cryogenic-seals

name	cryogenic-seals
description	Cryogenic seals — elastomers at low temperature, PTFE/Buna behavior, lip seals, O-rings, face seals, labyrinth seals for LN2/LH2/LHe, thermal contraction effects, cryogenic valve stem packing, ISO/ASME standards.
metadata	{"priority":7,"promptSignals":{"phrases":["cryogenic seals","cryogenic O-ring","cryogenic valve seal","low temperature seal","cryogenic packing","LH2 seal"],"minScore":3}}

Cryogenic Seals — Complete Skill

Seal Material Behavior at Cryogenic Temperatures

Elastomers

Critical issue: glass transition temperature T_g — below T_g, elastomers become rigid and brittle Standard elastomers unsuitable below their T_g:

Material	T_g [°C]	Min service T
Nitrile (Buna-N, NBR)	-40	-40°C (LN₂ — NOT suitable)
EPDM	-50	-50°C (limited)
Neoprene (CR)	-45	-40°C
Silicone (VMQ)	-65	-55°C (static only)
Fluorosilicone (FVMQ)	-73	-65°C
Fluorocarbon (Viton)	-20	Do NOT use cryogenic
FFKM (Kalrez, Perlast)	-20	Do NOT use cryogenic

None of the above are suitable for LN₂ (77 K = -196°C) or LH₂ (20 K)

Plastic and Composite Materials

Material	T_min	Notes
PTFE (Teflon)	4 K (-269°C)	Best cryogenic sealing polymer; hardens but retains integrity
PCTFE (Kel-F)	4 K	Better creep resistance than PTFE at cryogenic; preferred for valve seats
Filled PTFE (15% glass or carbon)	4 K	Improved dimensional stability; reduced cold flow
Polyimide (Vespel)	4 K	Structural seal applications; low CTE
PEEK	77 K	Marginal at liquid helium
Nylon	-40°C	Not cryogenic

Metals (Metallic Seals)

Material	Suitable?	Notes
304/316 SS	Yes	Spiral wound; ring joint; reliable
Copper	Yes (soft metal)	Crush seals; excellent conformance; deforms well at low T
Aluminum	Yes	Soft; crush-type seals; not LOX (ignition risk)
Inconel 625	Yes	Spring-energized seals; metallic C-ring/O-ring
Invar	Special	Low CTE — problematic gland design

O-Ring Seals (Low Temperature)

PTFE O-rings: Rigid at cryogenic T → cannot compress to fill gap → leaks unless spring-energized Spring-energized PTFE seals: PTFE jacket + metal spring (coiled, cantilever, or open-V) Spring maintains seating load even as PTFE contracts and becomes less elastic Suitable to 4 K; widely used for LHe and LH₂ service

PTFE shrinkage: CTE_PTFE ≈ 100×10⁻⁶/°C (room T to -100°C) — much higher than metals PTFE O-ring in SS gland: O-ring contracts more → reduced seating stress → leak at low T Solution: use metal gland designed to contract LESS than PTFE (close gap as T drops) Or: PCTFE (lower CTE: ~55×10⁻⁶/°C) — preferred for LH₂/LHe valve seats

Indium seals: Soft metal; melts at 156°C; ductile to 4 K; CTE matches many metals Used for ultra-high vacuum cryogenic connections (helium leak rates < 10⁻¹⁰ std cc/sec) Indium wire (0.5–2 mm): crush into knife-edge groove; excellent low T sealing Disadvantage: one-time use; expensive; requires precision groove

Valve Stem Packing (Cryogenic)

Standard PTFE packing: acceptable for LN₂ service (77 K) with spring-loaded followers Extended bonnet design: keeps packing at warm end of stem → normal temperature sealing Extended bonnet length: 100–300 mm; packing works at ambient T (PTFE, GFPTFE)

Live-loaded packing: Belleville disc springs maintain load on packing as PTFE creeps/cold-flows Packing stress: 7–14 MPa on PTFE packing → no creep relaxation over 10,000 cycles

Bellows-sealed valves: Metal bellows (welded SS) eliminates stem packing entirely → zero-leakage Rating: 1000–10,000 cycles; fatigue is limiting failure mode Ideal for LH₂ (zero leakage requirement) and high-value cryogens

Gland follower preload: Initial: 100–150% of operating requirement After cool-down: retighten if PTFE-packed (packing contracts)

Face Seals (Static)

Flat face seals: Metal-to-metal (SS knife edge on PTFE flat): 316 SS knife edge; 2.5–4 mm wide PTFE gasket Seating stress: 100–200 MPa on PTFE → sufficient for cryogenic Cool-down: re-torque bolts after initial cool-down (PTFE shrinks 2–3% → flange gap)

CF flanges (ultra-high vacuum): OFHC copper gasket on SS knife-edge; suitable to 4 K; leak rate < 10⁻¹² Pa·m³/s Standard for LHe dewars, accelerator cryostats; ISO 3669 / ASME B16.20

VCR/VCO fittings (Swagelok): Metallic gland + SS or copper gasket; rated cryogenic; leak rate < 10⁻⁸ std cc/sec Use for small-bore tubing (< 25 mm) in LH₂/LN₂/LHe instrument connections

Labyrinth Seals (Non-Contact)

Application: cryogenic turbomachinery (compressors, expanders, pumps) No contact → no wear at cold T; accepts differential thermal contraction Radial gap: 0.1–0.5 mm (cold clearance); must account for differential contraction during cool-down

Cold clearance design: Δgap = gap_hot - ΔR_differential_contraction ΔR = R_shaft × (α_shaft - α_housing) × ΔT [must remain positive after cool-down] Use materials with similar CTE or design gap to accommodate worst case

Leakage flow: ṁ_leak = C_d × A_tooth × P_in / √(R_gas × T) × f(P_out/P_in, N_teeth) [Fanno flow approximation] Each tooth reduces leakage: ṁ ∝ 1/√N_teeth for choked flow

Differential Thermal Contraction in Seals

Gland design example (PTFE O-ring, SS gland, from 300 K to 77 K): SS contraction: ΔD_gland = D × 2.98×10⁻³ (inward) PTFE contraction: ΔD_PTFE = D × 18×10⁻³ (inward — more than SS!) Net: PTFE O-ring shrinks INSIDE SS gland → reduced interference → potential leak Solution: spring-energized design maintains minimum contact force independent of thermal state

Standards

Standard	Scope
ISO 21011	Cryogenic vessels — valves for cryogenic service
ASME B16.20	Ring-joint gaskets (metallic)
CGA V-9	Metallic valves for use with compressed gases
ASTM F38	Gasket materials creep relaxation
ISO 15848	Industrial valves — fugitive emission testing
SEMI F47	Specifications for semiconductor facilities (cryogen)

Output

Provide: cryogen and temperature [K], seal type (O-ring/face/packing/labyrinth/bellows), seal material selected (PTFE/PCTFE/spring-energized/metallic), material T_g or T_min [°C] vs. operating T, differential thermal contraction ΔD [mm] between seal and gland, net seating stress at operating T [MPa] vs. minimum required, labyrinth gap cold clearance [mm], valve type (extended bonnet length [mm] / bellows cycles), leakage rate specification [std cc/sec or Pa·m³/s], re-torque requirement after cool-down, and applicable standard (ISO 21011, ASME B16.20, CGA V-9).