Check Valves for LNG & Cryogenic

For LNG submerged pump discharge lines (−160°C, Class 150–300, typically DN50–DN200), dual-plate check valves (wafer pattern) with spring-assisted closure are the preferred design. The spring assists rapid disc closure on pump trip, limiting liquid reverse velocity and water hammer. Dual-plate check valves are more compact and lighter than swing check valves — important for cryogenic piping where pipe support loads are already increased by thermal contraction. Materials: A352 LCC body (minimum design temperature −46°C, impact tested at −60°C); SS 316 disc and disc pin; Inconel 718 or SS 316 spring. BS 6364 cryogenic testing at −196°C (nitrogen test medium) is standard for LNG terminal check valves — confirms body integrity and seat leakage at operating temperature. For LNG loading arm check valves (DN150–DN400), API 6D-rated swing check valves with spring-assisted closure are common — the larger bore makes dual-plate less compact.

An extended bonnet (also called extended stem or cold box extension) on a cryogenic check valve keeps the mechanical components (hinge pin, spring, disc actuating mechanism) above the insulation boundary — at or near ambient temperature — while the valve body (and the fluid contact surfaces) are at the cryogenic operating temperature (−160°C to −196°C). This prevents three problems: (1) Icing and frost seizure — if the hinge pin assembly is at −160°C, condensation and frost from atmospheric moisture can ice up the moving parts, preventing the check disc from seating properly. The extended bonnet keeps these parts above the dew point. (2) Packing degradation — PTFE packing loses flexibility at cryogenic temperatures; keeping the packing area warm prevents packing stiffening. (3) Metal embrittlement — while LCC and SS 316L bodies are rated for cryogenic temperature, the extension keeps non-cryogenic-rated components (springs, pins, bonnets) at ambient temperature. BS 6364 check valve designs specify minimum extension lengths based on insulation thickness.

Both BS 6364 (British Standard — Testing of Valves) and API 6D (Annex F — Cryogenic Testing) are accepted cryogenic certifications for LNG service, and both are widely specified. The choice depends on the project specification and the operating company's standards: Shell, BP, and European IOCs commonly specify BS 6364; ExxonMobil and US-based operators more often cite API 6D Annex F. The test requirements are broadly similar: both require shell pressure test and seat leakage test at the actual operating cryogenic temperature (using liquid nitrogen, −196°C, as the test medium) and at ambient temperature (to confirm no permanent deformation occurred from thermal cycling). Key difference: BS 6364 specifies leakage in drops/minute (Class A = 0 drops, Class B = 5 drops/minute); API 6D Annex F specifies leakage in drops/minute for liquid and bubbles/minute for gas. If the project specification requires both, simply state 'cryogenic tested per BS 6364 and API 6D Annex F' — most major LNG valve manufacturers can provide both.