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Very Low NPSHA + Negative NPSHA
3

Very Low NPSHA + Negative NPSHA

Very Low NPSHA + Negative NPSHA

(OP)
As all of us know that you MUST procure a pump that having NSPHR ≤ its NPSHA.
I have following cases need your view:


(1) In case of having a pump with NPSHA = 1m, that make your choices very rare to be fulfilled with respect to the NPSHR that need to be offered by the Manufacturer. My question on this, do you think is it possible to attain such value keeping in our mind that a reasonable margin between (NPSHA & NPSHR) need to be kept.

(2) In case I have very low NPSHA (similar to what exampled in (1) above), what are the best pumps types can be suit for such case (Vertical or Horizontal, Centrifugal or PD , if PD which type of PD)?

(3) In case of suction lift (source lower than pump) & we have high VP (Vapour Pressure) for the liquid need to be pumped & Source pressure is ATM (atmospheric), this will probably lead us to negative NPSHA. what the solution in such case?

Mohd Yaseen

RE: Very Low NPSHA + Negative NPSHA

Firstly the pump doesn't have 1m NPSHa, it's the system that governs NPSHa, with the pump dictating NPSHr.

What is the product being pumped?
This will narrow the pump style and then allow you to approach manufacturers of this equipment.

Far too little information supplied for anyone to even start guessing what is required.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Very Low NPSHA + Negative NPSHA

2
To keep this simple
1) NO
2) PD pumps are better, but still not great. Can pumps are your only answer
3) Submersible pumps, can pumps or vertical axial pumps with inlet as far below the liquid surface as possible

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Very Low NPSHA + Negative NPSHA

There is no such thing as negative NPSH. Don't ever say that in front of other engineers. NPSH is representative of a value greater than 0.

RE: Very Low NPSHA + Negative NPSHA

Negative NPSHa makes no sense.

RE: Very Low NPSHA + Negative NPSHA

(OP)
Thank you For all the engineers who responds to my clarification


Dears spartan117/TenPenny

As you know;
NPSHA = Pressure @ Source - Vapour Pressure ± Suction/Lift Static Head (+ if flooded & - if lifting) - Losses (frictional+Potential)

Assume Crude Oil Fluid with S.G of 0.87
Normally the Losses play the least effect in this equation.
Assume Pressure @ source is ATM 1.01325 bar which is approximately equal to 0.1187 meter (1.01325/0.87*9.81).
Pumping temperature is 85C, & PV = 1.344 bar which is approximately equal to 0.1574 meter (1.01325/0.87*9.81).
In case my source is under the pump level that mean I have Lift Static Head (negative value). Assume just 0.5m.

That will lead us to Negative NPSHA.
Simply for any suction lift system (source is under the pump level), unless you having pressure at the source you will face the negative NPSHA regardless how the source is leveled compared to the pump.

Mohd Yaseen

RE: Very Low NPSHA + Negative NPSHA

Mohd,

Your post above contains a number of errors.

1) When working NPSH, work in absolute pressure and note this by saying bara or psia, not "bar"

2) assuming your pressures above are in bara, your calculation of head is out by about 100.
The formula when using bar and SG is m = (bar x 10.197)/SG https://www.easycalculation.com/formulas/pressure-...

3) If the vapor pressure of your liquid ( I assume PV means vapour pressure) is 1.344 bara, but it only has atmospheric pressure at sea level on top of it, then it won't be a liquid, but instead a violently expanding fluid. The only way this fluid can be a liquid at that vapour pressure is if the gas pressure above the liquid is the same or higher.

"Negative" NPSH cannot exist as a liquid. I think you really mean NPSHA is lower than NPSHR, in which case the liquid will be very difficult / impossible to pump as it will vapourise in the inlet of the pump, but you really need to understand you can't have negative NPSH.

but in general, if you have aliquid that is either in equilibrium with the pressure of the gas above it or very close, then there is no ability to "lift" the liquid or have any appreciable friction losses.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Very Low NPSHA + Negative NPSHA

"Assume Pressure @ source is ATM 1.01325 bar which is approximately equal to 0.1187 meter (1.01325/0.87*9.81)."

