A valve is a device that regulates, or controls the flow of a fluid (gases, liquids, or slurries) by opening, closing, or partially obstructing various passageways. In an open valve, fluid flows in a direction from higher pressure to lower pressure. Valves have many uses, including controlling water for irrigation, industrial uses for controlling processes, residential uses such as on/off and pressure control to dishwashers and washing machines for clothes as well as water taps in the home. Valves are also used in the military and transport sectors. In HVAC ductwork and other near-atmospheric air flows, the role of valves is more like a damper.
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An actuator is a device that uses a form of power to convert a control signal into mechanical motion, for example, from electric door locks in automobiles, to ailerons on aircraft. A few types of actuators include electric motors, stepper motors, electroactive polymers, screw jacks, servomechanism, solenoids, and hydraulic cylinders. Therefore, it can be said that a valve is a physical part of a system that opens and closes to dictate the amount of fluid being allowed to pass through, while the actuator tells the valve when to operate.
There are numerous applications of valves, ranging from water and sewage processing, mining, power generation, processing of oil, gas and petroleum, food manufacturing, chemical and plastic manufacturing and many others. In daily use, valves are used for plumbing, such as taps for tap water, gas control valves on cookers, small valves fitted to washing machines and dishwashers, safety devices fitted to hot water systems, and poppet valves in car engines.
Valves may be operated manually, either by a handle, lever, pedal, or wheel. Valves may also be automatic, driven by changes in pressure, temperature, or flow. These changes may act upon a diaphragm or a piston which in turn activates the valve, examples of this type of valve found commonly are safety valves fitted to hot water systems or boilers.
Why do Valves Need Paint Coatings?
Valves are often used in flow systems that contain toxic liquids or gases, perhaps re- active chemicals, flammable petroleum, high-pressure steam, or compressed air. Usually, valves are made from steel as it is the cheapest material and is very easy to cast or machine into various intricate shapes. However, various other materials are also used to cast valves. Some of the common materials are carbon steel, ductile iron and cast iron, stainless steels, brass and bronzes, plastics such as PVC. Carbon steel is the cheapest material, easily available and easy to fabricate in any shape or form. However, it has a strong tendency to corrode in even moist environments and can corrode heavily in aggressive environments such as onshore and offshore. Thus, alternative materials such as stainless steels, and bronze, etc. are used. Plastic valves are also becoming very popular, and they are free from any corrosion.
Corrosion of the valve is basically the deterioration of valve material due to corrosive fluid as well as due to its flow rate. Corrosion of valve occurs both from the inside as well from outside. Internal corrosion is mainly due to two reasons: corrosive fluid and its flow rate. Figure 2 shows how the flow rate affects the corrosion rate of the pipe material from inside. External corrosion of valve is equally important and occurs mainly from the high humidity and polluted environment or from the vapours released in the plant, specifically in chemical plants, petrochemical plants, or refineries, where gases such as SO2 and H2S will attack the valve surface and cause various forms of corrosion ranging from uniform, localized pitting and crevice, galvanic and stress corrosion cracking. External corrosion also occurs by the release of corrosive vapours from the leakage of valves.
Corrosion Protection Technologies
There are various kinds of valves available in the market with different corrosion protection technology. There is a good market with zinc coated valves. Zinc can either used as a primer to enhance the life of the valve by cathodic protection whilst zinc plated valves are well known for water and gas applications1 which may provide longer life. There are several kinds of paint coatings starting from alkyd to acrylic to epoxy which protect the external valve body from corrosion.
Two-component acrylic paint is used for valves that are exposed to atmospheric influences and UV. The coating meets all requirements in connection to excellent resistance to UV radiation, mechanical stress, and corrosion. Two component acrylic paint is used for valves exposed to atmospheric pollutants and UV. This coating can be used for all valves used in industrial applications involving high air humidity and aggressive atmospheres. Hard-rubber lining is a special coating used for valves in the chemical industry. It consists of a high-tech, premium- quality synthetic or natural rubber.
The strong adhesion of the material to cast iron is ensured by vulcanizing the rubber to the cast-iron substrate. The finished rubber lining is non-porous and exhibits great hardness as well as excellent chemical resistance. This coating is available e.g., as a lining for butterfly valves, the plunger valves, and slanted seated tilting-disk check valves.
Other superior quality valve coatings can be achieved by powder coating. Powder coating is an electrostatically applied coating on an oppositely biased valve body, followed by baking to achieve a finish consisting of a mixture of finely ground resin, pigments, and binders. Powder coatings have much longer durability and also have high flexibility. Powder coating is the standard technique for actuator casings, yokes, and cylinders, as well as for cast iron and carbon steel valve body assemblies.
In case of a valve used in a refinery, chemical or petrochemical plant, it must be coated with not less than an epoxy coating or further modified epoxy coatings such as Epoxy-modified by MIO, or glass flakes. They enhance the strength and durability of the coating and reduce the permeability providing longevity to the coated valve. In case the environment is very corrosive specialized coatings are used. Some of them are explained:
To take care of severe acidic or alkaline environment from pH 1-14 and also to stabilize the coating up to a temperature of 250°C, a nano-modified epoxy-based coating is available which can be applied either directly on the blasted surface or after partial cleaning using st2/st3.2 (st2/ st3 refers to SSPC methods of partial cleaning using wire brush, sandpapers or using power tools.) 2.
