Do pump manufacturers always publish NPSHR in absolute pressure? 1

[Mark0420]

I need some help from one of you pump gurus out there, but I hope this isn't going to start another three month long discussion on NPSH. I'm working with NPSH on a centrifugal pump for the first time in my life, but have read some posts with hundreds of responses. To avoid all those tangential discussions, hopefully I can ask some "yes or no" questions here and find the answers I'm looking for without evoking references to Star Trek and Black Holes.

Here we go . . .

1) If a centrifugal pump manufacturer calculates that the NPSH required (NPSHR) is 7 feet, is that always in absolute pressure?

2) If there is 20 feet NPSH available (NPSHA) at the suction flange and atmospheric pressure is 14.7 psi x 2.31 ft/psi = 33.96 feet, is there any way that water could be leaking out of the suction side shut-off valve?

3) 0 psia is a perfect vacuum. Is there any way pressure can be less than 0 psia?

4) If NPSHA is always in absolute figures, then wouldn't NPSHA = 0 be the same as 0 psia?

5) Can NPSH available (NPSHA) be negative as suggested in many, many posts? (I don't think negative absolute numbers are possible.)

6) Finally, if my "NPSHA = 20 ft head" application developed a hole in the suction line would it be sucking air into the system since atmospheric pressure is 33.96 feet of water?

Please, please give me some simple yes/no responses. Giving me examples of some cryogenic process operating in space won't help. I'm having enough trouble figuring out how to pump water out of my tank.

 

[katmar]

I guess I was guilty of diverting some of the old threads into Black Hole territory, so I will try to keep it concise and relevant here!

1) Yes - always absolute.

2) Yes - water could leak. NPSHA is less than the point pressure by the vapor pressure of the liquid. For example, Pressure = 35 ft, vapor pressure = 15 ft, => NPSHA = 35 - 15 = 20 ft. Because the actual pressure is above atmospheric you could have a leak.

3) No (It's so tempting to refer to Black Holes.)

4) No. See 2 above. Unless vapor pressure is 0, which I guess only happens if T(abs) is 0. You aren't designing a Large Hadron Collider are you? (Whoops, I promised not refer to Black Holes.)

5) No - NPSHA cannot be negative. For that to happen the vapor pressure must be higher than the point pressure and in that case you have a gas and not a liquid.

6) Maybe. Maybe not. See 2 above.

Sorry - that wasn't as concise as I hoped I could achieve. But I think we are still going to see lots of arguments against my points of view.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[BigInch]

No arguments. Zip. Zero. Nada.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[MortenA]

Katmar i think you would have to explain 2) again for me:

How could water leak if the pressure in the pipe is below the outside pressure?

A little extra for 3)

Your hydraulic package may calculate a pressure below 0 psia. This is just math. It has no physical meaning. It should tell you that you solution is "unreal".

Best regards

Morten

 

[katmar]

Morten, wrt 2) the pressure in the pipe is not below atmospheric. The pressure in the pipe is 35 ft while atmospheric pressure is 34 ft. This is only a small difference, but there would still be 1 ft of head to drive the water through the leak.

The NPSHA is numerically less than atmospheric because we have to subtract the vapor pressure from the point pressure to get NPSHA. By "point pressure" I mean the pressure you would see if you connected a gauge there. NPSHA is a less concrete concept and can't be measured directly.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[BigInch]

NPSHA is just a value that means something to a pump. For example, NPSHA could actually be comprized entirely of velocity head alone and have a pressure component of essentially zero, or v/v.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Mark0420]

Katmar,

If NPSH is always absolute, then my NPSHA of 20' is already an absolute pressure. Why would I add 15' to it to get 35'? If the absolute pressure inside the pipe is 20' and the absolute pressure outside the pipe is 35', wouldn't air enter the pipe rather than water leave the pipe?

 

[Mark0420]

Katmar, Also, when I'm calculating NPSH available I'm starting with atmospheric pressure above the liquid in my tank. Including atmospheric pressure in the calculation causes my final pressure calculation to be in absolute figures. Is this not correct?

