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Valves perform a wide variety of functions in a gas or liquid piping system. This includes flow control, flow on or off, backflow prevention, and pressure regulation and reduction. Depending on the needs of the application, an assortment of valve designs are available which include plug, globe, gate, butterfly, ball and many more.
However, one of the most important elements of a safe, durable, compatible and easy to maintain piping system is the material composition of the valve body. A valve can be subjected to extreme pressure and temperature conditions. It must be able to withstand the stresses and loads of aggressive and abrasive gases, slurries and liquids. Due to differences in composition, a valve material is often better suited to one project than another. In order to ensure that the best valve material is installed for the application, it is essential to recognize the characteristics, strengths and weaknesses of each.
Selecting the correct valve body material, be it PVC, steel or brass, keeps the piping system in better condition and prolongs its functional life. Users should also determine the pressure and temperature requirements at the intended valve 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, from HVAC applications to water treatment systems. Softer than steel and therefore easier to cast, brass has also been a preferred option in applications such as commercial and residential plumbing systems.
However, care should be taken with the use of brass valves: the FDA does not approve the use of brass valves for potable water and food and beverage applications, unless they are certified lead free.
In slightly acidic or alkaline solutions, brass can undergo dezincification, where zinc is lost and copper is left on the valve material. In mild cases, there is only a cosmetic change where the color of the valve changes from yellow to pink. However, severe dezincification can lead to weakening of the brass and sometimes puncture and rupture of the valve.
Brass is more expensive than PVC and is more corrosion resistant than plastic valves. It can withstand more heat than plastic, which makes brass valves an optimal choice for household plumbing systems. Additionally, due to the softer material, brass valves are often used in lighter industrial and municipal applications such as natural gas and HVAC applications.
Due to its chemical makeup, brass valves tend to accept welding better than stainless steel valves, making brass more versatile and less expensive to shape and shape. Brass valves can have a pressure rating of up to 3000 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 alloy of iron mixed with chromium and nickel, stainless steel valves are best used in high pressure and temperature applications and in harsh corrosive environments such as crude oil refineries, brewing systems, food processing and marine environments.
Corrosion is one of the main problems that can affect the material of the valve body. Corrosive properties can worsen when subjected to high temperature and pressure differences, leading to cavitation erosion. However, stainless steel is a valve material which is more resistant to wear than other materials under the correct conditions and may be able to resist this corrosion. This is due to the presence of chromium which gives stainless steel an oxidized and corrosion resistant surface.
The food and beverage industry is where careful selection of valve and pipe materials is especially important. Due to the various factors that must be taken into account, such as temperature, pressure and viscosity of the elements to which the piping systems will be exposed, the need for the most suitable products is crucial.
Stainless steel valves are commonly used in the food industry, especially direct contact valves. Due to stringent sanitary standards, corrosive cleaning agents are used to keep food handling systems clean and safe, and all materials used in these systems must have high resistance to these agents.
Additionally, stainless steel can be electropolished to smooth out surface imperfections that might otherwise facilitate bacterial growth, leaving an ultra-clean surface. Food and pharmaceutical units benefit from this sterilization feature of stainless steel valves.
Nuclear and fossil power plants involve critical applications where high temperature and pressure conditions prevail. Here, austenitic stainless steel valves are a better choice. Likewise, in pulp and paper manufacturing units, stainless steel valves are used for their high chemical resistance properties.
Although this is a more expensive option than brass valves, stainless steel valves are durable, resilient, and generally very resistant to corrosion. This gives stainless steel valves a longer life, making them an economical and low maintenance 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 mixture of carbon and iron and other alloying elements. Depending on the exact amount of carbon present in the carbon steel material, different grades are 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 in hardness, the material loses its malleability and ductility. Soft and medium grade carbon steel is malleable and can be designed to suit specific applications.
Carbon steel is used in a variety of industrial applications such as hydraulic processes, chemical plants, and power plants. In hydraulic applications, carbon steel valves are used for their high resistance to the corrosive tendencies of water and other liquids. Due to its high carbon content, it has very good mechanical properties and makes the valve device more strong and durable. Their exceptional tensile strength and impact resistance make carbon steel valves perfect for industrial installations where pressures are high.
Compared to stainless steel valves, carbon steel valves lack chrome, which makes them prone to rust, corrosion, discoloration, and stains. However, carbon steel valves are cheaper than stainless steel valves. So, if the process does not require high corrosion resistance, carbon steel valves offer an economical solution.
PVC
Polyvinyl Chloride (PVC) valves are lighter than brass and steel, cheaper to produce, and resistant to corrosion, making them useful in applications such as irrigation systems, plumbing pipes commercial and residential and chlorinated environments. PVC valves are resistant to sunlight, oxidation and have wider chemical compatibility with acids, bases, saline solutions and organic solvents.
However, PVC will deform at temperatures above 140 F and this is the maximum recommended operating temperature for PVC. Because the coefficient of thermal expansion of plastics is greater than that of brass and stainless steel, PVC is not suitable for high temperature applications. Likewise, their lower burst pressure properties exclude them from high pressure applications.
On the other hand, PVC valves work well at temperatures near or slightly below freezing; PVC tensile strength increases as temperature decreases and it has some ability to expand under high pressure. However, be careful to avoid physical impacts to the system, as the impact resistance of plastics decreases as the temperature drops and brittleness may appear to affect the piping materials.
Compared to metal valves, PVC valves are a greener option because their production requires less energy and they are recyclable. In addition, PVC does not corrode as much as other materials, resulting in a uniform flow rate over time.
PVC valves are relatively easy and inexpensive to install, have a long service life, have lower maintenance costs, and have better resistance to cavitation. This is the reason why plastic valves are also used in sewage treatment systems, some chemical treatments, food and pharmaceutical products, and petroleum refineries.
Choose the valve material carefully
The selection of the valve body material is as important as the selection of the valve type. To ensure that the valve body material can be used under suitable conditions for longer periods of time, users should understand the purposes and conditions under which the valve will be placed; thus, they can select the material appropriately.
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