View from the Other Side

Published: 3rd February 2017 | Issue 40 Share article:

Chris Warnett CEng MIMechE, is president of CPLloyd Consulting Inc. (www.cplloydconsulting.com.), providing marketing and applications expertise for the valve automation industry and its customers. He has over 38 years of engineering, sales and marketing experience in valve automation. Chris is the author of the Amazon.com best-selling book “Valve actuators”. The book can be found online at www.createspace.com/5327931

For the Want of a Stem Nut

It is a fact well known that all mechanical devices wear out eventually. For any machine, at the end of its life, the ideal outcome would be to have all parts wear out or fail at the same time, this would give the maximum operational life to the machine. But in our imperfect world, some parts are bound to fail before others.

Fortunately, with rigorous design discipline most of the weak links can be designed out or mitigated by redundancy, but there is inevitably one weaker link in the chain of reliability. If this single component has no redundancy and is susceptible to wear or neglect, then failure of the machine will occur. Such a situation is present in the ubiquitous rising stem valve.

The vulnerable component is one that is not even supplied by the valve manufacturer on an automated valve. It’s the lowly actuator stem nut and the failure of a stem nut could have dire consequences.

With apologies to Benjamin Franklin:

• For the want of due care the stem nut was lost
• For the want of the stem nut the valve was lost
• For the want of the valve the pipeline was lost
• For the want of the pipeline the process was lost
• For the want of the process the plant was lost …….

You get the idea; it culminates with the apocalypse. The simple valve stem nut is a very effective way of transforming the rotary motion and torque of a valve actuator into the linear motion and thrust needed to operate a sliding stem valve. But effectiveness does not necessarily mean efficiency.

The efficiency of the stem nut is a direct function of the helix angle of the thread. The steeper the angle the more efficient the mechanism. However, the tradeoff is in thrust generated and stability. The greater the angle the lower the mechanical advantage to produce thrust. Also the stem nut mechanism may become dynamically reversible above 6 degrees of helix angle. When reversibility occurs, the valve stem may move unintentionally. So, for it to be effective, the helix angle and the associated mechanical efficiency of the stem nut is deliberately kept low, nominally between 15% and 40% efficient.

For example, this means that an actuator generating an output of 1,000W and using a 30% efficient stem nut mechanism will only deliver 300W to the valve stem.

So what happens to the other 700W?

Unfortunately, most of that energy is transformed to heat at the contact point of the threads between the stem nut and the stem. This is not good news, heat tends to break down lubrication and less lubrication means more friction, which in turn increases the heat generated. The net result is accelerated wear at the stem nut.

If left unchecked, this spiral of deterioration will lead to complete failure of the stem nut. When the threads are stripped out of the stem nut there is a loss of control over the valves positon. For example, a gate valve may not be able to be opened or a heavy sluice gate may fall several meters into its seat.

And for the want of a valve … And so on.

Some valves are exposed to wind bourn dust or sand, these environments compound the vulnerability of the stem nut mechanism to rapid wear and possible failure.

The regular maintenance of the stem and nut mechanism is therefore a prominent necessity for plant service personnel. The visual inspection of the stem to ensure cleanliness and proper lubrication is essential. However, there is very little science employed in monitoring the rate of the stem nut wear.

Because the stem nut is inside the actuator mechanism and inaccessible because of the presence of the valve stem, it cannot be directly measured. Certainly the actuator and stem nut could be removed for an internal caliper measurement, but that is very time consuming, and the automated valve would have to be taken out of service, not a very practical solution.

One solution is to use one of the patented stem nut wear measuring devices that monitor the increase in stem nut backlash. These devices can be used by service technicians to directly measure the stem nut backlash on any valve in the field without removal of the actuator. This measurement then is fed into a spreadsheet to calculate the percentage wear. Regular measurement will indicate the rate of wear. Intelligent predictive maintenance can then be applied to the valve stem lubrication. In critical valves the replacement of the stem nut can be timed to ensure the valve is not taken out of service prematurely, which would incur unnecessary cost, yet no safety risk is taken for plant or personnel.

Despite the fact that so much attention is paid to factors such as; the connectivity and the constant monitoring of plant equipment, predictive maintenance, plant efficiency as well as health and safety legislation. It is surprising that there is so little done to monitor the actual condition of the lowly valve stem nut. There seems to be an assumption that all is well with the stem nut as long as the valve is functioning reasonable well. But should a stem nut fail, then the repercussions could be catastrophic, for the want of a stem nut…

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