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# Vapour Pressure in Equilibrium cancelling Vapor Pressure in Tank?

## Vapour Pressure in Equilibrium cancelling Vapor Pressure in Tank?

(OP)
Dear Experts,

I have on pump project in which End User have provided me the NPSHa calculation.
Pressure inside Vessel is 1 bar and Static Head of 0.3 m. In the calculation, they are cancelling the Pressure in Vessel with the Vapor Pressure because they mentioned that the Fluid is in Equilibrium so they cancel each other.

Please give your though if their assumption is correct as it will give the NPSHa value is way too low.
I though that we always calculate vapor pressure based on the liquid been pump at the temperature?

NPSHa = Vessel Pressure + Static Head - Friction Head (neg) - Vapor Pressure.

Application - Skim Pump

Razee.H
Noob Engineer

### RE: Vapour Pressure in Equilibrium cancelling Vapor Pressure in Tank?

First off list all pressure in absolute pressure.

Then provide some data.

Unless the liquid is essentially boiling at the pressure and temperature of your fluid, the argument above seems to be incorrect, but without actual numbers it is difficult to say.

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

### RE: Vapour Pressure in Equilibrium cancelling Vapor Pressure in Tank?

(OP)
Vessel pressure is 2 bar (a)
Head due to elevation is 1m
Vapor pressure according to end user is 2 bar(a) - mentioned that it is in equalibrium.
Friction losses - negliable

### RE: Vapour Pressure in Equilibrium cancelling Vapor Pressure in Tank?

what is fluid and temperature?

"Equilibrium" is a bit of an odd word and might mean different things to different people.

If they say vapour pressure at your storage temp = 2bara then I can believe them, but can you check?

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

### RE: Vapour Pressure in Equilibrium cancelling Vapor Pressure in Tank?

(OP)
Littleinch,

The temperature is around 105 degree F.
Unable to check since they asked me to use their value.

### RE: Vapour Pressure in Equilibrium cancelling Vapor Pressure in Tank?

What is the fluid?

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

### RE: Vapour Pressure in Equilibrium cancelling Vapor Pressure in Tank?

This is common with fractionating columns wher the fluid is literally boiling in the vessel. In that case, it is perfectly correct and appropriate for the pressures to cancel out. It is also true in propane storage bullets.

Johnny Pellin

### RE: Vapour Pressure in Equilibrium cancelling Vapor Pressure in Tank?

Agree with the last post, Essentially the fluid is at bubble point at the surface of the liquid level in the supply vessel. the only head providing NPSH margin is the height of the liquid column.

Can you provide some data on the pump that you are running, and its NPSHr curve? in a case like this where the margin is close it will be important to calculate the pipework losses between the pump and the vessel, as these will erode the margin. Further, and equipment between the pump and the tank (such as filters) should be designed to provide only minimum resistance. I would also consider differently pressure readings over any filter to ensure that it doesn't become blocked. A further layer of protection would be to establish performance measures for the pump. If it is a centrif, then DP, or power measurement. If it is a PD pump, then flow measurement. Set limits close to expected operational values and set to alarm or shut down if limits are reached.

Hope this helps, Happy to provide further help if needed.

### RE: Vapour Pressure in Equilibrium cancelling Vapor Pressure in Tank?

(OP)
JJ Pellin,

Thanks for the explaination. Yup i have read one of old thread about NPSH and it does cancel out. My application is Skim tank in which the client mention all fluids are mixed with Hc gas

### RE: Vapour Pressure in Equilibrium cancelling Vapor Pressure in Tank?

(OP)
Physichack,

My pump is PD pump with NPSHr 17 feets and NPSHa based on the liquid level to pump inlet is 2.2 feet.. it is far off.. the only solution that we can do for now is booster pump. We tried Install suction booster by bypass the high pressure fluid from discharge to suction side but this only able to reduce the NPSHr by 2 feet..

Regards
N.E

### RE: Vapour Pressure in Equilibrium cancelling Vapor Pressure in Tank?

5m NPSHr is a bit high for a PD pump but volatile fluids like this are difficult to pump.

Can pumps exist for this exact purpose and different pumps can go down to possibly 2-3m NPSHr.

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

### RE: Vapour Pressure in Equilibrium cancelling Vapor Pressure in Tank?

Now that you have given us a bit more information, we can start to understand your problem. You could select a PD pump (larger and slower) with a much lower NPSHr. A mixture of liquid well below boiling point with some hydrocarbon gas is much different that a homogeneous liquid at boiling point. PD pumps can operate with cavitation with no harm or damage to the pump. You may be able to operate well with very low or even zero NPSH margin.

Johnny Pellin

### RE: Vapour Pressure in Equilibrium cancelling Vapor Pressure in Tank?

Another option is a vertical can pump, which will allow you to accommodate more potential pressure head. This is also another common option for gas feed pumps with fluid in a critical state.

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