LPG pumping
LPG pumping
(OP)
Dear all!
I am facing a propane and butane bulk transfer system from horizontal bullets vessels towards rail tank wagons with no vapor return line. As far as I know in steady state the pressure inside the LPG vessels will depend only on their temperature and will be coincident with the LPG vapor pressure but during the bullet discharge there is a transient condition during which the pressure decreases below vapor pressure. Is it right? How can I calculate that transient pressure decrease?
Moreover if pressure decreases below vapor pressure we will have a problem with pump NPSH. Which are the usual solutions for this problem?
Thank you all for any help
I am facing a propane and butane bulk transfer system from horizontal bullets vessels towards rail tank wagons with no vapor return line. As far as I know in steady state the pressure inside the LPG vessels will depend only on their temperature and will be coincident with the LPG vapor pressure but during the bullet discharge there is a transient condition during which the pressure decreases below vapor pressure. Is it right? How can I calculate that transient pressure decrease?
Moreover if pressure decreases below vapor pressure we will have a problem with pump NPSH. Which are the usual solutions for this problem?
Thank you all for any help





RE: LPG pumping
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: LPG pumping
RE: LPG pumping
BigInch, I fail to see any J-T expansion effect. Can you clarify ?
RE: LPG pumping
Is it not the J-T cooling effect on the vapor expanding to fill up the increasing space above the liquid as liquid is drawn off the bottom? I thought that was why there is condensation (often freezing) that can appear on the outside of a butane lighter refilling can, but only on the portions of the can's surfaces that are above the liquid level. Of course the JT effect would be more pronounced on butane that actually vaporizes as it exits the tank.
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: LPG pumping
This is evaporation by pressure reduction. The J-T effect -mostly cooling but not always- is a throttling (flow) process. Right ?
RE: LPG pumping
I think evaporative cooling is part of the total cooling effect, but a significant portion must be due to J-T. My logic being that lowering pressure at constant enthalpy, whether it is done by taking the cap off the top, removing liquid from the bottom, or by running it through a throttle valve should make no difference. Would not the heat of vaporization be removed from the liquid, cooling that, but actually adding heat to the vapor? But, now you do have me wondering if I remembered correctly. Is it the can's surface above the liquid line that frosts, or the can's surface below the liquid level that frosts??? I'm out of butane right now, so I can't test it myself tonight.
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: LPG pumping
I still think there are no JT effects inside the bullets, and that all of the cooling comes from the flashing saturated liquid at reduced pressures.
RE: LPG pumping
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: LPG pumping
All the answers seem to be in here,
http://www.corken.com/media/training/cp281.pdf
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: LPG pumping
Although you are usually right on the money, this time I have to concur with 25362's questioning the existance of a Joule-Thomson effect on a liquid.
In the pure sense of the Joule-Thomson Effect definition, it involves the expansion of gas or gas mixtures - not compressed liquids. The effect of cooling inside an LPG tank (or "bullet") is purely the result of liquid vaporization - similar (but not the same) to the effect seen in an atmospheric cooling water tower. This is the same process that a refrigerant liquid undergoes when it is expanded across the usual expansion valve. I still correct many engineers who think that this, too, is an example of the Joule-Thomson Effect. I don't believe it is. Now, I can believe that I could be mistaken and I welcome any corrections, but all the J-T values I have ever collected are on gases/gas mixtures.
RE: LPG pumping
RE: LPG pumping
The JT expansion mentioned by Montemayor refers to fluids, i.e., it includes liquids. The interesting (counterintuitive) fact is, that all fluids appear to heat up on expansion as long as their reduced temperatures (=T/Tc) are lower than 0.75 or greater than 5.0, or reduced pressures (P/Pc) above 12. For mixtures, one can use pseudocritical properties.
RE: LPG pumping
If no pressure makeup, a vapor return line is essential, otherwise your pumps may cavitate frequently due to low pressure head in the suction and thus the low NPSHA. Purely relying on evaporization may not be adequate.
RE: LPG pumping
I've been told by experts that these pumps probably always cavitate but due to the low bubble energy severe damage to pump parts is seldom found.
RE: LPG pumping
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: LPG pumping
When analysing the process inside an LPG bullet being emptied, or filled, let's not forget that enthalpy is an extensive macroscopic property, i.e., depends on quantity of contents.
As for the issue in hand, the rollover (mass transfer) and heat transfer effects in the liquid take time to reach equilibrium, as implied by Renoyd.
RE: LPG pumping
Best regards
RE: LPG pumping
I haven't found such a source. The following threads may be instructive:
thread407-152358
thread798-129328
RE: LPG pumping
RE: LPG pumping
Probably a return of liquid from the pump to the bullet would induce the turbulence required for mixing and promote vertical convection currents to aid in "turn over" and minimize thermal gradients between the bottom outgoing stream and the liquid surface.