Selection of Greases for Motor-Operated Valve Stem / Stem Nut Lubrication

Abstract

In Motor-Operated Valves (MOV), the choice of a high-performance grease for stem/stem nut lubrication is required to guarantee long-term operability. Valve stem regreasing intervals and/or actuator limit switch resetting intervals of less than two years are no longer acceptable to most stations, because of the large number of valves in service. A joint MOV test program was launched in 1995 by Atomic Energy of Canada Limited (AECL), Electricite de France (EDF) and Ontario Power Generation (OPG) to select the best grease candidates available and address issues relative to valve reliability, increased maintenance costs, and environmental protection. The objective of this research was to identify greases that would be suitable for 4 to 5 years in service without regreasing. A thermal aging procedure was developed by OPG to simulate a five-year exposure to the in-service temperatures experienced by valve stem/stem nut arrangements. The grease candidates were initially screened using data from pin-on-disk wear tests and/or results of thermal aging tests. The thermal aging tests screened for grease consistency variations, acidity build-up, etc. Samples thermally aged for the equivalent of five years in service were then used for mechanical tests in a full-scale MOV test rig developed by AECL and EDF at the Chalk River Laboratories (CRL). The assessment of the stern/stem nut lubrication condition was mainly based on stem/stem nut friction coefficient measurements (value and stability of this value with valve stroke number). In this paper, the effect of grease thermal and mechanical aging on stem/stem nut lubrication is assessed, along with the effect of high temperature and high irradiation levels (accident conditions). The MOV test results are compared according to the type of base oil and thickener used in the grease. The effect of stem material and stem geometry on stem/stem nut lubrication is investigated. Results obtained for grease mixtures are also reported. Finally, grease specifications recommended to maintain adequate long- term MOV stem/stem nut lubrication are provided.