. I'd guess your actual RPM value is likely in the range of 100 to 400 RPM.

 

But to obtain this info, you'd need to either have an accurate specsheet or measure the diameter of the sheeves and determine the rotational speed of the pumkp shaft based on the Movement mechanics.. For example, if the motor is running at 1750 RPM's and has a small sheeve with a 6" Diameter, ever turn moves the belt 6" x pi inches. If the big sheeve is say 16" then every 6 inches of belt movement from the belt is 6 x pi / 16 x pi = 0.375 rotations. Thus you would have 0.375 x 1750 rpm = 656.25 RPM's at the pump.



#11 Sherif Morsi

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Posted 22 February 2017 - 07:30 AM

You ought to use the RPM that the pump is actually running at.

 

The motor may be spinning at 1800 RPM (likely not, I'd assume a synchronous or actual speed of ~1750 RPM) but it is quite rare that a motor directly drives a pump! Chances are there is some sort of Chain or belt driving the shaft that does the fluid pumping. I'd guess your actual RPM value is likely in the range of 100 to 400 RPM.

 

But to obtain this info, you'd need to either have an accurate specsheet or measure the diameter of the sheeves and determine the rotational speed of the pumkp shaft based on the Movement mechanics.. For example, if the motor is running at 1750 RPM's and has a small sheeve with a 6" Diameter, ever turn moves the belt 6" x pi inches. If the big sheeve is say 16" then every 6 inches of belt movement from the belt is 6 x pi / 16 x pi = 0.375 rotations. Thus you would have 0.375 x 1750 rpm = 656.25 RPM's at the pump.

 

Can you elaborate more on the calculation using the sheave dimensions? Like a more detailed example?






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