Key points
When it comes to temperature-critical processes, poor temperature control can be a result of a number of different factors. With experts highlighting the significant improvements possible through a different approach to control valve specification, Darren Silverthorn, National Controls and Metering Specialist at Spirax Sarco explores the huge benefits soft seats, and specifically materials like PEEK, can deliver in demanding food and drink manufacturing environments.
There are a number of options when it comes to material and components available to engineers, leading to benefits such as reduced costs, increased energy efficiency and improved uptime and productivity. However, selecting the correct solution for your specific process can be a daunting prospect.
Indeed, faced with this choice, it is common to default to familiar options – especially if the benefits of potential alternatives are not immediately apparent, or if you didn’t know such benefits were even possible in the first place. With that in mind, let me explain further.
Ensuring process uptime
Minimising product spoilage and wastage is a key challenge in the food and drink sector. With many manufacturers within the industry operating at close to maximum capacity, ensuring process uptime is crucial.
Poor temperature control can result in product spoilage, product burn-on and increased CIP (clean-in-place), which is a major frustration. Yet what many may not be aware of is that poor temperature control can often be caused by control valve leakage, which can be identified by symptoms such as those specified above.
However, the source of this problem may not always be easily identifiable, which is where control valve specification comes in. Through correct control valve specification, engineers could reduce leakage and improve control, while also making additional components like automatic isolation valves unnecessary. Taking this into account, let’s continue onto the topic of control valve leakage classifications.
Knowing leakage classifications
Class leakage refers to the 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.
Class IV and class VI are the most common leakage classes, and as the class number increases, so leakage falls concurrently. Numerous plug and seat materials are available to engineers looking to achieve the required leakage rate, with metal usually chosen for class IV and soft materials such as PTFE used for class VI.
The need for soft seats in food and drink
For food and drink applications, specifiers most often select metal seats and plugs. This is because they are produced in a wide array of stainless steel grades, the benefits of which are commonly known and understood. However, it is not possible to achieve class VI leakage with seats and plugs made of this material.
In general terms, the softer the material, the better the prevention of leakage. With this in mind, materials such as Polytetrafluoroethylene, or PTFE, are used for class VI leakage. Yet this material has possible drawbacks in this application, as it tends to be less resilient and therefore more prone to damage.
Engineers in the food and drink industry may be familiar with PTFE as a fluoropolymer material used for its sealing properties, chemical resistance and low friction coefficient.
Despite such appealing properties, PTFE can also be restricted by its robustness and maximum operating temperatures, which are important considerations for steam-using applications.
Because of this, it is understandable that engineers may default to hardier, commonly used materials like stainless steel, and offset increased leakage by installing automatic isolation valves.
Sophisticated PEEK solutions have been developed in response to this situation, and can offer a major window of opportunity for food and drink manufacturers looking to enhance their level of process control.
What is PEEK?
PEEK – or PolyEtherEtherKetone to give it its full scientific name – 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.
A key aspect of working with steam is having peace of mind about the reliability of the material being used to control it. Such properties show PEEK can provide this level of reassurance, combining the benefits of metal and PTFE seats discussed above, but without the compromises of these solutions.
Reducing valve requirements
Those specifying PEEK seats to improve process control and reduce leakage also no longer need additional automatic isolation valves. Whilst a dedicated isolation valve is crucial for safety and maintenance purposes, opting for PEEK essentially means reducing your amount of valves from three to two. This can be done without compromising safety through the use of all-encompassing double block and bleed isolation valve units.
By choosing this option, maintenance and process managers are reducing the total cost of ownership associated with maintenance, compressed air and electricity consumption, while also negating an additional valve’s upfront costs.
The improved process reliability realised by making such a move also reassures that leakage is much less likely, therefore reducing the resulting effects of product spoilage and downtime.
Steam is vital to thousands of food and drink applications, so it is easy to see why many specifiers have traditionally opted for metal over PTFE.
However, with PEEK having already proven itself in more abrasive and higher temperature environments adhering to specific regulations, it is continually growing in status as a challenger material worthy of consideration from maintenance and process engineers in the food and drink sector.
I would urge anyone responsible for control valves in this space to find out more.
