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Negative Pressure At Pump Suction Side

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#1 prasad54


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Posted 25 March 2009 - 05:39 AM

Hallo all,

I have come accross a old datasheet of a project i am working on and have got a negative suction pressure at the suction side.

The pressure drop through the suction side Pipe line is higher than the terminal pressure.

I would like to know what will happen pratically if we have a negative pressure at the suction side of pump.

Is it common to have negative pressure at the suction.

How should I proceed with this??

Thanks in advance.

#2 djack77494


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Posted 25 March 2009 - 07:25 AM

It is not at all uncommon to have a negative gauge pressure at a pump's suction. To clearly demonstrate, many sump pumps are horizontal pumps located at grade and sitting above (but close to) the sump they draw from. The pressure on the sump's liquid level is atmospheric, so the pressure at the pump's suction will be atmospheric less elevation gain less hydraulic losses. Not a problem if the system is adequately designed to accomodate this.

QUOTE (prasad54 @ Mar 25 2009, 01:39 AM) <{POST_SNAPBACK}>
How should I proceed with this??

I suggest you endeavor to master the concept of NPSH. If (when) you succeed, the answer to this question will be apparent.

#3 JoeWong


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Posted 25 March 2009 - 07:35 AM

QUOTE (djack77494 @ Mar 25 2009, 07:25 AM) <{POST_SNAPBACK}>
It is not at all uncommon to have a negative gauge pressure at a pump's suction...

Agreed with Doug. Always check if your system can tolerate air ingress via flange leak...



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Posted 30 March 2009 - 03:11 PM

Another common situation where there is a negative gauge pressure at a pump suction is on a distillation column reboiler circulation loop.

There is some information about NPSH-net positive suction head in Goulds Pump Manual in the section in the back about centrifugal pump fundamentals. Also find information in Cameron Hydraulic Data.

NPSHr is the required net positive suction head needed to prevent cavitation in the pump and will be on the pump curve.

NPSHa is the available net positive suction head for the system.

NPSHa must be >/= NPSHr at all times.

As noted by Joe, air entering into the pump suction can also be a problem. It is sometimes difficult to determine if a pump operating under vacuum and malfunctioning is cavitatiing because there is insufficient NPSH or if it has air in it from a leak.

Hope this helps.


#5 ziba


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Posted 15 June 2012 - 12:18 PM


I have problem calculating the NPSHA for a sump pump.
Sump is 0.7 m underground from top of the sump, LLLL is 0.5 meters.

sump is buried at 3 meters below ground , sump contains centrate water at 90 C so vapor pressure is 70.1, sump is at atmospheric p=95 kpaa
my calc NPSHA=(0.102(95-70.1-0)/0.965)+ VERTICAL DISTANCE - SAFETY MARGIN
Assumed suction loss=0
vertical distance, what number should i consdier, is it 0 as sump pump should be equal to LLLL level?
what happens if i ignore safety margin of 0.914 m? that we usually consider for other pumps.
sump may have solids so pump is on the other side of sump (opposite of weir).

Edited by ziba, 15 June 2012 - 12:27 PM.

#6 ziba


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Posted 15 June 2012 - 12:27 PM


I attached an excel file for convenience, please let me know if i am right or wrong on calcualtions?

how do i attach an excel file?

#7 Art Montemayor

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Posted 15 June 2012 - 02:27 PM


Go to "more reply options" at the lower right-hand section of the page before starting your post.
A new window will open and at the bottom of you message you will see a slot where you can browse and select your upload file.
After selecting your file to upload, click the "attach this file" option and the file will be uploaded into the posted message.

#8 ziba


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Posted 20 June 2012 - 11:51 AM


can u plz let me know how do i calculate NPSHA for sump pump based on levels

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#9 ziba


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Posted 20 June 2012 - 11:55 AM

how do you add your picture

#10 Art Montemayor

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Posted 20 June 2012 - 12:32 PM


You are "skyjacking" a 3-year old thread. You are only creating confusion and problems when you take over and change the topic of discussion taking place in someone else's thread. This thread was started by and belongs to Prasad54. Kindly start your own thread with this specific topic of yours. That way, all members can focus and respond to your query and your topic. Starting a new thread is free and costs no one any time or money. Our Forums are based on any member having access to starting their own threads.

I will delete your posts on this thread after you have read this message and started your new, personal thread in this Forum.

Thank you.

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