Can You Spec A Pump with Negative NPSHA/NPSHR 3

[Odom71]

I'm working on specifying a pump to transfer LPG from storage offsite. The thing that I noticed is that the NPSHA that I'm coming up with is extremly low. Infact, it is negative. I've scanned a specsheet for another pump in similar service and I noticed that it was specified with a NPSHA of 0. The NPSHR under the performance data is -2. I've never heard of negative NPSHA or NPSHR before. I was hoping that someone could advise if it is acceptable to specify negative NPSHA which would in turn require a negative NPSHR. If so, what is the practical limit before I play games like placing the pump below grade etc. to overcome this issue?

Thanks,
Odom71

 

[scalleke]

So Steve, What is the NPSH of a liquid while it is boiling in a reservoir that is at a pressure below vapor pressure?

Phase changes do not happen instantaneously. Pulling liquid out of a vessel that is at vapor pressure will make the vessel pressure drop below vapor pressure and depending on the flow rate of the liquid being pulled out and depending on the evaporation rate of the liquid in the tank it will take a certain time before vessel pressure is at vapor pressure again.



Best Regards
Scalleke

 

[TD2K]

Steve, 0 NPSH is NOT absolute vacuum. It means the fluid is at its boiling point, whether that is -40F or 500F.

 

[Scipio]

Scalleke & TD2K,
Agreed completely - smckenne, check your formula for NPSH. It has nothing to do with a vacuum, it has to do with the difference between vapour pressure and absolute pressure of liquids. As I've stated, you can have a pressure of 1000's of psig and still have a negative NPSH.

 

[smckennz]

Oops forgot to say ignoring vapour pressure, and for that matter the inlet velocity head.
NPSH is referenced to an absolute vacuum,but the vapour pressure is subtracted from it.
Imagine a tank with 10m H2O absolute head on the surface, negligible vapour pressure and friction loss. If the takeoff was 10m above the tank liquid level, the NPSHA would be 10-10 = 0m or a perfect vacuum. This could be measured near enough with a manometer or barometer. NPSH is measurable. Thats how they produce NPSH curves. You cant measure a negative Net Positive Suction Head because it doesnt exist.
Negative NPSH? Nah. Impossible real-world condition.
NPSHR > NPSHA? Possible but expect cavitation and decreased liquid flow rate.

Cheers

Steve McKenzie

 

[Assumptions]

Don't know why this post keeps comming up?

Yes you can!

But I wouldn't do it.

Rule of thumb that I have been using is you add a couple of feet to the NPSHR for safety/comfort/screw-ups.

So why would one start out tryng to screw up?

How much air is allowed into the system from leaks? Is there a min operating pressure? Have you looked into a canned pump, it is somtimes necessary.

If you have a two phase pump I would like to learn about it.
Good luck in your search.

 

[Assumptions]

To 25362

You sent me into some reviews because we/I have always calculated NPSHa on the non-recoverable losses.
Looking at one of the more recent texts "Transport Phenomena A Unified Approach" by Brodkey and Hershey, 1988, Chapter 10, page 451 when they calculate the available pressure they do not consider the kinetic energy term.
In my mind it stads to reason because if the pressure is taken at the wall and it is lower than the vapor pressure then one will have a vapor.

Taking the stagination pressure is not the lowest pressure in the system.

 

[25362]

To Assumptions, the NPSHa estimated at the pump suction nozzle as described above is the recognized formula, which is mainly used for evaluating "test results".
The v²/2g term, however small, is a component of NPSH when measured at the pump suction reference date line.

The other formula, mostly used in the design stages to estimate NPSH_a, is:

[(P over liquid surface-Vap. press.)/density] +/- Static elevation-friction drop.

Those are the definitions accepted in the pump business.

As for NPSH_r the requirements may be based on different criteria, depending on the specific pump duties.

I can only repeat the phrase attributed to Galileo: Eppur si muove.

 

[TNH]

It happens for some applications. Vertical canned pump is usually the answer. The pump is installed inside a pit/tank and length is decided in such a way that enough NPSH is made available because of the static head above the pump suction.

 

[gamve]

Not enterig into the NPSH debate, but there are mixed phase pumps availible that have compressor vane type primary stages that concentrate the gas back to a liquid that is then pumped by an ordinary closed shrouded impeller final stage.

 

[25362]

Going through the huge amount of messages this issue has commanded here and in other threads, I'm driven to think the "perpetuum mobile" (aka perpetual motion machine) has been finally discovered.

 

[escheir]

NPSH is defined as the excess of inlet total head over the head equivalent of the vapour pressure of the pumped liquid.
NPSHa is defined as the NPSH available at the pump inlet.
NPSHr is defined as the NPSH required at the pump inlet for operation free from unaccptable deficiencies caused by cavitation.

To achieve acceptable operation of a centrifugal pump the value of NPSHa must be greater, or at the very least equal to the value of NPShr.

Since the value of NPSHr is always > 0, the value of NPSHa can never be negative.

 

[scalleke]

To escheir:

Then why are there dual phase pumps available?

Scalleke

 

[escheir]

To Scalleke,

I was talking about a centrifugal pump.
What are dual phase pumps? do you mean vapor ejector?

escheir

 

[scalleke]

Dual phase or mixed phase. Pumps that can handle liquid mixed with gas.

Scalleke

 

[Scipio]

Isn't centrifugal vs. dual phase a redundant point since NPSHA is independant of the pump in question?

 

[Sammy2]

I agree, a canned vertical turbine pump is probably your best bet. I'd also look real hard at all of your piping line losses, you may be able to get an increase in NPSHA by reducing line losses.

 

[chanyuenlok]

PE/Steel Gas Transition Fitting can sustain negative pressure?

Anybody knows if a 50mm dia. PE100-steel gas transition can sustain negative pressure (7 bar from outside surface of the pipe / fitting) at a prolonged period? What is the effect? Any literature / information about it?

 

[RotaryG]

Odom,

I have not seen these many replies on any other post. Obviously you tickled something special in us.

Your calculations are wrong. Sorry to be blunt. By definition NPSHA cannot be negative. Your calculations are wrong does not mean you do not have a problem. Usually, when dealing with fluids such as LPG (at close to boiling point), you would be requiring a vertical pump in a can. Your calculations (though wrong) are proving this point. With a canned pump, you will ‘create’ adequate NPSH, by simply selecting right length of the can. If someone is selling you a horizontal pump, please be prepared to leave some change in the bank to change the pump later.





Regards,

Guru

 

[Odom71]

Dear RotaryGuru,

Thank you so much for your comment. Unfortunately, I must return the favor and advise you that you are WRONG. My calculations are spot on. It may not be sensible to have a negative NPSHA, which is why is posted the question since I am not a pump specialist. However, my calculations were correct in pointing out the fact that there is insufficient pressure available to efficiently get the material to a standard centrifugal pump. The bottom line is we simply have to modify the suction piping, or increase the pressure on sphere.

Thanks again for your reply.

Best Regards!

 

[iainuts]

Interesting thread! Didn't read everyone's question, but thought I'd comment. Cryogenic pumps often end up with a 'negative' NPSH when actually put into service. The result is decreased 'volumetric efficiency' due to the gas that is ingested. Pumps can be damaged by this, but not always. Some cryogenic recips are designed to be used in 2 phase lines, with vapor fractions as high as 90%. I've not heard of centrifugal pumps like this though. Can you improve the suction line insulation? Increase diameter? Do you know which affect contributes the most to the loss of NPSH (ie: elevation, heat, or pressure drop)? Have you focused your solution on the largest contributor to the loss of NPSH?

Best of luck,
Dave.