Teflon coating on valves is required whenever one wants to protect the valve from severe acidic vapours released from the plant as well as toxic fumes and leakage from valve to pipe joints. PTFE solenoid valves are commonly used in controlling and distributing processes, throughout OEM equipment for cleaning, mixing, blending, and etching. They are frequently used side-by-side with PTFE pumps, PTFE check valves and PTFE tubing, fittings, and other devices. Details for Teflon coating on ball-valves and vacuum valves can be seen in references.3, 4
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Internal Coatings on Valves
Internal valve surfaces can suffer damage due to high flow rates of the fluid, which leads to the erosion of the surface. If, in addition, there are impurities in fluid such as chlorides, oxygen or sulphur, this erosion is combined with corrosion leading to severe erosion-corrosion. As an example, see Figure 3 for a propeller in a water pipeline. In the water industry, valves are exposed to considerable stress such as high-water pressures, extreme temperatures, desert, or tropical climates. Valves therefore need to be made of a suitable material, with perfect technology and quality needs to ensure that the valves function reliably even after decades of service.
Thus, the coating should be made either by a hard material with good corrosion resistance or it can be made with normal steel with a hard coating. Several hard coatings are available in the market. However, the coating Belzona is a two-part ceramic-filled epoxy coating designed to provide erosion and corrosion resistance of metal surfaces. This solvent-free epoxy coating has outstanding chemical resistance and thanks to its high compressive strength it can also be used to create a perfect shim.
This material is easy to mix and apply without the need for specialist tools and it cures at room temperature, eliminating the need for hot work. It has longterm erosion and corrosion protection, excellent bonding to almost any rigid surface including steel, aluminium, copper, brass, and glass reinforced plastics with high compressive strength and excellent resistance against a wide range of chemicals. It can be used for internal coating for centrifugal and positive displacement pumps, heat exchangers, water boxes, and tube sheets, butterfly and gate valves, fans and Kort nozzles. (Also known as a ducted propeller, a Kort nozzle is a marine propeller fitted with a non- rotating nozzle for enhanced efficiency.)
Thermal spray gives a wide selection of hard coating solutions for the valve industry. The careful selection of coating materials is integral to the valve design process. Extremely hard coatings, using tungsten carbide or ceramic compositions can be achieved. Thermal spray coatings produce wear and corrosion resistance in valves.
Friction fluctuation can also be brought under control using such coatings. Ceramic and cermet materials such as chromium oxide for hard-coating valves are used in the chemical processing sector, and tungsten carbide and similar materials are used for the oil & gas industries. Metal-to-metal coating solutions for ball and gate valves are used in the petrochemical industry. Though, there are several thermal spray processes such as flame and arc spray and plasma, excellent results have been achieved using specialized high velocity oxygen fuel systems (HVOF) to achieve a high hardness and pore-free coating. Carbides are generally used for oxidation resistance as well as erosion–corrosion resistance. Arc spray techniques are used for depositing the tungsten carbide on metals as a porous coating. The HVOF technique is also used for coatings which are hard and abrasion resistant.
Summary
Valves are essential elements of a flowing system, which deteriorate with time. External corrosion of the valves occurs due to severe environment, especially C4 and C5 and toxic vapours emitting from chemical, petrochemical plants, or refineries. C1, C2, C3 C4 and C5 are the various categories of the extent of corrosion as per ISO classifications. Internal surface damage of the valves occurs due to erosion and corrosion as a result of the change in flow pattern as well as impurities present in the fluid. In order to avoid erosion and corrosion, one can either choose a material with high hardness and wear resistance or can create coatings which provide wear and corrosion resistance. There are a variety of coatings available ranging from simple alkyd, acrylic coatings for C1 and C2 environments to high performance coatings from simple epoxy to modified epoxy, Teflon and many other for the protection of external surface. For internal surface, both corrosion resistance and hardness of the coating is important.
Choosing the best thermal spray company for your equipment is an important yet difficult endeavor. Given how many thermal spray companies are out there competing for clients, there may be some who are willing to cut corners to provide their services at lower prices. But if this means poorer results, it is not what you want for your company. Read on for some factors to consider.
You are entrusting your equipment to this firm. You should find out what will happen to it. If a firm has a defined set of processes, it implies they’ve thought about the best way to do things, which usually means your part will be better cared for. If a firm understands what it’s doing, it should have these ready for prospective consumer evaluation and shouldn’t be afraid of a quality audit or even a shop visit. Take note if you detect tension on the line when you ask these questions. You should also make certain that their normal operating procedures contain processes for quality and coating inspections.
You may not need to ask this question if you already know they are ISO certified, but it never hurts to know whether they have the capability to undertake lab testing. You might inquire about how these firms assess the quality of their services. Do they use the right instruments to measure coating thickness regularly during the spray process? Do they have a coordinate-measuring machine to measure complex or spherical surfaces like valve balls? These tools and methods will guarantee that your part is returned to you with the proper dimensions.
It’s a great idea to verify the lab tests and equipment are of high quality. If the company does not do regular machine monitoring and maintenance, the results you want will be unachievable. This goes the same for all of their service and measurement equipment. This will have an impact not only on the quality of the equipment’s coating but also on its process durations and materials, thus costing you extra time and money. The employment of robotic technology is an easy approach to achieve uniform, high-quality thermal spray coating.
Some of the materials used in thermal spray processes, like aluminum oxide grit and chrome carbide powder, are classified as hazardous compounds and must be handled, stored, and disposed of in a specific manner. While this may not have a direct influence on the quality of your coating, it does reveal a lot about the individuals you’re working with. Material handling that is done correctly demonstrates a feeling of responsibility for the task that is being done. If they aren’t taking proper care to dispose of their materials, they might not be taking adequate care of your components either.
It might be difficult to determine who to entrust with your machine’s components. While a lower price may be appealing, you may be compromising quality. Working with a firm that can answer your questions with elegance and forethought speaks a lot about the level of service you will receive.
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