Introduction

The valve selection process requires careful evaluation of different variables and parameters. Everything from internal valve design, connection methods, and material selection affects valve performance in the long run. While several valve manufacturing materials exist in the market, each type and grade of material is optimized for specific fluid flow applications. The longevity of valves is vital for various industrial applications.

Valves must be fabricated using materials that can withstand pressure and temperature variations of the specific fluid application. Industrial valves, such as ball valves, butterfly valves, and gate valves, will have varying physical appearances, operating mechanisms, and performance characteristics. Fluid pressure and temperature variations stress the valve body and internal valve components, which may cause ruptures if the material is not strong enough.

Engineers analyze fluid properties such as purity, corrosiveness, abrasiveness, in-process temperature, pressure, and fluid viscosity when choosing valve materials. Some fluids react with the materials, leading to gradual wear and tear of internal and external valve parts. Another problem when using valves is the wire-drawing effect, which is the gradual wear of the valve disc and seat due to the erosive action of fast-moving fluids past the valve.

Engineers use several standards, including ASME B16.34 and EN12516, for valve designs, pressure rating, manufacturing and testing procedures, material combinations, and valve dimensions. Here is an in-depth look into the materials used to manufacture long-lasting valves that can complement asset reliability programs and provide impeccable performances for different fluid flow applications.

Valve Body Materials

The fluid pressure from adjacent pipes acts directly against the valve body. The valve body sustains the resultant fluid pressure. Here are some materials used to fabricate valve bodies.

Stainless Steel

Most chemical service valves are fabricated using stainless steel, which is resistant to chemical corrosion by certain fluid combinations. Stainless steel is available in several forms. Stainless steel type 304L provides optimum chemical resistance for pipelines conveying nitric acids and other highly corrosive fluids.

Stainless steel type 316 contains molybdenum, which enhances corrosion resistance and offers the valve body better performance for severe fluid service. Valve bodies made of these materials can accommodate a wide temperature range (-425^oF to 1500^oF) and can withstand fluid flow conditions that cause pitting and creep.

Inconel

This is an advanced valve material that is a blend of chromium and nickel alloys. This material composition increases the valve’s corrosion resistance and enables the valve body to withstand extreme process temperatures and pressure. Additionally, this material is resistant to oxidation and can be utilized for reactive chemical fluid service.

Monel

Monel is a nickel alloy that contains substantial traces of iron, copper, manganese, and silicon. Valves with bodies made of Monel are common in saline fluid applications. Monel has excellent corrosion resistance, making it suitable for fluid applications with high fluoride and hydrogen content.

Carbon Steel

Carbon steel is among the most popular materials for constructing valve bodies, since they are easy to manufacture. Carbon steel valves are suitable for low- to medium-pressure fluid processes.

Manufacturers use wrought carbon steel or forged carbon steel. Wrought carbon steel (WC) is available in three grades (A, B, C), and is classified as WCA, WCB and WCC with WCC being the strongest. Valve bodies made using WCB are suitable for low-temperature applications, where the service media is non-corrosive. WCC can accommodate fluid applications operating at temperatures below 800^oF. Such valve bodies can facilitate the flow of non-corrosive fluids and gases. Valves with carbon steel bodies are common in pipelines transporting compressed air as well as saturated and superheated steam. Exceeding the 800^oF temperature limit transforms carbon into graphite, impacting valve performance.

Chromium-Molybdenum Steel

Some fluid applications, such as steam service, experience high rates of erosion corrosion due to the extreme temperature and pressure. Manufacturing valve bodies for such processes requires materials that have superior corrosion resistance. Chromium-molybdenum steel is popular in pipelines transporting fluids with high sulfur content.

There are several grades of chromium-molybdenum steel, including WC9 and C5. C5 is prone to cracking when welded. Generally, chromium-molybdenum steel valve bodies are more difficult to cast but provide dependable and durable fluid service.

Alloy C-276, a nickel-chromium-molybdenum alloy, has superb corrosion resistance and is suitable for chemical pipelines. Other materials used for valve bodies include:

• Brass
• Titanium
• Polyvinyl chloride (PVC)
• Polypropene

Valve Closure Mechanism

The valve closure mechanism (ball, disc, or plug) is the most wetted component of the valve; it comes into direct contact with fluids bearing different properties. The durability of the flow control mechanisms depends on the total cycles that the valve can sustain. Actuating the valve moves the disc to a position that initiates or stops fluid flow, or changes its direction.

If the pipeline transports viscous fluids with impurities, the rate of wear increases. Valve manufacturers optimize its durability by selecting the most resilient materials to fabricate the disc. The material should be unreactive when in contact with the service fluids, should possess sufficient mechanical strength to avoid deformation due to stress exerted by the fast-moving service fluid, and should accommodate temperature variations in different fluid flow applications.

Sometimes, the valve is left in a partially closed position, where the disc and valve seat are near each other but not in total shutoff. This permits the flow of small quantities of fluids past the valve. When the valve is left in this position for long, the flowing fluid begins eroding sections of the disc and the valve seat in a phenomenon known as wire drawing.

The eroded portion (groove) appears like a thin stretch of wire. Wire drawing is more evident in applications with high differential pressure across the valve. Wire drawing leaves room for leakages and fugitive emissions at valve locations, posing a safety risk to equipment and personnel.

Depending on the type of the application, valve discs can be fabricated from stainless steel, Inconel, Monel, brass, chromium-molybdenum, carbon steel, or any other suitable valve material. The disc should not be overly bulky as it will demand larger actuation forces.

Valve Seat Materials

The service medium must be sealed as it passes through the valve and when the valve is in a closed position. Valves have specific leakage classes, determining the allowable leakage rates. As mentioned earlier, leaving the disc and the seats in an almost closed position exposes the valve to erosion due to the wire-drawing phenomenon. The seat materials must endure erosion-corrosion caused by fast-moving service fluids. The most popular sealing materials include the following.

Nitrile Rubber

Nitrile rubber (also known as Buna-N) is an elastic seat material that is used to seal soft-seated valves, which are chemically compatible with different fluids. This material seals against leakages around the valve body and possesses excellent abrasion resistance and sufficient strength that can withstand damage due to compressional forces.

Nitrile rubber is used for sealing valves in hydrocarbon, water, petroleum, oil, and grease pipelines. Nitrile rubber seats are not recommended for use in pipelines handling chlorinated hydrocarbons.

PTFE

This is a fire-resistant sealing material. It has remarkable resistance to corrosion and abrasion due to viscous fluid service. PTFE reinforced with fiber can withstand high fluid pressure and temperature. PTFE is suitable for pipelines transporting chemicals, acids, and viscous fluids. Ordinary PTFE can operate within a limited temperature and pressure range.

Metal Seats

These valves use metallic seats to seal the service media. They are suitable for high-pressure, high-temperature applications. They use copper-, chromium-, and nickel-based alloys, which are resistant to chemical corrosion and fluid abrasion.

Summing Up

Choosing the correct valve for any fluid application is a delicate process that demands due diligence and comprehensive analysis of the desired pipeline performance parameters. The process engineer must strike a perfect balance among the desired performance levels, process reliability, and manufacturing costs.

A durable valve reduces safety risks and performs optimally over a long time. Using the correct valve materials means that not only can the valve body and trim withstand the chemical properties of the service fluid, but the company can also enjoy uninterrupted fluid flow and effectively control its valve maintenance costs.