# Centrifugal Pumps

**URL:** <https://boards.straightdope.com/t/centrifugal-pumps/133658>\
**Category:** Factual Questions\
**Created:** [October 24, 2002, 10:55am UTC](https://boards.straightdope.com/t/centrifugal-pumps/133658 "2002-10-24T10:55:17Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\
**Post date:** [October 24, 2002, 10:55am UTC](https://boards.straightdope.com/t/centrifugal-pumps/133658/1 "2002-10-24T10:55:17Z")

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This question concerns NPSH: DO any SDopers know the answer to this?

If I change something in the discharge line, such as increasing flow or TDH, does that affect NPSH? In short, would you have to re-calculate the NPSH if you make changes to pump’s discharge line? (If I am not changing the suction side, then I would assume the available and required NPSH remain the same, correct?)

- Jinx

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**Author:** ![NutMagnet](https://avatars.discourse-cdn.com/v4/letter/n/ea5d25/32.png) [@NutMagnet](https://boards.straightdope.com/u/NutMagnet)\
**Post date:** [October 24, 2002, 11:42am UTC](https://boards.straightdope.com/t/centrifugal-pumps/133658/2 "2002-10-24T11:42:47Z")

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Without knowing all the specs: flow rate or velocity, operating pressure, line sizes, etc. changing the discharge could change NPSH, but more likely changes differential pressure.

there is a certain amount of backpressure with a specific discharge line. Enlarging it reduces the backpressure (unless it was zero to begin with), reducing it increases the backpressure. This would have an effect on differential pressure, and (I’m guessing) on NPSH.

From here:  
[http://www.pump-magazine.com/quizzes/quiz\_1/answer\_1.htm](http://www.pump-magazine.com/quizzes/quiz_1/answer_1.htm)

> [@](#):
>
> Usually the elevation between the gages is not as significant as is this example, and the flow velocities are usually lower (larger pipe), but, in general, it is important to remember to **account for all components** \* of the pump head, in calculations.

\*[sub]bolding mine[/sub]

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**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\
**Post date:** [October 24, 2002, 12:15pm UTC](https://boards.straightdope.com/t/centrifugal-pumps/133658/3 "2002-10-24T12:15:00Z")

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[Net Positive Suction Head (NPSH) and Total Dynamic Head (TPH)](http://www.rfmacdonald.com/techtalk/headcalc.html)  
The equations say no. I can think of several ways believing that might get you in trouble, but unless you’ve got a compressible fluid, or are on the ragged edge of cavitation, changing the output piping should only have minor effects.

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**Author:** ![cybersnark](https://avatars.discourse-cdn.com/v4/letter/c/b9e5f3/32.png) [@cybersnark](https://boards.straightdope.com/u/cybersnark)\
**Post date:** [October 24, 2002, 1:10pm UTC](https://boards.straightdope.com/t/centrifugal-pumps/133658/4 "2002-10-24T13:10:08Z")

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If you change the physical characteristics of the discharge piping, there shouldn’t be any changes to AVAILABLE NPSH. However, the REQUIRED NPSH as determined by testing performed by the pump manufacturer, is a function of flowrate.

Besides, how can you change the flowrate on the pump’s discharge side without also changing the flowrate on the suction side?