Material

Strength in a valve is its ability to withstand the internal stresses generated by containing and controlling the fluid under pressure. Strength can be measured in several ways, but the most common measure is by quantifying the metal’s tensile strength. Tensile strength is the resistance of the metal to stretch or break when pulled. The ability of the metal to stretch slightly is called “ductility,” and some ductility is generally useful in valve applications. But not all metals have good ductility. For example, cast iron is not ductile at all and it bends very little before it breaks. This lack of ductility is called “brittleness.” While brittleness is expected in cast iron, it is not expected and definitely not wanted in most valve metals such as cast steel.

Generally, the brittle cast irons are only used for lower pressures, particularly below 300 psi and in situations where water hammer (sudden pressure spikes) is not an issue. Higher pressures are reserved for the stronger and more ductile steel and high alloy valves.

The second major consideration in choosing a valve material is its corrosion resistance. Corrosion is the breakdown of a metal due to attack by various chemical reactions. We all have seen corroded bolts or rusted out fenders on a car. This rust and corrosion is a result of a chemical oxidation of the steel caused by a combination of oxygen and iron, with moisture helping to accelerate the process. In valve materials, basic exterior rusting of the valve is usually secondary to the corrosion going on within the valve due to the unique characteristics of the fluid contained inside it. Some fluids result in virtually no corrosive action to the inside of the valve. For example, steel valves in non-sour crude oil service could conceivably last forever, because the clean oil keeps the corrosion and oxidation from occurring, and the lubricity of the oil keeps the valve in tip-top shape.

Another important aspect of a valve material’s strength is that metals become softer and lose their strength as the operating temperature is raised. For example, a low-carbon-steel, grade WCB valve has an operating pressure of 285 psi at 100 degrees, but only 50 psi at 900 degrees!

The dangers of corrosion damage are particularly high in the chemical manufacturing industry where the issues of strong chemicals, high pressures and high temperatures cross paths. The harsh acids and other compounds can sometimes eat through metals such as iron and steel in a matter of days or even hours. The development of corrosion-resistant alloys was borne out of the necessity to help contain and control the flow of these products. These corrosion-resistant alloys are in the family of nickel alloys commonly known as stainless steels. Most of them are an alloy of chromium and molybdenum, plus other elements that combine to create their corrosion resistant armor.

Valve shells (bodies and bonnets) are usually manufactured from a combination of castings and/or forged or wrought components. The castings are made by pouring molten metal into a mold or pattern of the appropriate shape. The parts are then removed from the mold, cleaned up and machined as necessary. The forging process creates a component by shaping a red-hot piece of metal under high pressure in a forging press. This process yields parts that are free from the defects that often plague metal castings such as shrinkage and porosity. Wrought components are those that have been intensely rolled or squeezed through a mandrel, sometimes at room temperature and sometimes at very high temperatures. In valves, wrought components, which are usually round in shape, are found most often in stems or spindles. As cousins to forgings, wrought components also are devoid of the defects that often are found in castings.

You might wonder, if forgings and wrought components are so great, then why aren’t they used in all valves? The answer is simple—cost. Castings are much cheaper to produce than forgings. In a world where money is no object and ultimate quality is the only goal, all valves would be forged. But the casting process usually achieves the desired ratio of strength to cost, although defects inherent in the casting process have to be considered. And if an additional degree of strength or safety factor is required, the valve designer usually has only to increase the casting’s thickness. Although there are some challenges resulting from the current crop of imported steel castings, cast valves have earned their keep very well over the last 150 years or so.

Although valve shell material selection is very important, other components must receive the same care when it comes to materials selection. Of particular concern is the valve’s trim. Valve trim is loosely defined as the closure elements in a valve, including disc, ball and seats, as well as the stem or spindle, all of which are exposed to the fluid contained in the valve. Valve closure element materials selection is very important because of the need to consider both corrosion resistance and possible erosion, caused by the high velocity created as the valve is closed and opened. As you know from using a narrowed down nozzle on a garden hose, the water sprays out farther and faster through the narrowed orifice. This velocity is inversely proportional to the size of the opening or orifice. This same situation occurs in a valve as it is cracked open or nearly closed. The smaller opening creates a very high velocity that can actually wash away the metal in the areas adjacent to the narrow flow path.

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.

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.

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.