From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: LP Gas fill problem
Message-ID: <09nhavsvkt4c8m1bqp0jv8fruiisp7r6oe@4ax.com>
Date: Fri, 25 Apr 2003 03:12:23 -0400

On Tue, 22 Apr 2003 13:53:44 -0400, "Jon Porter" <jporter@netwalk.com> wrote:

>
>"Neon John" <johngdDONTYOUDARE@bellsouth.net> wrote in message
>news:k76aavs3orh76iq7o4ager6vtff1c49jdo@4ax.com...
>> On Mon, 21 Apr 2003 21:12:07 -0400, "Jon Porter" <jporter@netwalk.com>
>wrote:
>>
>> >Yep, it's for the 80% fill mark. I've seen a local propane jockey
>> >strain his pump when my tank got close to full before he opened the
>> >vent. I make a point of letting them know to keep the valve closed
>> >until the have the line hooked up and then open just before they start
>> >the pump.
>>
>> That's a prime indication that there is trapped air in the tank.  If
>> there is nothing but propane above the liquid, the pressure will be
>> exactly the vapor pressure of the propane at whatever temperature
>> prevails in the tank - right up until the tank is filled solid if
>> someone isn't paying attention.
>>
>Air in the tank? Makes sense, it seemed to be hardly used before I bought
>the thing. It was several years old at then.
>

Yup.  That old Gas Law of Partial Pressures.  Since the air is non-condensable
at room temperature, its pressure adds to that of the propane.

I recently used this property to move some propane between the 250 gal tank
owned by the gas company to the new 500 gallon one I bought to replace it.  I
just hooked my liquid withdrawal line from the 250 gal tank to the POL fitting
on the 500 gal tank after pushing enough CO2 into the 250 gal tank to push the
total pressure up about 50 lbs over the vapor pressure.  This moved about half
a tank of liquid in minutes. (before someone bitches, yeah, I purged the old
tank afterward so the next customer would not have a problem.)

If you have air in the vapor space it will gradually purge as it mixes with
and flows out with the propane gas during use.  During this time the appliance
flames may act like they're set too lean and/or blow out.  Doesn't last long,
as a whole tank full of air isn't all that much.

Incidentally, a similar problem can occur with auto air conditioners.  If the
owner tops off the system a few times and doesn't know enough to purge the
hose of air first (more difficult with R134a hoses), the air within the hose
will eventually end up in the condenser.  This will eventually raise the
pressure in the condenser until either the compressor loads down and skids the
belt or the high pressure cutout activates or something blows/burns out.  I
see this more often than I should now that consumer-operated blow-off cans of
refrigerant are again available on the store shelves.