Materials for water treating equipment

Ordinary carbon steel is by far the most important alloy in the oil and gas industry because it accounts for more than 98% of the construction materials used in produced-water systems. As a general rule, every attempt should be made to use steel, such as modifying the process with corrosion inhibitors in the fluid or coating the steel.

Material produced water systems

Selection of materials for produced-water-treating equipment must take into account:

• the pressure rating of the application
• the corrosivity and erosivity of the fluid
• the end location

The primary problem constituents in produced water are:

• salt
• hydrogen sulfide
• carbon dioxide
• sand

Proper material selection is critical for long operation life and minimal maintenance. A small increase in capital expenditure for an optimum material selection can greatly reduce the mean time to maintenance or failure, thus greatly saving on operating expenses.

Normal service materials

Ordinary carbon steel is by far the most important alloy in the oil and gas industry because it accounts for more than 98% of the construction materials used in produced-water systems. As a general rule, every attempt should be made to use steel, such as modifying the process with corrosion inhibitors in the fluid or coating the steel. Even though the chloride content can be higher than seawater, piping and vessel equipment used to treat normal produced water is usually manufactured from carbon steel because the oxygen level is very low. Carbon dioxide in the water stream may present a problem because it will form carbonic acid, which is corrosive to carbon steel, even in the absence of oxygen. As little as 1 ppm carbon dioxide in pure water will lower the pH to 5.49, sufficient for a corrosion problem. A corrosion inhibitor or coating must be used in this case to protect the carbon steel.

Piping Systems

The most commonly used material for piping systems in produced-water treatment is carbon steel. Piping systems for produced water are normally designed to the American Soc. of Mechanical Engineers (ASME) standard B31.3 for process piping and use ASME specification SA-106 carbon steel piping. Because of its strength, this material can withstand high pressures and is commonly available in diameters up to 48 in.

Steel pipe has excellent impact resistance and flexural strength. It is susceptible to both internal and external corrosion. When oxygen is excluded from the system, which is the normal case for produced water, internal corrosion may not be a problem. External corrosion is normally fought with a coating system. For pipe exposed to salt air, a three-coat epoxy-paint system is often specified. Underwater pipe may be protected by:

• a thin-film-epoxy
• coal-tar-epoxy
• extruded-plastic system

Thin film epoxy

Thin-film epoxy is more popular because of its greater toughness to potential handling and installation damage.

Vessel fabrication

Vessels and other equipment to handle produced water are most commonly fabricated from carbon steel. Small pressure vessels (< 48 in.) can be made from seamless pipe (SA-106), and larger pressure vessels are fabricated from rolled plate (SA-516). These grades of steel exhibit a yield strength of at least 60,000 psi, which can easily be fabricated into inexpensive, large, high-pressure-containing components. Nonpressure-vessel applications normally use carbon steel plate (SA-516 or SA-36).

Much like a carbon steel piping system, the vessels must be protected from internal and external corrosion. Produced water normally contains insignificant amounts of oxygen, which greatly minimizes internal corrosion. If oxygen is present, sacrificial anodes (normally of aluminum) can be used within a pressure vessel. External corrosion is normally combated with a coating system. In offshore, salt-air environments, a three-coat epoxy-paint system is most commonly used.

Materials for severe service environments

Severe service environments are encountered increasingly as oil and gas production embraces more difficult production situations. Materials have been developed to handle highly corrosive well products (CO₂ and H₂S) at high production temperatures and pressures. The two most common classes of metals used in highly corrosive applications are stainless steels and superalloys. These materials may be expensive; however, they may provide the only acceptable long-term solution.

UNS refers to the “unified numbering system” that is a standard practice of the American Soc. for Testing and Materials (ASTM) for commercial metals and alloys.

316 and 316L

The most common stainless steels used are 316 and 316L (UNS S31600 and S31603), providing moderate resistance to chloride and H2S and good resistance to CO₂. The L grade is used for components that require welding.

Duplex and superduplex

For higher chloride and H₂S resistance, duplex (UNS 31803) and superduplex stainless steel (UNS 32750) are finding increased use.

Incoloy* 625

Extreme conditions are being met by the use of Incoloy* 625 (UNS N06625) and 825 (UNS N08825), which are nickel-based superalloys.

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• Incoloy is a registered trademark of Special Metals Corp.

Materials for erosion protection

Material selection for erosion protection (i.e., sand) is dependent on the type of wear exhibited. There are two main mechanisms for wear—impact and sliding abrasion. Impact wear occurs where the particles directly impinge against the material surface from a near-perpendicular angle. These particles may hit the surface once and bounce off. Sliding wear occurs where the particles move parallel to the material surface and rub repeatedly against the material substrate. Each mechanism abrades the material surface differently; therefore, appropriate materials are suggested for each.

Impact wear

To accommodate impact wear, a material is needed that will absorb the impact without breaking. Leaded tees are used in piping to make 90° turns. The soft lead absorbs particle impact without wearing into the steel pipe. Elastomer rubbers provide good impact resistance but find limited use in produced-water applications because of dissolution in the presence of hydrocarbons. Nitrile rubber overcomes this hindrance because it displays a moderate resistance to hydrocarbons and can be used to internally line vessels or pipe.

