A brief introduction to the valve stem
The valve stem is an important component of a valve, used for transmission. It is connected to the actuator or handle at the top, and directly drives the valve core to move or rotate during the valve opening and closing process, thereby achieving the valve's opening, closing or regulation functions. During the valve's opening and closing process, the valve stem is not only a moving part and a force-bearing part, but also a sealing part.
At the same time, it is subjected to the impact and corrosion of the medium, and also rubs against the packing.
Therefore, when selecting the material for the valve stem, it is necessary to ensure that it has sufficient strength, good impact toughness, anti-abrasion property and corrosion resistance at the specified temperature.
The reasons for the fracture or deformation of the valve stem of a gate valve
The valve stem is a wear-prone component. When selecting it, one should also pay attention to the mechanical processing performance and heat treatment performance of the material.
Valve accessories
The valve stem of a gate valve usually breaks at the root of the upper and lower threads. This area has the smallest cross-sectional area and is prone to stress concentration and exceeding the limit. Especially when the working conditions deviate significantly from the design parameters.
For instance, in a certain power plant, there were multiple incidents where the pull-out groove on the trapezoidal thread of the valve stem of the DN150 electric gate valve broke. The investigation revealed that the pre-tightening nut of the valve cover was loose, causing the valve cover to move upward and the valve stem nut to get stuck. Clearly, this was an accident caused by the incorrect adjustment of the electrical assembly stroke and excessive protective torque.
Another type of valve stem fracture accident occurs at the moment of opening.
The symptom is that the gate has not yet separated from the valve seat, but the valve stem has broken at the top or bottom thread root. The cause is usually believed to be that the gate is stuck, but this is only a partial or secondary reason.
One important reason is the abnormal pressure rise in the chamber of the valve body after it is closed. That is, after the valve is closed, the fluid pressure in the middle chamber, which is sealed between the sealing surfaces on both the upstream and downstream sides, is much higher than the upstream pressure. There are two reasons for this phenomenon.
In the first case, the fluid in the middle cavity is heated and warmed by the upstream fluid, causing it to expand and resulting in a significant increase in pressure.
When the second gate plate is closed, the fluid is completely enclosed in the middle chamber and cannot flow out. The space in the middle chamber is further compressed by the valve stem. Due to the extremely limited compressibility of the liquid, the pressure will also increase sharply. This phenomenon is particularly likely to occur on the Z962 type main feedwater gate valves in power plants.
At the same time, the abnormal pressure increase usually increases exponentially, far exceeding the strength design limit of the valve stem. The deformation and bending of the gate valve stem usually occur when the electrical assembly of the electric valve is not properly debugged, such as when the closing torque is too large and no travel protection or misalignment is set, etc. This causes significant damage to the valve.
