Inlet and outlet direction of the stop valve
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As the title suggests: the inlet and outlet directions of stop valves. On some devices, the fluid enters from above and exits at the lower level, while on others it enters from below and exits at the higher level. How was this design determined? What are the differences?
Stop valves generally have the inlet at the lower level and the outlet at the higher level; when closed, the packing is not under pressure, which helps to extend their service life, and they are easy to open. A high-inlet and low-outlet design can effectively prevent internal leakage, but it requires a relatively high opening pressure; generally, stop valves with a DNA rating of >100 can use a high-inlet and low-outlet configuration.
Stop valves can be installed in two ways: low inlet and high outlet, or high inlet and low outlet. Small-diameter valves generally have low inlet and high outlet, while most large-diameter stop valves are designed with high inlet and low outlet.
For liquid pipelines, flow generally goes from low to high; for gas pipelines, it flows from high to low, with some exceptions due to design considerations. There is usually a concept of net positive suction head available
Generally, low-pressure systems use a low inlet and high outlet design； Another type is used in high-pressure equipment, taking leakage into account; however, the downside is that it is difficult to open the valves at high pressures, especially the outlet valves of high-pressure pumps, which require two people to operate, and the handwheels tend to get damaged
Single valves usually have low inlet and high outlet; a high inlet and low outlet configuration is not recommended； For the convenience of device operation and maintenance, two valves are sometimes used in series, with each configured in a different manner.
It is necessary to decide whether the stop valve should be installed in the correct orientation or reversed, depending on the operational requirements of the equipment.
Generally, medium and low-pressure stop valves have the inlet at the lower level and the outlet at the higher level; this prevents the packing from being under stress when shutting off, thereby reducing the likelihood of leakage, which is beneficial for production management. High-pressure valves are different: when the pressure is high, the force acting on the valve core becomes very great. Operating such valves is difficult when there is a difference in pressure between the inlet and outlet sides. In particular, medium and large-diameter valves of DG50 or higher require a lever in order to be operated, which makes it even more inconvenient to address any abnormal faults that arise. That’s why they are designed in a different way. There is no perfect solution in the world; the choice depends on the specific circumstances, and this is true even for valves.
Generally, the distinction is made within the DN250-DN300 range; for sizes smaller than 250, a low-inflow and high-outflow configuration is usually used, while for sizes over 300 or those with higher pressures above 200, a high-inflow and low-outflow configuration is chosen. Why is it that for large-diameter or high-pressure applications, the low-inflow, high-outflow configuration is chosen? This is mainly because, in such conditions, it is difficult to close the valve if the high-inflow, low-outflow configuration is used. Additionally, when dealing with high pressure and large diameters, using the low-inflow, high-outflow configuration can cause the valve stem to deform or bend over time due to water pressure, which affects the valve’s safety and sealing performance ； Thirdly, by choosing a high inlet and low outlet position, the diameter of the valve stem can be reduced, which also saves costs for both manufacturers and users.
In our design, we mainly consider pressure; in the case of high pressure, it generally means high input and low output