For chemical inertness and temperature range - the two things elastomers cannot deliver together.

PTFE is essentially unreactive. Strong acids, alkalis, solvents, oxidisers and hydrocarbons that would swell, harden, dissolve or embrittle a rubber seal have little effect on it. That removes the need to match a specific elastomer to a specific chemical, which matters where a line carries several fluids or where the process may change.

It also spans a very wide temperature range, remaining serviceable well below and well above what common elastomers tolerate, so it covers steam, hot oil and cryogenic duty.

And it does not leach. Elastomers can release plasticisers and absorb components of the fluid, which is unacceptable in high-purity, pharmaceutical and analytical work.

The cost is mechanical compliance, which is why the coupling design has to compensate.

A PTFE sealing element with a metal spring behind it that keeps it pressed against the sealing surface.

The reason it exists is that PTFE creeps. Under sustained compression it takes a permanent set, so a plain PTFE seal gradually loses the interference that makes it seal - particularly after thermal cycling, where differential expansion opens a gap that rubber would simply follow.

A spring-energised seal has a coiled or cantilever metal spring housed inside a PTFE jacket. The spring provides the resilience, continuously loading the PTFE against the mating surface and taking up wear, creep and thermal movement, while the PTFE keeps the wetted surface fully inert.

Encapsulated seals achieve something similar by moulding a PTFE skin over an elastomer core.

For anything cycling in temperature or pressure, or for long service intervals, one of these designs is worth specifying over solid PTFE.

Less so than elastomer-sealed couplings, and it should be weighed at selection.

An elastomer seal deforms and recovers each time the coupling is made and broken, which is what it is designed to do. PTFE has much less recovery, so repeated cycling wears it and any scoring or scratching of the sealing surface is not accommodated the way a rubber seal would accommodate it. Particulate contamination is also harder on a PTFE seal.

Spring-energised designs improve this considerably by taking up wear, and are the right choice where the coupling will be connected regularly rather than left in place.

Where the duty involves very frequent cycling and the chemical requirement is moderate, a chemically resistant elastomer such as a perfluoroelastomer may serve better and last longer.

State the expected cycle frequency when specifying. It changes the recommendation more than most people expect.

A wide one, but the limit is usually set by the body material and the coupling design rather than by the PTFE.

PTFE itself remains serviceable from cryogenic temperatures up into the range used for steam and hot oil, which is far broader than any common elastomer. In practice the assembly's rating is governed by the metal body, the spring material, any secondary seals, and the way thermal expansion affects the fit between components.

Pressure rating also falls as temperature rises, so a coupling rated at ambient will carry less when hot - the two figures must be read together from the manufacturer's curve rather than taken separately.

At cryogenic temperatures, differential contraction between the PTFE, the spring and the body is the design challenge, and couplings intended for that service are specifically engineered for it. Do not assume a high-temperature PTFE coupling is suitable for cryogenic use.

Wherever the fluid or the temperature rules out an elastomer, or where the seal must contribute nothing to the process.

Chemical transfer and process lines carrying acids, alkalis, solvents and oxidisers are the largest group. Semiconductor and high-purity fluid handling use them because PTFE neither leaches nor absorbs. Pharmaceutical and food processing specify them for inertness and cleanability. Steam, hot oil and thermal fluid circuits use them at the top of the temperature range, and cryogenic transfer at the bottom.

They also appear in laboratory and analytical work where a sample must not be altered by contact with the seal.

They are not the default for general air, water and hydraulic service - an elastomer seal is cheaper, more forgiving and better suited to frequent cycling. Specify PTFE when the chemistry or the temperature requires it, not as a general upgrade.