Control valves and pumps make up the “control element” category of the control loop. A control loop can only be reliable if the instrument, controller and control element are reliable. There is a considerable amount of information available on pump reliability. Most of it involves vibration and lubrication. Control valves are equally important for a reliable process. Both the mechanical and instrument portion of a control valve must be addressed in order to ensure valve reliability. I have concentrated on the mechanical side for this post.
Valve packing tends to be the most common mechanical failure with a control valve. As the stem moves axially on a sliding stem valve, or rotates on a rotary valve, through the packing can develop a leak. Historically, a common practice has been to routinely tighten the packing on valves. The problem with this is twofold. It will shorten the packing life and require a premature packing change. It also creates more work for the actuator and generally results in poor performance – the pressure needed to stroke the valve increases which causes the valve to overshoot the set point. A better strategy is to use a valve positioner which will alert you to needed packing maintenance. This will be presented in a future post.
Rather than packing, a bellows seal can also be considered. While these have often been used for hazardous service, designs and technology have greatly improved their performance. Eliminating packing, and subsequent increased packing friction, prevents leaks and improves performance. It should be noted a hydroformed, rather than welded design, should be used as it will provide an improved cycle life. A submerged bellows can be used in applications with erosive fluids. Non-metalic bellows are also available for chemical applications.
Cavitation and flashing can wreak havoc on a control valve. In order to mitigate these problems, a hardened plug, stem and seat are generally used. However, hardened materials can be quite expensive and making these parts out of a single material is cost prohibitive. Using a less expensive base material and then facing them with a hardened material is a cost effective way to achieve the desired performance. The stem, plug and seat should be the same material in order to prevent galling and sticking.
While is doesn’t relate directly to reliability, noise in a control valve is a common problem. While trims have been developed to mitigate noise, these can be expensive and decrease the flow capacity of the valve. This may require you to use a larger valve, which decreases performance. It is important to note that noise is developed downstream of a valve. If it is possible to move the valve close to a vessel (or outside wall for an indoor process) this can greatly reduce the noise realized in the process.
Mechanical reliability of a control valve is critical to overall process reliability. There are a number of mechanical concerns that should be addressed. As previously mentioned, future posts will address reliability considerations for the positioner. You will notice that many of same strategies that have been presented for instrumentation are quite relevant to process control valves.
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