Sliding wear

Material hardness is the main characteristic used to accommodate sliding wear. If the material substrate is harder than the abrading material, ideally, no wear will occur. The most common solid present in produced water is sand, which is primarily quartz (SiO₂). The two groups of materials used to handle sliding wear are hard metals and ceramics.

Hard metals

Metals are used in cases of moderate sliding-wear protection and where some toughness is required. The most economical hard metal is white iron, which is a high iron-carbide-content cast iron. White iron is used for slurry pump impellors and casings, and corrosion-resistant grades are available. A slightly less hard metal that is also commonly used is Stellite**, a cobalt alloy that finds large use in smaller, intricate-wear applications. Both white iron and Stellite are cast materials, difficult to machine, and not as hard as quartz.

Ceramics

The best commercial materials to handle this type of wear are ceramics. Ceramic materials are brittle (and, therefore, hard to work with) and expensive, but in cases of sliding wear, they provide the maximum erosion protection. The most common ceramic used is alumina (Al₂O₃), which can be cast into intricate sections or made into tiles for hand lining. This is the most common material for manufacturing desander liners. Additional common, though more expensive, ceramics are:

• silicon carbide (SiC)
• tungsten carbide (WC)

These materials provide 5 to 10 times higher wear resistance than alumina but are correspondingly more expensive. Diamond films provide the best protection possible but find limited use in large-scale erosion protection because of the cost. The ceramics listed above are harder than quartz.

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• Mark of Deloro Stellite, Swindon, U.K.

Materials for seawater systems

The selection of materials for equipment in seawater treating must take into account the pressure rating of the application, the corrosivity of the fluid, and the end-use location. Seawater contains approximately 3.5% salt (sodium chloride) and is slightly alkaline (pH 8). It is a good electrolyte and can cause severe corrosion. Corrosion rate is affected by:

• oxygen content
• liquid velocity and temperature
• the presence of biological organisms

In most oil and gas seawater-handling systems, oxygen content is the biggest concern in designing for corrosion protection because the dissolved oxygen greatly accelerates the corrosion rate. Typical pipe-transport velocities are in the range of 3 to 12 ft/sec, which will have a minimal effect on corrosion but will minimize the ability of fouling organisms to attach themselves to the piping materials. Generally, erosion resistance is not a major factor in handling seawater because the solids are not very erosive.

Metals for piping and vessels

For untreated seawater, exotic metals such as titanium, Hastelloy C, and copper-nickel (70-30 or 90-10) must be used to prevent severe corrosion. Stainless steel (Type 316 or 304) will exhibit severe pitting in a flowing seawater application. This can be reduced somewhat by upgrading to duplex stainless steel. Vessels can be made of standard carbon steel but must be lined with an internal coating to prevent corrosion. Common materials for internal vessel coating are neoprene rubber or glass-flake-filled, amine-cured epoxy. Both epoxy lining and neoprene are good to 200°F. Standard carbon steel piping can be used on seawater after removal of dissolved oxygen.

Plastic Piping

Plastic pipe is the most common material for low- to moderate-pressure raw seawater service. It has a pressure rating up to 450 psig, depending on the pipe size. Plastic pipe:

• is not susceptible to either internal or external corrosion in seawater service
• has a low friction factor
• has a light weight that makes it easy to install

In general, plastic pipe can be purchased in accordance with the following American Petroleum Institute (API) specifications:

• Spec. 5LE for polyethylene line pipe (PE)
• Spec. 5LP for thermoplastic line pipe [polyvinyl chloride (PVC) and chlorinated polyvinyl chloride (CPVC)]
• Spec. 5LR for reinforced thermosetting resin line pipe (RTRP)

Because of the superior strength and greater resistance to internal pressure and hydrocarbons, fiber-reinforced plastic (FRP) is the most commonly used material for this service. This pipe is available as a filament-wound fiberglass-reinforced vinyl ester material, usually with a resin-rich reinforced liner. It is available up to a 16-in. nominal pipe size and a pressure rating from 150 to 450 psig (this decreases with pipe diameter). FRP pipe has the disadvantage of being very brittle, which can lead to damage during installation. Ultraviolet (UV) light, or sunlight, can degrade the physical strength of FRP by attacking the resin-glass bond. Pigments or dyes are incorporated into the resin to form a barrier for UV penetration into the laminate, which maintains degradation to a surface-only attack. In certain cases, an overwrap, such as an organic veil layer, can be used to provide even greater UV resistance.

Materials for Steam Systems

Primary concern with steam transport systems are temperature and pressure. Corrosion is usually not a major issue in clean, dry steam systems because the dissolved solids and gases are removed before steam generation. Corrosion can be a problem if the steam is not fully dried and if the water content concentrates boiler water chemicals. Plain carbon steel is resistant to general corrosion by clean, deaerated steam up to 850°F. For higher temperatures, high-strength, low-alloy (HSLA) steel is needed, such as 2 1/4Cr-1Mo, which is good to 1,200°F. Generally, erosion resistance is not a major factor in these systems because steam from a boiler is free of solids. Proper material selection is critical for long operation life and minimal maintenance.

Noteworthy papers in OnePetro

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External links

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See also

PEH:Water-Treating Facilities in Oil and Gas Operations

Steam Injection

Water treating facilities

Produced water discharge or steam injection

Surface water treatment for injection

Water treating chemicals

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Hank Rawlins, PhD, PE