enthalpy 1

[gorkus]

I have propane in a tank at equilibrium with vapor .
I want to know how can I get amount of vapor formed at inlet pump.
If I know tank pressure and vapor pressure ,could I Know Enthalpy value?
Then Knowing this value I have to calculate enthalpy value when pressure drop.
with this new enthalpy value I Kno the amount of vapor formed.
Could you help me?

 

[25362]

As long as you have available NPSH (net positive suction head) the propane wouldn't evaporate in the suction line or at the pump suction eye.

This happens because the hydraulic "static" head from the tank to the suction would more than compensate the friction pressure drop of the propane flow in the suction pipe. Thus, you may still have vapor-free liquid at the pump suction, and in fact the liquid may be slightly "subcooled" in comparison with the tank contents.

 

[gorkus]

You are right,but I want to know amount of vapor formed in order to know what flow will be pumped by pump.
There are a loss of delivery of propane which is caused by this phenomenaof liquefied gases.
Then Ihave to be able to know this loss of delivery for a given pressure drop .
That is why i want to calculate the enthalpy value.

I work for a pump factory in Spain. We are starting to build glp vane pumps.

 

[25362]

If you get hold of a pressure-enthalpy diagram or a Mollier enthalpy-entropy chart on propane, you'd be able to locate the V/L composition at any desired pressure and temperature at the pump suction point. Si necesita mas explicaciones estoy a su disposicion. Buena suerte.

 

[Montemayor]

gorkus:

Go to the GPSA Engineering Databook. There you will find a complete and detailed Mollier Diagram for Propane. The enthalpy for pure Propane and a complete path for any process involving pressure or temperature can be easily traced on it.

You can also go to:

http://webbook.nist.gov/chemistry/fluid/

and obtain detailed propane enthalpy or 11 other thermodynamic properties. From this FREE database you can easily calculate any flash vapor formed by the free expansion of liquid propane (if that is what you are describing as your proposal).

25362 is correct in his statement that you cannot get any vapor at the pump's suction if you maintain a positive NPSHa above the pump. I think you know that already, but I believe there is a failure in communicating between English and Castellano thinking. I suspect what you have is a system where you are pumping liquid propane and subsequently flashing it downstream of your pump. You may want to calculate the amount of flash vapor formed and that is why you are seeking the Propane enthalpy figures which will enable you to make a simple mass balance and come up with the equation:

V/L = (hL -hF)/(hF-HV)
where,
hL = Flashed liquid enthalpy
hF = Feed liquid enthalpy
HV = Flashed vapor enthalpy

Is this what you mean?
I hope this helps.



Art Montemayor
Spring, TX

 

[Montemayor]

gorkus:

I've found that you've created 3 posts out of this one, common query. I also think I now know what your dilemma is, but what's going to happen is that the duplicated posts are going to be removed - and a lot of prior posts are going to be lost. It is Forum policy to eliminate double (& much more, triple) posts.

You may have to re-post your query and if so, I strongly recommend you re-compose your description of the problem. I suspect it has nothing to do with the enthalpy of Propane but a lot to do with the NPSHa of your pumping system. I know that it can't have anything to do with the NPSHr, since that value is experimental. But I write this because if you are to obtain some help here, it may have to be in a different posting - and I recommend you keep it in the Pump Engineering Forum - not here in the Heat Transfer & Thermodynamics Forum.

Buena Suerte

Art Montemayor
Spring, TX