Liquid Propane Use

[Lin]

I have a dual propane set up.  There are two removable tanks and one 45 gallon fixed tank.  The fixed tank is the type used to run a pickup on propane.  Although this was not considered when it was installed, the fixed tank's propane is drawn from the lower 1/3rd of the tank.  This would mean that it is drawing liquid as opposed to vapor.  Being unaware of the difference, I have run appliances using the fixed tank and had not seen any problem.  Since the tank is some distance from the appliances, is the propane vaporizing in the line after the regulator?  If so, is it okay to continue to use it that way?

[boogiethecat]

Hi,
The quick and safe answer to your question is "no".  Any propane appliance or regulator that is supposed to run on vapor has no means of supplying the heat needed to change liquid propane to vapor.

Changing liquid propane to vapor in a "vapor system" is  done inside the tank where the thermal mass of the liquid propane (and the tank itself) can supply that heat.  When you draw propane off through the vapor outlet, the liquid inside the tank boils a bit to create vapor to replace what you just used, and this lowers the tank temperature just a little.  

With high volume usage such as an vehicle engine would create, the tank temperature would lower quickly to the point that the propane was so cold it couldn't boil anymore (this happens once the tank temperature becomes equal to the boiling temperature of propane at atmospheric pressure, or -42c), so the tank pressure would actually go to zero, and the engine would quit even though there was plenty of propane still in the tank.  This is why you see propane tanks freeze when you're using a lot of gas.

 So to run the vehicle engine- the system takes liquid out of the tank instead of vapor (which does not result in a tank temperature drop, so the tank stays pressurized) and then uses a  water heated "vaporizer" in the line to vaporize the liquid propane- using radiator water to supply the heat necessary .  Without using some piece of gear like that between your tank and your appliance on a liquid-fed system, it's almost a certainty that the supply lines and/or the regulator will freeze in the process of being used.  If the regulator freezes and sticks, uncontrolled propane pressure could result and the appliance you're using that expects less than 1PSI coming in to it could suddenly be subjected to 200PSi, a blowout could occur and your vehicle could look just like that stick and staple fire that Oldmansax posted yesterday. Not a good thought...

My guess is that you have gotten this far because your usage has been small enough that, as you say, the lines between your tank and appliance, and the regulator, have been doing the vaporizing.  I'm guessing that with a stove burner or small flame like a refrigerator, it's not enough to cause a freeze. But its iffy at best and I'd strongly recommend you change the system to vapor.  A water heater or cabin heater would definitely draw too much and freeze things...

Most vehicle tanks I've seen that have liquid ports also have vapor ports at the top of the tank.  Take a look at yours and see.  If there is one, it's a simple matter of emptying the tank and re-hooking your line to the proper port, and sticking a pipe plug in the liquid line.
You could also find a liquid propane vaporizer and install it- they are super common in propane powered forklifts.  Search ebay for "propane vaporizer" and you'll find a lot of them, even some simple air heated versions that don't require radiator water lines.  If your usage is small enough, these will work well.  I personally favor Impco water heated units.  

If there is not a vapor port in your tank, and/or you don't want to add a vaporizer, the proper thing to do then would be to replace your tank with a properly ported one.  

But if you were like me you probably wouldn't want to do that... some other options (probably safe- definitely better than what you're now doing.... but don't say I told you to do it)  might be to put a tee under the tank filling neck  and take vapor off from there,  or even on the pee-valve (filling indicator valve) and use that.  If you do either of these be sure to use something really heavy like forged or stainless schedule 80 high pressure fittings that have no chance of breaking off....

Cheers
Boogie

[Lin]

Boogie,

Thanks for the detailed explanation.  There is a closed off port at the same level of the fill, so with luck that will be usable.  I guess the main issue is finding something to do with the 40 gallons of propane in the tank first.  I had thought of converting the generator from gas to propane and was told that required liquid.  From your explanation, I assume that the conversion parts would include a vaporizer at the carburetor. 

[Jriddle]

I used a tank like this for my shop air compressor. If I remember right it had both vapor and liquid ports and they were labled.

John

[boogiethecat]

Sounds like that port would be the ticket.

One thing I didn't explain is that if you do use a vaporizer and it's liquid heated, it'll only work when the engine it's hooked to is running. So that solution wouldn't work for other appliances although an air-heated vaporizer would be fine  for those.

There are three ways to empty the tank.
 One is to hook a pump to it and pump it's contents into another tank.  That requires a specialized pump though which you're not likely to find anywhere but a propane dealer, which might be a thing to look in to.

Method two is to just use the stuff over time until the tank is empty..

Method three is to just dump the propane via a long hose and a VERY safe area where there will be a lot of wind to blow it away and zero or fewer sparks or sources of ignition.  

An interesting thing will happen if you can dump it via a vapor port instead of the liquid port (which I don't think you could do with your current setup)- if you open the vapor valve all the way and get a major flow going, after a while, when maybe 1/4 or less of the volume of liquid inside has gone away, the tank will refrigerate itself so much that the pressure will fall to almost zero.  At that point the flow will almost stop even though theres still a lot of propane left inside.  Then it's time to change the plug to a valve , shut everything down and walk away knowing you've done it!  This sounds crazy but it's actually one of the methods some propane gurus use to repair a broken valve on a big tank... they just knock it off with a sparkless version of a sledge hammer, let the tank vent until it refrigerates, then change the valve to a new one.  It takes balls and planning to do this safely but If you're a bit crazy it's a tried and true method!!!

[Lin]

If a local propane dealer can help, I'll go that way.  I'm planning to drive the bus over to one on Monday and see what they say.  If that's not a go, I was thinking of draining it over time into portable tanks as I need it.  I suppose that it would not completely fill them, but an empty tank should be able to fill until the pressure is equalized.  I have a hose that I got a while ago at a flea market that looks like it was made for transferring propane from one tank to another.

[Jriddle]

Here is a pictrue of my tank and how it is set up for liquid and vapor when it was a propane tank and not my air compressor tank.

John

[Chopper Scott]

I used to live in the country and we had propane for heating. The tank had a slow leak in a fitting that required emptying the tank and our dealer came out and pumped it empty and repaired the leak. 

[Lin]

Thanks for the picture, Scott.  My tank is not marked like yours.  However, the port it is drawing from is toward the bottom.  There is a plugged port that is slightly higher than the fill that I hope will give vapor.  It is always possible that it is plumbed to the bottom of the tank like yours must be.

[blue_goose]

I made the mistake of not knowing there was a difference when I took the tank out of my van and put it in the bus.  When a lot of propane was being demanded the regulator would freeze up and stop the flow.  I talked to some people that were supposed to know and they said I could blow up the bus if I didn't change.  There were two places to put the output on the tank.  I had to empty the tank and change the output to the vapor and all was fine.
Jack