Non-metallic materials are commonly used for valve components. The selection of non-metallic materials for valve design and for application-specific conditions is critical to ensuring product reliability. Like metallic materials used in valve designs, to meet reliability target selection of non-metallic materials is important to fulfill the design function of the component in the intended applications. In this article, we address some key areas to aid in the design and selection of non-metallic materials.
INCORPORATING NON-METALLICS INTO VALVE DESIGN
From a design/manufacturing standpoint, when selecting a material you need to be clear on the function of the intended part. Is it a dynamic seal? Is it a static seal? Is it pressure boundary or pressure controlling component?
With respect to pressure boundary components in industrial valves, non-metallic materials are not as common as metallic materials. One reason we don’t see non-metallics utilized broadly in industrial valves is due to the lack of codes or standards providing acceptance and guidance for their use. Several industries will use non-metallic pressure boundary materials (such as fiberglass reinforced plastic composites) because these materials are better suited than metallic materials with respect to chemical compatibility. Composite materials can provide superior chemical compatibility in many acid services thus selecting the right material for the service can provide overall product reliability.
In addition, consider:
- Chemical resistance and physical properties
- Flex modulus and creep resistance
- Processing method for making the non-metal part: What expertise is required and what is the cost of the equipment?
From an application standpoint, pay attention to the conditions the valve will encounter:
- Chemical composition of the flow medium (critically important)
- Elastomer liners must withstand chemical attack and swelling
- Temperature limits of the material
- Solids content of the flow, if any (is there risk of abrasion?)
- Flow rates or velocity
PROPERTIES OF NON-METALLIC MATERIALS USED IN VALVES
Understanding material properties is critical for selecting the right material for the valve design function and compatibility for application. Both chemical and physical properties of the materials of construction should be understood to utilize a particular material.
The chemical properties of a material determine whether it can be used within a particular application include:
- Chemical compatibility/corrosion resistance/chemical resistance
- Electrical conductivity
- Food safety
The physical properties of the material need to be taken into account to ensure functional performance, including under a full range of operating conditions:
- Strength: tensile, yield, compressive
- Flexural modulus
- Compression set
- Deformation under load
- Hardness (Shore durometer)
- Volume change: swelling, especially due to the flow medium
- Abrasion resistance
- Extrusion resistance
- Temperature rating: performance within given temperature range (Figure 1 shows examples)
It also is important to know that no two materials of the same type may have the same properties. Here are two examples:
Elastomer properties are highly reliant on the “recipe” and “ingredient brand,” so changes to the brand of ingredient can result in unexpected material property changes.
Polymer mechanical properties are highly reliant on the processing (recipe) of the materials; material properties can be directly related to crystallinity of the material, which is dictated by the heat treatment. Any change in heat treatment will correlate to a change in mechanical properties.
COMPATIBILITY IS CRITICAL
Material manufacturers know their products well and provide chemical compatibility information, often in charts like the one shown in Figure 2.
Notice in the second column that FEP/TFE/PFA (fluoropolymer) material is shown as excellent for use with all the chemicals listed. That said, this type of material isn’t necessarily good with everything; an extremely challenging flow such as molten sulfur could cause it problems.
In a similar guide for rubber materials, instead of having corrosion rates as for metal, you’ll see swelling rates or degradation rates. Look for a material that’s going to be compatible with whatever is going to flow through that pipe.
Examples of incorrectly applied materials:
- PEEK dissolves completely in concentrated sulfuric acid at room temperature.
- EPDM elastomer swells and decomposes when hydrocarbons are present, even in trace amounts. Use nitrile or Viton instead, depending on the manufacturer’s recommendations.
- Polypropylene is excellent for dilute and concentrated mineral acids and bases, but chlorinated hydrocarbons cause it to swell and soften.
- EPDM elastomer can swell when steam is present.
- Swelling in an O-ring or flat gasket may actually improve static sealing, but in a dynamic seal of a butterfly valve liner, for example, swelling of the liner could result in tearing when the valve cycles.
TAILORED TO THE APPLICATION
From an application perspective, it is critically important to know the composition of the flow medium, temperature and pressure of the service. Information gathering may not be as simple as readying a valve data sheet. Often a conversation between the valve manufacturer and the end user is required to understand all possible conditions to ensure compatibility throughout the process requirements. Say you’re looking to put a rubber-lined valve in a certain service that has five different chemicals. The rubber lining material has to be good with all those chemicals. Even trace amounts of incompatible chemicals can cause problems, particularly swelling with elastomers.
What type of chemical resistance do you need for this valve? How long is this valve going to last? How many cycles?
Does the flow medium contain solids that make it abrasive? What is the flow rate? 20 to 25 ft/s (6.1 to 7.6 m/s) has been a common design velocity. Some manufacturers go to 30 ft/s (9.1 m/s) with a clean liquid (no solids) and with a gas you can go to sonic velocity.
Any time you have a lot of solids in the line, the slower the velocity the better. However, where the flow is 100% solids, as in bulk-solids hopper applications, valves can last a long time.
Finally, you have to be cognizant of the temperature limits of the material. The bottom line is you really need to consult the material manufacturer to get their recommendations for your particular application. With the right material in the right place, a valve can operate reliably over a long life, even when fulfilling a challenging application.