Stem Separation Failure in a Double Ported Globe Valve

Figure 1: Guide Bushing and Stem-Pin Location on a Double Port Globe Valve
Figure 1: Guide Bushing and Stem-Pin Location on a Double Port Globe Valve

Double port globe valve construction often incorporates upper guide bushings in the bonnet, a lower guide busing in the bottom flange, and a stem-to-plug connection as shown in Figure 1. A US power plant utilized double port globe valves as the normal level flow control valves for the intermediate-pressure heaters. The double ported globe valves were operated by diaphragm actuators and configured spring-to-close.

During plant operation, one of the double ported globe valves failed closed and did not respond to plant control signal to maintain a normal level. Inspection of the valve revealed the plug assembly separated from the stem which prevented the actuator from operating the valve. Further inspection revealed the stem threads were highly worn and the stem-pin was broken.

KEI performed a root-cause analysis of the stem connection failure. The cause was excessive plug and bussing wear that resulted from continuous valve cycling. The valve cycling was partly due to valve/system interaction.

KEI performed transient-flow CFD analysis to calculate the time-dependent forces acting on the plug and calculated the bushing and stem.

The wear calculations corroborated well with the observed plug/bushing wear. KEI concluded that the low hardness and wear properties of the bushing material and system-induced fluid forces resulted in accelerated wear of the bushing. High impact loads between the stem and bushing due to increased clearance from wear further accelerated the wear and resulted in stem connection failure.

KEI recommended options for harder and more wear resistant bushing material which would significantly increase the life of the valve.

Figure 2: Excessive wear between the guide bushings and plug-stem allowed the stem to catch in the bushing and overload the stem-to-plug connection in its degraded condition.

a) Busing Wear
b) Plug-stem Wear
Figure 2: Excessive wear between the guide bushings and plug-stem allowed the stem to catch in the bushing and overload the stem-to-plug connection in its degraded condition.