Valve stem dynamic stress and its safety design

When the stem valve disc by opening and closing position, must overcome the valve disc before and after the pressure difference generated the longitudinal force, and according to the requirements of regulating performances of the steam turbine, the valve disc play and must have a certain speed. Thus, the valve stem to withstand the pressure of the formation of the external load and valve plate, valve stem inertia force. Similarly, when the closure is also subject to external loads and inertia force. Visible, the valve in the open and close the valve, the valve stem is also the impact of production. In addition, the valve stem is also affected by the steam chamber and the flow of the valve before and after the disturbance of the steam flow. These shocks and disturbances, not to the cycle and planning, the degree is not a strong Yue Yang gauge. This makes the stem at random shock, random loading is very unfavorable conditions, the considerable dynamic stress produced in the valve rod. Therefore, in the juice of the stem, cut not available in simple tension calculation method to determine the structure size and material of the valve are, but should stem as affected by the rod of a random shock load to calculate the dynamic stress and according to random loading unlimited cycle bad not to undermine the principle of to select stem structure size and material.
It should be noted, the calculation, has been moving load and shock wave in the longitudinal bars in the overlapped to increase should impact force calculation, and should therefore not be in the strength check like static force calculation as the meter and dynamic load effects and also to increase the safety coefficient.
However, the above calculation is only the case when the impact of the maximum impact after the opening and closing. In fact, the valve in the process of work, but also due to the disturbance of the steam flow and the flow of the steam flow disturbance. The amplitude of the disturbance force is usually less than the impact force when the valve is opened or closed. Therefore, the disturbance of steam flow to the valve stem, and the random loading. Because of its random nature, the fatigue of the valve stem material is adversely affected. Therefore, in the security check, and random disturbance force (the force amplitude is greater than the open or close the impact of adverse effects and processing safety margin.