Structural design features and installation requirements of ultra-low temperature ball valves

The definition of low temperature valve in the petrochemical industry is defined according to the design temperature of the conveying medium. Generally, the valve applied in the medium temperature below -40 ℃ is called low temperature valve, and the valve applied in the medium temperature below -101 ℃ is called ultra low temperature valve .

Ultra-low temperature ball valves are mainly used in installations in liquefied natural gas, liquefied petroleum gas and air separation industries. These media are not only flammable and explosive, but also gasify when heating up or flashing, and their volume expands rapidly during gasification. If the valve that transports these fluids has a closed valve cavity and the structural design is unreasonable, it will cause the valve cavity to exceed pressure, resulting in medium leakage, or even valve cracking and accidents.

2. Design features of the main structure of the ultra-low temperature ball valve

Due to the particularity of the medium and environment used, the ultra-low temperature ball valve has significantly different characteristics from other types of valves in structural design.

2.1. Design of extended valve cover and drip plate design

The bonnet of the cryogenic valve adopts the design of the extended bonnet. The design of the extended bonnet should keep the valve operating handle and the packing installation position away from the low temperature area, which can not only avoid the cold burn of the valve operator caused by the low temperature of the medium, but also make the valve packing work at a normal temperature to ensure that the packing will not be affected. Frost damage caused the packing to break and fail. In addition, because the cold insulation layer of the general ultra-low temperature valve is relatively thick, the extended valve cover also ensures the space for cold insulation construction, and the packing gland is located outside the cold insulation layer. When adding packing and tightening the gland bolts, there is no need to damage the cold insulation layer. Some manufacturers use the drip plate design on the extended bonnet of the ultra-low temperature ball valve. The drip plate needs to be set on the outside of the cold insulation layer, which can prevent the condensed water droplets from falling on the cold insulation layer and the upper part of the valve body, protect the cold insulation layer and prevent the loss of cold energy.

2.2. Fire protection design and anti-static design

Since ultra-low temperature ball valves are generally used in flammable and explosive media, fire protection design and anti-static design are particularly important. They are an essential part of the structure of ultra-low temperature ball valves and are indispensable. The fire protection design of the ultra-low temperature ball valve is similar to that of the ordinary ball valve, and graphite sealing rings are used in the sealing of each link. The anti-static design adopts a spring-type structure to avoid the danger of explosion caused by the accumulation of electrostatic charges.

2.3. Valve cavity pressure relief design and valve sealing ring design

When the ultra-low temperature ball valve is used in flammable, explosive and easily gasified media, it has special requirements for the valve sealing structure. This is because the ball valve generally has a closed valve cavity in structure. Due to the rapid expansion of the volume when the medium is vaporized, the ball valve must consider the pressure relief requirements of the closed valve cavity in the design of the sealing structure. According to the structure, the ball valve is divided into two types: floating ball type and fixed ball type. Due to the different valve sealing principles of the floating ball ultra-low temperature ball valve and the fixed ball ultra-low temperature ball valve, there will be obvious differences in the pressure relief design.

The sealing of the floating ball valve relies on the pressure of the medium to push the ball to the downstream sealing surface, so as to meet the sealing requirements. Therefore, the sealing structure of the general floating ball low temperature ball valve is that the upstream sealing ring adopts a spring pre-tightening sealing structure, and the pressure relief adopts the method of drilling holes on the sphere. Due to the high pressure of the upstream medium, the general pressure relief direction of the valve is to use the upstream pressure relief. The spring preloaded valve seat can make the ball press against the downstream sealing ring under the action of the spring. The advantage of this sealing structure is that the sealing is more reliable, and the opening torque of the ball valve will also be reduced. Drilling holes on the ball can make the valve cavity communicate with the upstream of the valve, so as to avoid the partial pressure increase of the valve cavity due to the gasification and expansion of the medium.

The seal of the fixed ball ultra-low temperature ball valve is generally a two-way seal, that is, the upstream and downstream sealing rings must simultaneously play a sealing role. The upstream sealing ring uses a single-piston sealing valve seat, and the downstream sealing ring uses a double-piston sealing valve seat. The principle of the single-piston valve seat is the one-way piston-type action. In the design of the sealing pair, the difference in the direction of the acting force caused by the difference between the two directions of the sealing surface is considered, so that the sealing pair is in the pressure difference. Under the push of the corresponding direction, the piston movement in the corresponding direction (single piston effect) is generated.

3. Precautions for installation of ultra-low temperature ball valve

Because of the special structure of the ultra-low temperature valve, the corresponding valve structure characteristics should also be considered when installing the valve, otherwise it will affect the use of the valve and cause the valve to fail.

3.1. Stem installation angle

The ultra-low temperature ball valve cannot be installed horizontally, that is, the valve stem cannot be installed in the horizontal direction. This is due to the structural characteristics of the extended bonnet of the ultra-low temperature valve, otherwise the low-temperature medium will fill the extended part of the bonnet, causing the valve packing to fail, and the cold energy will be transmitted to the valve handle, causing personal injury to the operator.

3.2. Direction of pressure relief

As mentioned earlier, cryogenic ball valves generally relieve pressure in the upstream direction. What should be specially mentioned here is that this upstream direction is not always the opposite direction to the flow direction of the pipeline, to be precise, it should be the upstream direction when the valve is closed. For example, the installation direction of the shut-off valves at both ends of the regulating valve should be opposite due to the consideration of maintenance, and the pressure relief direction of the two shut-off valves should be opposite to the direction of the regulating valve. In this way, during the maintenance of the regulating valve, when the two shut-off valves are in the closed state, the pressure can be released in the direction of the connected pipeline respectively, otherwise the gas in the valve cavity will be released to the atmosphere, causing environmental pollution and explosion hazard. Therefore, when installing the valve, special attention should be paid to the requirements of the pressure relief direction of the valve. The direction of valve pressure relief should be marked on the process flow chart and reflected in the pipeline axonometric drawing.

3.3. The valve should be thoroughly dried

Since the ultra-low temperature ball valves are used in low temperature conditions, high requirements are placed on the drying of the valves. If there is stagnant water in the valve, the water will freeze at low temperature, and the volume will expand, which will generate pressure on the internal components of the valve, resulting in poor sealing of the valve and even structural damage. After the hydrostatic test, the ultra-low temperature valve must be strictly dried before leaving the factory. Usually, the ultra-low temperature ball valve valve is not subjected to the hydraulic test on site, but is replaced by the air pressure test. This is because the conditions at the construction site are limited. After the hydraulic test, the water accumulated in the valve is not completely removed, and the drying is not complete, which affects the normal use of the valve.

4. Conclusion

As the application of liquefied natural gas becomes more and more extensive, the requirements for the safety of natural gas pipeline systems will become higher and higher, and higher requirements will continue to be put forward for the structural design of ultra-low temperature ball valves. Special attention should be paid to the design of the pipeline system, the installation and operation of the valve, in order to ensure the safe operation of the system and reduce the pollution and harm to the environment and personal injury.

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