PTFE is often the first material suggested for extremes. It deserves the reputation at both ends, with some specific caveats.
High pressure, high temperature
Strengths. Continuous use to about 260 degrees C, chemical immunity to sour gas, brines, amines and solvents, no ageing in the sense that rubber ages.
Problems at temperature. - Strength and modulus fall sharply with temperature. At 200 degrees C virgin PTFE has a fraction of its room-temperature stiffness. - Creep and extrusion into clearances become severe. Use filled grades, tight clearances and back-up rings. - Thermal expansion is large and not recoverable. Allow for it in the gland. - Permeation of gases and solvents rises with temperature and pressure. - Rapid gas decompression. PTFE does not blister like a rubber, but gas permeated at high pressure can still cause damage in jackets over elastomer cores.
Design pattern. A spring-energised PTFE seal with a filled jacket, a metal back-up ring and a hard, smooth counterface. The spring is chosen for the corrosive fluid (Elgiloy or Hastelloy for sour service).
Cryogenic and HPLT
Strengths. Rubber turns glassy and brittle at its glass transition. PTFE stays tough far below that, to liquid-nitrogen and liquid-helium range. Spring-energised PTFE is the standard seal in LNG, liquid-oxygen (with the right grade and cleanliness) and hydrogen valves.
Problems at low temperature. - Thermal contraction. The seal shrinks away from its housing, then the spring must take up the gap. Energiser selection matters more than at room temperature. - Stiffness rises, so lip contact force changes. - Fill with care in liquid oxygen systems. Carbon and organic fillers can be unsafe; use grades approved for oxygen service and follow cleaning procedures.
Combining with rubber
Where gas tightness is needed over a wide span, a PTFE jacket with an elastomer or metal energiser can be paired with an HPLT-compound O-ring behind it. At very low temperature the rubber loses elasticity and the metal spring takes over.
Verification before use
- Soak or cycle test at the real minimum and maximum temperatures.
- Leak test with the real gas at pressure.
- For gas service above about 10 MPa, do explosive/rapid-decompression testing on any elastomer in the stack.
HPHT and HPLT are duty families, not ratings. State actual pressures, temperatures, fluids and cycles on the enquiry.
