LPG pumping

[carletes]

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

 

[BigInch]

Pumping liquid from the bottom of the tank, rather than tanking butane gas from the top, the pressure in the vapor space remains equal to the vapor pressure of the butane at the (avg?) tank temperature. Taking liquid will tend to reduce the tank temperature due to a combination of the Joules-Thompson expansion of the butane gas above and some evaporation of the liquid butane as the increased vapor space is filled. The proportion of liquid and vapor at any given time can be modeled by vapor-liquid equilibrum diagram. The vapor pressure reduction will correspond to the rate of the liquid drawoff and the roughly equal rate of vapor space increase, the temperature reduction and corresponding decrease in vapor pressure associated with the temperature reduction.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[dcasto]

see thread798-178139 too

 

[25362]

BigInch, I fail to see any J-T expansion effect. Can you clarify ?

 

[BigInch]

You don't smoke then.

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

 

[25362]

This is evaporation by pressure reduction. The J-T effect -mostly cooling but not always- is a throttling (flow) process. Right ?

 

[BigInch]

25362,

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

 

[25362]

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.

 

[BigInch]

Vale. Perhaps was thinking in reverse mode.... or perhaps not at all.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[BigInch]

Back to the original post,
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

 

[Montemayor]

BigIncher:

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.

 

[pipehead]

Not to split hairs, but during the pressure drop of a liquid near its bubble point, some molecules become a gas (flashing), those molecules that are gas next drop in pressure. So, in effect some propane molecules do go through the JT effect. Water on the other hand when sprayed does not readily produce water vapor by the change in pressure like in the cooling tower.

 

[25362]

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/T_c) are lower than 0.75 or greater than 5.0, or reduced pressures (P/P_c) above 12. For mixtures, one can use pseudocritical properties.

 

[Renoyd]

I want to know whether there are fluids flowing into the LPG bullets. If so, can they make up for the pressure decrease during the transfer process, or how do you control the operations of the LPG pumps.

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.

 

[25362]

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.

 

[BigInch]

Well, lowering pressure at constant enthalpy... is J-T, one way or another.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[25362]

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.

 

[carletes]

Any book o publication explaining how to calculate that transient until the equilibium is reached again during which the pressure inside the LPG vessel being emptied or filled is not coincident with equilibrium pressure?

Best regards

 

[25362]

I haven't found such a source. The following threads may be instructive:

thread407-152358
thread798-129328

 

[dcasto]

From experience only. We had verticle can pumps for loading trucks. We had 3 30,000 gallon bullets to pull from to load the trucks. We loaded through spray bars at 300 GPM. If we pulled from all 3 bullets at the same time our pumps would not cavitate. If we pulled from one bullet at a time, the pump would cavitate when the outside temperature was below about 60F. The fix was to use a steam coil to heat a small amount of recycled propane to make vapors to put back in the bullet tank. (you can do the math to calculate 100% vapor replacement). WE COULD NOT HAVE A VAPOR RETURN because of quality specifications and odorization.