The erosion resistance of a material must be considered along with strength and corrosion resistance when choosing trim materials. This situation becomes more critical in higher pressures, with their resultant higher velocities through small orifices. Special erosion-resistant alloys, called hard-facings, have been developed to combat this situation in steel valves. The most popular alloy for this service is a Cobalt-based alloy called Stellite. Stellite is extremely hard and also very corrosion resistant.
Even if a hard-facing is not used, the trim material is nearly always a material with higher corrosion and/or erosion resistance, such as bronze in iron valves and stainless steel in steel valves. There are exceptions to this rule, particularly in alloy valves such as stainless steel and bronze, where the trim is either integral with the body or disc, or the trim material has similar chemistry to the valve body.
The valve stem or spindle is also an important trim component, since it operates within the fluid flow area, as well as outside the valve body. The stem must transmit the force required to open and shut the disc or ball against the pressure of the flow. Since some of the stem is in the flow area, some attention must be paid to corrosion and erosion resistance; however, the most important characteristic is strength. The stem cannot break off while opening or closing.
Nearly all the metals used in valve manufacturing are listed in detail in material specifications. In the United States, as well many other countries, the American Society of Testing and Materials (ASTM) is the governing body for these standards. An ASTM material standard will contain the acceptable chemical composition of the material, its strength requirements, and often incidental information such as how it should be manufactured, heat treated and tested. Table 2 contains a list of some of the more common ASTM valve body component standards.
Specific Industries, Specific Material Choices
The red-helmeted fire hydrant is the visible tip of the municipal water industry. Water distribution valves usually only see relatively low pressures, and chemicals and high temperature are not an issue, so the materials choice is not difficult. For this reason cast or gray iron is the choice for most water valves, unless they are small in size, when the material of choice becomes bronze. By the way, those hydrants are just globe valves with long bonnet extensions. And they too are made of cast iron. In high-rise office buildings it is necessary to get water to the top floors, requiring the use of high-pressure pumps. This means that at ground level the related piping and valves might see 600-800 psi or more, which is beyond the capability of the iron valve’s working pressure. In this case, cast steel valves would be used instead.
Improvement in power plant design has always pushed the envelope of valve materials and construction. Power plant boiler temperatures and pressures have risen to levels that require very tough alloy steels. Chrome/Moly steels, particularly the 9 Cr-1Mo-V, C12A alloy, as well as some stainless steels, are used in most of today’s high pressure/temperature power plant applications. For lower temperature service in these facilities, cast steel such as ASTM A216, grade WCB is used.
Nuclear power plants have material requirements similar to those of fossil power plants. Today, the prime valve material for critical applications in nuclear power plants is austenitic stainless steel. One unusual fact about nuclear plant valve materials is that cobalt-based hard-facing alloys, such as Stellite 6, are not used, due to the potential for cobalt in the hard-facing becoming irradiated in radioactively hot areas and spreading to other cobalt alloys in the fluid stream. For these situations, non-cobalt-containing hard facings are used.
Many pulp and paper mill process applications require strong chemical resistance. As a result the austenitic stainless steels (300 series), with their high chemical resistance, are a frequent choice for tough paper and pulp processing applications.
The chemical industry creates unique valve challenges due to the vast array of corrosive environments found in chemical processing. While the carbon steels and basic stainless steels such as 316ss, work well in other industries, more corrosion resistance is often needed for these challenging service conditions. Austenitic stainless steels (300 series) such as 317, 321 and 347 are regularly utilized to meet those requirements. Additionally, nickel “superalloys” such as Hastelloy and Inconel are often found where the combination of high strength and very high corrosion resistance is required.
Modern oil refining and petrochemical manufacturing provide some of the toughest challenges for valve materials. The silver-hued cast steel valves reign, and are found by the thousands in every refinery. The most popular cast carbon steel is grade WCB. It is suitable for use in temperatures up to 800° F. For higher temperatures the ASTM A217 cast alloys such as WC6, WC9, C5 and C9 are often utilized. Their forged counterparts—ASTM A182, grades F11, F22, F5 and F9—handle the same work in smaller-sized valves. High temperatures, caustics, acids and volatile gases create opportunities for many non-commodity materials. Low carbon or “L” grade stainless steels are used often, as well as super stainless steels such as 317, 347 and Alloy 20. Very high temperatures often call for Inconels and high-carbon stainless steels such as 304H. Copper/nickel alloys are also found in very demanding refining situations, along with Hastelloys and duplex stainless steels.
Many oilfield valves are designed to withstand great pressures, although high temperatures are generally not a consideration. Everything from carbon steels to hardened martensitic (400 series) can be found in oilfield production valves.
An important material consideration in both refining and oil production occurs when the crude oil is very sour. This sour crude is laced with hydrogen sulfide, which is lethal to both humans and certain metals. The situation is so critical that NACE International, an international corrosion engineering organization, has developed material recommendations to help piping designers deal with this dangerous condition. Many materials are suitable for sour service; however, strict guidelines as to maximum material hardness and heat treatments must be followed in order to keep catastrophic failures from occurring.
Valve materials for extreme cold (cryogenic) conditions must also be selected carefully. Materials chosen must remain ductile at ultra-low temperatures, which is not a trait of plain carbon steels or cast irons.