- Published on Sunday, 19 October 2008 19:46
- Written by Greg Johnson
If you walk through any large PVF warehouse and look around, it seems that valves come in three flavors: black, silver and gold. These “flavors” are really iron, steel and bronze, which constitute the lion’s share of valve body materials used in today’s valve industry.
Valve designers also utilize dozens of other metals and alloys to accomplish their goal of effective valve design and function. So why do we need a huge variety of materials for valve construction? Why can’t that golden valve in the big box store work for everything? The answer is that valve materials, particularly valve body materials, are chosen primarily for two reasons—strength and corrosion resistance. And in valve material selection, one type does not fit all.
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.
Bodies and Bonnets
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.