Pressure testing new gas line


I ran a new gas line for a garage furnace. It is about 50 foot of 3/4 line I pressure tested with air at 30psi to find major leaks. I soaped all joints and did not see anything. It dropped to 28.5psi in 24hours.
From what I have read the normal test is 10psi for 10-30min with no drop. I am now going to let it sit at 10psi and see when/if it drops.
Any other suggestions?
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you should just plug it at both ends, dont depend on the furnaces shut off valve to hold pressure
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it is plugged at one end, and I have a gas checking valve on the other end, it has a shrader valve to add air to the system. There are no other valves or anything inline. Just pipe and connections.

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What is your normal gas pressure?
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wrote:

I have no idea
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I am having problems with the unions leaking. Thats probably why I had the pressure drop. I jacked the pressure up to 50psi and saw the unions leak. I found one had pits in the sealing surface - I replaced it. The other has a very small leak with 30psi or higher. I tightened the snot out of it. No leaks at 10psi. I will check for a drop in the morning.
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Watch out for temp changes
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Rick Samuel wrote:

That's why I use dry nitrogen for all my pressure tests. Whenever I do an HVAC install with my friend, we're usually having to test the refrigerant line set as well as the gas line so we just use nitrogen for all of it.
TDD
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dpb wrote:

Let me guess. Humid air. Temp drops. Water condenses out to pipe walls. Pressure drops more than expected.
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Bob F wrote:

In the summertime the air is indeed very humid here in Alabamastan. I like dry nitrogen because it is inert and has no effect on any of the materials I'm using. For Freon lines it's a no-brainer but a gas line with a little bit of moisture mixed with any trash left in the line can clog a screen in a gas valve. I have seen the screens in gas valves look like someone poured the contents of a vacuum cleaner in them. With high pressure nitrogen I can blow the line out, leave a suitable amount of pressure in it because the pressure I use for testing refrigeration lines would damage a gas valve. If we ever install lines out on a new construction site, it could be weeks before any equipment is set/installed outdoors so I like being able to walk up, put my pressure gage on a fitting and the pressure on a good line never seems to vary outside the tolerance of my gage. If put 150psi of nitrogen a line set and there are no leaks, I'll read 150psi a week later with the gage I have. I'm sure that a very precise laboratory instrument could measure the difference but with my "crude" gage I don't see any.
TDD
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IIRC the test the inspector wanted was 15 PSI 24 hours no leaks. With that said, your pipe should hold 30PSI with no leaks.
As someone else mentioned you have to be aware of temperature changes. Several things go on. First as a gas is compressed it gets hotter and expands. Let it sit in a pipe in November and it will cool and shrink, so it might be that your initial observation of 30 PSI was of warm air and your second observation was of a cooler pipe full of the same volume of air.
I would leave it at 30 PSI for several days and make a note of the ambient temps with the pressure observations.
Assuming you have natural gas, the service pressure is 11 inches of water or a little less than 1/2 psig.
Also don't forget to check for leaks at the fill valve.
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Roger Shoaf

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Roger Shoaf wrote:

11 inches is a lot higher than Alabama Gas uses for standard residential service. Whenever I've installed a NG generator, the specs called for 11 IWC requiring the customer to request a 2 pound pressure service from the gas company and the regulators for all the other NG appliances are usually adjusted for 3-3.5 IWC. The older farts working in the field for the local gas company have told me that the pressure on the typical 3/4" black pipe supply line coming off the meter and going into a home is around 6 IWC. If you were to connect 11 IWC pressure to most natural gas appliances, the gas valve may go into a safety lock out state. Many other gas companies will require a special request from a customer for pressures higher than 5 or 7 IWC going into their home.
TDD
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On Thu, 19 Nov 2009 16:37:05 -0800 (PST), theedudenator

Well, typical residential gas line pressure (downstream of the gas meter/regular) is about 0.2 psi, so I think you will be ok.
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theedudenator wrote:

It should hold pressure. If it doesn't, find the leak. Sometimes, that is tricky. Does it continue to drop? If so, temp drop is not the problem.
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At 30psi a union still leaks - small bubble.
It held 10psi fine for 2 days. Actually jumped from 11psi to 9psi depending on the temp.
I guess I will try some other unions.

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