Where does the propane go?

[D_E_Bishop]

Ol biscuit and I disagreed on whether or not is was possible for there to be air in a propane system basically with the tank valve shut and all the appliances turned off(valves shut). He said impossible but I say there is a gas(inert) in the system that won’t burn but will blow a match out.
As he said, it is a closed system and I agree, however, there is that gas flowing through the pipes that won’t burn. How is this possible? My system is all hard piping, no rubber hoses. I have never had an alarm that I didn’t want, i.e., testing of the alarm. Each appliance has at least three valves between the tank and the igniter and burner.
It makes no never mind, it is just that an inquiring mind wants to know. If I shut off the appliance, turn off the solenoid and shut the valve on the tank, where does the gas go? I’m sure there is a good answer, I just don’t think it is logical.
Any ideas from you experts?

[pnichols]

rhagfo wrote:
Personally I keep the system pressurized all the time. I believe far better for the valves, I have had several homes with gas never shout off the gas there either.

So do I.

Our RV's propane system has it's tank valve turned ON with the system pressurized all the time .... except: When traveling in tunnels in certain states.

FWIW, whenever I leave the propane tank valve OFF for any length of time, the cooktop burners are a bear to get to light afterwards.

[K3WE]

BB_TX wrote:
Propane is a liquid under high pressure and changes to a vapor when the pressure is reduced. It does not change back from a vapor state to a liquid state unless repressurized.

This is wrong.

There are temperatures where propane becomes a liquid at atmospheric pressure...The gas liquid thing depends BOTH on pressure AND temperature, not just pressure...and this goes for just about ALL gasses, including CO2 and Oxygen!

And, "propane" that we buy for our campers is not necessarily propane. In fact, in much of the Southern united states, "Liquefied Petroleum Gas" (what we ACTUALLY buy) can be predominately butane which will exist as a liquid at atmospheric pressure and cold-weather temperatures.

[K3WE]

donn0128 wrote:
He may be a retired something but gas specialist he aint. Leave my gas off for a couple of weeks turn it on and it takes several minutes for gas to refill the lined enough to light a burner. Been that way on every RV I have used or owned way back to 1977. Same RVS have been tested with no gas leaks! Where it goes? No clue. I'm not small enough to crawl thru the lines to watch my gas excape. I just know it is so.
BTW rubber hoses can and do leak. Even brand new.

ABSOLUTELY!

When I first turn on the LP, the stove won't light, the water heater won't light and the fridge won't light and the furnace won't light. The LP has largely dissipated from the lines and they are "full" of air.

It takes a solid 30 seconds (if not more) to get enough gas to the stove to keep a sustained flame...maybe a minute or more for the Hot water heater which is at the end of almost 30 feet of tubing.

[Harvey51]

Thank you, Chris - of course I have electric valves on the furnace and fridge. A senior moment for sure! Yes, I see my idea that a regulator might deliberately have a very slow leak is also wrong - over pressure protection requires a valve and large vent. Thanks for the tip to replace my 11 year old regulator. And hoses, I reckon.

It must be just little leaks in various places, with the electrically operated valves high on the list. The time it takes for the air in the lines problem to occur might be a useful indication of how leaky the system is.

I see an earlier discussion of this here:
https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/22835405/print/true.cfm

[Chris_Bryant]

Harvey51 wrote:
My low end MH does not have any electronic valves (the propane stove works when there is no electricity) but it still has the delay before the stove burner will ignite after not being used for a while. Could it be that the propane system is now deliberately designed to leak outside the house on concern that a regulator failure or leak could cause too high pressure on the low side - at the appliances? Perhaps the deliberate leak is a safety feature of the regulator. I know my regulator area sometimes has a faint smell of propane that I cannot detect by means of detergent bubbles. If so, no wonder the propane bottle enclosure must be ventilated.

Are you sure about that- unless you have a pilot type furnace and have to manually light the refrigerator, you have electrically operated valves. Water heaters are the only common pilot type appliances used these days.
The regulator should not leak *unless* there is a serious overpressure- it will vent in the case, but that is a sign it is bad. In normal use it should never vent. I would replace it, just as a matter of maintenance, if it's over 5 years old or so.

[nomad297]

Some systems leak. Some systems don't leak.

Bruce

[tll]

I love discussions like this. They make us all think deeper than what is needed on a typical day. I think that everyone has many valid points, but no one has really hit the nail on the head for us to all to say "yup I get it". I've been working on marine engines for 30 years, I understand fuel much better than the average person, but can't quite accept what has been said as the reason for the disappearance of combustible propane. Maybe we need to write a grant and get a government funded study to figure this out!

[Harvey51]

Fascinating discussion, folks!

"Until sufficient flow is established the air/fuel ratio is 'air rich'
Propane is there just not rich enough for amount of available air to make fire"

This makes sense. The system must leak - propane out, air in. When the pressures are nearly equal, molecules can migrate both ways simultaneously.

I'm also interested in the comment about systems not leaking much in the past. My low end MH does not have any electronic valves (the propane stove works when there is no electricity) but it still has the delay before the stove burner will ignite after not being used for a while. Could it be that the propane system is now deliberately designed to leak outside the house on concern that a regulator failure or leak could cause too high pressure on the low side - at the appliances? Perhaps the deliberate leak is a safety feature of the regulator. I know my regulator area sometimes has a faint smell of propane that I cannot detect by means of detergent bubbles. If so, no wonder the propane bottle enclosure must be ventilated.

