Jean-Paul Boyer, Massimiliano Franco, Nagendra Maddula, and Eugenio Sudati recently published an article in the November 2022 issue of LNG Industry. It is titled “The Key to Valve Selection for Challenging LNG Applications” and it describes key design parameters for choosing the best valves for LNG terminal operations. A summary of the article follows.
Liquified natural gas (LNG) has become a mainstay of the low-carbon energy market. The product is purified, sub-cooled, and liquified at approximately -160 degrees Celsius, then shipped around the globe via tankers. LNG terminals offload these ships into storage tanks, then vaporize the product into gas to be transferred to other sites and local users (Figure 1).
LNG terminal operations present punishing challenges for control and relief valves. Cryogenic temperatures, two-phase flow, very high pressures, and cyclic pressure and temperature swings are common across the facility, pushing equipment to the limits. Despite these conditions, valves must perform reliably, or the entire operation suffers.
LNG on/off valves
On/off valves (Figure 2) play a critical role across the entire LNG facility, isolating pieces of equipment and providing zero leakage, despite operating under very high pressures and cryogenic conditions. Automated valves divert LNG liquid and vapors around the facility, and they are often required for safety shutdown applications.
On/off valve components should be carefully chosen for very low temperature operation and employ special environmental packing designs to reduce emissions. The valves must not trap liquids, so C-ball or high-performance triple offset valves are commonly used. Top-entry body styles allow the valves to be maintained while still installed, dramatically reducing repair time and cost.
Safety emergency shutdown valves are common in LNG terminals, so SIL-rated valve assemblies and high performance, diagnostic positioners are often required.
LNG compressor anti-surge valves
Centrifugal compressors must be protected from a catastrophic condition called surge, which occurs at reduced flow rates. Surge is avoided by installing an anti-surge control valve (Figure 3) that routes the compressor discharge back to the suction to maintain a minimum flow.
The anti-surge valve is sized to pass a significant portion of the compressor flow, and it must act very quickly to protect the equipment. This valve usually incorporates high-capacity and high-precision positioners, dedicated linear actuators, pneumatic boosters, and air volume tanks.
LNG pressure relief
LNG storage tanks must keep the product temperature very low so LNG doesn’t gasify. If temperature control is lost, the LNG will start vaporizing and subject the tank to significant overpressure. In the event of this condition, or other issues, storage tank pressure relief valves (Figure 4) protect the tank from damage.
The authors discuss the requirements for these critical devices:
Very large capacity is a must for these valves, or each tank will require a large number of valves for adequate protection. High-quality, pilot-operated designs are necessary to provide high capacity and suitable tightness, and they can be specified with either pop or modulating performance to meet the application requirements.
Pilot-operated designs handle backpressure from flare headers, and wireless monitoring devices can be installed to detect open or leaking relief valves, helping plant personnel reduce product loss and environmental releases.
In addition to the tank relief vents, LNG terminals require a host of pressure relief devices to protect equipment from LNG thermal expansion and process upsets. Small thermal relief valves protect the piping and valve seals, while pilot-operated relief valves provide overpressure protection for tanks, exchangers, and process equipment. Relief valves in LNG service must handle two-phase flow and cryogenic temperatures, and they must seal very tightly, even when subjected to pressures near the setpoint.
The authors conclude with this advice to users tasked with specifying valves for LNG service:
When considering options, it is wise to consult your control valve and relief valve automation partners to evaluate the available designs and select the best option for a particular application. Careful selection can significantly extend service life, reduce downtime, dramatically reduce emissions, and improve plant profitability.