The low temperature valve is installed at normal temperature and works at low temperature, with a wide range of temperature variation. If the design and installation are not appropriate, it is easy to generate thermal stress or deformation. Meanwhile, the operation part of the valve is at normal temperature, and the circulation part is at low temperature. In order to reduce the cold loss, the stem is often made very long, which is easy to produce deformation and get stuck.
Low temperature valve often rotates flexibly at normal temperature, but it is tight at low temperature, even can't be opened. The main reasons why the valve gets stuck at low temperature are:
1) during installation, prestress is generated due to unreasonable configuration of the valve and the pipeline; or the cold compensation ability of the pipeline is poor, and the valve position changes at low temperature; or the valve lacks support, and deformation is generated at low temperature; or the valve is not fixed properly, and the cold box will deform at low temperature, which will affect the concentricity between the valve stem and the valve body.
2) in terms of design, because the material of the valve stem and the valve sleeve are different, the linear expansion coefficient is different. Generally, the stem is made of stainless steel, and the coefficient of linear expansion is 1.73 × 10-6℃-1; The valve sleeve is brass, and the linear expansion coefficient is 19.9 × 10-6℃-1, That is, the shrinkage of brass is larger than that of stainless steel, which may bite the silk buckle at low temperature. Especially when adopting the hidden rod structure and fine-tooth thread, the temperature variation range of the screw thread is large, and the gap between the screw thread is small, which is more likely to cause the phenomenon of biting.
3) during the operation, due to the incomplete heating of the valve, or the water flowing into the valve stuffing, it will freeze at low temperature, or close the valve too tightly at normal temperature, making the screw buckle bite, etc.
In order to prevent the phenomenon that the valve gets stuck, the design should adopt open-bar structure and thick-tooth trapezoid thread; there should be a firm support at the valve when installing, to prevent the valve stem from bending due to the displacement of the pipeline. The fixation of the valve and the cold box can adopt elastic connection to prevent the valve stem from deformation and being different from the valve body; during the period of naked cooling, check and adjust the installation condition of the valve under cold condition, when it is found that the valve gets stuck after cooling down, the fixed flange of the valve on the barrel shell can be adjusted to make it switch freely.
During the operation, the heating before starting should be thorough. When closing the valve, the principle is no air leakage, and do not use too much strength.
