Centrifugal compressors are main components of the gas liquefaction train. These are typically a multi-stage compression of refrigerants necessary to ensure that natural gas reaches its liquid state for storage and transport. The entire refrigeration cycle, in short, relies on robust compressors to ensure uninterrupted LNG production.
Globe valves have been used for many years on this application as an industry standard. With an accurate valve, material and actuator selection, and because of the non-symmetric design of the trim, systematic failure can be significantly minimized. The non-symmetric trim design, in particular, triggers different valve behavior, depending on which direction the differential pressure is applied to. In the preferred sealing direction of the valve, the pressure tends to keep the valve closed, allowing users to benefit from safer functioning during emergency shutdowns. In the opposite sealing direction, where the pressure tends to open the valve, it improves safety when the goal is to provide emergency blow off/vent operation. This function was the one specified by the end user for the hot gas compressor application.
Application: Hot gas bypass valves perform a major protection function by remaining usually closed until a signal to discharge pressure is produced. Such signal prompts the opening of the valve in order to recirculate the flow from the intermediate compressor to the compressor suction and avoid surging with potential mechanical failures.
Challenges: Centrifugal compressors in LNG liquefaction trains are used to pressurize the refrigerant gas mix which must avoid pollutant and dangerous emission of VOC. This represents an important challenge for several manufacturers, and fugitive emission testing for valves has become an important component for the choice of the most appropriate product. For example, Emerson has been awarded an order from a leading Oil & Gas end user for class 600, NPS 20 and NPS 24 Series 30,000 valves to be used for a hot gas bypass application in an LNG centrifugal compressor. Vanessa valves were chosen after close scrutiny by the OEM, the EPC contractor and ultimately the end user instead of the originally specified globe valves. Vanessa valves, well-known for tight shutoff performance and credentials in the reduction of fugitive emissions, largely exceeded the originally specified leakage class for globe valves.