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Control valves and LNG: Common, yet critical

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LNG Industry,

The LNG industry stands as a cornerstone in the global energy landscape, with control valves serving as indispensable components in its supply chain. The journey of LNG entails numerous stages, each necessitating precise control and regulation of fluid flow. Control valves emerge as essential elements, enabling efficient operations and maintaining safety standards across LNG liquefaction plants, transportation vessels, and receiving terminals. Understanding the diverse applications and types of control valves in the LNG industry is paramount for proper control valve selection and long-term operational reliability that optimises operations and upholds the integrity of the various steps in the LNG process.

LNG liquefaction

LNG liquefaction is a complex and pivotal process within the LNG supply chain, serving as the foundation for the entire industry. This process occurs at LNG liquefaction plants, strategically located near natural gas reserves or major pipeline networks. The primary objective of LNG liquefaction is to convert natural gas from its gaseous state to a liquid form, enabling efficient transportation and storage. This transformation involves several intricate steps, each requiring meticulous control and regulation, with control valves playing a crucial role in ensuring the smooth operation and safety of the liquefaction process.

At the heart of LNG liquefaction is the need to reduce the volume of natural gas while maintaining its energy density, making it economically viable for long-distance transportation. This reduction in volume is achieved through the application of cryogenic temperatures, typically around -162°C (-260°F), at which point natural gas transitions into a liquid state known as LNG.

The liquefaction process can be broadly categorised into three main stages: pretreatment, refrigeration, and condensation. During the pretreatment stage, raw natural gas undergoes purification to remove impurities such as water, carbon dioxide (CO2), and sulfur compounds (H2S). This purification step is essential to prevent corrosion and contamination within the liquefaction equipment. Specialised control valves with unique material combinations are utilised in the pretreatment stage of the LNG plant. During this phase it is common to remove CO2 and H2S with an amine contactor. The letdown valve at the bottom of the amine contactor needs specially selected materials such as duplex stainless steel or high nickel alloys to combat corrosion from the process fluid. In addition to the material selection, a proper valve design will utilise multi-stage trim to prevent cavitation and also have a gradual expansion in the trim flow area to prevent choking due to off-gassing during the pressure reduction. The result is a specially engineered valve suitable for the rigours of the application: corrosion, cavitation, and off-gassing.

Once purified, the natural gas enters the refrigeration stage, where it is cooled to cryogenic temperatures using a series of refrigeration cycles. These refrigeration cycles rely on the use of cryogenic refrigerants, such as propane or ethylene, to achieve the required temperature reduction. Control valves are deployed throughout this stage to regulate the flow of refrigerants, control pressure levels, and maintain precise temperature conditions within the liquefaction equipment. Some of the most critical control valves in this stage are those associated with turbomachinery protection. Compressor anti-surge control valves modulate flow to protect compressors from surge conditions that could lead to equipment damage or failure, safeguarding the integrity of the LNG trains. The compressor anti-surge valve is high capacity while also having very fast opening and response times, typically no more than 1 – 2 secs. Due to the high pressure drop ratios of the application, noise and vibration need to be mitigated using advanced valve trims based on pressure drop staging, frequency shifting and velocity management. High rangeability, often more than 100:1, is also required for when the valves are used at low capacity during compressor start-up. The cumulative requirements lead to specific valve designs tailored to each piece of turbomachinery.

The final stage of the liquefaction process involves the condensation of the cooled natural gas into LNG. This condensation occurs within specialised equipment, where the natural gas is subjected to low temperatures and high pressures, causing it to transition from a gaseous to a liquid state. Control valves play a crucial role in this stage, facilitating the precise control of flow rates while ensuring optimal pressure conditions. One of the most critical control valves in this part of the plant is the turboexpander bypass valve, or Joule-Thompson (J-T) valve. This valve needs to be suitable for low cryogenic temperatures while utilising specially designed multi-stage trims to safely manage the J-T phase change without excessive, damaging vibration. Special consideration needs to be made to the J-T valve, so it is sufficiently large for low inlet and outlet valve velocities.

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