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Compression fittings and LP gas

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Compression fittings used in LP

After seeing some of the posts regarding the use of compression fitting on LP lines, I did a little homework and decided to make this post.

WARNING - Use compression fittings at your own risk.

Compression fittings are legal and do meet code in some localities, HOWEVER, there are generally stringent conditions for their use, i.e. testing and approval of vibration, metallurgy, fitting and pipe/tube combinations etc. From what I've seen, the average Home Depot compression fitting will not be approved for use in any LP or natural gas, as the approvals I've run across are very specific to testing that manufacture has performed on their "Approved Connection System".

In layman's terms, the approve is conditioned on using a very specific manufactures pipe and their corresponding compression fitting, and its installed with their tools to their specification. (ref Georgia Trade union)


A double Flare fitting would be my number one choice for LP, or anything that I want to flare ( I own a 37 deg double flare and use it a lot in aviation). These look similar to a single flare, but if you look carefully, you can see the metal is folded back on itself to double its thickness in the flared portion. The only problem, Double flares require a special flare tool that is a little pricy for the average DIYer.

Single flare tools are a lot cheaper, ( If you do more than 2 flares a year, buy a good one) ALSO - Your everyday automotive and home depot fittings and flare tools are a 45 deg flare. Odds are, unless you hang around an airport or high pressure hydraulic, you wont ever see a 37 deg (JIS) flare, so forget I even mentioned it. :-)

Making a flare is easy, Basically;

1) cut the tube.

2) Deburr the inside and out.

3) Place the nut on the tube (in the correct direction of course) NOTE The quality of the flare is inversly proportional to weather you forgot to put the nut on :-)

4) Clamp the tube in the flare fixture. better tools have a guide for how far the tube sticks out, cheep ones don't. I make it so the tube is slightly above the top of the clamp. Looking at the clamp sideways, the tube just barely sticks up above the clamp.

5) drop of oil on the flare head and the tube.

6) Slowly, tighten flare head until its snugged into the flare fixture. (watch for any cracking of the tube while tightening the head)

7) Unscrew the head and release the tube from the fixture.

8) Inspect the flare for cracking.

9) use a fine file / paper to remove sharp edges from the flare.

There are a lot of YouTube videos on how to flare, google them. Two cautions, make sure you put the nut on before you make that perfect flare, AND - inspect the flare afterwards to make sure the tube did not split.


I'm one of those people who want to know the reasons, and generally, "Because its against the law" doesn't usually satisfy my curiosity. So, Here are some of the reasons on why its not a good idea to use compression fittings.

Consequences - A leak on a hydraulic line will make a mess, a leak in an LP line could make a very, very big mess, with explosive results.

Tube hardness - The softer the tube material, the more likely that the compression fitting will develop leaks and not seal. There are several grades of cooper tubing that range from soft to hard. LP and gas should be a harder type.

Tube thickness - Same as hardness. The thinner the wall the more likely the fitting will develop leaks.

Vibration - Compression fittings generally don't like vibration. any movement between the tube and the compression fitting will result in leaks.

Pressure - The higher the pressure, the more likely the failure

John Mc

88 Dolphin 4 Auto

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I totally agree John,

Compression fittings are a Mickey Mouse solution at best. Fine for water if you don't mind wet walls and floors! I got really lucky and inherited the best flare tool I've ever seen. It has rollers like a roller bearing, on the flare. Wow does it do nice smooth work! It also has a depth setting and stops going down at this point, but still turns, to smooth things completely.

I did the maintenance for 15 years, at 8 remote cabins that used propane for lighting, heat, water heating and refrigeration. Having worked with inspectors and several different propane companies, I learned a lot. One of the things was their insistance on using flare nuts that were designed for this usage. These looked chunky, more like a cap nut with the cap sliced off. Not the type that has a long snout the tube comes into. They are much less likely to split or strip out, when tightened down fully. We always pressure tested with a good gauge and air pressure, watching for leak-down, before hooking up propane. At the very least, after working on the system, check all fittings with a few drops of detergent in water, applied to each with a brush and watch for any bubbles.


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Harbor Freight has a very inexpensive flare tool kit.

Under $10. I have used it and it works well.

Good instructions JOhn.

I have had to redo a couple until I got it right.


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I do a lot of aviation fuel, hydraulic, etc., As I mentioned, the quality of the flare is inversely proportional to whether I forgot to put the nut on OR, if I put it on backwards.

When I inspect the flare, if it looks absolutely perfect, my heart races because I know I forgot to put the nut on .

JOhn Mc

88 Dolphin 4 Auto

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