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The cryogenic valve for LNG

Views: 212     Author: Site Editor     Publish Time: 2020-10-26      Origin: Site

Natural gas, as a clean and efficient high-quality energy source, plays an important role in optimizing the energy consumption structure, improving the atmospheric environment, and controlling greenhouse gas emissions. Liquefied natural gas (LNG) plays an increasingly important role in national production. As one of the important equipment in the LNG plant, the design, selection and production of cryogenic valves are very critical.



Valve Material

The normal working temperature of LNG valve is about -163℃. At this temperature, the metal materials will be cold brittle at low temperature, i.e. the strength and hardness will increase, and the plasticity and toughness will decrease significantly, which will seriously affect the safety of the valve. In order to prevent low stress brittle fracture of the material at low temperature, austenitic stainless steel is often used for valve body and valve disc. The valve body, bonnet, disc, seat and stem must be cryogenically cryogenically treated to transform austenite into martensite to reduce the impact of temperature on the sealing performance of ultra-low temperature valves.



Structure Characteristics of Cryogenic Valves

(1) Stem Extension Design

The purpose of lengthening the valve stem is to raise the stuffing box so as to keep the packing working at normal temperature and ensure the sealing performance of the packing. In the low temperature state, the anti leakage performance is gradually reduced, and the leakage is easy to cause the ice at the joint of the packing and the valve rod, which affects the normal operation of the valve rod. Moreover, the movement of the valve stem is easy to scratch the packing and cause leakage, so the anti leakage performance gradually decreases. The extension height is the distance from the top of the valve body to the bottom of the stuffing box. The shape of the extended stem structure is convenient for winding the cold insulation material to prevent the loss of cold energy.


valve rod

(2) Drip Plate Structure

The extended stem valve provides a drip plate. This drip plate can prevent condensed water from flowing into the heat insulation area, and it can more effectively ensure the working environment of the stuffing box, thereby avoiding adverse effects.


(3) Cryogenic Treatment

All the important parts related to the sealing performance of the valve need deep cooling treatment. The method is to place the parts at a low temperature (such as liquid nitrogen) of -196℃ for a certain period of time, then take them out and put them at the ambient temperature, so as to restore them to the normal temperature. The goal is to stabilize part dimensions and eliminate stress due to tissue changes. Because most materials will undergo permanent metallographic changes when they are cooled below 0°C for the first time, their volume will expand or shrink. After the cryogenic treatment, when the part is cooled to below 0°C for the second time, the amount of permanent metallographic structure change is very small, resulting in a small volume expansion or contraction.



In a word, the safety and reliability of cryogenic valve equipped in LNG plant is higher than that of liquid oxygen valve and liquid nitrogen valve.

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