Does a gas pressure test lose pressure without leaks?

Question

Context

I am adding one branch with two sub-branches to my gas line to supply a fireplace and a range stove.

I am testing the branch for leaks. I have a cap on one side of the branch, a valve shut off at the fire place, and placed a gauge at the stove output to measure pressure drop. All joints have been generously spread with gas sealant.

Test

• I started the test early afternoon for 4 hours.
• Pressure at 5psi with my nail gun compressor and a 15psi gauge on the gas line.
• Dropped 1/10th of 1 psi in about 4hours (ie 0.1 psi).
• Temperature and outdoor pressure were stable.
• I cannot find any leaks on the branch and two sub-branches.

Questions

1. Does gas pressure hold over time or does it lose pressure? (I am thinking of a balloon that loses pressure over time).
2. Is it possible to lose pressure without leaks? If so why? (I have thought about climate change i.e. outside temperature and pressure)
3. For leaks, I am using a liquid leak detector found at my local supplier. What better techniques to use to detect small leaks? (I tried soap and water also)
4. Are small leaks ok?

Answer 1

1/10th PSI (1:50 change) might well be temperature-related (the compressor heats air as it compresses it, and that cools down to room temperature when in the pipe) but simply waiting longer should confirm that. 1:10 change (0.5PSI) is more significant than expected without a drastic temperature change.

A balloon is porous (at the scale of helium molecules) and helium leaks out through it. A gas pipe should not be porous at the scale of air or gas molecules, so any continuous drop in pressure is a (perhaps small) leak somewhere. But changes (both down and up) from temperature differences do happen.

So if you check tomorrow and it's down more, you have some sort of leak. If it's pretty much the same, you don't.

You may need to figure a way to check the closed valve at the fireplace.

Small leaks are Definitely Not OK in gas pipes.

Not really home-owner friendly, but the combination of filling the pipe with helium and using a detector that can sense helium is a "better leak detection" method for troublesome leaks. Generally the bubbly stuff and a stable gauge reading are adequate, as that's an expensive rig you generally need to hire with an operator.

Answer 2

Physics!

PV = nRT

P is pressure (5psi), V is volume (static [if you don't have a leak!] - the amount of gas your length of pipe holds), nR is also constant. T is the temperature (in Kelvin).

So, pressure is directly correlated to temperature, and to lose 0.1psi from 5 (2%), would require your temperature to drop by 2%. 20C is 293K, so 2% is likely to be less than 6C or 11F. If you think your temperature is changing that much, you probably don't have a leak... I wouldn't risk my life on it though.

Answer 3

No , it should not leak. there may be pressure changes caused by temperature as answered. A small leak in a confined space like a wall can eventually cause an explosion as the gas accumulates and mixes with air.