Fig. 1. Much of a control valve's TCO is typically spent on maintenance.
Fig. 2. SPIRA-TROL valves enjoy up to 30 per cent longer stem seal life than most other valves.
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How to pick control valves
Almost any organisation can benefit from using valves that keep their operational expenditure to a minimum. The key lies in sourcing valves that will give the lowest life cycle cost. Companies should avoid buying decisions based on the initial price of a valve and look instead at their total cost of ownership (TCO). Much of a control valve’s TCO is typically spent on maintenance (Fig.1). The cost of an unplanned shutdown of a process to replace a poor quality valve that has failed will far outweigh any capital costs saved at the time of purchase. So it is vital to buy valves that will give long service life and low maintenance costs, since these tend to outweigh any difference in initial cost. Valves that can be maintained and restored to duty quickly help to limit production delays. Easier, faster maintenance also frees up engineering resources to be deployed elsewhere. Look out for valves that can be maintained without removing them from the line and the ability to replace components without using special tools. When the pressure is on, the last thing maintenance staff need is to spend time hunting for a misplaced tool. Check for self-aligning, clamp-in-place internals. These are unlikely to cause problems by seizing in place, which can be an issue with screw-in components, requiring the valve to be removed from the pipeline and incurring a lengthy plant shutdown. Keep it simple Not only do simple maintenance procedures reduce training costs, they also help to ensure that any maintenance work done will restore the valve to its full operating capability. Complex maintenance routines carry a higher risk of valves not being re-assembled correctly, potentially reducing their performance. This can cause poor process efficiency and even lead to further maintenance to correct the error. Control valves should deliver years of reliable service before major maintenance, such as replacing the stem seal, is required. This figure is typically about five years, but could be ten years or longer depending on the application. Valves on lines carrying fluids with suspended particles may need maintaining more frequently than valves in less harsh applications. Longer service intervals Some valves have design features to minimise the impact of arduous conditions, such as wipers to prevent particles getting into the valve stem. These offer much longer service intervals. Hard trim materials and a design that keeps high flow velocities away from the valve body can minimise internal erosion and further extend valve life. It may not happen often, but occasionally there may be a major change in process parameters that requires new valves. Or a process line may shut down completely. In such situations, control valves that use a modular design carry a cost advantage because they can be reconfigured to suit new demands relatively easily. Without doubt, this could easily save the cost of purchasing several new valves. Modular valves also help to cut spares stockholding costs because components can be used with several different valves. Accurate control is important for the efficiency of any process, but it can also affect a valve’s maintenance costs. Smooth control reduces the amount of valve movement and wear, as well as reducing associated costs, such as compressed air consumption. High quality, well-designed valves can help to achieve smooth process control under all conditions. Smooth control leads to reduced energy use, improved product quality, less stress and longer life. Take time to look beyond the valve specifications and review the supplier and the level of support it provides, particularly when specifying valves. The biggest impact of a poorly chosen valve will be poor process performance, leading to low productivity and poor product quality. The running costs of a badly specified valve may also be excessive. A valve that's too large for a given duty, for example, will operate more closely to its seat and experience more trim wear and poor control. Equally, a valve body that is too small may constrict the flow and lead to high velocities, reducing its life. An incorrectly sized and selected valve may also be subject to cavitation, which can rapidly wear the valve body and internals, or create excessive noise at high flow. So it makes sense to use the vendor’s technical expertise to help get it right. Pick a vendor that has the in-house expertise and experience to provide support whenever needed. Another aspect of the support service is rapid delivery. Fast delivery is not only essential when a plant failure or change in production occurs, but also eliminates the need for an organisation to hold several spare valves in stock. One low-cost solution Extended life and simple maintenance are the two key virtues of Spirax Sarco’s new SPIRA-TROL two-port general service control valve. Meeting the needs of the modern flexible process plant, Spirax Sarco’s extensive site and laboratory testing has shown that SPIRA-TROL valves enjoy up to 30percent longer stem seal life than most other valves (Fig.2). Coupled with simple maintenance procedures, this durability helps to achieve a very low TCO. The Spirax Sarco SPIRA-TROL range is available as EN versions from DN15 to DN100 sizes and as ANSI versions from 1.2-in to 4-in sizes. They can be used with electric or pneumatic actuation, fitted with proprietary positioners and therefore easily interfaced with an external control system. Specifying valves – a quick checklist When specifying valves, be sure to ask the following questions. Get a ‘yes’ to all or most of these questions and you are well on the way to reducing your site’s total valve costs:
Rick Plummer is Spirax Sarco's UK Engineering Manager. For more information, visit www.spiraxsarco.com/uk |
