In rotary-shaft valves, the surface area of the closure member is generally quite large as compared to the surface area of the bearings and bushings that guide the closure member. Because of the relatively small guiding surfaces, the pressure drop ratings are often lower than the ANSI Class inlet pressure ratings. Manufacturers publish pressure drop limits that must be observed in order to prevent disk bending, excessive torque on the shaft, shaft bending, and shaft shearing as shown in Figure 9.
In most sliding-stem valves, the ratio of the guiding surface area to the surface area of the valve plug is quite high; accordingly, sliding stem valves are typically rated for higher pressure drops than rotary-shaft control valves. If the pressure drop exceeds the manufacturer’s published rating, the fluid forces that act on the valve plug can result in stem bending, plug and stem vibration, and seat wear as shown in Figure 9.