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Pump Npsha And Suction Pressure

pump npsh suction pressure

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#1 Leonard W

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Posted 25 January 2015 - 11:22 PM

Hi,

 

I have a fundamental question regarding NPSH of pumps, in short, are NPSHa and suction pressure of a pump the same? I have searched through the forum and found numerous entries on NPSH but could not find one that answers my query.

 

While,

 

NPSHa = absolute pressure above liquid surface ± Suction Static - Friction Loss - Vapour Pressure

 

This is easily applicable for a pumping system which consists of an elevated open tank and a pump downstream of the tank's discharge. However, if a pump's suction pressure is provided by a booster pump upstream, how should then the NPSHa be calculated?

 

Please see the simplified pump system diagram as attached. I would really appreciate it if someone could provide answers on the following queries:

1) What is the difference between 'suction pressure' and 'NPSHa'?

2) Referring to the attachment, what should be the NPSHa for each of those pumps? (lift pump, booster pump and main pump)

 

Please help. Thank you.

Attached Files



#2 breizh

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Posted 25 January 2015 - 11:48 PM

http://www.engineeri...head-d_634.html

 

Leonard W,

Consider the link attached to support your query .

 

Hope this helps

Breizh



#3 samayaraj

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Posted 26 January 2015 - 12:55 AM

NPSHa = absolute pressure above liquid surface ± Suction Static - Friction Loss - Vapour Pressure

 

This is easily applicable for a pumping system which consists of an elevated open tank and a pump downstream of the tank's discharge. However, if a pump's suction pressure is provided by a booster pump upstream, how should then the NPSHa be calculated?

 

Leonard,

 

If pump 2 gets feed from pump 1, NPSH A for pump 2 = Pump 1 discharge pressure + static head - loss - VP

 

 

 

Please see the simplified pump system diagram as attached. I would really appreciate it if someone could provide answers on the following queries:

1) What is the difference between 'suction pressure' and 'NPSHa'?

2) Referring to the attachment, what should be the NPSHa for each of those pumps? (lift pump, booster pump and main pump)

 

 

Pump suction pressure/ suction head = Pressure above liq surface / discharge pressure of booster pump + static head - loss

 

NPSH A = Pump suction pressure/ suction head - VP

 

With this, you can calculate the NPSH A for each pumps.

 

 

#Samayaraj


Edited by samayaraj, 26 January 2015 - 01:37 AM.


#4 breizh

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Posted 26 January 2015 - 01:20 AM

Samayaraj ,

Pay Attention  to the definition , it's not the pump suction pressure to consider but the suction head (m)  - VP expressed in (m CL).

hope this helps

Breizh 


Edited by breizh, 26 January 2015 - 01:30 AM.


#5 samayaraj

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Posted 26 January 2015 - 01:36 AM

Breiz,

 

Thanks for you comment. In general I said as pressure. While calculating we convert it into head. Ya its suction head (m) - VP (m). I've corrected it.

 

 

#Samayaraj



#6 fallah

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Posted 26 January 2015 - 01:59 AM

Leonard W,

 

1) Pump suction pressure= Pressure above liquid surface+/- static pressure of liquid column-pressure loss in the suction line

While: NPSHa=Suction head-corresponding head of liquid vapor pressure

 

2)  NPSHa (Lift Pump)=15.44 m

     NPSHa (Booster pump)=27.44 m (assume 4 barg as discharge pressure))

     NPSHa (Main pump)=79.44 m (assume 8 barg as discharge pressure)



#7 Leonard W

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Posted 26 January 2015 - 04:09 AM

Thank you all for the inputs. I have studied the contents from the link provided by Breizh which has provided clear explanation of suction head and NPSH. Samayaraj's input is also consistent with most materials that I found on NPSH. 

 

With that, I would like to summarise my understanding of this topic and hope that you can verify if my understanding is correct. 

 

 

If pump 2 gets feed from pump 1, NPSH A for pump 2 = Pump 1 discharge pressure + static head - loss - VP

 

 

2)  NPSHa (Lift Pump)=15.44 m

     NPSHa (Booster pump)=27.44 m (assume 4 barg as discharge pressure))

     NPSHa (Main pump)=79.44 m (assume 8 barg as discharge pressure)

 

According to replies from Samayaraj and Naser Fallah, I have come to understand that the basic formulas should be,

 

Suction head = pressure above liquid surface / discharge pressure from upstream pump ± static head of liquid column - head loss 

NPSHA = Suction head - liquid's vapour pressure (m)

 
Since most texts only explain NPSH of a single pump and tank system, I was really looking to substantiate that the NPSH of a pump downstream of a booster pump should include 'discharge pressure from upstream pump' in the calculation. Is this conclusion correct?


#8 samayaraj

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Posted 26 January 2015 - 11:32 AM

Ya you're correct.

#Samayaraj




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