The Issue
Inadequate lubrication of MOVs has led to multiple failures and prompted the Nuclear Regulatory Commission to issue NRC INFORMATION NOTICE 2010-03:FAILURES OF MOTOR-OPERATED VALVES DUE TO DEGRADED STEM LUBRICANT
This notice outlined five MOV stem lubrication events at nuclear plants that pointed to issues with the grease lubrication of these interfaces. Presentation of this information at the MOV User’s Group meeting in 2011 noted that stem lubricant degradation can affect the efficiency of actuator torque conversion.
“Lubricant must be applied to the stem in an effective manner at appropriate intervals to ensure that the coefficient of friction between the stem and stem nut remains within the design assumptions for the MOV,” the report stated. “Improper or inadequate stem lubrication can result in excessively high friction that can cause the torque switch in the MOV to trip the motor actuator and stop the valve operation. Inadequate lubrication can also cause excessive wear and degradation of the MOV stem nut such that the actuator cannot move or control the valve stem.”
The NRC again raised the issue at the MOV User’s Group in January, 2013 when they referred to an LER at Comanche Peak that cited an April 2011 failed surveillance on a PORV block valve, where the failure was found to be due to inadequate lubrication of the stem and stem nut over a 10 year period due to inaccessibility.
The Solution
To date, the only practical way of relubricating the stem and stem nut of an MOV requires stroking the valve back and forth or diassembly. There is now a more efficient option offered by MRG Labs called the StemThiefTM.
After significant design and testing, MRG Labs introduced the StemThief at the 2013 MOV User’s Group meeting. A simple and straightforward design, the patent-pending StemThief uses the stem protector mounting threads in the actuator housing to introduce a device that creates a seal against the stem nut. This allows new fresh grease to be pumped into the stem/stem-nut thread area, purging out the old grease and particulates, establishing a new lubricant film for the threads. The relubricator tube section of the StemThief is sized to slide over the stem without impacting it, and a Zerk fitting mounted on the tube allows the new grease to be introduced with a manual lever grease gun.
This device has been tested in an SMB-0 Limitorque actuator, with a G2 fit for the stem threads. Additionally, a “worst-case scenario” was simulated by placing the assembled stem and stem nut, with a generous coating of MOV Long Life NLGI 1, into an oven at 215 deg Celsius (420 deg F) for 14 hours to dry out the grease, and create a degraded grease and varnish layer on the surfaces. Even after this degradation, when the stem was allowed to cool to room temperature and reinstalled in the test stand actuator, the StemThief successfully displaced most of the old grease, and established a new grease coating on the threads.
Grease Analysis
To address the other issues raised in IN 2010-03, a grease sample can be obtained from the purged grease at the far end of the stem nut. A special kit is available from MRG that follows the guidance of ASTM D7718, “Standard Practice for Obtaining In-Service Samples of Lubricating Grease.” to gather a 1 gram sample of the discharged grease. This sample can be analyzed using the Grease Thief®. With the one gram of grease, which is close to the amount that is effectively coating the stem threads in the interface area, analysis can provide insight into particulate
levels, both wear and contaminants, consistency changes (hardening or softening from the
new fresh grease baseline), and oxidation of the grease. The condition of the grease, as well
as wear of the threads can be monitored in this manner, and the NRC guidance to “assess
lubricant performance in MOV applications” can be systematically and objectively achieved.
StemThiefTM– US Patent 9,360,130
StemThief 