Control valves should be properly matched to the chemicals they come into contact with. Kieran Bennett, Bürkert field segment manager for hygienic/food & beverage applications explains.
A significant change increasingly seen in the valve market, particularly for hygienic applications, is the focus on the chemical composition of the materials used in valve construction. Today, perhaps half all users of process valves require a materials certificate, while nearly all diaphragm valve users consider it a necessity. In some cases, such as the pharmaceutical sector, every valve that comes into contact with the process media requires full documentation and traceability.
This increased requirement is unsurprising. Not only does it give the customer reassurance, but it meets increasingly strong regulatory compliance demands. In sectors such as food and beverage and pharmaceutical, a valve user needs to be able to prove the chemical composition. For the manufacturer of a PTFE diaphragm valve, for example, this means having traceability to the original batch of manufacture of the chemicals and the quantity used.
The chemical composition of a valve, mainly consisting of its body, seal and potentially the diaphragm that creates a barrier with the media, is an important factor for the quality and safety of the end product. Food and beverage manufacturers need to prove that there’s no shedding of traces of valve components, which can contaminate the end product or affect its taste or odour. The suitability of valve materials for long-term use with the media they come into contact with is fundamental.
The other main concern in valve material specification is resilience to the chemicals the valve comes into contact with. For this reason, proving origin is also an important factor for customers to validate valve quality. For example, a manufacturer can be asked to provide a 3.1 material certificate to identify the carbon content of a stainless steel valve body. Lower quality valves can use stainless steel with a higher carbon content, generally making it more brittle and less robust, so validation is a sensible request.
Assuring chemical compatibility can also be a matter of experience. In many dairies, nitric acid is used as part of the clean in place process. In a ball valve, typically two carbon graphite seals are used in the end caps. However this material isn’t suitable for long-term use with nitric acid, and the seals will degrade, reducing the life of the valve. This is a commonly overlooked example; instead Bürkert would use an angled seat valve with a PTFE seal, suitable for use with nitric acid and capable of achieving a seven million cycle lifetime. It is important to select the body and seal materials most effective for use with the chemicals in each specific application.
PTFE is commonly used for seals and diaphragms, as well as valve bodies. It is an ideal choice for clean in place applications. Its carbon-fluorine bonds make it inert and therefore ideal for use with reactive and corrosive chemicals. It is, however, susceptible to cold flow, which is distortion under the stress of high temperature or temperature fluctuations. Advanced PTFE can instead be specified if the application includes extreme temperature changes or frequent sterilisation.
Alternatively for diaphragm use, GYLON is a third generation PTFE, which is used in the a wide range of applications. With increased resistance to stress, it can be used with greater fluctuations in temperature as well as higher temperatures, and has a longer life. EPDM is also frequently used for diaphragms and seals because of its resistance to ozone and hot water, as well as FKM which is often selected for its resistance to oil.
Valve bodies are frequently brass or stainless steel, or a suitable hard plastic variant with sufficient impact resistance. Polypropylene and polyethylene are commonly selected for their resistance to various organic solvents, acids, bases and salts while polyamide is appropriate for use with grease, oils, waxes and fuels. Meanwhile, PPS is ideal for use in high temperatures above 200°C.
The chemical composition of a valve’s components and the media flowing through it are critical factors for a wide range of applications. The challenge can be complex, so engaging the right application expertise is well advised.