Performance of MOV Stem Lubricants at Elevated Temperature (NUREG/CR-6750)
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Manuscript Completed: September 2001
Date Published: October 2001
K.G. DeWall, J.C. Watkins, M.E. Nitzel
Idaho National Engineering and Environmental Laboratory
Idaho Falls, ID 83415-3129
J.E. Jackson, NRC Project Manager
Division of Engineering Technology
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
NRC Job Code Y6593
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 lubricant 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 run at ambient plant temperatures, usually in the 70 to 100°F range. 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 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.
The results from an earlier accelerated aging test are presented. The accelerated aging test identified the concerns the led to the elevated temperature testing. To evaluate elevated temperature performance, five different lubricants on four different valve stems and stem nuts were tested. The test series included collection of baseline data at room temperature, single step temperature tests where the temperature of the test setup was elevated directly to 250°F, and step testing where the temperature was elevated in steps to 130, 190, and 250°F, then returned to 70°F. This research produced the following conclusions:
- The physical characteristics of each lubricant change with increasing temperature, changing the frictional performance of each stem and stem nut.
- The consistency of the stem/stem nut coefficient of friction from one stroke to another changes significantly with increasing temperature.
- The stem/stem nut coefficient of friction can increase significantly at elevated temperature.
- The end of stroke friction behavior is highly dependent on the unique stem/stem nut tested, the lubricant, and temperature.
- Each individual stem and stem nut combination has unique characteristics with regard to variation between strokes, elevated temperature performance, and end of stroke friction behavior.