Paint and coating selection of valves In many cases, paint on new valves is only a temporary coating designed to delay external corrosion during valve storage or before the final paint or coating is applied to the entire piping system after installation.
Except for the black valves in the waterworks, the valve standards hardly keep silent on the paint and coating of the new valves, calling the paint “manufacturer’s standard”. The updated version of the American Petroleum Institute (API) 600 added that the paint should be silver or aluminum. Over the years, an API 600 valve manufacturer has painted its black Valves a pleasant light blue.
Although valve factory OEM paint may be basic, today’s end users have detailed paint and coating requirements for many valves they buy. These requirements go far beyond the simple silver or black coating that wraps the valve when it leaves the factory.
But what are the paints and coatings used?
Most of us know that paints are usually related to thinners. This diluent indicates that the paint or coating contains more than one compound or chemical component. Most of them are organic compounds composed of carbon based components. They contain four main elements:
Pigments that not only provide coloring but also contribute to the corrosion resistance and overall strength of the coating or paint mixture
Additives, including thickeners, UV stabilizers and defoamers
Resin, composed of acrylic acid and epoxy resin, can combine all components to form a homogeneous substance
Solvent affecting drying time and viscosity of coating mixture
Paint and coating selection of valves
The most basic industrial coatings are varnish and enamel. These polymer types constitute the common vanilla of the industrial painting world. For residential and commercial applications, water-based acrylic resins are preferred. In some countries with very strict regulations on volatile organic compounds (VOC), aqueous acrylic emulsion (lotion coating for short) is used for industrial application.
Specific coatings have dozens of different components. Each end user may have many different requirements, depending on the specific weather exposure and plant environmental impact. All these different coatings have two requirements: the first is proper surface treatment, and the second is the need for primer.
The primer is the interface between the final paint or coating and the substrate. They provide temporary corrosion resistance during transportation and storage. The most popular type of industrial primer is the primer with anticorrosive zinc base.
Proper surface treatment is essential for the successful application of any coating. The surface may be chemically or mechanically prepared. The chemical preparation process creates an etched surface that provides more bite force for the coating to be applied. Mechanical surface treatment includes the removal of surface defects by water or dry spray technology. In rare cases, old-fashioned grinding or brushing is still used. Different paints require different surface treatments to properly adhere.
Grease, oil, dirt and rust are removed before any coating is applied.
Detailed surface preparation procedures and requirements are specified in the SSPC documents. SSPC also issued a series of standards and recommendations for all types of industrial coatings and coatings.
SSPC and NACE International (NACE) have different degrees of painting and coating surface treatment specifications (Table 1).
The two most frequently cited surface treatment methods are SSPC SP6 (commercial blast cleaning) and SP10 (near white blast cleaning).
Thousands of valves and actuators are damaged every year due to improper protection during coating surface preparation. Linear valves appear to have suffered the most damage, especially externally threaded and yoke (OS & Y) bonnet valves. The right angle rotary valve cannot be protected from damage during the surface treatment stage. Some areas that are frequently damaged during surface treatment blasting include those listed in Table 2.
Acrylic fiber. Acrylic coatings for protective coatings are based on organic or water-based solvents. Waterborne acrylic emulsion coating is also known as lotion type coating. They are usually used when there is a VOC release problem during the coating process.
Alkyd resin. Alkyd resin is a fatty acid modified polyester resin. Most alkyd coatings dry due to the chemical reaction between fatty acids and air.
Epoxy resin. Epoxy resin coating is a resin composed of two parts. When they are combined, a chemical reaction will be formed between the epoxy resin and the curing agent for curing the epoxy resin. Epoxy resins are usually very tough and corrosion-resistant coatings, but their service life is very short. Most epoxy coatings also have poor UV stability.
Polyurethane. Like most epoxy resins, polyurethane is a two-part coating. They exhibit a smooth surface and are often used as final coatings for epoxy and zinc primers.
The first question that needs to be answered when selecting a coating is “why should the equipment be coated?” In piping systems and valves, the answer is usually corrosion resistance rather than overall attractiveness.
The second and most important question concerns the environment to which the coating will be exposed, for example:
Is there any chemical smoke nearby?
What is the working temperature?
Is there any possibility of chemical splashing or spilling?
Will the coating cover the insulating material?
Will the coating be cleaned? What cleaning process and chemicals will be used in the cleaning process?
Will the coating be applied to the existing coating?
Other questions about the correct choice include: where the coating will be applied, in the workshop or on the site; What is the expected service life of the coating?
The last question is: what is the budget of the coating? This issue often trumps other issues, depending on budget, life expectancy and other considerations.
Real world examples
The coating specification consists of three parts, which constitute a complete coating process. They are: 1) pretreatment and surface treatment, 2) paint or coating application and 3) paint or coating curing. The paint or coating is transformed into a solid film by the curing process. This curing can be carried out at room temperature or in an oven.
The paint or coating requirements of the end user are different from those of the valve manufacturer. The following are some example paint / coating specifications for these two entities:
Petrochemical plant, general piping system, including valves:
Surface treatment: SP10
First layer: phenolic epoxy resin
Second layer: phenolic epoxy resin
Offshore facilities, process equipment and pipelines:
Surface treatment: SP5
First coat: solvent inorganic zinc
Second layer: silicone acrylic
Ball valve manufacturer, standard finish:
Surface treatment: SP3 (equal)
First coat: phenol modified alkyd resin paint
Second layer: synthetic resin aluminum paint
Gate, globe and check valve manufacturer:
Surface treatment: SP6 or SP3
First coat: modified alkyd resin primer
Second coat: silicone modified alkyd resin paint
Compact gate, globe, and check valves designed in accordance with API 602 are usually unpainted at the factory. However, they usually have a corrosion resistant (e.g., phosphate) coating to prevent corrosion until they are installed and the piping system receives its final coating.
The development of dozens of paint and coating options has helped to extend the service life of installed valves. This means that potential leakage caused by external corrosion is almost eliminated. In addition, with the continued focus on unorganized emissions, it is certainly a good thing to reduce leakage concerns.