NDE & inspection
Simplifying stem nut inspection
23 April 2012A new tool provides a new method for checking stem nut thread wear, a common cause of motor-operated valve failure.
Stem valves are used in a variety of applications in nuclear power plants, including safety systems. There could be as many as 400 motor operated valves (MOVs) with rising stems in a nuclear plant unit; as many as 40% may be safety-related.
On rising stem valves, a stem nut–typically made of a bronze alloy material–transfers motor-operated or manual valve rotational motion (torque) to axial movement (thrust), of a hardened stainless steel valve stem. Over a period of time, stem nut threads will wear when opening or closing the valve; eventually this could lead to valve failure.
A number of factors can affect the extent of stem nut wear, including the type of material it is made from, its operating conditions (such as loads, number of rotations, frequency of valve strokes) and its lubricant.
Excessive stem nut wear represents a potential common-cause failure mode that could impact all rising stem valves. When a stem nut fails to operate, it will prevent valve operation and may cause an inaccurate display of valve position in a control room. For motor-operated valves that are electrically interlocked, the situation can also result in a costly or catastrophic failure event.
In 2003, following a failure of a main steam safety relief valve at Hatch unit 1, the NRC issued an information notice concerning stem nut wear (IN 2003-01). The Hatch licencee (Southern Nuclear Operating Company) believed the failure of the valve was related to the manufacturing tolerances of the valve stem and piston assembly and to the lengthy service time (20 years) without adequate inspection and maintenance.
This emphasizes the importance of preventative maintenance activities in identifying, quantifying and minimizing MOV stem nut wear. Periodic stem nut removal and inspection or measuring thread backlash may be beneficial to prevent failure. Further, it is important for the plant maintenance programme to detail proper stem cleaning and lubrication procedures.
Stem nut inspection
Inspection of stem nuts is often a costly and time-consuming process as it requires the valve to be taken out of service. Stem nut removal itself could take 2-8 hours (4-16 man hours), according to Chuck Reames, SNAP business manager at The Shaw Group, and workers could be exposed to radiation.
Diagnostic testing typically involves either stem position detection devices, or expensive strain gauges. The former are used to indicate that the valve is in the correct position. It will tell if a stem nut has failed, but not when it will fail.
Two Crane Nuclear diagnostic systems, the Viper 20 and Votes Infinity, can detect stem nut wear. Votes Infinity was launched in 2010, and has more advanced electronics and software than the Viper 20 system launched in 2000. Both systems monitor and analyse many aspects of motor-operated valve and check valve performance.
The systems measure forces acting on the valve stem itself with a strain gauge. Product manager George Smith says that by measuring (in milliseconds) the time from when the stem nut is pulling to when the stem nut is pushing on the stem (the ‘zero force plateau’), it can determine the clearance between the valve stem and stem nut threads. Smith admits that the method cannot deduce whether an increasing zero nut plateau measurement comes from stem nut wear or from other factors such as a loose stem nut locknut.
But he adds that a loose locknut is as much a risk as stem nut wear; if the locknut backs out of the top of the valve, it could make the valve inoperable. Regardless of cause, an increasing zero plateau trend ‘still means that we have to go and look for where the problem is,’ he says.
Snap solution
Construction and industrial services provider Shaw Group’s industrial maintenance engineering team has developed an innovative new tool to measure stem nut wear that does not require removing the valve from service. The Stem Nut Analysis Protractor (SNAP) tool was invented at Louisiana Offshore Oil Port (LOOP) and has successfully measured stem nut wear on over 250 valves. A comparison of SNAP data with manual measurements using destructive tests with calipers found that the SNAP tool and method measured wear within about 10% (tending to overestimate wear) for nuts with wear ranging from 10% to 88%. While SNAP has not yet been trialled in any nuclear power plants, co-inventor of the tool Chuck Reames says that Shaw is now developing SNAP into a service business.
The SNAP tool measures the relative axial distance between the stem nut threads and the valve stem threads. From this it is possible to quantify the remaining stem nut thread metal. The tool mounts on the stem nut and measures stem nut free rotation (backlash) as the valve is opened slowly. An indicator shows stem movement and gauge indicates the percentage of worn stem nut thread. It is possible to back-calculate the remaining stem nut thread thickness from the percentage value. At the moment there is one constraint: the SNAP tool can only be used on valves where the stem is pointing straight up.
Operators can use the percentage wear figure to determine whether or not a stem nut should be replaced. Replacement really depends on the type of services the valve is in and the plant preference, Reames says. At LOOP, stem nuts with SNAP test results of 40% put on a watch list and replaced at 50%.
Smith at Crane Nuclear points out that past stem nut failures have been caused by poor lubrication. For motor-operated valves, he recommends MOV Long Life lubricant from Canadian manufacturer Forsythe. He adds that the Votes Infinity system can determine the valve's coefficient of friction (based on direct measurements of stem diameter, stem thrust and torque), and monitor changing trends. According to Smith, the coefficient of friction of a well-lubricated stem nut is 0.1, and for metal-to-metal rubbing is 0.2.
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This article was published in the April 2012 issue of Nuclear Engineering International magazine.
