Why should a stop valve have the inlet at a lower level and the outlet at a higher level? Under what circumstances is it necessary to buy at high prices and sell at low prices?

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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 extends their service life and reduces the likelihood of leakage. Additionally, this allows the packing to be replaced or added with the valve closed, facilitating maintenance. Many people think that globe valves always have fluid entering from the bottom and exiting from the top, but this is not the case. Under normal circumstances, globe valves have fluid flow from low to high; however, there are some special cases where the flow direction is 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 large diameter, 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 effect. Because when the diameter is greater than 100 mm, the flow rate increases and the pressure per unit area becomes very high; therefore, to achieve sealing when using a low-inlet and high-outlet configuration, a large counteracting force is required. As a result, the valve stem must be enlarged, and either an impact handwheel or a high-torque worm gear mechanism must be used. It increases the volume of the valve and the product cost, with poor results. High inlet and low outlet pressure helps with sealing (the medium’s own pressure enhances the sealing effect after it is closed), but it is difficult to open it because the pressure of the medium has to be overcome. “It facilitates sealing; the medium pressure ensures a tighter seal of the valve, and the stress conditions on the valve stem are improved. 2. Two globe valves are connected in series on the bypass pipeline; the second globe valve should have flow from high to low. To ensure the tightness of the valves throughout a maintenance cycle, valves that are operated frequently need to be equipped with two globe valves in series. 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 shut-off valves in the direction of medium flow are referred to as the primary valve and the secondary valve. 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 allow for the replacement of the valve packing during operation, the secondary valve requires to be installed in a direction with \"fluid entering from above and exiting from below\". 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 promptly cutting off the fuel supply. 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-cut 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-cut valve operates, and thus the desired goal 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 weight 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 high pressure and large diameters, 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.