[12thgenusa]

D.E.Bishop wrote:
In summary, the liquid propane in the tank becomes a vapor when released, it then becomes a flammable gas when it is further atomized when passing through the orifice in the nozzle and then oxygen is introduced in the burner tube and passes into the burner as a flammable gas.

Not quite.

Propane exists as both a liquid and gas in the tank or bottle. Since the valve is on the top, gas only is drawn off for use in the system. The pressure in the tank is the same for a given temperature whether the tank is full or nearly empty and can vary from about 100 to over 200 psi depending on temperature. The regulator drops the pressure to a constant 6 ounces per square inch. The gas is metered through the burner valve and mixed with air in the burner tube. There are no different properties of "static" vs "flowing" propane vapor.

Old-Biscuit wrote:
That is because LP regulator has opened to provide 0.4 psi in downstream system......but then closes back down until demand is required.
Upon demand the little bit of propane in system is not sufficient for ignition (or maintaining ignition ) until higher flow is established, which happens when reg senses demand and drop in line pressure.
Lighting off stove top burners helps establish flow and demand which places regulator fully in service vs static state when not being used.

The regulator definitely controls flow rate while maintaining constant pressure, but this narrative doesn't explain why a burner will light almost instantly if it has been used recently, but not if it has been a few days or more. The only reasonable explanation is that the low pressure gas in the manifold downstream of the regulator slowly leaks off over time. I guess someone would need to leave a manometer attached to the system and monitor it over time.

[Chris_Bryant]

OK, I'll explain again how air gets in the LP lines- and it is air when you have to leave a stove burner on for 30 seconds before it will ignite.

30 years ago, all the LP appliances had manual, positive shutoff valves. These systems would hold pressure nearly forever- they just didn't leak.

Now, however, we have electronic controls, using LP valves that simply are not near perfect at sealing. As far as safety goes, they don't leak anywhere near enough to be dangerous, but they leak, none the less. Below is a cutaway of the type of valve that is used-(with one exception) Note that in the direction of flow, only the spring pressure and pressure of the LP is keeping the valve closed- and with the valves we use, the springs are not strong at all, thus they don't draw much current to operate. Also note that there is no resistance to pressure flowing in the opposite direction- they are like check valves.

So- the LP leaks out, the line pressure actually goes below atmospheric pressure, because LP gas is very temperature sensitive, thus air is drawn back in to the system.



* The difference in the valve illustrated is that it is for a household appliance, is 24 volts, and has an extra strong spring, so has a "booster" coil to pull it open. Lots more watts used, but less leakage (if any).

[Old-Biscuit]

BB_TX wrote:
But with your reasoning, if the line still contained propane (i.e. it was not somehow lost when the burner valve and tank valve were closed) then that propane in the line would have been repressurized and recondensed as soon as the propane tank valve was reopened, and the subsequent flow should have been immediate propane to the burner that would have ignited. But it does not. Not even a half hearted attempt. Not until a minute or so of flow for the line to refill with the new propane coming from the tank. And that initial flow is enough to blow the flame on a match, so it is not something that slowly builds.

I don't mean to be overly argumentative, just trying to understand the phenomena.

Until sufficient flow is established the air/fuel ratio is 'air rich'
Propane is there just not rich enough for amount of available air to make fire
Once demand is established with reg open enough to allow high enough flow and maintain line pressure then proper air/fuel ratio is met...FIRE

(go to higher elevations and you run into a fuel rich environment due to thinner air...no fire)

[Oaklevel]

Its good to know there are so many experts here.

[BB_TX]

But with your reasoning, if the line still contained propane (i.e. it was not somehow lost when the burner valve and tank valve were closed) then that propane in the line would have been repressurized and recondensed as soon as the propane tank valve was reopened, and the subsequent flow should have been immediate propane to the burner that would have ignited. But it does not. Not even a half hearted attempt. Not until a minute or so of flow for the line to refill with the new propane coming from the tank. And that initial flow is enough to blow the flame on a match, so it is not something that slowly builds.

I don't mean to be overly argumentative, just trying to understand the phenomena.

[Old-Biscuit]

BB_TX wrote:

Old-Biscuit wrote:
After a period of non use one does not purge air from propane system.......one establishes flow on a static system and places a demand on LP Regulator.

It's as simple as that.
......

Not that simple. On mine there IS a flow when the burner valve is first opened (as indicated by the match flame being blown about) but ignition does not occur for a minute or more after that flow starts.

That is because LP regulator has opened to provide 0.4 psi in downstream system......but then closes back down until demand is required.
Upon demand the little bit of propane in system is not sufficient for ignition (or maintaining ignition ) until higher flow is established, which happens when reg senses demand and drop in line pressure.

Lighting off stove top burners helps establish flow and demand which places regulator fully in service vs static state when not being used.

[Me_Again]

And once a new a propane tank is purged for the air in it, it CAN be filled without opening the bleed valve. I had a guy in a marina in Canada overfill my boat tank once by filling it without using the bleed valve, pumping propane into until it would take no more and it was venting gas as it warmed up in hot weather under my fly bridge! Chris

http://www.propane101.com/propaneliquidandvapor.htm