sealing threaded joints in pipe feeding a gas fire.

Gas PTFE tape. It's a thicker version of normal PTFE tape, which usually comes in a yellow plastic reel. You can use the gas tape for normal plumbing too.

SteveW

There is a slightly thicker PTFE for gas work - usually in a yellow container rather than white. It applied starting at the back of the thread and wound toward the free end with a 50% overlap.

There is a relevant BS as well: BS EN 751?3: 1997

The instructions on mine warn you to only use just over one turn so I don't see how you get 50% overlap.

It depends on the length of the thread. With longer pipe threads you will need several turns. You want no more tape thickness than a double layer, hence 50% overlap.

The procedure as described in the standard:

"Wrap a length of PTFE tape clockwise round the male threads starting from the only partially cut thread on the pipe of the test assemblies described in 7.2.1.1 to give a double layer covering, using minimal sufficient tension for the PTFE tape to take up the form of the thread. The end is torn off. For coarse threads 50 % overlap is recommended (see figure 1)."

Figure 1 is:

Its in the shed somewhere but I haven't used it for over a year.

The above BS is the standard cited in Tolley's

BS 6891:2005 "Installation of low pressure gas pipework of up to 35 mm (R1¼) in domestic premises (2nd family gas) ? Specification"

Basically reproduces the above guidance.

(there is also British Gas publication IM/16 that describes the process, but I don't have a copy of that to hand to quote).