1. Field of the Invention
The present invention relates to acme thread wear, particularly stem nut thread wear for a valve having a threaded valve stem and a threaded stem nut that moves the valve stem responsive to a rotation of the stem nut. More particularly, the present invention relates to a method and apparatus for detecting stem nut thread wear for any valve that has a threaded stem operated by rotation of a stem nut and wherein this apparatus is not an as-manufactured part of the valve to be inspected.
2. General Background of the Invention
A motor operated valve or MOV equipment can employ an externally-threaded rising stem. This rising stem is operated by an internally-threaded rotating stem nut. One of the problems that plagues pipelines, nuclear plants, water treatment plants and other such facilities is the failure of the stem nut while it is in service.
A MOV stem nut is typically made of a softer material (such as a bronze alloy) than the material used to make the stem (usually carbon or stainless steel). Stem nut thread wear is preferential to having wear occur on the valve stem (as the stem nut is typically much easier to replace than the valve stem).
With use, the stem nut threads will wear when opening and closing the valve. The wear can be influenced by several factors. These factors can include the stem nut material, normal operating loads, and maximum loads for example.
Other factors that influences wear are the number and frequency of valve strokes, stem nut manufacture, threaded length, stem-to-stem-nut fit, valve stem condition, lubrication and environmental contamination.
On rising-stem valves, a stem nut transfers rotational motion (torque) of a motor operated gearbox or manual valve to axial stem movement (thrust). Stem nut failure will prevent valve operation and may cause valve position indication to be incorrectly displayed in a control room. If the motor operated valve is electrically or mechanically interlocked, its failure can also interfere with the operation of other plant equipment resulting in a potentially costly or catastrophic failure event.
Unfortunately, most MOV users do not take a proactive approach to maintaining their stem nuts because they are difficult to access and intrusive to remove and inspect. Also, removing a stem nut will cause the valve to be inoperable and out-of-service during the inspection. In most cases, a failure may take many years to occur promoting an “out-of-sight, out-of-mind” approach to maintaining them.
Even if stem nut removal for inspection is being performed, measuring the stem nut thread wear on a removed small-diameter stem nut can be very difficult as space is limited inside the stem nut to get a direct measurement with dial calipers.
Excessive stem nut thread wear represents a potential common cause failure mode that could impact all rising stem valves. The unexpected failure of stem nuts, and resulting consequence, emphasizes the importance of improving conditions, monitoring maintenance practice activities, and identifying, quantifying and minimizing stem nut thread wear. It is important for any maintenance program to also detail proper stem cleaning and lubrication procedures.
The valves to be tested are not always gate valves, but any valve using a threaded stem and nut.