| 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).