Well, as my father would say, "there's your problem Harry".

I suggest you double check how to calculate NPSHa.

RE: Very Low NPSHA + Negative NPSHA

(OP)
LittleInch

1) I think whether its absolute or gauge this will not affect the calculation cause I am considering that VP & Source Pressure will finish each other.

2)right (Kpa should be used instead of bar).

3) yes may you have logic in this, but currently in our plant we have our crude with VP = 0.95 & as our source is ATM that will lead both of them to cancel each other (approximately). In our case we have suction head, so we don't have an issue, but what the case if the suction is lift suction (source below the pump).

No I don't mean the difference between the NPSHA & NPSHR (this another matter).



TenPenny
Yes may I have mistaken (using bar instead of KPa), so it should be 11.87 meter instead of 0.1187 meter, nevertheless this has nothing to do with the NPSHA calc cause the idea that Source Pressure will be finished with Vapour Pressure (this is approximately, so please don't get back with highlighting the minor difference between both of pressures as an issue).

Mohd Yaseen

RE: Very Low NPSHA + Negative NPSHA

Mohd,

The difference between gauge and absolute definitely matters. If you don't understand why, you need to talk to a more senior engineer at your firm.

RE: Very Low NPSHA + Negative NPSHA

(OP)
Dear LittleInch

even if we consider a water for example with low VP (0.4 bar @70C about 4 meter), & the source (below pump) is ATM (1.01325 bar) (about 10 meter) & with considering the line losses (0.2bar about 2 meter).

Considering 1 meter as least (margin between both NPSHR & NPSHA)

Let us assume Suction Static Lift = 3 meter this will lead us to have NPSHA = 1 meter & consequently NPSHR must be = 0 which isn't exist.

Mohd Yaseen

RE: Very Low NPSHA + Negative NPSHA

Mohd,

1) Yes it does - that's how it works.
2) good
3) If you mean Vapour pressure is 0.95bara then you have some serious storage issues as this can't normally be stored in a floating roof tank and the vapour handling for a fixed roof tank will be significant.

Often with NPSH you have only a few metres to play with so using accurate data is essential, your previous post had the vapour pressure as 1.3bara...

If the pump inlet is higher than liquid level at that sort of vapour pressure then beyond about 1m you will be trying to pump gas - tends not to work very well and your pump will complain and disintegrate if you do it for too long.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Very Low NPSHA + Negative NPSHA

(OP)
RVAmeche

Absolute = gauge + ATM
(Pa (kPa a) = Pr (kPag) + Patm (kPa a) = 100 kPa a at sea level)

so in my case

I have assume Vapour Pressure will equalize Source Pressure.

Mohd Yaseen

RE: Very Low NPSHA + Negative NPSHA

(OP)
LittleInch

Right, I know my case is little tough.

Just to note (in your respond to point (1) above, yes it does if there is difference but in case of ASSUMING equilibrium between both what will affect the calculation?)

Mohd Yaseen

RE: Very Low NPSHA + Negative NPSHA

Mohd,

Looking at your latest post at 14.10

Vapour pressure water at 70C is actually 31.1 KPa, so actually 3.2m, atm pressure 10.3m

So with a 2m friction loss and 3m lift, you're looking at 2.1m


That is very low and yes, you will have a lot of problems finding a pump to do this. Therefore something has to change.

normally it is the location of the pump inlet relative to the liquid level so either a vertical axial pump, submersible pump or just a pump located at a lower level. Or pump cooler fluid.

I'm not sure what your point is / question is anymore?

You are far better to use bara, even if the gas pressure is the same as the vapour pressure - it means you don't miss anything and if one or the other changes you don't miss it.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Very Low NPSHA + Negative NPSHA

(OP)
Dear LittleInch

Thanks a lot for your feedbacks & cooperations.

Thanks also for all other contributors whom give inputs on this discussion.

Mohd Yaseen

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