Back to Basics
- Published on Tuesday, 30 April 2013 14:58
- Written by Tim Moran
Although plastic valves are sometimes seen as a specialty product—a top choice of those who make or design plastic piping products for industrial systems or who must have ultra-clean equipment in place—assuming these valves don’t have many general uses is short-sighted. In reality, plastic valves today have a wide range of uses as the expanding types of materials and good designers who need those materials mean more and more ways to use these versatile tools.
The advantages of thermoplastic valves are wide—corrosion, chemical and abrasion resistance; smooth inside walls; light weight; ease of installation; long-life expectancy; and lower life-cycle cost. These advantages have led to wide acceptance of plastic valves in commercial and industrial applications such as water distribution, wastewater treatment, metal and chemical processing, food and pharmaceuticals, power plants, oil refineries and more.
Plastic valves can be manufactured from a number of different materials used in a number of configurations. The most common thermoplastic valves are made of polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC), polypropylene (PP), and polyvinylidene fluoride (PVDF). PVC and CPVC valves are commonly joined to piping systems by solvent cementing socket ends, or threaded and flanged ends; whereas, PP and PVDF require joining of piping system components, either by heat-, butt- or electro-fusion technologies.
Thermoplastic valves excel in corrosive environments, but they are just as useful in general water service because they are lead-free1, dezincification-resistant and will not rust. PVC and CPVC piping systems and valves should be tested and certified to NSF [National Sanitation Foundation] standard 61 for health effects, including the low lead requirement for Annex G. Choosing the proper material for corrosive fluids can be handled by consulting the manufacturer’s chemical resistance guide and understanding the effect that temperature will have upon plastic materials’ strength.
Although polypropylene has half the strength of PVC and CPVC, it has the most versatile chemical resistance because there are no known solvents. PP performs well in concentrated acetic acids and hydroxides, and it is also suitable for milder solutions of most acids, alkalis, salts and many organic chemicals.
PP is available as a pigmented or unpigmented (natural) material. Natural PP is severely degraded by ultraviolet (UV) radiation, but compounds that contain more than 2.5% carbon black pigmentation are adequately UV stabilized.
PVDF piping systems are used in a variety of industrial applications from pharmaceutical to mining because of PVDF’s strength, working temperature and chemical resistance to salts, strong acids, dilute bases and many organic solvents. Unlike PP, PVDF is not degraded by sunlight; however, the plastic is transparent to sunlight and can expose the fluid to UV radiation. While a natural, unpigmented formulation of PVDF is excellent for high-purity, indoor applications, adding a pigment such as a food-grade red would permit exposure to sunlight with no adverse effect on the fluid medium.
Plastic systems have design challenges, such as sensitivity to temperature and thermal expansion and contraction, but engineers can and have designed long lasting, cost-effective piping systems for general and corrosive environments. The major design consideration is that the coefficient of thermal expansion for plastics is greater than metal—thermoplastic is five to six times that of steel, for example.
When designing piping systems and considering the impact on valve placement and valve supports, an important consideration in thermoplastics is thermal elongation. Stresses and forces that result from thermal expansion and contraction can be reduced or eliminated by providing flexibility in the piping systems through frequent changes in direction or introduction of expansion loops. By providing this flexibility along the piping system, the plastic valve will not be required to absorb as much of the stress (Figure 1).
Because thermoplastics are sensitive to temperature, the pressure rating of a valve decreases as temperature rises. Different plastic materials have corresponding deration with increased temperature. Fluid temperature may not be the only heat source that can affect a plastic valves’ pressure rating—maximum external temperature needs to be part of design consideration. In some cases, not designing for the piping external temperature can cause excessive sagging due to lack of pipe supports. PVC has a maximum service temperature of 140°F; CPVC has a maximum of 220°F; PP has a maximum of 180°F; and PVDF valves can maintain a pressure up to 280°F (Figure 2).
On the other end of the temperature scale, most plastic piping systems work quite well in temperatures below freezing. In fact, tensile strength increases in thermoplastic piping as temperature decreases. However, impact resistance of most plastics decreases as temperature falls, and brittleness appears in affected piping materials. As long as the valves and adjoining piping system are undisturbed, not jeopardized by blows or bumping of objects, and the piping is not dropped during handling, adverse effects to the plastic piping are minimized.
