for plumbing or tubing- why use a flare nut? I don't understand the purpose of it...?

Question

I don't understand why you'd use a nut that transitions upwards in diameter from that of the tube... why not use a normal nut?

Answer 1

Wow srmainman, you almost had nothing right about that entire answer, except for the fact that flare fittings are used in gas lines.

First of all, copper is NOT approved for natural gas use. Only limited LPG use is permitted. Natural gas contains sulfur which reacts with copper to form copper sulfide flakes. These flakes can clog gas valves and thermocouple-operated pilot valves, causing an explosion hazard if they are not able to close properly.

Second of all, compression fittings are NEVER allowed under ANY circumstances for gas use, and they are useless for hydraulics due to the high pressures involved. There's a reason hydraulics ONLY use flare fittings. Because flare fittings are extremely high strength connections. The tubing itself is more likely to fail than a flare joint. And by the way, compression fittings are only found in DIY huh? They are not permitted by code? Then why is every plumbing valve under a sink or toilet connected to the fixture with a COMPRESSION FITTING?? Don't comment on things that you obviously know nothing about.

Flare fittings work because instead of having a tiny point where the two parts come together, it creates a sealing surface that is many times larger (the entire 'face' or domed surface of the male flare is the sealing surface), and the flare nut puts pressure on the female part over the entire surface.

**EDIT: lpgnh3 - I clarified my error.. I did mean the pilot valve which is operated by the thermocouple. And the problem I was referencing isnt just the clogging/blockage of the pilot valve/orifice, but that in a worst case, a flake could stop the valve from closing if the pilot goes out.

sensible man - You have a point there, that local codes allow different things. But for the most part natural gas lines are only allowed to be black iron or stainless steel flex (because of the copper sulfide problem). Same applies for water. Flares are almost always required on underground connections in copper because of their strength. However, around here, the first joint inside is allowed to be sweated. YMMV

Answer 2

To form a seal. There are two kinds of fittings flare and compression. The former you know because of the flare you mention. The compression uses a small tube that just fits over the tubing and when the nut is applied, the compression fitting is squeezed down and deforms to grip and seal between the fitting, the nut and the tubing.
If you just used a nut, in the real world, it is likely that a small stream of water under 60-120 psi would make its way along the tubing and leak. And because of the varying pressure, it is very likely that the tubing would work back through the nut and like a cork, pop out.

Answer 3

Taz pretty much summed it up but, as usual, there are always some codes that allow things that other areas do not. In Ohio, copper (soft tubing) was permitted for gas lines such as gas grills. The line was required to be steel pipe when it entered the home though. On water service lines (from the street to the house) code was 1" or larger copper tubing under ground. The only joints allowed were flare fittings. No sweat or compression joints.
I agree that some answers here are from people who don't have a clue.

Answer 4

Gas Flare Fittings

Answer 5

Actually they are called flare nut wrenches. They are similar to a 6 point box wrench but have a section cut open to allow you to pass it over the line and onto the nut. You will need to go to Sears or a tool supply with a large selection of tools, they are not too common but are available.

Answer 6

Great answer Taz, for the most part anyway. Please edit your answer in regards to the thermocouple though. The way it reads now one would assume that you are implying that there is a flow of gas through the thermocouple, which would be incorrect. My assumption is that you’re referencing the pilot valve. You’ll have flow through the pilot valve, pilot tubing and pilot orifice that could be clogged. But the thermocouple is a thermo-electric device that has no direct contact with the flow of gas.

Very good answer none the same, deserves “Best Answer” in my book.

Answer 7

I was going to correct the dude who knew not what he sayeth, but scrolled down and saw Taz had already handled it. I am getting increasingly frustrated by the people who answer questions when they don't have a clue what they are talking about! Is it just for the points? Someone please explain it to me. I have been either a handyman or a contractor for a long time and still skip over questions I don't have an answer for!

Answer 8

Someone take away my points for not answering. But I have to comment. Taz and Martin, thumbs up to both of you. Taz gave the right answer.
Edit: Thanks Ipgnh, good clarification.This is the kind of good information I like to see on YA.
I work for a company that builds hydraulic testing equip. ( electrician, but do my share of the plumbing too )

Answer 9

i think its for a compression fitting, it alolws you to seal a line of high pressure water without stripping and soldering. when the pressure of the water goes through the pipe - it reverses on the flare and seals it.