Brett Hofman, additive materials engineer for Emerson, recently published an article in the November 2021 edition of Processing. The article helps demystify the design process for selecting proper valve component materials and is titled Select The Right Control Valve Materials. It is summarized below.
When faced with a bewildering number of options, it can be very challenging to choose the best materials of construction for control valves. Each valve component may have a different set of critical property requirements, and there can a whole host of processes which might be acting to degrade or destroy them. Brett explains the problem:
“There are many reasons for control valve component degradation, including erosion, adhesive wear, flashing, cavitation, corrosion, temperature extremes, and others. Several of these challenges often occur simultaneously, so it is important to identify and understand each problem.”
Erosion physically removes material from a part due to particulate in the process fluid. Adhesive wear results when metals rub against each other. Flashing damage (Figure 1, left) can occur when a liquid passes through a valve and the downstream pressure is below the vapor pressure of the liquid. The boiling liquid tends to wear the metal over time. Cavitation (Figure 1, right) involves boiling liquid as well, but in this case the pressure recovers as it moves through the valve and collapses the vapor bubbles. The resulting microjets and shock waves can inflict significant damage.
Figure 1: Flashing damage (at left) can be significant, but cavitation (at right) is usually much more destructive. Cavitation damage also makes a valve much more susceptible to corrosion.
Corrosion comes in many forms and is chemically induced (Figure 2). General corrosion occurs with uniform attack on a metal, such as the rusting of steel or iron. Pitting corrosion is a localized attack that leaves deep pits in metal that might otherwise be unaffected in other areas.
Figure 2: General corrosion (at left) attacks all surfaces of a part evenly, while pitting corrosion (at right) attacks localized areas, often leaving the rest of the part unscathed.
Brett describes other forms of corrosion:
The list of corrosion types goes on and on, including stress corrosion cracking (SCC), crevice corrosion, intergranular corrosion, galvanic corrosion and many others. The hardest part of battling corrosion is understanding what chemical process is in play because, in many instances, there are several types of corrosion occurring simultaneously.
Identifying Key Parameters
Each valve component is designed to meet specific performance criteria, and that knowledge figures heavily in the material selection process. Some of the typical material properties include strength, wear resistance, thermal expansion, corrosion resistance, and creep resistance. These critical material properties vary significantly from part to part (Figure 3).
Figure 3: This figure illustrates how each valve component requires very different critical material properties. Note that this figure is generalized and may vary based on valve design and process conditions.
Strength (or hardness) measures how a material resists cutting, scratching, or bending. Wear resistance indicates how well a material absorbs energy. Thermal expansion and corrosion resistance are self-explanatory. Creep resistance is a solid material’s ability to avoid slowly deforming over long periods of stress and high temperatures. The best material for a particular application depends on how that component is being used in the valve.
Know your material options
The number of materials options for control valve components is expansive, and the breadth of proprietary and generic names often leads to confusion. There are over 20 versions of “Hastelloy” metals and at least six different alloys called “Inconel”. When referring to alloys, it is often best to use a UNS number or ASTM standard.
It is also important to understand how a particular metal protects against corrosion so it can be applied appropriately. Some materials employ an oxide layer that provides passive corrosion resistance. These materials tend to work well in oxidizing environments but work poorly in reducing environments, which attack the oxide layer. Other materials are inherently inert and are less reactive in a variety of environments. Figure 4 lists a variety of materials, along with their various strengths and weaknesses.
Figure 4: This table is a small sample of the many materials available, and the wide range of corrosion, wear, and erosion resistance offered by each.
Brett offers the following advice for choosing the best material:
Clearly, the number of options is huge, and the price differential from one alloy to another can be significant. When faced with a difficult material selection decision, it is advisable to discuss the options with your valve vendor. Often, several alloys may work, and the best choice for your particular application may be a combination of valve design and valve component material selection.
Brett Hofman is an additive materials engineer for Emerson, researching how to realize the potential of additive manufacturing technologies in Emerson’s products. He previously held the role of Materials Engineer for Emerson’s flow control products, providing materials technical support on a global level to various departments across the company. He graduated from Iowa State University with Bachelor of Science degree in materials engineering in 2016.
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