Usually what we call the gate valve accessories, also can with its pressure, heat resistance, high and low temperature, corrosion resistant (configuration) a variety of materials, with bi-directional sealing function, wear resistance and long life and other advantages more and more widely used in all kinds of industrial system, open and closed, the medium for the system's stability and control play an important role. However, it is necessary to pay attention to the fracture of the valve stem.
Stem fracture in gate valve accessories usually occurs at the root of the upper and lower threads, where the area is the smallest, especially prone to stress concentration and overshoot. Especially when the working conditions are deviated from the design parameters. For example, when the power plant has repeatedly had the DN150 electric gate valve open, the valve stem ladder type thread return knife groove to break the accident.
After investigation, it is found that the valve cover pretightening nut is loose, the valve cover is removed, and the stem nut is stuck, which is obviously the accident that the electric loading schedule has been adjusted and the protection moment is too large. Another type of gate valve stem fracture occurs in the open moment. It is shown that the gate has not been removed from the seat and the stem is broken at the top or bottom thread.
The stem fracture in the gate valve accessories is usually considered to be a lock, which is partly or secondary. An important reason is the abnormal increase of pressure after the closure of the valve body, that is, when the valve is closed, the fluid pressure in the middle cavity between the two sides of the upper and lower reaches is much higher than that of upstream pressure. There are two reasons for this phenomenon. First, the fluid in the middle cavity is heated by the upstream fluid heating, which causes the pressure to rise sharply.
Gate valve stem fracture in the accessories, but also because the gate closed moment fluid sealed in the lumen, unable to drain, lumen space is stem further squeeze, because the compressibility of the liquid is very limited, also can make pressure surge, this phenomenon is especially easy to occur in power plant Z962 class main feed gate valves. At the same time, the abnormal increase of pressure generally increases geometrically, far exceeding the limit of stem strength design in gate valve accessories. In view of this, it is necessary to avoid the above situations in the design and use of the stem in the gate valve accessories.http://www.pump-casting.com/