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A stop valve is a device used in fluid systems to control the direction, pressure, and flow rate of fluids; it is used to enable or prevent the flow of media (liquids, gases, powders) within pipes and equipment, as well as to regulate their flow rate. During the construction process, the way in which the stop valve is installed directly affects its proper operation in the future. 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. Meanwhile, 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 medium pressure and temperature 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 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 with large diameters, 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 efficiency. 2. 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. 3. Two globe valves are connected in series on the bypass pipeline, with the second globe valve requiring a flow direction of \"high to low\". To ensure the tightness of the valves over one 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 pipe, 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 allow for the replacement of the valve 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. 4. Electromagnetic quick-stop valve The function of the electromagnetic quick-stop 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 desired 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, together with the forces generated by the weight of the weight and the lever, still remains. In cases of large diameters and high pressures, it is difficult to close the valve when fluid enters from below and exits from above. If this configuration is used under high-pressure and large-diameter conditions, the valve stem is prone to deformation and bending due to the water pressure, which affects 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. The valve body of a globe valve is equipped with clear arrow markings at the time of manufacture; when installing such a valve, the flow direction of the medium should be in line with the arrow on the valve body.