 

[vpl]

A clarification on Q5: When people refer to a "negative NPSH" what they are actually saying is "my net positive suction head available is less than the net positive suction head required (or NPSH_r - NPSH_a = -#)."

Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[Mark0420]

VPL, Thanks for that. Learning the venacular will help me keep from looking stoopid. ;-)

 

[katmar]

If NPSH is always absolute, then my NPSHA of 20' is already an absolute pressure. Why would I add 15' to it to get 35'?

Remember that the 15' was the example vapor pressure I used. Don't think of it as adding 15' to the 20'. The NPSHA is not the starting point in the calculation. You have to work the other way around. The actual pressure of 35' is the parameter that you can measure (or calculate) and then you derive the NPSHA from that by subtracting the 15' of vapor pressure.

If the absolute pressure inside the pipe is 20' and the absolute pressure outside the pipe is 35', wouldn't air enter the pipe rather than water leave the pipe?

If the absolute pressure in the pipe were 20' then air probably would leak in, but in my example the absolute pressure in the pipe is not 20'. The NPSHA is 20'. The absolute pressure in the pipe is 35' and the absolute pressure outside the pipe is 34'. The pressure inside the pipe is higher than that outside so the water leaks outwards.

Also, when I'm calculating NPSH available I'm starting with atmospheric pressure above the liquid in my tank. Including atmospheric pressure in the calculation causes my final pressure calculation to be in absolute figures. Is this not correct?

Yes it is, but it is potentially confusing to put it that way because you are using a specific case to prove the general rule. You are using atmospheric pressure as the specific case. The pressure (expressed in absolute terms) above the liquid in the tank is added to the static height (and the friction loss and vapor pressure are subtracted) to give NPSHA. It does not matter what the pressure in the tank is - it could be vacuum, atmospheric or high pressure. You express that pressure in absolute terms and add it to the static head.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Mark0420]

Katmar,

Okay, I'm feeling really dense here and I appreciate your patience.

My original question was: If a centrifugal pump manufacturer calculates that the NPSH required (NPSHR) is 7 feet, is that always in absolute pressure?

Your answer was: Yes - always absolute.

In your last response you said that the NPSHA = 20, but that this is NOT absolute pressure: If the absolute pressure in the pipe were 20' then air probably would leak in, but in my example the absolute pressure in the pipe is not 20'. The NPSHA is 20'. The absolute pressure in the pipe is 35' and the absolute pressure outside the pipe is 34'. The pressure inside the pipe is higher than that outside so the water leaks outwards.

Let me apply some numbers for this example. If I calculated the 20' NPSHA starting with atmospheric pressure above the liquid in my tank . . .

For instance

NPSHA = Hp + Hz - Hf - Hvp

Hp = 14.7 psi = 34' water (atmospheric pressure)
Hz = 6' of water in tank
Hf = 16' friction loss through pipe and connections
Hvp = 1.73 psi = 4' water (120F approximately)

NPSHA = 34 + 6 - 16 -4 = 20' ft of water

1) I think my 20' of water is already in absolute terms because I started out using 34' for atmospheric pressure rather than 0.
2) I think my pump impeller is seeing 14' of water vacuum since atmospheric is 34'. (-14' gauge + 34' = 20' absolute)
3) I think since my pump needs 7' NPSHR that this will work regardless that the pump is actually seeing 14' of vacuum.
4) I think that when manufaturers publish numbers for their pumps such as "NPSHR = 7", that these are indeed absolute numbers and that this pump will work with a 27' of water vacuum, assuming no safety factor (-27' gauge + 34' = 7' absolute).

Is there anything that I've said here that isn't correct?

 

[katmar]

Mark, I think I see where our misunderstanding is coming from. In your last response you quoted me as saying "the NPSHA = 20, but that this is NOT absolute pressure". That is not what I said. That is your paraphrasing of what I said.

