As water utilities are looking for ways to do more with less, some manufacturers are investing in research and development to improve the product by using new materials to extend product life and value. A simple choice of material can impact the design life, function, and performance. One change that has all three beneficial outcomes for water distribution is moving from Nitrile (Buna-N) to Ethylene Propylene Diene Monomer (EPDM) for valve seats.
By Timothy Fallon, Pratt Technical Sales Manager – Mueller
Nitrile rubber or Buna-N, also known as nitrile butadiene rubber (NBR) and acrylonitrile butadiene rubber, is a synthetic rubber derived from acrylonitrile (ACN) and butadiene. Buna-N is a general-purpose oil-resistant polymer that has good resistance to solvents, oil, water, and hydraulic fluid with good compression set properties. It also has acceptable resistance to abrasion and good tensile strength in laboratory tests. The resistance to solvents, particularly oils, has made Buna-N a popular elastomer candidate for use when collecting raw water, where pollution and illegal dumping have allowed petrochemicals to infiltrate freshwater supplies. Buna- N elastomers are also used in waste-water applications where resistance to solvents and oils is important. These properties are a reason Buna-N has been a widely used choice for elastomeric seats used in multiple types of valves including butterfly, rubber seated ball, and resilient seated check and gate valves in North America.
Seal Degradation
It has always been the mission of water utilities to provide safe drinking water, but as contaminants are on the rise, municipalities are having to do more testing and look at a broader spectrum of treatment solutions. One treatment that is becoming more common and in increasing quantities is the use of chloramines to disinfect drinking water. As this has been standard practice in many parts of the world, analysts have discovered that increased chloramine use leads to substantially higher rates of valve seat failure.
Chloramine usage has been cited to be a cause of Buna-N elastomer degradation, characterized as mass change. This occurs because the rubber swells in the presence of chloramines; mass change is the result of excessive water absorption. It is believed that the other failure modes experienced include: increased susceptibility to breaking stress and strain, along with increasing hardness, where the rubber simply loses its elastomeric properties.
When the integrity of any elastomer is compromised, chemicals or additives can leak into the drinking water. When certain standard minimum concentrations are detected, the rubber compound is designated unworthy for use in drinking water applications, per the guidelines set forth through NSF/ANSI 61: Drinking Water System Components – Health Effects. To ensure the rubber compound is safe for use in drinking water or wastewater applications, the rubber compound should be thoroughly evaluated to the NSF standards as appropriate.
Shifting to a Safer and More Reliable Elastomer
As the trend in Chloramine treatment increases in North America, a new approach will be needed to ensure drinking water is safe and that valves continue to perform their important function without having to send out crews to replace failing seats, which could result in a service disruption. Ethylene Propylene Diene Monomer or EPDM is not only becoming a suitable replacement to Buna-N, but given the changing water environment, a superior performing elastomer in general. However, not all rubber compounds perform the same. There are some Buna- N compounds that perform very well under adverse conditions, as noted in AWWA M75 – Elastomers for Waterworks: Pipes, Valves, and Fittings. The same can be said for Styrene-butadiene rubber (SBR), natural rubber, and neoprene rubber.
The most common way to confirm the integrity of the specific rubber compound is by testing it under lab conditions. Many formulations of peroxide cured EPDM are favored as less susceptible to chloramine degradation followed by many formulations of Buna-N, Styrene-butadiene rubber (SBR), natural rubber, and neoprene rubber. As new methods of disinfection come into play, manufacturers of water infrastructure will need to adapt and innovate products to better meet the needs of the municipal water industry.
Looking Forward
The decision as to what rubber compound to use is commonly based upon past practice and often outdated specification language. Given significant research and its mandate to continuously innovate the functionality and extend the product life of water infrastructure solutions, many companies are moving to EPDM seats as the standard seat configuration for butterfly valves. EPDM is also readily available and globally sourced, reducing the challenges of supply chain issues that have arisen since the start of the pandemic.
As materials improve and industry standards evolve, utilities often adopt these changes when they upgrade facilities or perform routine maintenance that requires replacement parts. The latter is where utilities have an ongoing ability to adopt better designs and materials, increase the design life of existing infrastructure, and reduce the frequency of maintenance as a result. Knowing what new materials and design improvements are available for new builds, or plant upgrades, is also key to ensuring utilities are well-positioned to handle the challenges of today and the evolving landscape of delivering safe drinking water to all citizens.
