Why Do Globe Valves Above DN200 Adopt the "High Inlet and Low Outlet" Structure?

"High inlet and low outlet" means that when the globe valve is opened, the medium flows in the S-shaped flow channel of the globe valve from above the valve disc to below the valve disc along the direction of the valve. Since the flow channel is in the shape of a horizontal S, the upward convex part is called "high" and the downward concave part is called "low". Therefore, if the medium flows from the upward convex part to the downward concave part, it is called "high inlet and low outlet".

When the nominal diameter DN of the globe valve is ≤ 200mm and the nominal pressure PN is ≤ 25MPa, the "low inlet and high outlet" structure is usually adopted, that is, the medium flows from below the valve disc to above the valve disc. Then why do globe valves with DN > 200mm and PN > 4.0MPa adopt the "high inlet and low outlet" structure?

The disadvantage of the medium entering from below the valve core is that the driving torque of the valve is relatively large, about 1.05~1.08 times that of entering from above. The valve stem is subjected to a large axial force and is prone to bending. When both the nominal diameter and pressure increase, these disadvantages become particularly prominent, significantly affecting the normal use of the valve. Therefore, this "low inlet and high outlet" structure cannot be adopted. For this reason, the mode of medium entering from below is generally only applicable to small-diameter globe valves (below DN50), and globe valves above DN200 all adopt the mode of medium entering from above. Globe valves usually adopt the mode of medium entering from above. The disadvantages of the medium entering from above are exactly the opposite of those of entering from below.

When DN > 200mm, the "high inlet and low outlet" structure is selected. The valve disc of such globe valves is of a double-disc structure. Because when the diameter is larger than 200mm, due to the increased flow rate, the pressure per unit area is very high. When the "low inlet and high outlet" structure is adopted, a large opposite force is required to ensure sealing. This necessitates an enlarged valve stem, and the use of an impact handwheel or a high-torque worm gear device is mandatory, which increases the volume of the valve and the product cost, and the effect is not good. The "high inlet and low outlet" structure helps to improve sealing (the self-pressure of the medium enhances the sealing effect after closing), but it is relatively difficult to open because it is necessary to overcome the medium pressure. In summary, this structure is beneficial to sealing: the medium pressure makes the valve seal tighter, and the stress condition of the valve stem is improved.