NRC test results and operations experience provide insights for a new gate valve stem force correlation

Abstract

The Idaho National Engineering Laboratory (INEL) is performing research in support of the efforts of the U.S. Nuclear Regulatory Commission (NRC) regarding the ability of motor-operated valves (MOVs) to perform their design basis function, usually to close against specified flow and pressure loads. Before 1979, the nuclear industry was not aware that they had major problems with their MOVs. However, the Three Mile Island (TMI) accident revealed the seriousness of this issue when the failure of a PORV block valve (a motor-operated gate valve) contributed to the severity of the accident. The inability of the auxiliary feedwater gate valves at Davis-Besse to reopen after they were inadvertently closed in 1985 further confirmed the seriousness of this issue. This paper presents the results of testing sponsored by the NRC to assess valve and motor operator performance under varying pressure and fluid conditions. This effort included an examination of the methods used by the industry to predict the required stem force of a valve, and research to provide guidelines for the extrapolation of in situ test results to design basis conditions. Years ago, when most of these valves were originally installed, the industry used a set of equations to determine analytically that the valves' motor-operators were large enough and the control switches were set high enough to close the valves at their design basis conditions. Our research has identified several inconsistencies with the industry's existing gate valve stem force equation and has challenged the overly simplistic assumptions inherent in its use. This paper discusses the development of the INEL correlation, which serves as the basis for a method to bound the stem force necessary to close flexwedge gate valves whose operational characteristics have been shown to be predictable. As utilities undertake to provide assurance of their valves' operability, this ability to predict analytically the required stem force is especially important for valves that cannot be tested at design basis conditions. For such valves, the results of tests conducted at less severe conditions can be used with the INEL correlation to make the necessary prediction.