NPSHa under vacuum conditions

[gorkus]

we are going to pump from a tank under vacuum.0.9 barg

In order to calculate NPSHa we use follow formula.

NPSHa=Hs+h+Ha-Hv where:

Hs: static suction head (m)
h: suction pipe loss (m)
Ha:surface liquid pressure ( m)*
Hv: liquid vapor pressure at pumping temperature.(m)

* when the tank is open,the value of Ha is atmospheric pressure( 10.3m), but when liquid in the sucction tank is under vacuum the Ha= -9 m but do i have to add the atmospheric value as well?

many thanks in advance.

 

[rzrbk]

The surface liquid pressure needs to be an absolute pressure. In this case the surface liquid pressure would be 1.3m instead of -9m. The atmospheric pressure is included this way.

 

[gorkus]

so when the surface liquid pressure is equal the liquid vapor pressure at pumping temperature.( saturated liquid)
Both values cancel each other.
In this case in order to work out NPSHa value I dont add atmospheric pressure value?? it is ok??

thes

 

[gorkus]

So when the surface liquid pressure is equal the liquid vapor pressure at pumping temperature.( saturated liquid)
Both values cancel each other.
In this case in order to work out NPSHa value I dont add atmospheric pressure value?? it is ok??

 

[vpl]

gorkus posted this in a separate thread, and I think it really belongs here:

Sorry¡¡ there is a mistake in the formula.is negative the pipe loss

NPSHa= Hs-h+Ha-Hv

Patricia Lougheed

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

 

[rzrbk]

In your formula, 'Ha' should be in absolute pressure. (Assuming that the vapor pressure is also in absolute units.) This is gage pressure + atmospheric pressure.

In the case of an open tank this would be Ha = 0m + 10.3m =10.3m.
In the case of a tank at -9m vacuum this would be Ha = -9m + 10.3m =1.3m.
In the case of a tank at 30m positive pressure this would be Ha= 30m +10.3m =43.3m.

 

[timbones]

Gorkus, if I understand you correctly, you have saturation conditions in the vessel you are pumping out of. In this case you are correct, the Ha term and vapour pressure cancel eachother.

So using the same definition of terms as you have used (assuming Hv=1.3 m)

NPSHa = Ha - Hv + Hs - h
NPSHa = 1.3 - 1.3 + Hs - h
NPSHa = Hs - h

Often in these cases it's difficult to get enough static head (Hs) to satisfy the requirements of the pump. Your choices are to raise the height of your vacuum vessel or stick the pump in a hole. The most common I have seen is to use a vertical canned suction pump.

I notice your pressure is 0.9 barg, which is not a vacuum. Did you mean 0.9 bara?

 

[RotEquipEngr]

The pump will not know or care if it pumping under a vacuum. You will have to figure your NPSA in abs. pressure. Even if your NPSH comes out to a negative number, the partial pressure of the tank under vacuum will compete with the static pressure of the liquid, but it will not take it away completely. On our vacuum tower the suction line on the pump is under a vacuum, or 10 psia; the pump is on a variable speed driver, so we have no issues with the pump performance.

 

[BigInch]

OPEN or CLOSED Tank NPSHa

Tank Pressure measured by gage, convert to Head.
(for a vacuum reading USE the minus sign)

+Atmospheric Pressure, (always absolute), convert to head.

Static Head, (negative if liquid level is below pump)

-Pipe Friction Head Loss. (usually negative, sometimes zero)

-Vapor Press, convert to head.
(always absolute, but always subtracted for NPSHa)



BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[25362]

The subject of NPSHA has been discussed many times. I suggest you visit the following threads to get ideas:

thread407-6587
thread407-9873
thread407-12387
thread407-34369
thread407-41042
thread407-45276
thread407-51468
thread407-59805
thread407-61239
thread407-61919
thread407-85933
thread407-130418
thread798-63268

 

[BigInch]

Forgot one,
thread407-179720

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[25362]

BigInch, thanks.

 

[25362]

I forgot to mention that the added thread, mentioned by BigInch, is this one.

 

[quark]

eh! 25362,

I am unable to find whether you are very serious type or this is a coup-de-grace

 

[BigInch]

we're having fun. How about you?

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[25362]

Quark, my comment had no ulterior purpose, it was intended as a light-hearted jeu d'esprit, as BigInch aptly described. :-D

 

[quark]

I thought conspicuous humor is oxymoron. Nevertheless, count me in for the fun party.

 

[MortenA]

Getting back to the subject: Its difficult to meet NPSHr for a centrifugal pump when your source is under vacuum (and cheating in the calcs wont help you when you "plug it in" ;-) )


Your only "upside" is to have a high vapour pressure or a high liquid column. This i have in fact used in a degasser. The vessel was (more for piping reasons) elevated aprox. 5 metres and this would enable me to meet the require NPSH when using large suction pipe diametres.

Best regards

Morten

 

[davsy]

MortenA

Meeting NPSHr when the source is under vacuum does not really have to be a problem. As you said further down your post sufficient liquid column is one way to overcome the problem as is a low NPSHr pump.

There are centrifugal pumps on the market with particularly low NPSH requirements.