Bleeding air from LP/propane tank

[AlienImpulse]

Sent: 8/4/2004 8:28 PM

I had a problem with air in my LP tank. Someone told me to open the valve and let air out of the tank. I have done this, and all seems to be working ok now. What I'd like to ask, is how do you know when to shut the bleeder valve? Should you bleed it until LP starts to squirt out? Or is this a hit or miss process?
I was told to let it bleed for an hour or two, and that was the only instruction I got. I bled it for about an hour, but it seemed like it was mostly air still coming out. 

[70winnie]

Sent: 8/4/2004 9:03 PM

From what I've read, what you saw is not air but in fact the LP vapor.  The tanks are filled under pressure, so there should be no air inside of them.  And as you let more out of the bleeder, you should only continue to see vapor (or not see it, since it's a gas instead of a liquid).

If the tanks were overfilled, the opposite would be true:  you'd see actual liquid propane escaping from the bleeder until the liquid was below the dip tube, at which point you'd see bursts of vapor intermixed with the liquid.

In other words, you should never see liquid coming from your bleeder -- it means the tank was overfilled.  If you see liquid, continue to bleed the tank until vapor starts to come out.

Source consulted: Trailer Life's RV Repair & Maintenance Manual

[AlienImpulse]

Sent: 8/4/2004 10:29 PM

Actually I saw nothing when bleeding it. Just seemed like air. If I saw any vapor at all I would have assumed that any air had vented. So should I continue to vent it until I see vaopr? If you hold your hand over the escaping air, wouldn't it feel a bit moist if it were vapor? My hand was completely dry. I think I might just leave it alone, seeing as my appliences seem to be working fine now!

Thanks!

[70winnie]

Sent: 8/4/2004 10:52 PM

I don't think you should see the vapor... it's apparently not like steam or clouds.  I think it would only feel moist if it was water vapor, not LP vapor.  I would leave it alone, like you said.   If it ain't broke...

[denisondc]

Sent: 8/5/2004 1:41 PM

I think any air inside a propane tank would mix fully; that is to say I doubt that you can bleed one gas (air) from a tank without bleeding the other gas (propane) along with it. If you open the valve on the tank to bleed it, what comes out will feel cool because it just expanded. You may see some mist, which is probably the condensed water vapor from the air outside the tank; due to the chilling effect of the cold propane blowing past.
When my propane has been shut off -at the tank valve- over the winter, I open the valve on the tank, then hold the button depressed on the back of the fridge for a couple of minutes, while the propane purges out the air that made its way into the tubing over the months - downstream from the regulator. I try to light the burner every few seconds, until I hear the -poof- and see the pretty blue flame. Same for the water heater.
For the first lighting of the stove before each vacation, we turn on a burner and keep clicking a sparking igniter next to it. In about a minute the first gas arrives, and after few seconds more the flame stays lit.
Liquid propane would vaporize in a fraction of a second - leaving behind the mist of water vapor condensed or frozen by the expanded (cold) propane.
When I open the valve on a propane tank connected to the regulator on the Winny, I open it all the way. I think the valve is designed to not leak from the packing gland if it is fully opened.

[DaveVA78Chieftain]

Sent: 8/5/2004 3:36 PM

Propane tank pressure is around 250lbs.  Propane is kept under pressure in order to keep it stored in it's liquid state.  Very little air would get in the tank and any that is would mix with the propane gas and be expelled during normal operation because of the high pressure.  There is a POL valve in the propane system whose purpose in life is to drastically reduce flow if the system incurs a leak (like say during god forbide an accident).  Fire protection requirement.  Always turn gas supply on slowly so the pressure can equalize across the POL valve otherwise, it will drive you crazy trying to figure out why you have no gas flow.  Really affects the water heater and furnace.  To reset it, turn gas supply off for 1 minute, then turn back on very slowly allowing at least 1 additional minute for the POL valve to equalize.
Air can easily get into the system on the low pressure side (appliance side).  High pressure side however basically flushes air on through mixed with propane such that you would never know it's there.  The regulator at the tank reduces the 250lb pressure to around .5lbs (11 inches water column).  Typically done in 2 stages (ergo the 2 stage regulator).   If you have a low presure side leak or disconnect an appliance hose/connection for some reason, air can easily (rather does) get in the lines.  This is the air that people refer to that needs to be purged.  Becasue it is under low pressure (11 inches water column), it can take a short bit to purge all the air out of a line.  May take a minute for a stove burner to light, several tries of the furnace or fridge, etc.  If you have several seperate piping legs, you may have to purge each one seperately.  If you a lucky like Dave is, you can purge most of the lines like he did.  However, the gas controls on the back of your fridge will have to be as easily accessable as his.  Many (more likely most) are not.  There are dozens of fridge designs.

Dave
Sidebar:  11 inches of water column is the amount of pressure needed to push a column of water (in a clear tube) 11 physical inches higher up a tube above the current ambient pressure level.  Take a fairly long 1/2 inch clear tube shape it into a U with one leg much longer than the other and put water in it.  Connect the shorter end of the tube to say the stove burner pipe.  Turn on the gas and it should push the water up the long side of the tube at least 11 inches higher than the at rest ambient pressure level.

[HeavyHaulTrucker]

Sent: 8/6/2004 10:21 PM

After its first filling, a propane tank should never have any air trapped in it.  Have you ever noticed -- prior to filling -- the attendant unscrews the relief port a bit?  Well, when being filled, any trapped air is forced out of the valve opening (this only happens on first fill, after assembly).  After the first fill, there should never be anything other than LP gas in the tank -- the only thing that varies is the tank pressure; an "empty" tank will still have 1-5 psi, but it will still be all LP gas.

Most likely, if you were not getting gas flow, you simply opened the tank valve too quickly; if the pressure rises too fast at the POL valve, it locks down (which is what it is designed to do, in the case of a catastrophic leak).

John