Double-walled Atmospheric Tank for liquid Propane 1

[mylexicon]

Gents,
Suppose having Double walled Storage Tanks containing refrigerated Propane (-47°C) at Atmospheric Pressure. Nitrogen blanketing NOT an option.
My question is regarding the thermodynamic behaviour of propane inside such tanks? Is this correct to assume that when the tank is partially empty, the empty half of the space contains vaporized propane? If so, upon refilling this partially empty tank with more liquid propane, what happens to the vopor at the top half?
Does it turn back to liquid phase upon rising of liquid level and increase of pressure?
Does this system require a relieving device? If the relief system is a must, does it absolutely have to relieve into a closed off piping system or relief tank? Is venting into the atmosphere an option for propane?

thanks a bunch

drno

 

[BigInch]

If the tank also contains liquid propane, the "void space" contains propane vapor at the vapor pressure

http://www.teecoproducts.com/Catalog/Q5.htm#4

particular to the temperature of the system at any given time. When the tank is filled, the void space is reduced and a new equilibrium between the gas volume and liquid volume is established. When the tank is empty of liquid, the pressure inside follows that typical of gases and increases with temperature.

A relief valve is necessary. Normal propane cylinders, such as those used with portable grills have pressure relief valves set to 375 PSI. PRV settings for ASME (American Society of Mechanical Engineers) tanks are 250 PSI. DOT engine fuel cylinders use PRV settings of 312.5 PSI.

Ontario Propane Association info here,

http://www.propane.ca/CleanTruth/

A leading LPG pump mfgr has all the rest of the information you might need here,

http://www.corken.com/publications/training.html

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[25362]

To find the answer to your query multiply the vapor densities (at equilibrium) by the corresponding "empty" volumes before and after refilling, and the differences will tell you about the mass of vapor that has liquified.

BTW, the equilibrium temperature of pure propane at 1 ata is -42.1[sup]o[/sup]C, not -47[sup]o[/sup]C. So, assuming the data is correct, the gas would contain a small amount of lighter hydrocarbons.

 

[Montemayor]

drno:

In response to each of your questions:

1. Yes, you are correct in assuming the empty half of the space contains vaporized propane. Upon refilling this partially empty tank with more liquid propane, the vapor at the top vapor space reverts back to it’s previous state when it was in equilibrium with saturated liquid propane at -47 [sup]o[/sup]C. Of course, it will have to be cooled or, if not, then vented to the atmosphere or to a Vapor Recovery Unit (VRU).

2. The residual vapor can turn back into a liquid. However, you must furnish the required latent heat of condensation (assuming the vapor hasn’t gotten superheated while residing in the tank’s vapor space. As I previously stated above, you must cool it if you don’t want to vent it.

3. As BigInch says, you require a Pressure relieving device. Let’s all go back to basics here. You are dealing with a SATURATED LIQUID. I don’t mean to “shout” but some engineers seem to follow a secret desire to avoid any thing related to Thermodynamics. In this application that isn’t an option. This is basically what BigInch and 25362 are also saying: you are storing a liquid that is at its saturation point --- it is on the “verge” of boiling and creating bodacious quantities of vapor that will quickly raise the tank’s internal pressure. That is one of the prime reasons why you MUST protect the tank, the people around the tank – and yourself as well. A PSV is not a “must” for this tank; it is legally mandated and enforced in all of the civilized world. To avoid using a PSV on this tank is a major crime.

4. The required PSV does not have to relieve into a closed piping system. I don’t know what you mean by a “relief tank” and I certainly doubt any such thing is legally allowed. You may relieve to the atmosphere if local regulations permit it – but you must take into consideration that these are very cold and heavy vapors and could easily cause a worse hazard to the immediate environment due to the fact that it is a combustible fuel and it will tend to precipitate to grade level and could seek (and find) an ignition source in the area.

Inquire and investigate what your local regulations call for and decide what is the safest method of discharging the relief vapors.

Good Luck.

 

[mylexicon]

I thank you all for caring to reply and your helpful comments. very useful, indeed.

drno