11 replies to this topic

[Shaily123_]

Why pump shut off pressure of a pump is calculated as 1.2 times the differential head+ suction head

[fallah]

Why pump shut off pressure of a pump is calculated as 1.2 times the differential head+ suction head

 

Hi,

 

This is just an estimation of shut off pressure when the pump's discharge pressure at no flow isn't available...

[Shaily123_]

Sir
I want to know that why we take the differential head factor as 1.2, why not 1.3 or 1.5..??

And I want to know that which factor to be considered to define the pump shut off head pressure
eg pump shut off head is design as design pressure of downstream equipment or set pressure of PSV

[fallah]

I want to know that why we take the differential head factor as 1.2, why not 1.3 or 1.5..??

And I want to know that which factor to be considered to define the pump shut off head pressure
eg pump shut off head is design as design pressure of downstream equipment or set pressure of PSV

 

Hi,

 

The 1.2 factor is an experimental estimation value to calculate the shut off head with having in hand the head of the pump at design point.

 

If downstream equipment/piping is designed per pump's shut off pressure no need to consider a PSV for blocked discharge; otherwise a PSV should be considered where downstream is at design pressure lower than pump's shut off pressure.

[Chemitofreak]

Hi,

 

Based on my experience it is 1.25.

 

If you use the factor of 1.25, there is high probability (90-95%) that the shut-off head specified by the vendor will be less than what you have calculated.

 

If you use a value more than 1.25, you end up in doing conservative calculation. If you use a value less than 1.25 there is high chances of you making a change in the design pressure of the downstream piping after receiving the vendor curves.

 

Revert in case you require additional clarification.

Edited by Chemitofreak, 01 February 2019 - 02:03 AM.

[Chemitofreak]

 

I want to know that why we take the differential head factor as 1.2, why not 1.3 or 1.5..??

And I want to know that which factor to be considered to define the pump shut off head pressure
eg pump shut off head is design as design pressure of downstream equipment or set pressure of PSV

 

Hi,

 

The 1.2 factor is an experimental estimation value to calculate the shut off head with having in hand the head of the pump at design point.

 

If downstream equipment/piping is designed per pump's shut off pressure no need to consider a PSV for blocked discharge; otherwise a PSV should be considered where downstream is at design pressure lower than pump's shut off pressure.

 

 

In addition, you can also ask the vendor to trim the impeller, to meet the shut-off pressure calculated by you.

 

The factor of 1.25 is good enough based on my experience.

[fallah]

Hi,

 

Based on my experience it is 1.25.

 

If you use the factor of 1.25, there is high probability (90-95%) that the shut-off head specified by the vendor will be less than what you have calculated.

 

If you use a value more than 1.25, you end up in doing conservative calculation. If you use a value less than 1.25 there is high chances of you making a change in the design pressure of the downstream piping after receiving the vendor curves.

 

 

Hi,

 

The 1.2 or 1.25 is an medium value/factor, as i mentioned, just for shut off pressure estimation and in most cases the vendor data sheets for relevant pumps would be received before pump's downstream piping construction; hence no matter you take the 1.2 or 1.25 value because on the other hand if you take 1.25 it might you will be much higher than the vendor's shut off pressure and you may have to reduce the estimated design pressure for downstream to avoid much CAPEX due to over-design...

Edited by fallah, 01 February 2019 - 08:30 AM.

[breizh]

https://www.cheresou...sure-to-be-125/

 

Hi ,

What fallah wrote a few years ago . The factor 1.2 or 1.25 is an estimation , not more than that .It will have to be confirmed with the curve provided by the vendor .

 

suction head should be : maximum suction head.

 

note : useful link for centrifgal pump :

 

http://www.mcnallyin...2-html/2-7.html

 

hope this is helping you.

 

Breizh

Edited by breizh, 04 February 2019 - 02:05 AM.

[Takechi]

Dear expert,

 

What is the definition of pump shutoff pressure between

1) Intersection of no flow on Y axis in pump performance curve  or

2) Intersection of no flow on Y axis in pump performance curve + suction pressure

 

Thank

[fallah]

 

What is the definition of pump shutoff pressure between

1) Intersection of no flow on Y axis in pump performance curve  or

2) Intersection of no flow on Y axis in pump performance curve + suction pressure

 

 

Hi,

 

In general and in delta P vs Q curve; 2 is correct...

[breizh]

Hi,

No flow means X=0 !

Shut off head =maximum head . 

https://www.quora.co...aulics of pump.

 

good luck.

Breizh 

Edited by breizh, 31 August 2020 - 01:41 AM.

[Subhendu]

Pump shut-off pressure is finally to be provided by the pump vendor. However, we calculate the shut-off pressure so that we can fix design pressure of the associated equipment. We report the calculated shut-off pressure in the pump datasheet with a note "Pump shut-off pressure shall not exceed .....(our calculated value), such that there is no surprise in the later stage of the design. In most of the cases, pump vendor will be able to meet the shut-off pressure estimated by us.

 

Now regarding estimation of pump shut-off pressure it varies from company to company. Few FEED companies estimate the shut-off pressure very conservatively as Max suction pressure + Max differential pressure, while most EPC companies prefer to do it as maximum of the following two

1. Maximum suction pressure + Normal differential pressure
2. Normal suction + Maximum differential pressure

What I have seen across many company, a factor of 1.25 is usually applied over the differential head to arrive at the maximum differential pressure. Please remember, the factor of 1.25 is applicable for motor driven pump. In case of turbine driven pump, this factor to be modified to 1.25 x 1.05² =1.38 (as per affinity laws) considering 5% over-speeding of the turbine.