Valves serve a wide variety of functions within a gas or liquid piping system. This includes controlling the flow rate, turning the flow on or off, preventing backflow, as well as regulating and relieving pressure. Depending on the needs of the application, an assortment of valve designs is available that includes plug, globe, gate, butterfly, ball valves and many others.
However, one of the most important elements of a safe, durable, compatible and easily maintained piping system is the composition of valve body material. A valve can be subjected to extreme pressure and temperature conditions. It should be able to withstand the stress and loads of aggressive and abrasive gases, slurries and liquids. Because of the differences in composition, one valve material is often better suited for one project than another. In order to make sure that the best valve material is installed for the application, it is vital to recognize the characteristics, strengths and weaknesses of each of them.
Selecting the appropriate valve body material, be it PVC, steel or brass, keeps the piping system in better condition and extends its functional lifespan. Users also need to determine the pressure and temperature requirements at the valve’s intended position.
Brass Valves
Brass, an alloy of zinc and copper, is a popular valve material used in applications ranging from the oil and gas industry, HVAC applications, to water treatment systems. Softer than steel and thus easier to mold, brass has been a preferred option in applications like commercial and residential plumbing systems as well.
However, one has to be careful about using brass valves: the FDA does not approve of brass valves being used for drinking water and food and beverage applications unless they are certified lead-free.
In slightly acidic or alkaline solutions, brass can undergo dezincification, where the zinc is lost and copper is left behind on the valve material. In mild cases, there is only a cosmetic change where the color of the valve turns from yellow to pink. However, severe dezincification can lead to the weakening of brass and sometimes to perforation and breaking of the valve.
Brass is more expensive than PVC and can resist corrosion better than plastic valves. It can withstand more heat than plastic, making brass valves an optimal choice for home plumbing systems. Furthermore, due to the softer material, brass valves are often used in lighter industrial and municipal applications such as natural gas and HVAC applications.
Because of its chemical composition, brass valves tend to accept welding better than stainless steel valves, making brass more versatile and less expensive to shape and form. Brass valves can have a pressure rating of up to 3,000 pounds per square inch (psi).
Steel Valves
There are many grades of steel, but the two most common types used in industrial applications are stainless steel valves and carbon steel valves.
Stainless steel valves
An iron alloy mixed with chromium and nickel, stainless steel valves are best used in high pressure and temperature applications and harsh corrosive environments like crude oil refineries, brewing systems, food processing and marine environments.
Corrosion is one of the main issues that can affect the valve body material. Corrosive properties can become aggravated when subjected to high temperature and high-pressure differences, leading to cavitation erosion. However, stainless steel is one valve material that is more wear-resistant compared to other materials under the correct conditions and can be capable of withstanding this corrosion. This is due to the presence of chromium that gives stainless steel an oxidized, corrosion-resistant surface.
The food and beverage industry is where a careful selection of valve and piping materials is particularly important. Due to the different factors that need to be taken into account, such as temperature, pressure and viscosity of the elements piping systems will be exposed to, the need for the most suitable products is crucial.
Stainless steel valves are commonly used in the food processing industry, especially direct contact valves. Due to the rigorous sanitary standards, harsh corrosive cleaning agents are used to keep the systems clean and safe to handle food, and any materials used in those systems need to have high resistance to those agents.
Moreover, stainless steel can be electro-polished to smooth surface imperfections that otherwise could facilitate bacterial growth, leaving an ultra-clean surface. Food, beverage and pharmaceutical units benefit from this sterilizing feature of stainless steel valves.
Nuclear and fossil power plants involve critical applications where high temperature and high-pressure conditions prevail. Here, austenitic stainless steel valves are a better choice. Similarly, in pulp and paper manufacturing units, stainless steel valves are used for their high chemical resistance properties.
Though a more expensive option than brass valves, stainless steel valves are durable, resilient and usually highly resistant to corrosion. This gives stainless steel valves a longer service life, making them a low-maintenance, cost-efficient option. Stainless steel valves can have pressure ratings up to 10,000 psi.
Carbon steel
Carbon steel, also known as low-alloy steel, is a mix of carbon and iron and other alloying elements. Depending on the exact amount of carbon present in the carbon steel material, there are different grades available, ranging from mild to medium to high and ultra-high carbon steel. The higher the carbon content, the harder the valve material tends to be. However, as it gains hardness, the material loses malleability and ductility. Mild- and medium-range carbon steel is malleable and can be designed to fit specific applications.
Carbon steel is used in a variety of industrial applications such as hydraulic-based processes, chemical and power plants. In hydraulic-based applications, carbon steel valves are used for their high resilience to the corrosive tendencies of water and other liquids. Because of its high carbon content, it has very good mechanical properties and makes the valve device stronger and durable. Their outstanding tensile strength and impact value make carbon steel valves perfect in industrial plants where there are high amounts of pressure.
When compared to stainless steel valves, carbon steel valves lack chromium, making them prone to rust, corrosion, discoloration and staining. However, carbon steel valves are cheaper than stainless steel valves. So, if the process does not require high corrosion resistance, carbon steel valves provide an economical solution.
PVC
Valves made from polyvinyl chloride (PVC) are lighter than brass and steel, cheaper to produce and corrosion-resistant, which makes them useful in applications like irrigation systems, commercial and residential plumbing pipes and chlorine environments. PVC valves are resistant to sunlight, oxidation and have broader chemical compatibility to acids, bases, salt solutions and organic solvents.
However, PVC will warp at temperatures above 140 F and that is the maximum recommended operating temperature for PVC. As the coefficient of thermal expansion for plastics is greater than brass and stainless steel, PVC is not suitable for high-temperature applications. Similarly, their lower burst pressure properties rule them out from high-pressure applications.
On the other hand, PVC valves work well in temperatures close to or slightly below freezing point; the tensile strength of PVC increases as temperature decreases and it possesses some ability to expand under elevated pressure. However, take care to avoid physical impacts to the system, since the impact resistance of plastics decreases as the temperature falls and brittleness may appear to be affecting the piping materials.
When compared to metal valves, PVC valves are a greener option as it takes less energy to produce them and they are recyclable. Also, PVC does not corrode as much as other materials and this results in an even flow rate over time.
PVC valves are relatively easy and cheap to install, have a long service life, incur fewer maintenance costs, and have better resistance to cavitation. This is the reason why plastic valves are also used in wastewater treatment systems, some chemical processing, food and pharmaceuticals and oil refineries.
Choose Valve Material Carefully
Selecting the valve body material is as important as selecting the valve type. To guarantee that the valve body material can be used in suitable conditions for longer periods of time, users need to understand the objectives and the conditions where the valve will be placed; thus, they can select the material appropriately.