Choosing the proper valve material
Appeared in Applied Automation on 6/11/2016
Engineers must consider flow media, pressure, flow velocity, and the intended purpose when selecting valves used in high-pressure water hydraulic circuits.
High-pressure water hydraulic systems have been widely used since they were developed in the late 1700s. The first systems were designed as a means of harnessing the power-generating potential of water, an application still widely used today. Water hydraulic systems have evolved over the years into a multitude of uses in the manufacturing, mining, and drilling industries (see Figure 1). These safe, reliable systems offer consistent efficiency when properly designed and maintained.
Of course, proper design requires a lot of thought, especially when it comes to specifying the components that make up these types of systems. Valve selection is of critical importance, and a key aspect of that selection process is understanding how the materials used in a valve under consideration will have a long-range impact on valve, and overall system performance.
Establishing the parameters
When engineers select valves used in a high-pressure water hydraulic circuit, the required function of those valves is what ultimately guides their choices. Flow media, pressure, flow velocity, and the purpose of a specific valve are the key parameters that will direct the engineer toward a certain type of valve.
For example, if a valve is needed to extend or retract a cylinder in a 750-ton press in a water hydraulics system, there will be a certain group of directional control valves that will fit this application. From there, the engineer can begin the process of deciding on a specific valve based on cost, ease of maintenance, or useful life of a particular valve model.
Beyond these basic operating parameters, the engineer will also turn his or her attention to valve materials. Considering the example of the valve for the 750-ton press, the engineer must decide which additives might be contained in the system media for possible incompatibilities with the valve. For example, if the flow media contains glycol for fire resistance, the body, seals, and packing components that make up the valve must be specified with an understanding of the possible effects that glycol may have on those valve components (see Figure 2).
Materials don't always play nicely
Understanding these material incompatibilities is crucial, particularly for those parts of the valve that will come into direct contact with the flow media, called the "wetted areas," which will vary for different types of valves. While the focus here is on water-based hydraulic systems, the approach for specifying materials in systems that use other media is the same.
The term "water hydraulics system" actually is a bit of a misnomer because systems that operate with pure water are very uncommon. The majority of water hydraulics systems are water-based, meaning that they contain a heavy concentration of water mixed with a small percentage of some other additive, typically water-soluble oil. The oil is added to improve the lubricity of the water so that it will reduce wear on seals, bushings, valves, and pumps because water by itself is a poor lubricant. Other additives can be added as well, such as glycol to improve fire resistance or biocides to inhibit bacterial growth.
For example, a water-glycol mix will degrade a polyurethane or polyester seal quickly. Similarly, chlorine present in some municipal water sources has been known to cause galvanic crevice corrosion in some valve applications. Valve, seal, and gasket manufacturers typically publish incompatibility charts that list common substances and how they interact with their materials of construction.
This is valuable information for any engineer. Of course, it's well known that water on its own typically leads to corrosion and will affect valve components over time. But it is also imperative that the engineer understand the cleanliness of the flow media. Properly maintained closed-loop systems typically will not experience media cleanliness issues. However, for open-ended systems, such as those used in descaling operations in steel mills where process water is recycled through the system, particles in the water have a harmful effect on the wetted areas of valves. These particles will impinge on those surfaces and cause excessive wear. Valve design and selection of materials of construction must account for these types of media characteristics.
Temperature of the flow media and the operating environment is another factor to be considered. Corrosion will occur at in increased rate at higher temperatures. In addition, temperature can cause different materials to expand and contract at different rates, which can cause leakage issues, undue binding stress on valve components, or seizure.
Common materials of construction
Components, such as valve spools, seats, and bodies usually are offered in materials that provide a varying range of corrosion, temperature, and wear resistance. Materials commonly used in high-pressure water applications include:
- Ductile iron: Used for its low cost and availability, ductile iron offers the additional benefit of readily absorbing shock. On the negative side, ductile iron offers poor corrosion resistance.
- Brass and bronze: Brass and bronze valves also offer low cost and are readily available. An added benefit over ductile iron is improved corrosion resistance.
- 316 stainless steel: Known for its excellent corrosion resistance, 316 stainless steel valves are more expensive than cast iron, brass, and bronze valves.
- Monel: A nickel-copper alloy Monel offers superior corrosion resistance. It is typically used for cladding of valve trim parts.
- Inconel: An alloy of nickel, chromium, and iron, Inconel is used for handling corrosive media at higher temperatures.
Seals, gaskets, and packing typically are constructed of the following:
- Polyurethane: This durable material is suited for temperatures up to 200 F, pressures up to 6,000 psi, and offers excellent abrasion resistance.
- Viton: Viton provides excellent chemical resistance and fares well in high-temperature applications.
- PTFE: An extremely low friction material that can function well over a wide range of pressures and temperatures.
- Glass-filled PTFE: PTFE with glass added to improve mechanical properties, such as wear resistance and heat transfer.
- PEEK: Polyetherketone is well-suited for use with steam applications. It generally offers higher temperature ratings and has good corrosion resistance.
Making reasoned choices guided by knowledge
Application parameters and flow media incompatibilities generally will drive valve material decisions, with cost coming in as a concern immediately behind. In most cases, the lowest cost valve that performs the intended function, and is constructed of materials that are compatible with the flow media, should be selected. There are some cases, however, where the service or replacement schedules of certain valve components can be extended by using a more expensive material of construction.
That additional cost may be worth it, especially in applications where scheduled downtime must be minimized. Designing a high-pressure water hydraulics system can be a daunting task for even the most experienced engineer with a wealth of hydraulics experience. There are numerous decisions to be made, and the majority of those decisions must be made while adhering to a budget. However, if engineers methodically go through the process of understanding the operating parameters of the system and the requirements of the various system components, they will be on their way to designing a safe, reliable, and efficient system.
Mickey Heestand is vice president and senior mechanical engineer at Hunt Valve. His leadership role includes oversight of the company's welding and nondestructive test procedures, welder workmanship training and examination, and contract engineering reports. In 1998, he was granted a U.S. inventor patent (US 5769123 A) for developing the cylinder actuated descale valve.