TYPES OF THERMOPLASTIC VALVES
Ball valves, check valves, butterfly valves and diaphragm valves are available in each of the different thermoplastic materials for schedule 80 pressure piping systems that also have a multitude of trim options and accessories. The standard ball valve is most commonly found to be a true union design to facilitate valve body removal for maintenance with no disruption of connecting piping. Thermoplastic check valves are available as ball checks, swing checks, y-checks and cone checks. Butterfly valves easily mate with metal flanges because they conform to the bolt holes, bolt circles and overall dimensions of ANSI Class 150. The smooth inside diameter of thermoplastic parts only adds to the precise control of diaphragm valves.
Ball valves in PVC and CPVC are manufactured by several U.S. and foreign companies in sizes 1/2 inch through 6 inches with socket, threaded or flanged connections. The true union design of contemporary ball valves includes two nuts that screw onto the body, compressing elastomeric seals between the body and end connectors. Some manufacturers have maintained the same ball valve laying length and nut threads for decades to allow for easy replacement of older valves without modification to the adjoining piping.
Ball valves with ethylene propylene diene monomer (EPDM) elastomeric seals should be certified to NSF-61G for use in potable water. Fluorocarbon (FKM) elastomeric seals can be used as an alternative for systems where chemical compatibility is a concern. FKM also can be used in most applications involving mineral acids, with the exception of hydrogen chloride, salt solutions, chlorinated hydrocarbons and petroleum oils.
PVC and CPVC ball valves, 1/2-inch through 2 inches, are a viable option for hot and cold water applications where the maximum non-shock water service can be as great as 250 psi at 73°F. Larger ball valves, 2-1/2 inches through 6 inches, will have a lower pressure rating of 150 psi at 73°F. Commonly used in chemical conveyance, PP and PVDF ball valves (Figures 3 and 4), available in sizes 1/2-inch through 4 inches with socket, threaded or flanged-end connections are commonly rated to a maximum non-shock water service of 150 psi at ambient temperature.
Thermoplastic ball check valves rely on a ball with a specific gravity less than that of water, so that if pressure is lost on the upstream side, the ball will sink back against the sealing surface. These valves can be used in the same service as similar plastic ball valves because they do not introduce new materials to the system. Other types of check valves may include metal springs that may not last in corrosive environments.
The plastic butterfly valve in sizes 2 inches through 24 inches is popular for larger diameter piping systems. Manufacturers of plastic butterfly valves take differing approaches to the construction and sealing surfaces. Some use an elastomeric liner (Figure 5) or O-ring, while others use an elastomeric-coated disc. Some make the body out of one material, but the internal, wetted components serve as the system materials, meaning a polypropylene butterfly valve body may contain an EPDM liner and PVC disc or several other configurations with commonly found thermoplastics and elastomeric seals.
Installation of a plastic butterfly valve is straightforward because these valves are manufactured to be wafer style with elastomeric seals designed into the body. They do not require the addition of a gasket. Set between two mating flanges, the bolting down of a plastic butterfly valve must be handled with care by stepping up to the recommended bolt torque in three stages. This is done to ensure an even seal across the surface and that no uneven mechanical stress is applied on the valve.
Metal valve professionals will find the top works of plastic diaphragm valves with the wheel and position indicators familiar (Figure 6); however, the plastic diaphragm valve can include some distinct advantages including the smooth inside walls of the thermoplastic body. Similar to the plastic ball valve, users of these valves have the option to install the true union design, which can be especially useful for maintenance work on the valve. Or, a user can select flanged connections. Because of all the options of body and diaphragm materials, this valve can be used in variety of chemical applications.
Like with any valve, the key to actuating plastic valves is determining the operating requirements such as pneumatic versus electric and DC versus AC power. But with plastic, the designer and user also have to understand what type of environment will surround the actuator. As previously mentioned, plastic valves are a great option for corrosive situations, which include externally corrosive environments. Because of this, the housing material of actuators for plastic valves is an important consideration. Plastic valve manufacturers have options to meet the needs of these corrosive environments in the form of plastic-covered actuators or epoxy-coated metal cases.
As this article shows, plastic valves today offer all sorts of options for new applications and situations.
1 Lead Free refers to the wetted surface of pipe, fittings and fixtures in potable water systems that have a weighted average lead content ≤0.25% per the Safe Drinking Water Act (Sec. 1417) amended 1-4-2011 and other equivalent state regulations.