1. The services inside an LNG terminal
An LNG terminal is a collection of cryogenic services that look similar on a piping and instrument diagram but behave differently in operation. The main groups are ship loading and unloading through the loading arms and manifolds, storage tank send-out to the vaporisers, boil-off gas handling and compression, and the vapour return line.
Each service has its own duty cycle. Loading and unloading is intermittent and fast, with large valves cycled between open and closed on every ship call. Send-out is continuous and steady. Boil-off gas duty is continuous but low in flow, and it often sits at a different temperature because the gas has already been warmed.
Ball valves appear across all four groups because quarter-turn operation, high flow capacity and tight shut-off suit the isolation duty. What changes between them is the cryogenic design, the fire-safe requirement and the fugitive emission class.
2. Cryogenic requirements that repeat across the terminal
The starting point for any LNG valve is the temperature. Liquefied natural gas is handled at deep cryogenic temperature, and designs are commonly specified down to -196 C so that the valve covers the full range and can be tested with liquid nitrogen.
That requirement drives the body and trim materials, the bolting and, above all, the stem sealing arrangement. At cryogenic temperature, a standard bonnet places the stem seal at the cold end, where the packing or lip seal loses resilience. The extended bonnet moves the sealing element away from the cold zone.
The testing standard is BS 6364, which covers the cryogenic test procedure and its acceptance criteria. J-VALVES runs an in-house cryogenic laboratory using liquid nitrogen down to -196 C, so the low temperature test is performed in the plant rather than subcontracted.
Shell tests at 1.5 times the rated pressure and seat tests at 1.1 times are applied before the cryogenic test.
3. Boil-off gas and vapour return duty
Boil-off gas service handles gas rather than liquid, and it usually sits at a warmer temperature than the bulk liquid in the tank. The valves still need low temperature capability, but the sealing behaviour differs because the medium is compressible.
Vapour return lines balance pressure between the ship and the shore tanks during loading. Flow is continuous while the transfer is running, and the line changes temperature when the ship connects and disconnects, so the valves must tolerate a thermal transient without losing seat tightness.
Fugitive emissions matter more in this group than in liquid duty. ISO 15848-1 provides a test class for stem sealing performance, and a terminal operating under an emission limit should specify that class on the datasheet rather than leave it to the supplier.
Small bore valves in this duty are often specified with metal seats, for fire-safe behaviour and for tolerance of the transient temperatures.
4. Loading arm and manifold service
| Service | Typical valve | Special requirement |
|---|---|---|
| Ship loading and unloading | Trunnion mounted ball valve | Extended bonnet, cryogenic test to -196 C |
| Vapour return | Trunnion mounted ball valve | ISO 15848-1 stem sealing class |
| Send-out to vaporisers | Trunnion mounted ball valve | Fire-safe design to API 607 or ISO 10497 |
| Boil-off gas | Small bore ball valve | Metal seat, low temperature trim |
| Tank isolation | Top entry ball valve | In-line seal replacement without line breaking |
Loading arm and manifold valves are large, often trunnion mounted, and they are cycled on a schedule set by ship arrivals. Reliability and shut-off tightness matter more here than the ability to modulate flow.
Fire-safe design is a standard requirement in this area, because the terminal handles a flammable medium and a valve may be exposed to a pool fire. API 607 and ISO 10497 define the burn sequence and the acceptance criteria that follow it.
5. A specification checklist for terminals
Build the LNG valve datasheet around the following points: size and pressure class, cryogenic design temperature, body and trim materials, extended bonnet dimensions, seat and seal materials, stem sealing class, fire-safe standard, and the documentation package.
Materials are usually selected from the austenitic stainless grades and from low temperature alloys with verified impact properties. Aluminium bronze to ASTM B148 or B62 appears in some seawater side and utility duties rather than in the cryogenic path, where stainless steel dominates.
Documentation should be agreed at enquiry. EN 10204 3.1 or 3.2 certificates are common on terminal projects, and third-party witnessed inspection can be arranged through SGS or TUV. Where the project procurement requires it, the valves are also CE marked before release.
6. Frequently Asked Questions
Why are extended bonnets standard in LNG terminals?
An extended bonnet keeps the stem seal outside the cold zone, so the packing or lip seal stays within its working temperature while the valve body sits at cryogenic temperature. Without the extension the seal is chilled, resilience is lost and leakage follows. The extension also allows insulation to be fitted around the body without burying the seal and the operator.
What fire-safe requirement applies?
LNG terminal valves in flammable service are normally specified fire-safe to API 607 or ISO 10497. Both standards define a burn sequence, measure leakage after the fire, and confirm that the valve still operates. A secondary metal seat or a graphite seal is the usual design answer. The requirement should be named on the datasheet rather than assumed.
Which materials are used for LNG service?
LNG service uses austenitic stainless steels and other low temperature alloys with verified impact properties, because carbon steel turns brittle at cryogenic temperature. Body and trim materials are selected with impact testing at the design low temperature. Aluminium bronze to ASTM B148 or B62 appears in some seawater and utility duties rather than in the cryogenic path.
