NPSHa under vacuum conditions
NPSHa under vacuum conditions
(OP)
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.
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.





RE: NPSHa under vacuum conditions
RE: NPSHa under vacuum conditions
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
RE: NPSHa under vacuum conditions
Both values cancel each other.
In this case in order to work out NPSHa value I dont add atmospheric pressure value?? it is ok??
RE: NPSHa under vacuum conditions
Patricia Lougheed
Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.
RE: NPSHa under vacuum conditions
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.
RE: NPSHa under vacuum conditions
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?
RE: NPSHa under vacuum conditions
RE: NPSHa under vacuum conditions
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
RE: NPSHa under vacuum conditions
The subject of NPSHA has been discussed many times. I suggest you visit the following threads to get ideas:
thread407-6587: ZERO NPSH PUMP
thread407-9873: How is the NPSH(R) determine?
thread407-12387: NPSH
thread407-34369: Centrifugal Pump Operating with NPSH << NPSHR
thread407-41042: Vacuum Tower Overflash pumps
thread407-45276: NPSHr (H2O vs Flowing Fluid) and Calculations
thread407-51468: NPSHa
thread407-59805: Can You Spec A Pump with Negative NPSHA/NPSHR
thread407-61239: what is positive suction pressure n negative suction pressure?
thread407-61919: Pump Cavitation Remedy
thread407-85933: NPSHR and viscosity
thread407-130418: Does direct recycling to suction improve NPSHa?
thread798-63268: Liquid CO2 Pumping
RE: NPSHa under vacuum conditions
thread407-179720: NPSHa under vacuum conditions
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: NPSHa under vacuum conditions
BigInch, thanks.
RE: NPSHa under vacuum conditions
I forgot to mention that the added thread, mentioned by BigInch, is this one.
RE: NPSHa under vacuum conditions
I am unable to find whether you are very serious type or this is a coup-de-grace
RE: NPSHa under vacuum conditions
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: NPSHa under vacuum conditions
Quark, my comment had no ulterior purpose, it was intended as a light-hearted jeu d'esprit, as BigInch aptly described.
RE: NPSHa under vacuum conditions
RE: NPSHa under vacuum conditions
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
RE: NPSHa under vacuum conditions
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.