The Expanding Reach of Plastic Valves

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.

PLASTIC’S PROPERTIES

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.

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.

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.

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.

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.

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.

Tim Moran is the industrial plastics product manager for NIBCO Inc., a manufacturer of thermoplastic piping products (www.chemtrol.com), based in Elkhart, IN. He is a member of the Water Environment Federation. Reach him at morant@nibco.com.