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NPSHA

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gt5879c

gt5879c

Civil/Environmental
Oct 31, 2006
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I am bit confused on the NPSHA calculation. The equation I am using is this:

NPSHA = Ha +/- Hst - Hf - Hvp

Where,
Ha = ambient pressure
Hst = static head from liquid surface to pump centerline
Hf = friction loss a entrance through piping/fittings
Hvp = vapor pressure

Ok, so I know on suction lift stations, the Hst value is the distance from the water level in the WW to the pump centerline, which is a negative value.

My question is this: on a submersible station, is the Hst value still the distance from the water level *in the WW* to the pump centerline or is it the static head (WW water level to discharge water level)?

Thanks!
 
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gt5879c,
Hopefully someone more experienced in pumps will answer this. I just spec pumps for sewage wetwells.
The Hst value is the distance from the water level in the wetwell to the pump centerline.
The NPSHa is at the entrance to the pump. What happens after the discharge doesn't matter for the available NPSH.
What happens after the discharge will affect your required NPSH by determining where the pump will run on the curve.
If you have a standard type lift station open to the atmosphere and are using standard type submersible sewage pumps with no suction piping, you should not have a problem with NPSH.
 
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Static head is on the pump suction side. So, it is the distance from the pump suction to the liquid level in the wet well.
 
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If your pump station has submersible pumps as indicated, there is no suction piping and the liquid level is always above the pump intake. Therefore the liquid is not lifted to the pump and NPSHA (Net Positive Suction Head Available) doesn't come into play because NPSHR (Net Positive Suction Head Required) is zero. In simpler terms, NPSH only applies when the pump is 'pulling' liquid to lift it. With submersible pumps, the pump is 'pushing' the water to lift it.
 
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Net positive suction head (NPSH) problems are eliminated with submersible pumps. However, the available submergence should be specified.
 
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Something does not sound correct to me. Are you saying that cavitation is never a problem in flooded suction pumps and that submersibles cannot cavitate?
 
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tkall,
There are other things which will cause a pump to cavitate.
If the pump runs too far to the right, it will cavitate.
If there is not enough submergence, a vortex will form and the pump will cavitate.
However, there are typically not NPSH-specific problems with submersible pumps open to the atmosphere with no suction piping.
 
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jgailla:
I think it's all the same thing as far as what the pump sees. Static lift is just a term in the equation. You can zero it out but you still have the other terms, right?
 
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tkall,
I am not even close to being an expert on pumps, but:
If you're talking about damage to the impeller, then yes it is the same thing.
Cavitation is caused by the pressure being less than vapor, no matter what physically outside the pump is causing it.
However, NPSH, vortexing, and running right on the pump curve are all caused by different things and should be checked differently.
It does make a difference in design or forensics if the cause is too high of a flow or vortexing or NPSH.

bimr, can you add to this?

tkall,
If you go to the Pump Engineering forum you will find a lot of discussion of NPSH issues.
There are a lot of very educated people on that forum. BigInch in particular, not to slight anyone else.
 
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To clarify, adequate suction pressure at the submersible inlet is necessary for the pump to perform as designated. This suction pressure (absolute) converted into water head is called the NPSH req. and is shown on the pump performance curve as one of the pump characteristics.

On the other hand, actual suction pressure (absolute) converted into water head is called NPSH av. It is defined as:

NPSH av. = Is (Submergence of Impeller) + Pa (atmospheric pressure) – Pv (vapor pressure) –Ps (friction losses in suction pipe of dry pit)

The NPSH required shall not exceed NPSH available in the continuous pump operating rants. Therefore, the NPSH of the submersible pump should be checked during engineering design.

The minimum submergence of the pump impeller is set by the pump manufacturer. This varies from 15” at 1000 gpm to 115” at 100,000 gpm. The minimum submergence is for vortex control.
 
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Therefore, the NPSH of the submersible pump should be checked during engineering design.

Isn't this what I said?

Jgailla, sure, there are many things that need to be investigated when one is in the design phase of a project. But just because the pumped liquid is not being "lifted" is not necessarily sufficient justification to ignore NPSHA and NPSHR. If I am wrong about this thats ok...I am a pretty good student and I do take instruction well.
 
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tkall,
I must have misunderstood you.
I agree with you, NPSHa and NPSHr should always be checked in design.
My point was that when using a pump with no suction piping in a submersible sewage lift station, NPSH should not be a problem, which doesn't mean it shouldn't be checked.
 
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gt5879c,

To answer your question directly: yes, the Hst value is still the distance from the water level in the WW to the pump centerline.

Regards,
Allie
 
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