We established right at the beginning that NPSH is always measured in absolute terms. So if we say "The NPSHA = 20'" we mean that the NPSH is 20 ft in absolute terms. When I said "The absolute pressure is 35'" I meant that the pressure as measured by a pressure gauge (what I called the point pressure) is 35' in absolute terms. Perhaps I did not make this clear enough because I obviously confused Morten as well. The point pressure is what you would get from Hp + Hz - Hf in your terms. This is the actual, absolute pressure that you would measure at the suction flange of the pump. This is the pressure that was 35' in my example. It is higher than the external pressure of 34' and water could leak out.

Although the NPSHA is referred to in the units of absolute pressure, it is not an actual pressure that exists anywhere, or that could be measured if you could connect a pressure gauge at any point.

A useful way of thinking of NPSHR is that it is the extent by which the pressure at the suction flange of the pump must exceed the vapor pressure to ensure that you do not get cavitation. Re-arranging your equation you can get
NPSH + Hvp = Hp + Hz - Hf
This shows that Hp + Hz - Hf must always exceed Hvp by a constant offset, and that offset is called NPSH.

In the eye of the pump impeller there will be local pressures that are below the pressure measured at the pump suction flange. If these local pressures go below the vapor pressure of the liquid the liquid will flash (boil) and you will get cavitation when that vapor collapses as the fluid moves to a higher pressure zone in the pump. The pump manufacturer specifies the NPSHR so that there is sufficient pressure at the pump suction so that when the pressure decreases further in the eye of the impeller the pressure never gets down to the vapor pressure. I hope this makes the mechanism clearer.

Addressing the four points at the end of your last post:

1) I think my 20' of water is already in absolute terms because I started out using 34' for atmospheric pressure rather than 0. Yes it is.
2) I think my pump impeller is seeing 14' of water vacuum since atmospheric is 34'. (-14' gauge + 34' = 20' absolute) I don't like the term "14' of water vacuum". I prefer -14' gauge, but it is always better to work in absolute pressures. I know mechanical engineers don't like it, but chemical engineers love it that way. The only thing gauge pressure is good for is specifying pressure vessels. Perhaps I am biased?
3) I think since my pump needs 7' NPSHR that this will work regardless that the pump is actually seeing 14' of vacuum. Yes.
4) I think that when manufaturers publish numbers for their pumps such as "NPSHR = 7", that these are indeed absolute numbers and that this pump will work with a 27' of water vacuum, assuming no safety factor (-27' gauge + 34' = 7' absolute). Yes.


Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[BigInch]

Hz should be equal to the height of fluid's surface above the centerline of the pump. Is that what your 6' dimension is?

If it is, you are correct on all points.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Mark0420]

Katmar, Thanks again for your patience. That makes it clear in terms I can understand. Perhaps with a few more applications I'll gain a better understanding of the equation. Right now it's kind of like velocity cubed. I can use it in an equation, but can't really visualize it!

 

[Mark0420]

BigInch,

Yes that is correct and thank you also for your response. My post didn't start as big a discussion as some of the last NPSH posts did. Perhaps that's because I started it at 3:30AM!! Hope I can sleep better tonight after getting answers from you guys.

 

[BigInch]

Take Katmar's advice and you'll sleep like a baby, if you always use absolute pressure. Nitey nite.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[rmw]

May the force be with you. But make sure it is all standing above your pump.

rmw

 

[rmw]

Mark0420,

Don't worry about having to try to get your head around the concept of NPSH. When I was a young engineer I didn't get it right away either. I don't know why, because once it clicked, it seemed so simple, but it did not click right away. That frustrated my Chief Engineer to no end. How many times did he send me packing with the words "because I sit in here and you sit out there" (in the drawing room.) We were in the evaporator (watermaker) mfg business and it mattered big time to us as the NPSH for our pumps determined how high we had to build our units. He was a PHD in ME and he must have thought I was a dolt. (I was a dolt.) But one day it just clicked. I was in his office and I can remember the look on his face like it was yesterday. It just clicked and I have had a grip on it ever since. He was as proud as a brand new father. Now then, I'd like to dig him up from the grave and tell him where he was wrong about heat exchangers, but then again that is another topic.

rmw

 

[BigInch]

He did have it right. Only needed a lot of reassurance.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/