On Sat, 20 Dec 2003 02:24:11 GMT, "George E. Cawthon"
wrote:



We are all a little confused (us non-propane dealers). I've
had three new tanks recently and none of them were purged
the way it says on the tag. One of them gave me a little
trouble that eventually went away. All of them cost me
about $10 for a purge, nobody here does it for free. Most
around here charge a minimum fee which is for filling a 20#
tank. But, purging is for safety and reliability of the
appliance usage. It has nothing to do with the possibility
of the tank blowing up. Air in the tank when it is filled
isn't going to increase the pressure over what it would be
if filled without any air. After all, the valve is open to
atmosphere pressure when filling. And when you use the
tank, you use gas, not liquid, so the air comes out which is
the problem. At least that's my take.


Your take is wrong. Due to environmental issues, the tank is NOT
vented to the atmosphere when filling by weight. The valve is
connected to the filler hose, the valve opened, and the pump started.
There is NOTHING in the tank but propane - either in liquid or vapour
state. The tank is filled to 80% max capacity with liquid. The other
20% is pure pressurized propane.
Now, if the tank is NOT purged, it starts out full of air at
atmospheric pressure. It is compressed to 5 atmospheres pressure if
below the boiling point of propane, and comprises a flammable mixture
inside the tank at some temperature between there and normal ambient.
Not likely a big safety issue - but there IS a point where an
explosive mixture exists inside a non purged tank.

When the tank is "used up" it does not have air in it if it started
out purged - and would have VERY little even if not purged - assuming
the valve did not freeze etc. So refilling the tank does NOT require
venting.
A "brown valve" on a vehicle tank also eliminates the requirement to
vent the tank when filling. As soon as the liquid level reaches the
bottom of the tube (80% mark) the fuel flow is shut off.
Venting of propane tanks at fillup is currently illegal in many areas
- as it should be - for both safety and environmental reasons.