Why should a stop valve have the inlet at a lower level and the outlet at a higher level? Under what circumstances does high inflow occur alongside low outflow?

Thread Content

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 subjected to any stress; thus, they are not exposed to the pressure and temperature of the medium for extended periods, which helps to extend their service life and reduce the likelihood of leaks. Additionally, this allows the packing to be replaced or added with the valve closed, facilitating maintenance. Many people think that globe valves always have flow from low to high, but this is not the case. Under normal circumstances, globe valves have flow from low to high; however, there are some special cases where they have flow from high to low: 1. High-pressure globe valves with a diameter greater than 100 mm – due to the poor sealing performance of valves of this size, this approach is used so that, when the valve is closed, the pressure of the fluid acts on top of the valve disc, thereby enhancing the valve’s sealing ability. 2. Two globe valves connected in series on the bypass pipeline; for the second globe valve, it is required that the flow direction be “from high pressure to low pressure”. To ensure the tightness of the valves throughout a maintenance cycle, two globe valves connected in series must be installed for valves that are frequently opened and closed. For bypass systems, the function of this bypass is as follows: ① To balance the pressure before and after the main pipeline valve, thereby making it easier and less labor-intensive to open the valve and reducing wear on it ； ②Low-flow pipe warming during startup ； ③On the main feedwater pipeline, the feedwater flow rate is controlled to regulate the boiler’s pressure rise rate during the boiler hydrostatic test. The bypass stop valves in the direction of medium flow are referred to as the primary valve and the secondary valve, respectively. When the unit is operating normally, both the primary valve and the secondary valve are closed, and they are in direct contact with the medium. To prevent the gasket between the secondary valve housing and cover, as well as the packing around the valve stem, from being exposed to the medium and temperature over extended periods, and to enable the replacement of the valve’s packing during operation, the secondary valve requires to be installed with the fluid flowing in from the higher side and out from the lower side. 3. Boiler exhaust and vent stop valves: These valves are used only during the boiler startup and water filling process; they are not operated frequently. However, poor sealing can lead to losses of fluid. To improve sealing performance, some power plants install such stop valves in a manner that fluid flows from higher to lower levels. 4. Electromagnetic quick-shut valve: The function of the electromagnetic quick-shut valve is to close quickly, thereby shutting off the fuel supply promptly. The structure of the electromagnetic quick-stop valve is similar to that of a globe valve; if fluid enters the electromagnetic quick-stop valve from the bottom and exits from the top, the force exerted by the fuel on the lower part of the valve disc is very large, whereas the weight of the plunger in such a valve is much smaller than this force. Therefore, if the working fluid enters the quick-shut valve from below, the torque generated by the weight is less than the torque generated by the fuel pressure; as a result, the fuel cannot be cut off when the quick-shut valve operates, and thus the intended purpose cannot be achieved. If the working fluid enters from above the quick-acting valve, then once the valve operates, the pressure behind it drops rapidly; as a result, the force exerted by the fuel on the lower part of the valve disc quickly becomes zero. The force exerted by the fuel on the valve disc is then balanced by the force generated by the weight of the hammer and the lever. Generally, in large-diameter applications and under high-pressure conditions, it is difficult to close the valve when using a low-inlet, high-outlet configuration. If this configuration is used under such conditions, the valve stem is subject to water pressure over time, which can cause it to deform or bend, thereby affecting the valve’s safety and sealing performance ； By choosing a higher inlet and lower outlet, the diameter of the valve stem can be reduced, which also saves costs for both manufacturers and users.