To control and cut off the medium, valve operation will be done through the stem to make the disc move. Valve stem is the main part of the valve, if the valve is installed in harsh conditions, the valve stem also easy to cause damage.
1. The stem is easy to occur spot corrosion and abrasion by packing, which is one reason for leakage at packing. The solutions are the following :
a:Reduce the content of Cl- and S- ions in the packing.
b:The use of corrosion retarding packing
c:Improve the degree of valve stem finish .
2.Because the disc is suspended in the fluid, the flow force acts on the disc and transmits the force to the valve stem. If there is vibration, there will be a great force acting on the stem and the stem will be broken. The large size diameter stem of throttle valve has been broken before .
3. The disc has the function of cutting off the fluid and controlling the fluid, which requires a larger driving force. In addition, when the valve is opened and closed, the stem also bears the force caused by the expansion force, so that the stem is broken, which should be given enough attention.
4.Wedge gate valve stem and disc free connection,when valve full open, gate is set above of the valve body channel, do not withstand the impact of the fluid. However if the gate valve is working as a regulating valve, globe valve or bypass valve, it is worked as a control in the half-open throttling state. The half-open gate valve is impacted in the flow fluid, it will cause intense vibration. The end of the stem was broken by wear. It can be seen from the above incident that wedge gate valves cannot be used in the adjustment of throttling and intermediate opening under the condition of large differential pressure.
5. Small size globe valve used at high temperature and high pressure, when used under throttling, the pressure difference between the inlet and outlet is quite large, the disc vibrates under the action of the flow medium, and bears the lateral pressure of the medium. The stem is subjected to large bending stresses, causing the stem to bend without action, or causing bite or abrasion to the stem guided bushing portion. In special cases, because the structure superimposed with high frequency vibration can break the stem, the prevention strategy is to use the disc guide structure, so that the stem does not bear abnormal bending stress and vibration (usually with the shell guide or body guide) .
6.When Large diameter globe valves (DN300-DN400) closed, most of valve stem is inside the body. In quickly opening, the stem of the back seal start to work, the stem is exposed in the atmosphere slowly cooling contraction. However ,the lower stem receives upward force at the back seal, restricting its free movement. The stem engages with the stem nut to limit axial movement. Therefore, with both ends of the stem fixed, shrinkage causes tension. Fracture is caused by stress concentration at the maximum variation of the section, or at the stem packing part where the high temperature medium contacts.
In order to prevent leakage in the packing part, an inverted seal was designed. But when the force of rotation or impact of the hand wheel, it easy to produce a large pull force and the stem will be fracture.In contrast, when large size globe and wedge gate valves are rapidly closed ,the stem enters the high temperature medium and the stem is gradually heated and expands. The valve is closed and the lower stem and stem threads are constrained. Due to heat expansion, stem bending, in the rapid close valve, large diameter high temperature valve should take measures to release heat expansion.
