Why should a stop valve have the inlet at a lower level and the outlet at a higher level?

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A globe valve, also known as a stop valve, belongs to the category of valves with forced sealing and is a type of shut-off valve. Based on the connection method, there are three types: flange connection, threaded connection, and welding connection. China’s “Three Standardizations for Valves” stipulates that for globe valves, the flow direction must always be from top to bottom; therefore, there is a specific installation direction. This type of isolation globe valve is highly suitable for cutting off, regulating, and throttling. Due to the relatively short opening and closing stroke of the valve stem in this type of valve, as well as its highly reliable shut-off function, and because the change in the valve seat opening is proportional to the stroke of the valve disc, it is highly suitable for regulating flow rate. 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 bonnet, 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 of time. This helps to prolong their service life and reduce the likelihood of leakage. Additionally, this allows for the replacement or addition of packing while the valve is 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. Generally, for stop valves, the inlet is at the bottom and the outlet at the top. However, there are some special cases where the inlet is at the top and the outlet at the bottom: 1. For high-pressure stop valves with a diameter greater than 100 mm, since large-diameter valves have poor sealing performance, this arrangement is used so that when the valve is closed, the medium pressure acts on the upper side of the valve disc, thereby enhancing the valve’s sealing capability. 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 a bypass system, the functions of installing such a bypass are: ① To balance the pressure on both sides of the main pipeline valve, thereby making it easier and less labor-intensive to open, and reducing wear and tear on the main pipeline valve ； 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. In the direction of the medium flow, the bypass stop valves are referred to as the primary valve and the secondary valve. During normal operation of the unit, both the primary and secondary valves remain closed; both of them are in direct contact with the medium. To prevent the gasket between the valve body and bonnet of the secondary valve, as well as the packing around the valve stem, from being affected by the medium and temperature over a long period of time, and to allow for replacement of the valve packing during operation, the required installation orientation for the secondary valve is “high inlet and low outlet”. 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 an electromagnetic quick-acting valve is similar to that of a globe valve. If, in the electromagnetic quick-acting valve as well, the working fluid enters from the bottom and exits from the top, then the force exerted by the fuel on the lower part of the valve disc is very large; meanwhile, the counterweight of the electromagnetic quick-acting 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 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 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 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.