Rubber turns glassy
Below their glass transition temperature, elastomers become hard and brittle. For standard NBR, FKM and EPDM this happens between about −20 °C and −60 °C, far above LNG or liquid nitrogen temperatures.
Cryogenic Sealing Solutions
High‑Performance Seals for Cryogenic Service
Explore NOVUM PTFE, modified PTFE and PCTFE seals for LNG, liquid nitrogen, liquid oxygen and industrial gas equipment.
Why cryogenic service is different
What Makes Cryogenic Service Hard to Seal
A seal that is tight at room temperature can leak once the line is cooled below about −150 °C – often through several of these factors at once.
Thermal contraction
Polymer seals shrink several times more than the stainless steel around them, so contact stress falls as the system cools. See how it opens a seal below.
Cold flow under load
Virgin PTFE creeps slowly under sustained seat load and does not spring back like rubber, so a seat can lose its seal over time.
Thermal cycling
Cool-down, service and warm-up repeat throughout the equipment's life. The seal has to keep its shape and sealing force through every cycle, not just pass a single cold test.
Thin fluids and helium
Liquefied gases have very low viscosity, and cryogenic valves are commonly leak-tested with helium. Porosity in the seal and scratches on the mating face become leak paths.
Oxygen service
Liquid and gaseous oxygen raise the risk of ignition. Contamination with oil, grease or other hydrocarbons has to be kept out of the seal and its housing.
Good to know: cryogenic valve tests such as BS 6364 check seat leakage again after the valve returns to ambient temperature, so the seal has to recover after cool-down as well as seal when cold.
How thermal contraction opens a seal
Assembly
The seal is fitted with interference against both faces at room temperature.
Cool-down
From room temperature to −196 °C, PTFE contracts by about 2%, stainless steel by about 0.3%.
Gap opens
On a 50 mm seat that is roughly 1 mm against 0.15 mm, and the interference is lost.
Leak path
Low-viscosity liquid or test helium passes through the gap unless the seal is kept loaded.
Typical linear contraction values from room temperature to −196 °C.
Seal types
Seal Profiles for Cryogenic Equipment
The NOVUM cryogenic grades are moulded and machined into these profiles to customer drawing.
Valve seats and seat inserts
Soft seats for ball, gate and butterfly valves in LNG and industrial gas service.
Backup rings
Support the primary seal and close the extrusion gap as clearances open during cool-down.
Joint seals
Static seals for flanges, bonnets and body joints on cryogenic valves and vessels.
Stem seals
Machined sealing rings for valve stems and extended bonnets.
Pump and coupling seals
Custom rings for cryogenic pumps, expanders and transfer couplings.
Liners and structural parts
PCTFE components for liquid oxygen and liquid nitrogen equipment.
Material portfolio
NOVUM Materials for Cryogenic Service
Four grades from the NOVUM engineering thermoplastics range, each suited to a different part of the low-temperature envelope.
Virgin PTFE · Free-flowingAPI 6A qualified
NOVUM P 01
For precisely moulded seats, seals and backup rings.
- Good mould filling and weldability
- Good electrical and mechanical properties
- API 6A sour gas qualification at 10% H₂S and 200 °C
Virgin PTFE · Compression mouldingHigh-load grade
NOVUM P 02
For larger seats and parts machined from moulded stock.
- Low mould shrinkage and good flow
- Developed for high-load applications
- Good weldability
Modified PTFE · Free-flowingLow-deformation grade
NOVUM M 03
For low-temperature service above the cryogenic range, such as ethylene and LPG.
- Chemically modified for lower deformation under load than standard PTFE
- Good mould filling behaviour
- API 6A sour gas qualification at 10% H₂S and 200 °C
PCTFE · CTFE homopolymerHigh-strength grade
NOVUM PCT 01
For cryogenic valve seats under high load and liquid oxygen and nitrogen equipment.
- Harder and stronger than PTFE
- Good dimensional stability and chemical resistance
- Low moisture absorption
Technical data
Material Properties
Specification values from ISMAT technical data sheets.
| Property | NOVUM P 01Virgin PTFE | NOVUM P 02Virgin PTFE | NOVUM M 03Modified PTFE | NOVUM PCT 01PCTFE |
|---|---|---|---|---|
| Hardness ASTM D2240 | 50 Shore A min. | 50 Shore A min. | 55 ± 5 Shore A | 75–80 Shore D |
| Density ASTM D792, g/cc | 2.155 ± 0.01 | 2.155 ± 0.01 | 2.14–2.17 | 2.10–2.15 |
| Tensile strength MPa min. | 25 ASTM D412 | 25 ASTM D412 | 25 ASTM D412 | 30 ASTM D638 |
| Elongation at break % min. | 250 ASTM D412 | 250 ASTM D412 | 300 ASTM D412 | 100 ASTM D638 |
Operating temperature range
NOVUM P 01
NOVUM P 02
NOVUM M 03
NOVUM PCT 01
Cryogenic range begins at about −150 °C
Ranges from ISMAT technical data sheets; actual limits vary with seal design and application parameters.
