LPG NPSHA CALCULATIONS AND SOLUTIONS

[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

 

[Montemayor]

I would never recommend such a liquified gas transfer operation. You are trying to save a few nickels by not installing a vapor return line and causing yourself a lot useless grief.

When you try to pump a liquid (regardless of whether it is saturated liquified gas or a supercooled liquid) you are, in effect creating a liquid piston effect on the target vessel if you don't vent or allow for vapor exit. Yes, it is true that eventually pressure and temperature will predominate and the vessel will reach its thermodynamic equilibrium saturated pressure. But you are depending entirely on the walls of the vessel to transfer the heat of vapor compression taking place in the vapor space - something that is inherently very inefficient due to the poor gas film heat transfer coefficients and dependency on natural convection. If you are lucky, your flow rates are low, and your tank relatively large, then you can get away with it. But why risk it? It is just so much more logical and secure to employ a return vapor line that every installation I've ever seen or heard about uses it.

You seem to be more worried about your source tank pressure. I'd be more concerned about the reliability of the basic transfer system. With a vapor return there is nothing to be concerned about hydraulically or thermodynamically.

 

[25362]

I suggest reading thread798-129328 and

http://www.freepatentsonline.com/3962882.html

 

[djack77494]

carletes,
Take Art Montemayor's suggestions very seriously. Think of your rail tank wagon. It might have been sitting outdoors in the sun and warmed up to a fairly high temperature. Now if you pump a liquified gas into it, you can get a rapid pressure increase as heat energy from the warm wagon walls is used to vaporize your LPG. There would be no problem with a vapor return line, but without one you must be counting on a low flowrate or large receiver volume, as Art indicated. The only other thing between this operation and potentially overpressurizing your equipment is your relief valve. Do you really want to operate like this?
Doug

 

[MJCronin]

carletes..

As always, Mr. Montemayor provides sage advice.....

A vent line is required under the configuration you propose.

Some questions:

1) Is this your first experience with LPG and pumping systems ?

2) Has your boss or someone in a similar role assured others that there WILL BE NO VAPOR RETURN PIPING !!!!! (While, of course, you will be responsible for system operation)

3) Are there many MBAs in your organization ?

3) In what country will the installation be ?

Regards

-MJC

 

[carletes]

Hello again,

I was not considering the installation of a vapor return line after reading some recommendations such as the following link, about not installing a vapor return but using spray filling in the vapor space.

http://ts.nist.gov/WeightsAndMeasures/upload/LPG-Chptr2May-05.pdf

Anyway when you say "low flowrate or large receiver volume", could you give me any suggested value? If you know any book or documentation discussing this topic it would be very helpful.

Regarding supply vessel I am quite concerned about the transient pressure decrease during the discharge and the way/method to calculate it: any suggestion or document to review?

Finally to solve the NPSH problen during that transient and after reading the interesting thread link by 25362, I see that there are two possible methods: increasing vessel heigth or using pressurized nitrogen to increase vessel pressure. One of my work mates has provided to me with a table (source unknown) which says that during the discharge of 75% of the propane stored in a vessel at 38ºC, there is a pressure drop of 2,38 bar. This pressure drop makes not practical to increase vessel heigth (it would mean to elevate the vessel 20 m or more) and unfortunately the maximum available nitrogen pressure at the plant is 8 barg so I can't use it to pressurize the vessel. Do you know of any other solution?

Thanks again.

 

[25362]

Carletes, the drop in pressure in the tank is surely accompanied by a cooling effect. The tank contents would be in equilibrium, thus the height above the pump is still the factor overcompensating the suction line friction drop.

 

[carletes]

25632,

I suppose that the pressure decrease will be accompanied by a cooling effect at vapor space, but I am not so sure that the temperature of the liquid being pumped will be affected instantaneously...

Answering to MJCronin it is my first experience with LPG pumping and it is for a project in Europe.

Regards,

 

[dcasto]

There are lots of truck and some rail loading facilities with no vapor return lines using spray bars. I useually install a verticle can pump with about 30 psi pressure over line line loss to get the liquids through the srayers. However I did run into a sticky problem where we were loading 8000 gallon trucks at about 300 gpm from 30,000 bullets. Then plnt operations wanted to make refrigerant grade propane into 1 bullet, and regular grade into the other two. They batched out of 1 bullet and made into the other just in case they had an upset, they could rerun. When we tried to load out of one bullet at a time, the pumps vapor locked about 1/2 way through. Turns out the the bullet tank could not come to equalibrium quick enough and the propane was bubbling in the 6" line to the pump. If we slowed down to 200 to 250 gpm, we could make it through a load, except in the winter. The solution was, we took the minflow 1" line with a RO in it and ran it through a steam heater. The vaporized propane would replace enough of the vapor space as the liquid was drawn down.

Why, not vapor return, we did not want to contaminate our tanks with odorant and we had an automated load system. We dialed in 8000 gallons, and we got 8000 gallons, and not 8000 gallons less some vapor return amount.

 

[25362]

Carletes, I didn't mention instantaneous cooling of the liquid. Your friends' example spoke of the situation after 75% removal, which I interpreted as a new equilibrium point. A certain reduction in flow rate may allow equilibrium to reappear. Dcasto and Montemayor gave you suitable advice.

 

[carletes]

Thank you very much for all the answers.

I see that the installation of vapor return line or a recirculation line into the supply vessel (according to dcasto experience) highly depends on pumping rate, vessels volumes, ambient temperature... If the pumping rate is very low I suppose that there is enough time for the system for being at equilibium all the time but if the pumping rate increases it wil not, but I can't find a way to calculate or estimate thi.

Do you know of any method/book/paper/document/specification for calculating/estimating the pressure decrease at supply vessel and pressure increase at receiving vessel during the pumping in order to define the necessary system configuration?

Regards

 

[25362]

This link may be of help:

http://ts.nist.gov/WeightsAndMeasures/upload/LPG-Chptr2May-05.pdf

you may find there statements against the vapor balance line.

 

[carletes]

Hello again,

Related to this matter, does anybody know of recommended LPG loading rates into rail cars and trucks depending of the type of filling or where to find them? (i.e bottom, with or without vapor return, etc)

I have been looking for them with no result.

Thank you.