Valve Materials: Do There Have to Be So Many?
When it comes to choosing the right valves for your project, there’s a lot to consider. Valves serve many purposes and industries. And within each industry, they have numerous different functions. So while all CPV valves are manufactured to meet or exceed the highest quality standards, they’re not all the same.
Several factors come into play when making a decision about which valve will meet the requirements of the job for which they’re needed. Besides knowing the function or the action it will perform, it’s necessary to know the type of fluid or medium that it will be used for, the temperature of the substance, the pressure and flow rate, and the size of pipes or other equipment it will be connected to.
Armed with this information, you’ll be able to narrow down the options and select the product that will best suit your needs and provide the most return on investment.
One of the most important decisions you’ll make during this process is selecting the materials for the valve body and trim. Valves can be composed of materials ranging from cast iron to bronze to PVC. In valves like the ones CPV manufactures, there are typically two or more different metals in a single valve.
You may wonder why we don’t always use the “best” material for everything. It doesn’t quite work that way.
What’s best for one application might be the worst possible choice for another. And the “best” might not be the most cost-effective.
Factors Affecting Body Material Selection
Once you know what type of valve you need, the next step is to determine what metal or material will best suit the application.
Let’s look at some factors that will help guide this decision.
Resistance to Corrosion
Corrosion is the act of destroying, weakening, or wearing away a surface. It affects both the exterior and the interior of the valve.
In the external environment, air and moisture can lead to corrosion. In materials such as iron and steel, we see this in the form of rust. Many CPV customers have more than just the usual air and humidity to consider. Their businesses may operate in or near salt water. The harsh external environments to which their equipment is subjected accelerates the rate of corrosion. If you’ve ever owned a vehicle and lived near the sea or in a cold climate where the roads are frequently covered with salt to melt snow and ice, you know that when salt combines with water and air, rust appears even faster on an automobile’s steel parts.
But exterior corrosion in the form of rust isn’t the only concern. When a substance is flowing through a valve, corrosion can occur from the inside as well. The rate and degree of internal corrosion are dictated by the type of substance flowing through the valve. Sweet crude oil for example (crude oil containing low amounts of sulfur), is non‐corrosive and actually helps to protect the metal surfaces it contacts. Strong acids like sulfuric acid, on the other hand, can eat through some metals in a matter of hours.
Strength of the Material
There are two general measures of strength that play a role in valve material selection. Tensile strength is its stretchability or resistance to breaking when it’s stretched. If it can be stretched a little without breaking, it’s ductile. Copper is an example of a metal with good ductility, which allows it to be stretched into a wire. If a material doesn’t stretch, it’s brittle, like a ceramic plate that shatters rather than bends when it collides with a hard surface.
A valve’s body has to be able to withstand the pressure from inside. That’s where its tensile strength comes into play. A metal such as cast iron may seem like a good choice, but it’s brittle and cracks under too much pressure. In operations where high pressure is a factor, the material used must have good ductility. Knowing the pressure during operation is essential in selecting the right valve material.
Operating temperature also affects a metal’s strength and is another important factor in determining its suitability for a valve. In general, as the temperature increases, the metal softens and weakens. If it’s too cold, the wrong metal becomes brittle. So the temperature limits of the valve material must be aligned with the temperature at which it will be operated.
For example, a low-‐grade carbon steel can handle 285 psi at 100 degrees F, but that number drops to merely 50 psi at a temperature of 900 degrees F. Long exposure to high temperatures can also change some carbon steels to graphite. Depending on the particular metal, the tolerance range, as in the case of cast iron, may be as limited as ‐20 to 410 degrees F. On the opposite end of the spectrum, certain stainless steel alloys have a broad range, from as low as ‐425°F to as high as 1500°F.
Resistance to Erosion
With each component of a valve serving a unique function; and therefore being subjected to somewhat different conditions, special materials are usually required for a valve’s trim (the closure elements or parts such as the stem, seat, and disc). In addition to strength and corrosion resistance, the trim is more susceptible to erosion.
Similar to corrosion, erosion is wear caused by a mechanical action or the repeated application of localized stress. In other words, a substance breaking down a solid material due to pressure and repeated action, rather than a chemical reaction.
Sandblasting is a form of erosion. Dirt or other abrasive particles contained in a substance will contribute to metal erosion.
The effects of erosion are amplified on trim closure elements. As a valve closes, the velocity of the substance flowing through it increases, like putting your thumb over the end of a running water hose to make the water spray out with more pressure. This increased velocity causes wear in the form of erosion.
CPV Manufacturing is committed to making only the best quality valves and fittings. We design them with versatility in mind so that our products are perfect for many industries, purposes, and settings including the harshest of environments. Whatever your needs, you can trust CPV to provide the highest quality products to satisfy them.