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Getting the balance right

20 January 2020

There are many contributors to poor temperature control – for example leakage. However, significant improvements could be made here by using a different approach to control valve specification, says Darren Silverthorn

Industry faces a plethora of choices when it comes to materials, component options and the consequential benefits they can each bring – such as improving uptime, improving productivity reducing costs, increased energy efficiency and the like. The abundance of choice can make it a daunting task to decide upon the right solution for a particular process, and it is not uncommon for engineers to default to a familiar option if it is unclear that there are alternatives which could make improvements. 

One of the main challenges in the food and drink industry is to minimise product spoilage and wastage. With many companies operating near maximum capacity, process uptime is key. Poor temperature control can result in product spoilage, product burn-on and increased clean-in-place (CIP). Poor temperature control is often caused by control valve leakage and can be identified by these symptoms. However, the root cause of the problem is not always crystal clear and this is where control valve specification has a lot to answer for. By reducing leakage, many processes could not only benefit from better control, but also subsequently negate the need for additional components such as automatic isolation valves. 

Best in class 
Class leakage refers to the leakage classifications defined by ANSI FCI 70-2 – an industry standard followed by control valve manufacturers. For globe valves, it defines the amount of leakage allowed between the plug and seat when the valve is closed. The most common leakage classes are class IV and class VI, and the higher the number, the lower the leakage. Different plug and seat materials are used to achieve the required leakage rate with metal typically used for class IV and soft materials such as PTFE used for class VI.

Metal seats and plugs tend to be the default for food and drink applications as they are produced in a variety of stainless-steel grades, of which the benefits are widely understood across all industries. However, with a metal seat and plug it is not possible to achieve class VI leakage. 

In general terms, the softer the material, the better the prevention of leakage, therefore for class VI leakage, materials such as Polytetrafluoroethylene, or PTFE are used. But these materials tend to be less robust and therefore more prone to damage. 

Engineers operating within the food and drink industry will be familiar with PTFE – a common example of a fluoropolymer material used for its sealing properties, low coefficient of friction and chemical resistance. Although PTFE is recognised for the properties mentioned above, it is restricted by its maximum operating temperature and robustness. Both important considerations for steam-using applications.

For these reasons, many will default to more robust and commonly used materials, such as stainless-steel, and will prevent any increased leakage by installing automatic isolation valves. It is in response to this that the more sophisticated PEEK solution has evolved, and where a major window of opportunity has opened for food and drink manufacturers to enhance their process control. 

What is PEEK? 
PEEK – or PolyEtherEtherKetone – combines exceptional wear, chemical and heat resistance with the ability to provide class VI leakage. It can be continuously exposed to fluid temperatures up to 250°C and a maximum differential pressure of 19 bar. 

When working with steam, you need to know that the components responsible for controlling it are reliable and these properties show PEEK can provide the reassurance required. Put simply, it comes with all the combined benefits of metal and PTFE seats discussed above, but with zero compromise.

Specifying a PEEK seat to reduce leakage and provide better process control, will also remove the need for additional automatic isolation valves. Although, it’s important to remember that a dedicated isolation valve is always needed for safety and maintenance, so essentially it is possible to reduce from three valves to two. This reduction can be made without compromise on safety through the use of all-encompassing double block and bleed isolation valve units, such as Spirax Sarco’s SafeBloc solution. 

By making this move, maintenance and process managers are not only removing the upfront cost of the additional valve, but also the ownership cost in the way of maintenance and compressed air or electricity consumption. By increasing the reliability of the process, it also enables peace of mind that leakage is much less likely occur, therefore reducing the resulting effects of product spoilage and downtime.  

With steam being crucial to thousands of food and drink processes, it is easy to see why metal has always been the go to choice over PTFE. However, with PEEK having already proven itself in more abrasive and higher temperature environments adhering to specific regulations, it really is becoming the challenger material for maintenance and process engineers in food and drink to consider. I would urge anyone responsible for control valves in this space to find out more. 

Darren Silverthorn is national controls and metering specialist at Spirax Sarco.

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