There is more to valve materials than just castings and forgings. Some of the key components are not metallic at all; these include packing, gaskets and seals. The two most common packing materials in use today are graphite and Teflon. Graphite is excellent for most service conditions and is good for temperatures up to about 1500° F, depending upon the degree of oxidation created by the contained fluid. Teflon has a maximum temperature of 400-500° F, depending on which Teflon compound is used. Like non-stick Teflon cookware, Teflon packing is very slippery and has minimal friction.
Teflon is also used in most soft-seated ball valves as a seating material. It can be compounded with graphite, glass powder or other elements to increase its maximum temperature, erosion resistance and strength. When harsh working environments dictate, ball valve seats are made of elastomers other than Teflon, such as PEEK and TFM.
Valve materials do run the gamut from Aluminum to Zirconium, with new alloys and compounds filling in the letters of the alphabet all the time. Although these new and somewhat obscure materials are on the valve designer’s plate, it will be hard to ever eliminate those three popular flavors of steel, bronze and iron.
VALVE MATERIALS: A SHORT HISTORY
More than 2,000 years ago, the Romans made bronze plug valves for use in their fresh water distribution systems. As advanced as the Romans were they couldn’t keep their technological advances from disappearing under the cloak of the dark ages. It wasn’t until James Watt and others began their experiments into steam power in the late 1700s that valve technology began to surface again.
During this period, most valves were made out of the same materials as the pipe and boilers of the time, and that was iron. Iron was relatively easy to cast and was used for numerous piping components. In the mid 1800s, brass foundry productivity improved to the point where most of the small valves (1/2 to 2 inches) were made of bronze.
Bronze and Iron Rule
The 19th century came to a close with bronze and iron as the materials of choice for valve construction. These materials could even handle the “extra heavy” 250 psi steam working pressures of the period. Since the Bessemer Converter jump-started the steel age in the 1860’s, steel castings had been working their way into American industry. The turn of the century saw steady increases in power plant steam pressures as superheated steam came into vogue and the capabilities of iron valves and fittings were approaching their practical design limits. Initial attempts to solve this materials issue resulted in the development of a much stronger cast iron called ferro-steel, also called semi-steel. This material was a cast iron that had been mixed with steel scrap during the melting process.
The semi-steel was only a stopgap as cast steel valves began to appear in all the major manufacturer’s catalogs during the first decade of the 20th century. By 1950, cast steel would become the primary valve construction material for the steam generation industry.
Materials for Valve Trim
The material of choice for valve trim was bronze until the introduction of Monel in 1906. Within a decade or two, Monel became the severe service trim material of the valve industry. Monel held that position until air-hardenable, martensitic stainless steels (400 series) became popular just prior to World War II. Following the war, Stellite, a cobalt alloy, took the position of the best severe service valve trim material.
Demand for Alloys
Meeting the production needs of World War II fostered much technological advancement in American industry, including valve design. The race for synthetic rubber, 100 octane gasoline and other valuable products needed for the war effort created a demand for alloys that could handle the pressures, temperatures and corrosion created by these processes. Valves of austenitic stainless steel (300 series) helped handle production in these plants, and these materials are still a staple today.
As pressures and temperatures continued to rise in steam power plants and refinery process equipment, the plain carbon steels were not hearty enough, so alloys containing chrome and molybdenum were developed, such as the 1-1/4, 2-1/4, 5 & 9 chrome/moly alloys. Today the ultimate metal for super-heated, power generation valves is C12A, an alloy of 9% chrome, molybdenum, vanadium and other elements.
Importance of Elastomers
Probably the most important valve material to come out of the 20th century was not a metal at all, but an elastomer called Teflon. Created by DuPont in 1938 and perfected in the late 1940s, this material gave life to the soft-seated ball valve industry. It is safe to say that without Teflon, there would not be the huge ball valve industry that exists today.
The development of nickel alloy and superalloy castings such as Hastelloy and Inconel during the past 40 years have helped valve manufacturers meet the pressure temperature rating and corrosion resistance requirements found in many of today’s critical process environments. Metallurgists are continuing to improve these unique materials to meet tomorrow’s fluid handling challenges. And tomorrow’s ultimate valve material might possibly be a graphite composite, containing no metal at all.
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