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.
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.
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.
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.
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.
Post time: Dec-24-2019
