Where does the propane go??

[nbounder]

Well, this question was posted (not by me) on the RV.NET forum. After 22 zilllion responses, only one was plausible. It's a famous question : You park the rig at home after a trip. A few hrs ago, you boiled some water for tea. You now shut the propane tank off. Obviously, the gas pipes are full of propane, and the supply valve and the stove valves are all shut tight. 3 wks later, you turn on the propane & try to turn on the stove. Something is coming out of the burner, but it isn't propane - it will not ignite. After 'bleeding off' whatever the heck is in the pipes, the stove lights and all is well. The question - what happened to the propane that was in the pipes??
Just to blunt the simplistic answers - it isn't air - the pipes were closed tight on both sides , & air cannot enter the system. If it was air, what happened to the propane? There's no 'automatic' leak built into any valves - that would result in explosions. The system is air tight. What ever is coming out (bleeding) of the burner is not combustible, and does not contain to any noticable degree any odorant that identifies the gas. Where did the propane go??

[RV Wizard]

The lp gas leaks out over time. The system is NOT without leaks. It is to be without leaks greater than noticeable pressure drop from 8" WC in three minutes. Air then occupies the space the gas once did.

[Dick & Pat]

The LP must leak out over time....it always takes 5 or 6 tries to ignite the refrigerator; assume it would be the same for stove or water heater, too, if we lit them first.

[chasfm11]

The air system on our brakes is supposed to be tight, too, but it leaks pressure down over the course of several days. As indicated, the propane plumbing system isn't completely tight because many of the joints aren't completely sealed. The small amount leaking out and being replaced by air isn't detectable.

I would say, however, based on your description that your leak rate is higher than mine. Plumbers use a combustion gas detector (I bought one to fix a gas dryer). You might consider getting one and using it around all of your propane joints. You might find one that is leaking faster than others. I use mine every time I open up a propane line and find it to be very sensitive.

[Gary RVRoamer]

Mine must be real tight - any and all appliances always light instantly when I turn them on in propane mode. No matter how long it has sat around.

[RVDude]

The wizard is correct, it leaks - and it leaks in two different ways at the same time. Note, 3 weeks seems a little on the fast side, but certainly not problematically so.

First, here's an analog. Fill a balloon up with air (be it either Mylar or rubber), in time it simply "leaks" out right? But you say (think) that it is all closed up and tight right? Well, it's not all closed up tight. Same as your LP system. There are two sources of leakage. First is a physical leak, the valves and joints are simply not "perfectly" sealed. Simply can not be. Most of the disappearance is coming from these joints. Second, and even more amazing is that leakage does actually occur through the pipes (and balloon walls) themselves - though at a far slower rate. Read on for the science content.

Here's the science. It's called "permeation" where the molecules of LP (or whatever) actually permeate the molecular structure of whatever is containing it. You think that metal pipe is solid eh? Well, at a molecular level it is far from solid. Holes all through it. The LP (and balloon air/gas) actually "leak" or permeate right through the walls of the vessel containing it. Mylar balloons are less permeable than rubber (Mylar has a denser molecular lattice structure than rubber) and that is why Mylar balloon last longer. Permeation is also how reverse osmosis (water filters) works. The problem of permeation is why it took so long to get ketchup in plastic squeeze bottles, science had a heck of a time inventing a plastic that would not allow the oxygen in the atmosphere to permeate the walls of the plastic and hence discolor/spoil the ketchup. Glass is naturally far more impervious to oxygen permeation. Lots of other food was able to be plastic packaged after that oxygen IN-permeable plastic was invented. If you recall, that was not all that long ago! Also note that an air filled balloon permeates far slower than a helium filled balloon because air molecules are much much larger than helium molecules and hence helium will permeate much faster (the smaller molecule can more easily find its way through the molecular structure of the walls holding it in).

So in summary, the LP in your "seemingly closed" system does simply leak BOTH from the fittings/valves (as these can never be "perfectly" sealed) AND it leaks right through the pipes themselves (permeation).

Hope this helps.... You can post it over in rv.net and settle that discussion too.

[GaryKD]

Hi RVDude,
I don't think a discussion is ever settled on RV.NET. That is why I am on iRV2 and quit going "over there".

[EngineerMike]

RVDude- what pushes the propane out of the pipes after the ~.5psi or so has dropped off? I.e. once there is no push to push the propane out, and no push to push air in (inside pressure = outside pressure), what motive force causes propane to exchange for air?

[Dick & Pat]

Quote:

Originally Posted by GaryKD Hi RVDude, I don't think a discussion is ever settled on RV.NET. That is why I am on iRV2 and quit going "over there".

My first, and only, post was deleted over there;
Haven't gone back since

[Latitude]

Hey Jeff

GREAT Explanation!!! Thanx Rick

[nbounder]

Engineer Mike - you are SOOOO correct. After the propane in the pioes is down to atmopheric pressure, I guess either a Magic Genie comes in in the dark and sucks the remaining propane out, then blows in air, or that explanation is all wet. The RV.net last post gave a real scientifically proper arguement concerning the proper air/gas ratio for propane combustion, and that seemed to me, a graduate engineer, to be the most plausible.
Assuming RV roamer is offering a valid fact - that his doesn't have this prob, then the permeability of copper pipe, etc etc, dosen't fly, at least with me.

[RVDude]

The "problem" - that of delayed ignition of the appliance - has already occurred when the LP ratio in the line has decreased to a point where ignition can not occur. Delta pressure only effects the rate of flow in/out of the system. Combustion requires a defined concentration of fuel (LP) and air.

mike - Once the LP pressure reduces inside the pipes there isn't sufficient LP in the lines to fuel combustion (the LP to air ratio being delivered to the appliance orifice is far too much air) and you get the ignition delay until LP gas re-fills the lines thus increasing the LP ratio at the orifice to enable combustion. It doesn't take much to upset the LP/air ratio so that combustion can not happen. So it takes just a little loss from the LP lines to interrupt combustion until the LP concentration being delivered to the orifice sufficiently increases.

The loss of LP is caused by regular ol' leakage and permeability. Nothing makes a perfect seal. And permeability is basic molecular physics. BTW - pressure difference only increases the flow through leaks or permeability. Even when the pressure completely equalizes flow will occur in both directions - albeit exceedingly slow. Review the fundamental physics of enthalpy and entropy.

[Gary RVRoamer]

Permeability is a fact, but the effect of permeability in a iron or copper pipe is negligible at best. The relative size of the molecules as well as pressure differential affects the rate of exchange. Tiny molecules like hydrogen and helium pass through other materials relatively easily, while big compound molecules like O2 do not. Propane is a relatively large molecule [C3H8] and black iron pipe relatively dense, so I doubt if permeability is a significant factor compared to leakage at joints and valves.

A three week leak down to the point where combustion will not take place sounds to me like a fairly serious leak. Might even be big enough that a sniffer tool could find it. I think I would be re-doing the major joints (e.g. regulator) and use gas-rated teflon tape on them. Not a high priority thing, but something I would do as time permits.

[RVDude]

roamer - Totally agreed and the gist of my first post on this topic. 3 weeks is a tad fast, may or may not be sniffable, worth monitoring, and is certainly a result of a leak in a joint or valve at that rate.

I would also second that redoing all the joints - if the guilty joint(s) or plumbing turns out to be un-sniffable - is a very worthy endeavor.