Propane tank pressure

Milan

Member
There are charts and graphs available on countless web sites that show the typical pressures inside a propane tank at various different temperatures.

What I am looking for and cannot seem to find, is any information on what the pressure can go to during the filling process when liquid is pumped into a tank.

Undoubtedly, it can vary according to temperature as well, but, I have wondered how fast a tank can be filled, does the pressure go close to opening the pressure relief valve?

When I take a 20 pound tank to be filled, it only takes a few seconds, it seems the liquid is just screaming into the tank, I wonder what the pressure goes to.
 
The gas vapors compress back into liquid propane. Like ammonia, propane has been used for refrigeration until better refrigerants were developed.
 
Good question! Here's the deal:

During pumped liquid filling pressure inside the tank rises only slightly above normal pressure "at rest", regardless how fast liquid propane is pumped in.

Reason: At any given temperature, when gas pressure exceeds equilibrium pressure for that temp, excess gas almost instantly condenses into liquid. This reduces pressure back to equilibrium pressure for that temperature.

Thus you can safely "scream" liquid propane into the tank without significantly increasing the internal pressure.
 
Actually more ammonia was used. Really LARGE systems still use ammonia cause it is SO efficient. Meat packers, Ice cream, frozen foods, etc. cause you can knock it down to -20* with no trouble. Only trouble is if you have so idiot hit one of the pipes with a fork lift! Run like hell cause it will kill you!
 
Bob, I thank you for the reply, I appreciate it.

I think you are right, because in the instance of filling that 20 pound tank, it was last summer, it was hot out, probably well into the 90's and the pressure relief valve did not open.

It was hot out, and the bulk tank they were filling from was undoubtedly just about as hot as it ever gets in the direct sun.

So, the static pressure was probably at the highest it can get, yet the relief valve did not open.

What I do not understand is something I found on this web site: http://www.propane101.com/propanedeliveryquestions.htm where they talk about trucks making bulk deliveries to larger tanks using two hoses, one to carry vapors back to the delivery truck.

They said, "The vapor return hose does not recover liquid propane from your tank during the filling process. It only recovers excess pressure. See "Vapor Return Valve" for details about this propane delivery process"

When I clicked on "Vapor Return Valve" (that was a link) it took me to a page where they said, "The vapor return valve is used primarily to relieve the pressure in the tank being filled to prevent overpressure and potential activation of the safety relief valve during periods of hot weather. The vapor hose from the delivery truck is connected to the tank's vapor return valve during the filling process and the excess pressure is vented back to the truck."

From my own past experience with watching both small (20 pound) and larger (1,000 gallon) tanks being filled, this has never been a concern, never been mentioned.

So, I am really curious now as to how rapidly the gas can condense to a liquid and how high the pressure inside the tank really goes during filling.

Again, thanks for your reply.
 
How rapidly vapor condenses into liquid inside an LPG tank depends a bunch of things: temperature of the liquid, temperature difference (if any) between the vapor and liquid, surface area of the liquid in the tank, temperature of the tank shell enclosing the vapor space, ect. So how high pressure rises during filling is difficult to figure.

Now it would seem by using a vapor return line to equalize vapor pressure between the nurse tank and the tank being filled, a faster liquid flow rate can be achieved. Or it permits the use of a smaller (lower pressure) liquid transfer pump.

Just my educated guess however...
 
Two lines are used as much for safety as anything, we do not like LP vapor floating around a plant with hydrogen, acetylene and propylene setting around. It can make a big mess:) If your LP tank starts blowing, off the filler screwed up big time and should be called on the carpet for it. Things go boom quickly from venting gas and a little spark.
 
(quoted from post at 20:44:22 03/02/15) How rapidly vapor condenses into liquid inside an LPG tank depends a bunch of things: temperature of the liquid, temperature difference (if any) between the vapor and liquid, surface area of the liquid in the tank, temperature of the tank shell enclosing the vapor space, ect. So how high pressure rises during filling is difficult to figure.

Now it would seem by using a vapor return line to equalize vapor pressure between the nurse tank and the tank being filled, a faster liquid flow rate can be achieved. Or it permits the use of a smaller (lower pressure) liquid transfer pump.

Just my educated guess however...


You can even fill without a pump, just gravity. The tank being filled just needs to be lower than the filling tank.
 
NFPA 58 says all DOT cylinders( the small tanks) and ASME tanks( home heat) need to have a max 250 psi pressure relief valve. The pumps on Lp bobtails wont work against too much head pressure, hense the 2 hoses. One returns the vapor that is displaced by the liquid filling the tank. The tank pressure is first equalized with the bobtail before pumping + ot -. then pumping occurs.. 20 lb bottles should have a OPD valve on them. It allows them to be filled to 80% capacity . This allows room for expansion if filling is done when it is cold & the next day it is 90 degrees. I couldny say what the pressure goes to when you have ur Bar B que tank filled. I am sure it isn't in danger. As long as the re-inspection is done on them. Be careful with older Lp bottle that may have escaped the retrofitting with OPD valves, maybe 14 years ago. They should be fixed. One note, If one ever cooks off on you, it is doing what is supposed to do. Leave it alone and stay away. the issue is more often after it is done venting if it closes all the way. Sometimes dirt etc get in there and wont let them close. If you are worried about it on hot days, keep them in the shade or turn the sprinkler on them.
 

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