why are propane fittings lefthanded thread?

Answer 1

The web site below provides support for the answer above.

Answer 2

The Compressed Gas Association (CGA) has designated unique connection fittings for most industial and commercial compressed gasses. This is to prevent accidental cross-connections where a hazard or contamination could be created. Not only are the thread hands different, (RH and LH) the thread forms are different as well as the size and shape of the fittings. Each gas usually has its unique fitting design which is designated by its CGA number. A few compatable gasses may have the same connection designation. Many compressed gas cylinders are likewise color coded according to the flammability or inertness of the gas.

Answer 3

The simple answer is that lots of people were seriously harmed by mistakenly mixing fuel and oxidizer hoses.

While propane fittings are not normally the issue, the standard in the U.S. has become that all oxidizer (i.e. oxygen) fittings are right-hand threads, and all fuel (propane, acetylene, etc.) fittings are left-hand thread.

This keeps exhausted apprentices (like myself many years ago) from blowing themselves up. The worst thing to do is apply pressurized oxygen to an acetylene-saturated hose. Bad things happen in a hurry.

Answer 4

It makes you pay attention when you're screwing them on. Seriously. They don't want you accidentally threading the tank up to a garden hose.

It also goes a long way to make specialized proprietary fittings for that purpose. That way you can only hook propane up to propane accessories.