MOV Stem Lubricant Aging Research (NUREG/CR-6806)
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Manuscript Completed: September 2002
Date Published: March 2003
K. G. DeWall, J.C. Watkins
Idaho National Engineering and Environmental Laboratory
Idaho Falls, ID 83415
J. E. Jackson, NRC Project Manager
NRC Job Code W6593
Division of Engineering Technology
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
This report documents the results of recent tests sponsored by the Nuclear Regulatory Commission (NRC) and performed by the Idaho National Engineering and Environmental Laboratory (INEEL). These tests address the effectiveness of the lubricants used on the threaded portion of the valve stem, where the stem nut turns on the stem. Recent testing indicates that an elevated temperature environment can lead to significant increases in the friction coefficient at the stem/stem-nut interface. Most valve actuator qualification tests are performed at room temperature. Similarly, in-service tests are performed at ambient plant temperatures, usually 70 to 100°F. Since design conditions can lead to valve operating temperatures in the 200 to 300°F range, it is important to know whether a temperature induced increase in friction at the stem/stem-nut interface will prevent the required operation of critical valves. Lubricant aging is another phenomenon that might have deleterious effects on the thrust output of a valve actuator. Laboratory experience and field experience both indicate that after long periods in elevated temperature environments, the lubricants may lose their lubrication qualities.
This research effort was performed to address the effectiveness of the lubricant used on the threaded portion of the valve stem. The effectiveness of this lubricant can greatly impact the thrust output of the valve actuator and reduce the margin for ensuring motor-operated valve (MOV) performance at design basis. Our analysis looked at the aged performance of two lubricants on one valve stem. It also looked at a new lubricant on one stem to determine its load and end-of-stroke friction behavior, elevated temperature, and aging performance. The following conclusions are based on this work.
For Chevron SRI and Mobil Mobilgrease 28, lubrication aging does not appear to degrade the performance of stem and stem nut interface. For the single stem tested (Stem 2), the stem and stem nut friction did not increase during the aging period. In some cases, the final friction values for both the hot and cold tests were lower than the initial hot and cold values.
On the single stem tested, the MOV Long Life lubricant's performance was similar or an improvement over that of other lubricants previously tested. MOV Long Life frictional performance, including end-of-stroke friction behavior, was stable and repeatable over a wide load range. Elevated temperature resulted in a lower friction coefficient than that observed at room temperature but resulted in greater rate-of-loading. Stem nut friction appeared to be stable over the simulated aging period.
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