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Performance over cost

Published: 27 November 2018 - Sarah Mead

Tameson says ensuring your valve’s housing and seal material will hold up to your media and environment will lower total cost. The company outlines the material selection process.

Once you have selected the appropriate valve type for your application and sized it correctly, you need to specify the materials of the valve. Specifically, for many valve types such as solenoid valves, you will need to choose the valve housing and seal material. Understanding the characteristics of your media and the environment that the valve will be in is crucial to specify the correct material and ensure valve longevity.

Since there are many valve types and configurations on the market, the top five housing material options and top four seal material options offered by valve suppliers are considered. To narrow down the selection, start with the media that will be passing through the valve to ensure that it will hold up against corrosion and temperature fluctuations (environment temperature as well). For the housing, choosing between a plastic and metal often comes down to durability, but keep in mind that plastic is cheaper and could offer the needed protection.

There are special circumstances to consider for each application (metallic taste, pressure, concentration, etc.) that should be taken into account.

Housing material options

The housing material comes into contact with both the media and the environment. The top five main valve housing material options are explained below.

PVC (PolyVinylChloride) – PVC is cheap and suitable for basic media and has some corrosive protection against salt solutions (seawater), organic solvents, and most acids and bases. However, it is not resistant to chlorinated hydrocarbons and aromatic media. The temperature range (-15°C to 60°C) generally limits its applications and it is less suited for high pressure applications compared to metal housings.

Nylon (Polyamide – PA) – Nylon is a cost effective solution and is suitable for greases, oils, salt solutions (sea water), hydrocarbons, and organic solvents. However, acids and alkalis will corrode the valve. It has a slightly larger temperature range (-40°C to 85°C), but is not suitable for high pressure applications.

Brass – Being a metal, it is more durable than the plastic material housings and has a higher temperature rating (<-100°C to 300°C). It is most often used for drinking water, but is also suitable  for neutral and non-corrosive media like air (compressed) and oil. However, it will corrode with salt solutions (seawater), acids, distilled water, and chlorides.

Nickel Plated Brass – This increases resistance to temperature (<300°C), abrasives, and pressure.

Therefore, it can be used for salt solutions (seawater), hydrogen sulfide,  weak organic/inorganic acids and alkalis, as well as carbon dioxide. However, if the nickel layer is scratched and the brass is exposed, rust will occur.

Stainless Steel (304/316) – Stainless steel valves are heavy duty and have good chemical resistance, large temperature range (<-100°C to <300°C), and are adequate for high pressure

applications. Typically, the temperature ranges of the seal or actuator limit the applications. However, it is still not suitable for hydrochloric acids, bromine, chlorides, and household bleach. Stainless steel 316 offers more protection than 304, but costs more.

Seal material options

The seal (o-ring and diaphragm) material generally only comes into contact with the media and is a rubber to form a proper seal. The four most common materials are discussed below.

NBR (Nitrile rubber) – This offers high resistance to wear (tearing) and compression. It is compatible with solvents, alcohol, and oil products from 0°C to 80°C. However, temperature fluctuations can negatively affect performance and it is unsuitable for polar solvents and brake fluid.

FKM (Viton) – FKM is widely used due to its resistance to compression set, large temperature range (-10°C to 120°C), and good mechanical properties. It is often used for acids, animal and vegetable oils, halocarbons and aromatics, and oils and solvents. However, it cannot be used for hot water and steam, as it starts to swell above 90°C - 100°C (still suitable for oil up to 120°C).

EPDM (ethylene propylene diene monomer rubber) – EPDM offers good resistance to temperature changes (-10°C to 130°C), making it suitable for steam, alcohols, acids/alkalis in low concentrations, ketones, and brake fluids. However, it should not be specified for applications dealing with aromatic hydrocarbons, solvents, oil, or grease.

PTFE (Polytetrafluoroethylene Teflon) – PTFE is the most mechanical and chemically resistant seal, but comes at a higher cost. The material is harder than NBR, FKM or EPDM and cannot be used as a flexible membrane. In ball valves it is often used as valve seat. In solenoid valves it is typically used in high pressure or extreme temperature valves. It allows for a temperature range of -30°C to 180°C.

Ultimately, you must understand your applications and know your options to keep the valve running, and more importantly the system.

Tameson
www.tameson.co.uk



 
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