Load Sensitive Behavior (LSB) is a phenomenon identified in the 1990’s through a series of dynamic flow loop tests conducted by INEL on motor operated gate valves that were closed under torque and limit switch control during various dynamic flow and pressure conditions. The tests showed that the operator thrust required to close the valve under the dynamic conditions of both flow and pressure can exceed a switch setting that was established under static pressure conditions only, which may result in valve motor shutoff prior to fully seating the gate valve. For motor operated gate valves with a safety function to close during dynamic flow conditions that are torque switch closed are particularly prone to LSB if the torque switch setting has little design margin over the required torque under static pressure alone. LSB typically occurs when the valve disk enters the flow stream. The differential pressure across the disk increases as more disk area is exposed to the flow that pushes the disk against the downstream guides and/or seat. Consequently, this increases the resisting drag on the disk and subsequently the loading on the stem nut and worm gear.
Further investigation revealed that the following factors also influence the impact of LSB for an application where valve safety function is required during either closing and/or opening under dynamic conditions: the type of stem lubricant, the stem to stem nut lubrication factor for predicting the required thrust load under dynamic load conditions, and the valve factor used in sizing the motor actuator and predicting the required stem thrust. In addition, when sizing a motor actuator for an application, the following questions should be considered: Is the valve factor and stem lubricant factor based on dynamic load testing? How is the valve disk guided throughout the valve stroke? Is the valve actuator wired for torque switch or limit switch closure and is there adequate margin, or does the motor actuator wired to bypass the torque switch while completing the valve stroke?
This paper investigates design features and general operation, especially design factors related to LSB that affect performance, of certain motor actuator operated, rising stem, pilot operated, balanced disk globe valves designed by Flowserve Corporation specifically for Westinghouse AP1000® nuclear power plants. These valves are designed to provide the safety-related function to open upon demand. Motor trace data from bench testing under nitrogen flow are presented as well as prototype dynamic testing under severe blowdown conditions conducted during plant startup. These tests confirm that maximum required actuator opening thrust occurs early during the valve stroke before the start of process fluid flow and that flow tends to assist the actuator during much of the remaining opening stroke. Typical design factors used in the design methodology are described, particularly for the sizing of motor actuators to address LSB for the AP1000® valve application.
The objective of the paper is to demonstrate, through description of the design process, hardware features, and performed testing, how the subject motor actuated, pilot operated, and balanced globe valves can reliably perform their required safety function to fully open on demand during the severe blowdown event. In addition, a focus is placed on the understanding of LSB for the application and how adverse effects are avoided.