Measuring Stem Nut Wear Using the SNAP Process

Other factors contributing to stem nut thread wear include the stem nut material, normal operating loads, maximum loads, number and frequency of valve strokes, number of stem nut rotations during valve stroke, stem length, stem to stem nut fit, valve stem thread condition, lubricant use and environmental conditions.

On rising stem valves, a stem nut transfers motor-operated or manual valve rotational movement (torque) to axial stem movement (thrust). When a stem nut fails, it prevents valve operation and may cause an inaccurate control room display of valve position, potentially leading to a costly or catastrophic failure event.

Motor operated valves are used to remotely open and close valves in industrial plants. Some valves have more critical applications than others, and failure could cause a serious environmental or safety event, cross-contaminate a product or shut off a city’s water supply. If stem nuts are not periodically inspected, actual wear will be unknown until failure occurs.

Depending on its size and location, stem nut removal could take eight to 16 work hours. If the threads strip, stem nut replacement could take twice as long. A major downside of stem nut removal for inspection or replacement is the valve will be inoperable and out of service during such time.

MOV diagnostic testing began in the nuclear power industry where the Nuclear Regulatory Commission (NRC) requires proof of operability of safety-related MOVs; however, measuring stem nut wear is not yet required. Many nuclear plants understand their vulnerability with stem nut wear and are using diagnostics or direct inspection to mitigate exposure.

MOV diagnostic equipment is used to retrieve stem thrust, torque, motor current and switches data. Diagnostic readings, combined with analysis of several variables, allow technicians to estimate stem nut wear. This method presents accuracy issues because it is difficult to determine exact motor speed and reading can’t filter out other non-thread wear movements.

Unfortunately, other industrial MOV users may not be as proactive in their approach to maintaining stem nuts, mainly due to high MOV diagnostic costs or the intrusive nature and cost of visual inspection of stem nut threads.

The Stem Nut Analysis Protractor (SNAP) process is a cost- and time-effective alternative because it provides a fast, accurate and non-intrusive method to quantify stem nut thread wear. The SNAP process does not require removal of the stem nut, and the valve can remain in service while conducting the analysis. It directly measures the backlash between the stem and stem nut threads on a rising-stem valve, which nullifies the effects of a loose stem nut lock nut and stem rotation.

The SNAP process converts the stem nut backlash to an angular value and displays it in percent wear. As stem nut wear increases, so does the amount of free rotation.

The SNAP process takes 15 to 60 minutes to perform and can be used to measure stem nut wear on almost any rising-stem valve using a stem nut. Testing has been performed on valves with stem sizes from 1 1/8-inch to eight inches in diameter.

Since this process was developed at LOOP LLC, more than 300 stem nuts have been tested. There have been no stem nut failures on any of those tested and about 25 stem nuts were found which measured 50 percent or greater wear.

The scale used on the protractor shows wear in three sections. Green, or zero to 30 percent, is an acceptable range of wear. Amber, or 31 to 50 percent, indicates the stem nut should be placed on a watch list. Red, or 51 to 100 percent, flags the stem nut for replacement. This system works well for plants using a majority of ANSI 300 slab gate valves. Clients may prefer a different acceptance scaling when used with other ANSI ratings or valve designs.

Chuck Reames is business manager at The Shaw Group, Inc. which developed/co-invented the SNAP process (Patent Pending. Covered by U.S. Patent Publication No. US-2012-0060745-A1). Reames has gained extensive experience in MOV diagnostics and analysis in the nuclear power industry and was working at LOOP LLC Reliability Department when this process was developed. For more information on the stem nut analysis process, contact Reames at chuck.reames@shawgrp.com.