Design features of cryogenic valves
#1. Material selection of cryogenic valve
The working medium of cryogenic valves is not only low temperature, but also mostly or, and has strong permeability, so it determines many special requirements for valve materials. The mechanical properties of steel at low temperature are different from those at room temperature, and the important indicator of low temperature steel, in addition to strength, is its low temperature impact toughness. The low-temperature impact toughness of the material is related to the brittle transition temperature of the material, and the lower the brittle transition temperature of the material, the better the low-temperature impact toughness of the material.
Metal materials with a central cubic lattice such as carbon steel have low temperature cold brittle phenomenon, while metal materials with a face-centered cubic lattice such as austenitic stainless steel are basically not affected by low temperature.
The materials of pressure-resistant parts such as low-temperature valve body and valve cover are usually made of ductile materials with good low-temperature strength, and factors such as weldability, machinability, stability and economy should also be considered. When designing, three low temperature levels of -46°C, -101°C and -196°C are commonly used. -46 °C low temperature grade generally uses low temperature carbon steel, -101 °C and -196 °C low temperature grade generally use 300 series austenitic stainless steel, this stainless steel has moderate strength, good toughness and good processing performance.
#2. Structural design of petrochemical cryogenic valve
(1) Bonnet structure design
A prominent feature of cryogenic valves is that their bonnet is generally a long-neck structure, which is also clearly stipulated in GB/T24925 “Technical Conditions for Cryogenic Valves” “The bonnet of cryogenic gate valves, globe valves, ball valves and butterfly valves should be designed according to different use temperature requirements into a long-neck bonnet structure that is convenient for cold preservation, so as to ensure that the temperature at the bottom of the stuffing box is kept above 0 °C”.
The design of the extended bonnet structure is mainly to keep the valve operation handle and stuffing box structure away from the low temperature area, which can not only avoid frostbite caused by the operator due to too low temperature, but also ensure that the stuffing box and pressure jacket are used at normal temperature to prevent the sealing performance of the packing from being reduced and extend the service life of the packing. Because at low temperature with the decrease of temperature, the elasticity of the packing gradually disappears, and the leak-proof performance decreases, due to the medium leakage causing the packing and the valve stem to freeze, affecting the normal operation of the valve stem, but also due to the valve stem up and down movement and scratching the packing, causing serious leakage. Therefore, the cryogenic valve must adopt the structure of the long neck bonnet.
In addition, the long neck structure makes it easy to wrap the insulation material and prevent the loss of cold energy. Since cryogenic pipelines generally have a thick thickness of the insulation layer, the long-neck valve cover is convenient for cold insulation construction, and the packing gland is outside the cold insulation layer, which is conducive to tightening the gland bolts or adding packing at any time without damaging the cold insulation layer when needed.
(2) Valve body
The valve body should be able to fully withstand the expansion and contraction caused by temperature changes. Moreover, the structure of the valve seat part will not be deformed due to temperature changes.
(3) Valve disc
The gate valve adopts flexible gate plate or open gate plate; The flat seat and needle valve of the globe valve adopt a plug-shaped disc. These forms of construction maintain a reliable seal regardless of temperature changes.
(4) Valve stem
The valve stem needs to be chrome-plated, nickel-plated and phosphorus or nitrided to improve the surface hardness of the valve stem, prevent the valve stem from biting each other with the packing and packing sleeve (gland joint), damage the sealing packing, and cause the stuffing box to leak.
The selection of gaskets should consider the low temperature properties of the gasket material, such as compressive resilience, pretension, fastening pressure distribution and stress relaxation characteristics.
(6) Stuffing letter and filling
The stuffing box cannot be in direct contact with the low temperature section, but is set at the top of the long neck valve cover, so that the stuffing box is far away from the low temperature and works in a temperature environment above 0 °C. In this way, the sealing effect of the stuffing box is improved. In the event of leakage, or when the low-temperature fluid directly contacts the packing and the sealing effect decreases, grease can be added from the middle of the stuffing box to form an oil seal layer to reduce the pressure difference of the stuffing box as an auxiliary sealing measure. The stuffing box mostly adopts a two-stage packing structure with an intermediate metal isolation ring. However, some adopt other types such as general valve stuffing box structure and double stuffing box structure with valve stem self-tightening.
(7) Upper seal
The cryogenic valves are equipped with a sealing seat structure, and the upper sealing surface should be surmounted with cobalt-chromium-tungsten cemented carbide, and ground after finishing.
(8) Sealing surface of valve seat and valve disc (gate plate).
The closing parts of the cryogenic valve adopt cobalt-chromium-tungsten carbide overlay welding structure. The soft seal structure is only suitable for cryogenic valves with temperatures above -70 °C due to the large expansion coefficient of PTFE and brittleness at low temperatures, but PTFE can be used for cryogenic valves at -162 °C.
(9) Medium flange bolt
The bolt should have sufficient strength, because the bolt works under repeated loads, and often breaks due to fatigue. Because the bolt is easy to cause stress concentration at the root of the thread, the bolt with a full-thread structure is used.