Published July 25, 2024
Polymer materials of construction expand capabilities of valves for specific applications.
By Alan Black, Equilibar
Choosing the optimal material of construction for a valve can be a consequential decision when designing fluid control systems. Some materials can handle corrosive chemicals while others are suitable for high temperatures. Some offer environmental advantages and others are cost effective. In advanced, complex applications, multiple factors are usually involved, so it is valuable to have a wide selection of materials.
Metals have been used to make valves since the Romans fabricated bronze plug valves to use in their waterworks, if not earlier. Even today, the majority of valves continue to be made of some type of metal, with materials ranging from inexpensive die cast to aluminum, brass, stainless steel and finally high performance high-nickel alloys such as Hastelloy and Monel.
This single-use valve is made of USP Class VI polymers with a reusable stainless steel cradle and is a particularly good choice for applications in biopharmaceutical manufacturing because the valve body can be gamma irradiated for sterilization prior to use.
At the same time, modern plastics such as PEEK, PVC, PTFE and polypropylene play an increasingly important role in valve design. These materials can allow valves to perform in service conditions that would have been impossible a few decades ago, including many pharmaceutical, semiconductor and chemical processes that have impacted countless lives around the world. Plastic valves will no doubt continue to enable cutting edge innovation even as scientists and engineers deal with the environmental challenges that current polymers sometimes present.
Most commercial valves use polymers for their static and dynamic sealing, including the main closure seat. For static or sliding seal applications, polymers or elastomers (polymers with viscoelasticity) are often selected due to their ease of use, robustness to a wide range of tolerances and generally good leak tightness.
For the main valve seat closure, polymers are also robust to a wide range of tolerances and provide for Class VI or bubble-tight shut-off, which is adequate for most industrial and chemical applications.
While metal/metal seals and shut-off are considered to have the tightest sealing performance and are especially useful when working with high vacuum or small molecules like hydrogen or helium, they are more difficult to work with, requiring much more force and much tighter tolerances. As a result, metal/metal seals are usually used only where required.
Plastic bodies are not as widespread as polymer valve seats. They are usually specified for one of the distinct advantages they offer, including:
At the same time, these advantages must be weighed against potential disadvantages of plastic valve bodies. Depending on the specific polymer, these may include:
When an industrial process requires high pressure and/or extreme temperature together with excellent corrosion resistance, a polymer lined metal valve is often the solution.
Polytetrafluoroethylene (PTFE), perfluoro alkoxy (PFA), and polyvinylidene difluoride (PVDF) are high value fluorinated materials that resist aggressive chemicals, but they are structurally weak, soften with temperature and are too expensive to be used in large installations. These complex lined valves are typically cast from carbon steel and lined with intricate sleeves of fluorinated polymer and seals.
Not surprisingly, certain industries tend to value the advantages of plastic valves more than others. These include the following:
Chemical processes using all-polymer piping and fluid systems are the most common type of industrial application using plastic valves. For example, injection-molded trunnion ball valves are dominant in plastic piping systems. Such systems are selected for their low cost and excellent chemical resistance.
Many polymers are used in these systems, including:
Three single use polymer back pressure regulators provide transmembrane pressure control in a Single Pass Tangential Flow Filtration (TFF) biopharmaceutical application.
Source: Equilibar
The high cost and complexity of sanitizing valves and other fluid components has led to a large movement toward single use components and tubing in biopharmaceutical and medical applications. The ability for gamma and other high-energy radiation to penetrate polymer components, as well as the hygienic nature of injection molding, has caused valves made from certain polymers to be increasingly selected in these applications. Polymers should be selected considering gamma stability and typically include polyethylenes, polypropylenes and other materials capable of being rated as USP Class VI and ISO 10993. Pressure ratings for these liquid valves are typically not greater than 60 psig, and they are often used below 10 psig, making these applications well suited for plastic valves.
Residential potable water systems frequently use polybutylene (PEX) or acetal (POM) valves to meet cost, purity,and reliability requirements. Residential drain systems frequently use PVC valves and piping in larger sizes for cost reasons.
A big user of high-value polymer valves is that of ultra-pure applications where metal ion contamination is a concern. Ultra-pure water (UPW) is surprisingly aggressive as a chemical. During the purification process, trace metal ions are removed. As a result, the UPW creates a high affinity to strip metal ions out of even high value stainless steel piping and valves. Therefore, the use of piping and components made of inert plastics such as PVDF, polyether ether ketone (PEEK), or olefin (polyethylene, polypropylene) is a requirement for maintaining ion-free liquids in a wide variety of semiconductor, photography and other sensitive processes.
A low flow back pressure regulator in polyether ether ketone (PEEK) can be a good selection for ultra-pure water applications involved in semiconductor manufacturing.
Virtually all segments of modern manufacturing, including the valve industry, are grappling with increasing scientific evidence showing the harmful effects of accumulated PFAS and microplastics in water supplies and living organisms around the world.
As a recent article in this magazine explained in detail, the issue of forever chemicals — as these materials are known — is extremely complex. Fluorinated polymers like PTFE, PFA and PVDF have certain properties that our modern technology could scarcely survive without. In addition, these materials are key to green energy innovation such as hydrogen applications that promise to have a positive environmental impact. They are also key to the production of life-saving medications.
The challenge for valve engineers going forward will be to balance competing needs and use the most appropriate materials for each application. The mindful consideration of factors such as end-of-life recycling and disposal will become increasingly important, as will the adoption of emerging technology capable of destroying forever chemicals in the environment or otherwise mitigating their harmful effects.
Physicist Alan Black is lead applications engineer and product manager for Equilibar, part of the Richards Industrials family. He specializes in using computational modeling to predict valve performance, with a special focus on hydrogen fluid control and aerospace applications.
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