Liquefied natural gas (LNG) enables natural gas to be more readily transported across the global to better match supply with demand. The cryogenic process has unique requirements for safe, reliable and efficient operations.
In a Valve magazine article, Enhanced Safety in LNG Applications, Emerson’s Jean-Paul Boyer highlights the importance of pressure safety valves (PSVs) in maintaining safe operations as more of these LNG facilities come on-line across the globe.
Jean-Paul opens noting that for safety and environmental protection:
…most safety valves on an LNG installation discharge to a closed piping system and to a dry flare. Without any moisture in the piping system, the dangerous freezing risks at the seat of the valve are eliminated.
He explains that many:
…other valves and equipment also discharge into this same header system so that the pressure in the header is always fluctuating. This variable backpressure is imposed on the PSV and will act directly on its disc, adding itself to the original setpoint (opening pressure) of the valve. This is not acceptable.
The typical solution to this situation is that:
…balanced bellows safety valves are commonly used across all industries. A metallic bellows is mounted on top of the disc of the valve to protect it from the header backpressure, so the safety valve opens at the correct pressure, whatever the variations of the header pressure.
But on LNG applications this creates highly hazardous situations. To better address this:
…the volume inside the bellows, connected to the bonnet of the valve, must be freely vented to the atmosphere, which is accomplished via a vent in the bonnet. As a result, only atmospheric pressure acts on the bellows, on top of the disc and around the spindle and guide. As these parts cool down with the process temperature—which can be as low as -260ºF (-162ºC) on LNG processes—the air moisture will condense and freeze at the lowest point: inside the bellows on top of the disc…
Pilot operated safety valves (POSVs) used in LNG applications:
…use the system pressure as a closing force so that the seat tightness is at its maximum close to the set pressure—completely opposite from a spring-loaded valve. With the addition of a properly engineered soft seat, the tightness on cold or cryogenic service will be reinforced to avoid any leakage or even icing risk, while reducing maintenance costs.
Read the article as Jean-Paul explains selection and installation considerations of true modulating POSVs fully balanced against backpressure, and how they act as reliable safeguards for LNG processes.
Visit the Pressure and Safety Relief Valves section on Emerson.com for more on these pressure relief devices in cryogenic and non-cryogenic process applications. You can also connect and interact with other pressure relief valve experts in the Valves, Actuators & Regulators group in the Emerson Exchange 365 community.
