How to pick control valves with low life cycle costs
Major processing sites often have valve populations running into the thousands. These organisations have well-developed processes for sourcing valves with low running costs to keep their OPEX (Operational Expenditure) to a minimum. More modest operations with fewer valves can benefit from a similar approach, thus making substantial year on year maintenance cost savings.
The key lies in the proper sourcing of valves that will give the lowest life cycle cost. The cost of prematurely shutting down a process to replace a poor quality valve that has failed, will far outweigh any capital costs saved at the time of purchase. So, in short, companies should avoid basing their decision solely on the initial price of the valve and look at the total cost of owning the valve.
The following criteria serve as a basic guide to purchasing valves with a low total cost of ownership. If a company employ them, there is a good chance that they will be rewarded with lower maintenance costs.
Aim to get best value for money
It has been estimated that more than 70% of a control valve’s TCO is spent on maintenance. Purchasing valves that will give long service life and low maintenance costs can far outweigh the initial attraction of a “value engineered” valve, with a lower capital cost.
Search out valves that are quick to maintain
Valves that can be maintained and restored to duty quickly help to limit costly production delays, particularly when the maintenance is unplanned. A quick replacement of the stem seal, for example, means less downtime and less lost production. Rapid maintenance procedures also mean less time is spent on valves, enabling engineering resources to be deployed in other areas.
Even when the maintenance is planned, this is often scheduled during plant shutdowns when maintenance personnel have a heavy workload to achieve in a short time. Minimising the time spent on valves reduces the need for costly overtime pay or the hiring of thirdparty contractors, incurring extra costs.
Points to watch out for when sourcing valves include maintenance that is possible without removing the valve from the line and the ability to replace components without special tools. When the pressure is on, the last thing maintenance staff need is spending time hunting for a misplaced tool.
Look also for self-aligning, clamp-in-place internals. Valves with screw-in components can suffer from the seats seizing in place, requiring the valve to be removed from the pipeline and incurring a lengthy plant shutdown.
Insist on simple maintenance procedures
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.
Size does matter
Ensure control valves are correctly sized. Approximately 50-70 per cent of control valves are oversized which leads to capital, operational & maintenance over expenditure. Make sure that the purchasing parameters are not decided upon with over enthusiastic or compounded safety factors.
Look for long maintenance intervals
Buyers of control valves expect several years of reliable life without major maintenance being required, such as replacing the stem seal. 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 or operating in dusty atmospheres may need maintaining more frequently than valves in less harsh applications.
Valves that have design features to minimise the impact of such conditions, for example by including wipers to prevent dust and particles getting into the valve stem, are likely to offer much longer service intervals throughout their life. Hard trim materials and a design that diffuses the discharge from the plug and seat and reduces internal flow velocities to minimise internal erosion, further extends valve life.
Ensure it’s easy to change the valve’s duty
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.
Consider the less obvious benefits of accurate control
Accurate control is of course important for the efficient running of any process. The use of positioners to provide precise valve movement contributes significantly to higher productivity and lower energy costs with their environmental benefits, as well as reduced maintenance. Accurate and smooth control tends to reduce the amount of valve movement and wear, as well as decreasing running costs such as compressed air consumption.
High quality valves that are well designed using modern design tools such as Computational Fluid Dynamics, can achieve smooth fluid flows under all conditions. Smooth flows lead to reduced internal wear and longer service life.
Good technical support to rely on
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 is too large for a given duty, for example, will operate more closely to its seat and experience more trim wear. Equally, a valve 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 and noise in the flow, which can rapidly wear the valve internals.
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.
Tel: 01242 521361
Published in Valve User Magazine Issue 20
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