Knowledge sharing on stop valves
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Why are stop valves designed with low inlet and high outlet? The stop valve is designed with the inlet at a lower level and the outlet at a higher level, in order to minimize flow resistance and make it easier to open the valve. At the same time, when the valve is closed, the gasket between the valve body and the valve cover, as well as the packing around the valve stem, are not under stress; thus they are not exposed to the pressure and temperature of the medium for long periods, which helps to extend their service life and reduce the likelihood of leakage. Additionally, this allows the packing to be replaced or added with the valve closed, facilitating maintenance.
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High-temperature and high-pressure stop valves usually have the inlet at a higher level than the outlet, as this is considered more conducive to achieving a secure seal. Are there any clear regulations regarding this installation direction?
The closing element of a high-temperature and high-pressure globe valve is a plug-shaped disc, whose sealing surface is either flat or conical; the disc moves in a straight line along the centerline of the fluid. The modes of movement of the valve stem include lift-type and lift-rotary types. Globe valves are classified as valves with forced sealing; therefore, when the valve is closed, pressure must be applied to the valve disc to ensure that the sealing surface does not leak. When the medium enters the valve body from beneath the valve disc, the resistance that the operating force must overcome consists of the friction between the valve stem and the packing, as well as the thrust generated by the pressure of the medium. The force required to close the valve is greater than that needed to open it; therefore, the diameter of the valve stem must be sufficiently large. Otherwise, the valve stem may become bent. In recent years, since the emergence of self-sealing valves, the flow direction of the medium in globe valves has changed to enter the valve chamber from above the valve disc. Under the pressure of the medium, less force is required to close the valve, while more force is needed to open it; as a result, the diameter of the valve stem can be reduced accordingly. At the same time, under the influence of the medium, this type of valve is also more airtight. China’s regulations on valve design specified that for globe valves, the flow direction should always be from top to bottom. When a high-temperature and high-pressure globe valve is opened, the flow rate reaches its maximum when the opening height of the valve disc is 25% to 30% of the nominal diameter, indicating that the valve has reached its fully open position. Therefore, the fully open position of the globe valve should be determined by the travel of the valve disc. Globe valves operated by hand wheels or handles can be installed at any position in the pipeline. The flow direction of the medium should be consistent with the arrow indicated on the valve body.