Mysterious Propane "Leak"

[JimK-NY]

Sorry about the length but I hope with enough details someone can explain what is a mystery to me.

After 5 years or so my propane pigtails had hardened. Since the regulator was the original from 2004, I decided I should replace it also. What motivated both was what seem to be very slow leak. I left one of my half empty propane tanks with the valve open but without using any propane for several months. When I finally checked the tank was empty. I put in a new tank, charged the propane lines, ran both the stove and the furnace. I shut off the tank and noticed the regulator showed red after a couple hours. I switched the regulator to the other tank, opened and then closed the valve and again the regulator showed red in a couple of hours.

After changing the regulator and pigtails. I did a careful leak test. No soap bubbles and absolutely no propane odor. This time, I used the stove and furnace and then shut the valves. I expected to see a green indicator on the regulator for many hours. Instead it changed to red in well under 2 hours. I switched over and had the same results with the other tank. I did several checks and the pattern repeated with about 1 1/2 hours until the regulator changed from green to red. I checked for odors and leaks everywhere with no problems.

It has now been several days since I replaced the regulator and pigtails. Last night I did another test and the regulator was still green after 4 hours when I went to bed. Today I did another check and it took 8 hours for the regulator to show red.

Any ideas that can explain this or point to additional testing?

[3_tons]

For the uninitiated, here’s the (above referenced…) ideal gas law, but then again this seems to be delving more into abject nuance than application…Bottom line with gasses remains when temperature goes up pressure goes up (this was my only point…), though I have no idea if this is the true cause of the OP’s particular issue:

https://sciencetrends.com/in-pvnrt-what-is-the-r-constant/

3 tons

[JimK-NY]

PV=nRT applies to all gases including propane. I showed the calculations above for the variation in temperature that occurred for my last leak test. Temperature for the universal gas law is expressed in degrees Kelvin. A drop of 10 degrees F was only about a drop of a bit over 1% in Kelvin degrees so the pressure would have dropped by that same small amount.

The tables cited by 3tons are for vapor pressure over liquid and would not apply for a leak test with no liquid in the system and the tank valve closed. The tables are quite useful for other purposes. As we can easily experience vapor pressure of gaseous propane over liquid propane changes greatly with temperature. This reflects a decrease in the rate at which gas can be produced from liquid propane. The tables show a substantial drop in the ability of various sized propane tanks to provide gas for heating or other purposes. The drop is so bad I long ago stopped trying to use a propane BBQ in the winter months.

[Bobbo]

JimK-NY wrote:
Thanks, that is an interesting web site. Those temp/pressure tables really do not apply to this topic. They are about vapor pressures over liquid propane, not about temperature changes in pressure for propane in a gas state.

Ummmm, "vapor pressures over liquid propane" IS "propane in a gas state." The material BELOW the "propane in a gas state" is liquid propane.

[wolfe10]

JimK-NY wrote:
Thanks, that is an interesting web site. Those temp/pressure tables really do not apply to this topic. They are about vapor pressures over liquid propane, not about temperature changes in pressure for propane in a gas state.

????

PV=nRT.

[JimK-NY]

Thanks, that is an interesting web site. Those temp/pressure tables really do not apply to this topic. They are about vapor pressures over liquid propane, not about temperature changes in pressure for propane in a gas state.

[3_tons]

JimK-NY wrote:
A drop in temperature would decrease the propane pressure. The drop is insignificant. During my testing, the temperature dropped from about 60 F to about 50F. Pressure of a gas is directly related to absolute temperature in degrees Kelvin. 60 degrees F is about 493 degrees Kelvin. 50 degrees F is about 487 degrees Kelvin. So the drop in temperature and the drop in pressure were insufficient at only a bit over 1%.

Not to get tooooo far into the weeds, but here’s a temp vs pressure chart:

https://www.propanewarehouse.com/helpful-information/lp-gas-properties/

3 tons

[JimK-NY]

A drop in temperature would decrease the propane pressure. The drop is insignificant. During my testing, the temperature dropped from about 60 F to about 50F. Pressure of a gas is directly related to absolute temperature in degrees Kelvin. 60 degrees F is about 493 degrees Kelvin. 50 degrees F is about 487 degrees Kelvin. So the drop in temperature and the drop in pressure were insufficient at only a bit over 1%.

[3_tons]

Oh, so I had misinterpreted your ‘made it faster’ for made the leak faster…

3 tons

[StirCrazy]

3 tons wrote:

StirCrazy wrote:

3 tons wrote:
“So how does the leak test go from under 2 hours to 6 hours and now to over 14 hours without any changes?”

However, note that the temperature is what changed…

3 tons

getting colder should have made it faster not longer.

How so?? From my perspective, getting colder would decrease pressure, thereby increasing the leak down time.

3 tons

increasing.... faster..., same thing isnt it ? 😉 point being that the colder temps should have made the propane shrink, a miniscule amount, but I would have expected the red flag to come on faster , not longer.

[3_tons]

wolfe10 wrote:

3 tons wrote:

How so?? From my perspective, getting colder would decrease pressure, thereby increasing the leak down time.

3 tons

True, but it would also "shrink" the metal, perhaps allowing for more clearance in the component that is leaking.

Well, considering the principle of Occam’s Razor it would seem that a mere change in ambient temperature might have a much greater effect on a liquified petroleum gas (resulting in a decrease or increase in internal tank pressure) than the shrinking of metal components…This is exactly why tanks are fitted with Overfill Prevention Devices which provide sufficient unused volume for expansion (due to changes in ambient temps) to prevent over pressure…

3 tons

[wolfe10]

3 tons wrote:

How so?? From my perspective, getting colder would decrease pressure, thereby increasing the leak down time.

3 tons

True, but it would also "shrink" the metal, perhaps allowing for more clearance in the component that is leaking.

[3_tons]

StirCrazy wrote:

3 tons wrote:
“So how does the leak test go from under 2 hours to 6 hours and now to over 14 hours without any changes?”

However, note that the temperature is what changed…

3 tons

getting colder should have made it faster not longer.

How so?? From my perspective, getting colder would decrease pressure, thereby increasing the leak down time.

3 tons

[StirCrazy]

3 tons wrote:
“So how does the leak test go from under 2 hours to 6 hours and now to over 14 hours without any changes?”

However, note that the temperature is what changed…

3 tons

getting colder should have made it faster not longer.

[time2roll]

JimK-NY wrote:
So how does the leak test go from under 2 hours to 6 hours and now to over 14 hours without any changes?

Keep going and this should extend to a few day or a week+. 😉

At these levels the only way is to start isolating items.