Control valves in liquified natural gas applications must operate reliably under high pressure and cryogenic temperatures to achieve gas separation and liquefaction. This is especially true for the main cryogenic heat exchanger (MCHE), where the closely-coupled Joule-Thomson (J-T) control valves feeding the MCHE help optimize the operation of the most critical area in the entire process.
My article in the Apr 2023 issue of Gas Processing and LNG is titled Optimized Joule-Thomson Control Valves are Critical for LNG Operations, and it shows why it’s important to choose the right control valves in these applications
Tight temperature control required
LNG liquefaction of natural gas requires process temperatures of -250°F or less, typically accomplished using a combination of turbo-expanders and J-T valves. The process uses the J-T effect to cool the gas by reducing the process gas pressure very quickly. As the author relates:
The turbo-expanders in the process are more efficient than J-T valves when running at a steady state, but the J-T valves are installed in parallel to replace the turbo-expander when it is out of service, and to handle the increased cooling demands found during startup and transition conditions.
FIG. 1. This figure shows the more critical control valves in a typical LNG plant. The valves marked “6” are the J-T control valves, with an emphasis on the critical “warm” and “cold” J-T control valves feeding the main cryogenic heat exchanger.
The turbo-expander reduces pressure to cool vapors, but flow through it is often limited. In these cases, the J-T valve is used to provide additional cooling, and some LNG facilities rely solely on the throttling performance of the J-T valves.
The J-T valve must be controlled very carefully to avoid process excursions, upsets and inadvertent shutdowns, or to achieve increased LNG throughput. It is not uncommon for the J-T control valves to receive and respond to setpoint changes as small as .05%.
Design considerations
J-T valves must withstand cryogenic temperatures, large pressure drop-induced vibration, and multi-phase fluid conditions—but still control to very small setpoint changes. This requires many special features, which are all provided by the Fisher valve shown below:
FIG. 2. This NPS 16-in. optimized proprietary J-T control valveacapable of controlling down to .05% step changes at -320°F.
Real-world results
An LNG facility replaced its existing MCHE J-T control valves with optimized Fisher J-T control valves.
This allowed the higher-order controls provided by the process licensor to be returned to automatic, optimizing startup, shutdowns and grade changes. The net economic impact was production equal to one additional LNG tanker for export sale, a value upwards of $200 MM. Efficiency also improved since the process could quickly and automatically respond to ambient weather conditions safely, allowing the plant to run at design capacity without risking trips.
Visit the Joule-Thomson Control Valve section on Emerson.com for more on technologies to help you optimize operation of the MCHE.
