NPSHA when taking in account the velocity head
NPSHA when taking in account the velocity head
(OP)
Could you please, let me know how to calculate the NPSHA if I wish to take in account the velocity head in the suction tank?
- For suction lift:
NPSHA= ha - hvpa - hst - hfs -v2/2g
- For positive (flooded) suction:
NPSHA= ha - hvpa + hst - hfs +v2/2g
Are those calculations right? (+ or - v2/2g)? In which case should I consider velocity head different than zero?
Thanks a lot
- For suction lift:
NPSHA= ha - hvpa - hst - hfs -v2/2g
- For positive (flooded) suction:
NPSHA= ha - hvpa + hst - hfs +v2/2g
Are those calculations right? (+ or - v2/2g)? In which case should I consider velocity head different than zero?
Thanks a lot





RE: NPSHA when taking in account the velocity head
So the short answer is that you can ignore the velocity head in both cases.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: NPSHA when taking in account the velocity head
Why do you think in this case is taking in account the velocity head?
thanks a lot
RE: NPSHA when taking in account the velocity head
The situation you describe is where you have an actual installation you can go out and measure. If you were working off drawings and you wanted to calculate the NPSHa you could calculate the velocity head and deduct it along with the other losses (which would give you the pressure a gauge would see), and then add it back in as Cameron advise. I'm a lazy guy, so rather than deduct it and then add it back in I just ignore it.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: NPSHA when taking in account the velocity head
NPSHa = Total Head - vp, or Static Head only - vp?
NPSHa is Total Head- vp.
Many engineers simply neglect velocity head to arrive at a conservative calculation, however IF velocity head was neglected in the calculation of NPSHa (only static head and fitting losses were considered), velocity head at the suction flange can/could be added into that result. Adding velocity head would give a less conservative result for the NPSHa calculation, but its theoretically valid to do so and you can include it, if you choose to do so.
Total head (HGL) at any point along the pipe is the sum of static head + velocity head. NPSHa is the Total Head at the suction flange - vapor pressure, so you don't subtract v^2/2g at the tank and then add it back in at the suction flange. Since NPSHa is really Total Head at the suction flange -vapor pressure, when calculating NPSHa from the tank level you can leave out all consideration of the velocity head term and subtract only pipe and fitting losses from the initial water level.
NPSHa = s + v^2/2/g - vapor pressure
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RE: NPSHA when taking in account the velocity head
1) For a new installation, the NPSHA in the pump is:
- For suction lift:
NPSHA= (ha - hst - hfs)-v2/2g - hvp
- For positive (flooded) suction:
NPSHA= (ha + hst - hfs) -hvp (more conservative since I dont take v2/2g in account)
2) For an existing installation:
- For suction lift:
NPSHA= Pressure gage - v2/2g - hvp
- For positive (flooded) suction:
NPSHA = Pressure gage - hvp (more conservative without v2/2g)
Did I understand this right? This issue with the velocity head is giving me hard time...
RE: NPSHA when taking in account the velocity head
However, as explained by the experts, its value can be neglected in most cases. See, please, the following values of v2/2g for three different linear velocities;
for 3 fps, v2/2g ≈ 0.141 ft of liquid
for 2 fps, v2/2g ≈ 0.062 ft of liquid
for 1 fps, v2/2g ≈ 0.016 ft of liquid
RE: NPSHA when taking in account the velocity head
RE: NPSHA when taking in account the velocity head
Here's a revised diagram with the NPSHa equation,
This equation adds the v^2/2/g term, however, as Artisi suggests the velocity head is seldom of any significance and most engineers safely ignore this term. I always ignore it myself, unless doing an accurate study and I feel I must incude it to accurately document the model.
I would also suggest that if you want to make such a model to a very high accuracy standard, and you want to include the velocity head, you should also consider other possible terms that are typically neglected, such as subtracting the distance from the suction flange to the top of the pump's impeller, for example, and to be sure to use the vapor pressure of the fluid at the temperature at which the fluid enters the pump, which would therefore also require some thermal considerations.
http://virtualpipeline.spaces.msn.com
RE: NPSHA when taking in account the velocity head
BigInch's diagram highlights the v2/2g situation very nicely by showing that some pressure head is lost to velocity head as the liquid is accelerated into the pipe, but also that some velocity head is converted back into pressure head as the liquid slows down in the reducer (the green section gets narrower). This begs the question - if you are going to add the velocity head back in, which velocity do you use? At the start of the pipe or at the pump suction? If you were using BigInch's formula to do a rigorous calculation you would have to take the loss of pressure head to static head at the start of the pipe, plus the recovery of velocity head to pressure head at the reducer, into the HL (Pipe Friction) term. And then you would add back in the velocity head based on the suction size to get NPSHa. All this converting back and forth comes to a net zero in the end, so it is easiest to simply omit the velocity head from the friction loss calculation and then don't add it back in again at the end.
The question I would like to raise concerns the distance between the centerline of the suction and the top of the impeller. As far as I can recall, all the pump curves I have used have specified the NPSHr at the centerline of the suction. Why would you concern yourself with the distance to the top of the impeller? Surely NPSHr and NPSHa should be relative to the same elevation?
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: NPSHA when taking in account the velocity head
RE: NPSHA when taking in account the velocity head
If the NPSHr curves are referenced to the CL, then there would be no reason to include the height to the top of the impeller. I doubt if I can find the reference for including that, as it has been at least 10 years since I first recall seeing it, however I'll make a few test trenches to look for it.
So, when ignoring velocity head, HGL is set by default and I would have to revise the formulas below the diagram above. We must strike the v^2/2/g term completely.
It seems Mother Nature already had this figured out, since v^2 never results in a negative value!
http://virtualpipeline.spaces.msn.com
RE: NPSHA when taking in account the velocity head
and Leaving in Hp, to cover the case where NPSHr curve reference might be unknown(?). Or strike that too if you like.
Rev 2. HGL reference.
Including pump and discharge line to tank.
http://virtualpipeline.spaces.msn.com
RE: NPSHA when taking in account the velocity head
I think NPSH is one of the most discussed subjects in all forums. See, for example:
thread407-163007: Pump nsph
thread407-171080: Insufficient NPSH and Airbound
thread378-175054: NPSHa Units
and the references therein.
RE: NPSHA when taking in account the velocity head
With reference to the above submittal
NPSHA= (Pt-Pv)(rho*g) + Hs - Hl (1)
Note: velocity head is already accounted for.
The above is based upon definition of NPSHA=
(P-Pv)/(rho*g) + V^2/(2g) (2)
But Pt/(rho*g)+ Hs = P/(rho*g) + V^2/(2g) +Hl (3)
Combine (2) and (3) to yield (1)
Hs is indicated in your first illustration.
Regards
RE: NPSHA when taking in account the velocity head
If you started from some node where you only knew velocity and pressure, you could calculate the HGL at that point to begin a sequential NPSH calculation, or as suggested previously, v^2/2/g is normally so small it is not likely to have any significance and it would be conservative to ignore it anyway.
http://virtualpipeline.spaces.msn.com