One of the most important components of an automated threaded stem valve is the ubiquitous stem nut, a relatively simple and unsophisticated mechanical device that converts the rotary motion of an actuator into the linear stem movement needed to open or close one of the many types of gate valves or sluice gates.
The stem nut also converts the torque from the actuator into a direct linear force to overcome the pressure in the pipeline and the other forces that are resisting the movement of the valve.
On sluice gates, the stem nut performs a similar function, but the linear force generated is usually required to overcome the weight, friction and other forces on the gate to lift it to the open position.
Stem nuts can also be specifically designed to provide a locking effect. This allows the actuator to be dormant, leaving the stem nut to hold the stem safely in position regardless of any forces exerted by the valve or sluice gate.
The locking effect comes from the irreversibility of the stem nut mechanism. However, the down side of this effect is a very poor mechanical efficiency in terms of power conversion at the stem nut. This efficiency is typically about 40% for a non-reversible stem and nut combination.
For example, this means that for every 1 kW of power that an actuator delivers to a stem nut, only 0.4 kW goes into moving the stem. The other 0.6 kW is dissipated, most of it going to heat generated in the nut. This heat breaks down the lubricant on the stem and promotes wear on the stem nut and stem.
To mitigate this wear, the stem nut is often made of a metal dissimilar to the stem. The common selection for a stem nut is one of the bronze variants such as aluminum bronze. Stems are usually one of the many stainless steels, chosen to resist corrosion. The stem nut is commonly the faster-wearing component, and the stem is invariably longer than the nut, so it has more material to be worn out. But examining the stem for wear does not tell you how quickly the stem nut is wearing.
Frequently, threaded stems are exposed to the elements. This means that the effects of sun, rain, wind and ice, as well as dust and grit, impact the quality of the lubrication on the stem. Clearly this item is an important maintenance consideration. A survey of end users earlier this year conducted by CPLoyd Consulting confirmed the most common maintenance concern for users of automated threaded stem valves is the lubrication of exposed valve stem threads.
The Maintenance Stepchild
But who is responsible for the stem nut? Although it is may have been supplied by the actuator manufacturer, it becomes something of a maintenance stepchild once installed.
The valve or sluice gate stem is often visible and can be examined relatively easily and regularly. However, this is not the case with the stem nut. This item can only be properly examined if the actuator is removed from the valve, something that is time consuming and disruptive to plant operations.
The stem lubrication is usually the responsibility of the site maintenance crew. They are able to monitor the need for cleaning and re-lubrication of their stems depending on the environment and the frequency of valve operation.
Actuators don’t need a lot of maintenance, possibly a bi-annual PM (preventive maintenance) check. But there are very few PM programs that require the measurement of wear in the stem nut because removing the actuator is impractical. So this critical component falls into the gap of responsibility between the parties that conduct the regular valve and actuator maintenance.
The failure of a stem nut on a critical valve could cause a hazard for personnel as well as the environment. Fire, chemical spillage, co-mingling and other hazards are compounded by the loss of production and revenue involved in a failure.
For large sluice gates the physical impact caused by dropping a gate several feet could crack the structure around the gate as well as the gate itself. For tunnels and buried pipelines this could be catastrophic.
There have been many advances in the diagnostic capabilities of valve actuators. They are able to monitor torque demand through the valve travel, as well as obstructions and many other faults. But even smart actuators have no way of monitoring the wear on the stem nut. An approximation could be computed by counting the total number of output turns of the actuator once installed. But the rate of wear would have to be known for each specific application and even then it would be only an estimate of the wear on the nut.
It is possible to measure stem nut wear without removing the actuator. The principle involves determining the backlash between the stem and the nut. In order to perform this measurement, the valve stem and stem nut movement both have to be very accurately measured during a reversal of motion. This requires precise data from the valve as well as the actuator. It would be extremely difficult to incorporate sensors for both these measurements into the standard automated valve, especially given the variety of designs for both valves and actuators and the fact that they are usually manufactured by separate companies.
The downside risk involved in ignoring the wear rate of the stem nuts on important valves and gates is overwhelming when compared to the time needed to examine these important components.
The measurement of stem nut wear should be a fundamental element in all PM programs—eespecially on critical valves and sluice gates.