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Achieving Predictable Valve Performance for Safety Applications

Achieving Predictable Valve Performance for Safety Applications

A focus on having the proper specificati...

Putting Servo Valves Back to Work

Putting Servo Valves Back to Work

Industries as varied as paper production...

Corrosion and Fouling: Is There a Solution?

Corrosion and Fouling: Is There a Solution?

According to a 1998 study released by the ...

Valves with All the Trimmings

Valves with All the Trimmings

The term valve trim has been around for ...

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Industry Headlines

Leading Economic Indicators Rose in January

Friday, 23 February 2018  |  Chris Guy

The Conference Board Leading Economic Index (LEI) for the U.S. increased 1.0% in January to 108.1, following a 0.6% increase in December, and a 0.4% i...

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Valve Positioners Offer Improved Control Valve Performance

Valve Positioners Offer Improved Control Valve Performance

Monday, 19 February 2018  |  David Matherly

From time to time, we are re-posting well-received or particularly valuable articles that have previously run on VALVEMagazine.com so that those who m...

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Industry Headlines

MRC Global Announces Fourth Quarter 2017 Results

3 DAYS AGO

MRC Global’s sales were $903 million for the fourth quarter of 2017, which was 26% higher than the fourth quarter of 2016 and 6% lower than the third quarter of 2017 due to seasonality. As compared to 2016, all sectors increased driven primarily by midstream and upstream.

MRC Global's fourth qu...

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Curtiss-Wright Acquires Dresser-Rand Government Business

4 DAYS AGO

Curtiss-Wright has entered into an agreement to acquire the assets that comprise the Dresser-Rand Government Business (Dresser-Rand), a business unit of Siemens Government Technologies, for $212.5 million in cash.

Dresser-Rand designs and manufactures mission-critical, high-speed rotating equipment sol...

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U.S. Refiners Increasingly Turning to Export Markets

3 DAYS AGO

“U.S. gasoline consumption has leveled off as the stimulus provided by low and falling oil prices between 2014 and 2016 has faded, so refiners are increasingly turning to diesel and customers in emerging markets,” Reuters  reports .

“In geographical terms, the fastest growth in co...

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$900M Power Plant Planned for Indiana

3 DAYS AGO

Vectren Energy Delivery of Indiana is proposing to install an additional 50 megawatts (MW) of universal solar and build an 800 to 900-MW natural gas-fired generation facility in addition to other critical investments that will significantly change the way the company generates power for the region .

Th...

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Leading Economic Indicators Rose in January

1 DAY AGO

The Conference Board Leading Economic Index (LEI) for the U.S. increased 1.0% in January to 108.1, following a 0.6% increase in December, and a 0.4% increase in November.

“The U.S. LEI accelerated further in January and continues to point to robust economic growth in the first half of 2018. Whi...

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IHS Markit Manufacturing Index Rose in February

2 DAYS AGO

U.S. manufacturers reported a strong upturn in business conditions during February, which continued the positive trend seen at the start of 2018. At 55.9, up from 55.5 in January, the seasonally adjusted IHS Markit Flash U.S. Manufacturing Purchasing Managers’ Index (PMI) pointed to the fastes...

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The A to Z of Valve Materials

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.

Withstanding Stresses

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.

Corrosion Resistance

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.

Valve Trim

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.

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