Selection guide
Choosing the Right Grade
How the four grades compare for common cryogenic and low-temperature service requirements.
| Requirement | NOVUM P 01 | NOVUM P 02 | NOVUM M 03 | NOVUM PCT 01 |
|---|---|---|---|---|
| LPG and ethylene down to −104 °C | Rated | Rated | Rated | Rated |
| LNG, liquid oxygen, argon and nitrogen −162 °C to −196 °C | Rated | Rated | Not rated | Rated |
| Holding shape under high seat load | Good | Good | Very good | Excellent |
| Low friction and operating torque | Excellent | Excellent | Excellent | Good |
| Cost efficiency | High | High | Very good | Premium grade |
Temperature rows follow the published data sheet ranges. Other rows are relative ratings; confirm by testing in your application.
Applications
Across the Cryogenic Value Chain
From liquefaction to the point of use, liquefied gases pass through similar equipment.
Liquefaction
LNG trains and air separation cold boxes: process valves, expanders and cryogenic pumps.
Storage
LNG tanks, bulk storage vessels and dewars, with their fill, vent and isolation valves.
Transfer and bunkering
Loading arms, ship-to-shore transfer lines, tanker couplings and LNG bunkering systems.
Regasification and distribution
Send-out pumps, vaporiser inlets, pipeline valves and pressure regulation.
Industrial and medical gases
Liquid oxygen, nitrogen and argon distribution, cylinder filling, cryostats and biobanking.
Low-temperature petrochemicals
Ethylene, ethane and LPG plants, storage and gas carriers.
Design guidance
Designing the Seal for Cryogenic Service
The same material can pass or fail depending on how the seal and housing are designed. These points are worth checking early.
Size the seal for the cold
Calculate interference, clearance and contact stress after contraction, not only at assembly.
Keep the seal loaded
Polymer seals do not recover like rubber. A spring, live loading or system pressure maintains contact stress as the seal shrinks.
Control extrusion gaps
Clearances change as metal parts cool. Backup rings and close-fitting glands stop the seal flowing into the gap. Why backup rings matter
Keep stem seals away from the cold
Extended bonnets, as described in MSS SP-134 and ISO 28921-1, place stem seals in a warmer zone above the liquid.
Specify a fine surface finish
Helium tests reveal scratches and tool marks on seats, balls and stems that a liquid test at ambient temperature would miss.
Inspect PTFE at a stated temperature
PTFE passes through a crystalline transition near 19 °C that changes its dimensions. Measure parts at a controlled 23 °C.
FAQ
Cryogenic Sealing: Common Questions
What temperature is considered cryogenic?
Temperatures below about −150 °C are generally classed as cryogenic, which covers LNG, liquid oxygen, liquid argon and liquid nitrogen. Service between about −50 °C and −150 °C, such as ethylene and LPG, is usually described as low-temperature.
PTFE or PCTFE: which is better for a cryogenic valve seat?
PCTFE is harder and stiffer, so it holds its shape under high seat loads and is widely used for LNG and industrial gas ball valve seats. PTFE is softer with lower friction, so it conforms more readily to the ball and gives lower operating torque.
Are NOVUM grades suitable for liquid oxygen?
PTFE and PCTFE are among the most widely used non-metallic materials in oxygen service. Suitability for a specific oxygen application depends on pressure, flow velocity, cleanliness and design as well as material, so share the oxygen service conditions and cleaning requirement with your enquiry.
Which standards apply to cryogenic valve testing?
BS 6364, ISO 28921-1 and MSS SP-134 are the most common. BS 6364 measures seat and body leakage with helium while the valve is immersed in liquid nitrogen at −196 °C. ISO 28921-1 covers isolating valves for design temperatures from −50 °C down to −196 °C, and ISO 15848-1 covers fugitive emissions from stem and body seals.
ADVANCE with ISMAT
Discuss Your Cryogenic Application
Tell us about your operating conditions and our engineers will recommend a grade and seal design.
Share with your enquiry
- Medium and minimum and maximum temperature
- Pressure and number of thermal cycles
- Seal type, drawing or sample
- Valve test standard, if any
- Cleaning requirement for oxygen service
Disclaimer: The information on this page is general technical guidance and is not a service-life, pressure or temperature rating. Values are specification limits from ISMAT technical data sheets and do not by themselves indicate the performance of a finished part; evaluate parts under actual service conditions, particularly where failure may result in injury or damage.