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As printed in VALVE Magazine, June 2001, with the title, "Plastic Valves Meet Niche Market Needs"

Intro to Plastic Valves:

The Why, When & Where of Plastic Valves

Mention plastic valves and most people picture the low cost, bright white valves connected to their swimming pool or aquarium. In reality, many plastic valves are well-designed, rugged products that sometimes cost more than the metal valves they supplant. Wherever valves routinely fail due to corrosion, or when purity concerns require exotic alloys that are cost prohibitive in most cases, very high quality plastic valves are specified...but with some important exceptions that must be considered.

The first thermoplastic valves were the result of corrosion-prone process industries searching for an alternative to constantly replacing metal valves. One plastic valve pioneer got started when a manufacturer of dry cleaning equipment was searching for a replacement air-operated shut-off valve used to deliver an extremely corrosive chemical. The manufacturer was constantly replacing valves and was in danger of losing customers, and noticed that the plastic filter units it was using showed no ill effects. The filter manufacturer declined an invitation to develop a replacement valve, but permitted a staff engineer to work on the project. Word of his successful plastic design spread, and an industry was launched.

Plastic piping systems have a distinct advantage in applications with either highly corrosive or ultra pure liquid media. The high quality plastic valves used in corrosive chemical and ultrapure semiconductor manufacturing are technologically equal to the best metal valves. In these applications it is generally agreed that plastics usually do a better, more cost effective job.

in this particular application photo, the plastic valves are the only thing not affected by the corrosive atmosphere and process.  These happen to be normally-closed (spring return) air operated shut-off valves; flow in the photo is from right to left.  The internal flowpath is a globe pattern; a volcano-type orifice with a plug.  The spring force keeps the plug down; air pressure (when actuated) forces the valve open...

In the photo at right, the large dark grey PVC shutoff valves are just about the only items not affected by the highly corrosive atmosphere. These heavy-duty valves are only remotely related to the cheap white "swimming pool valves" most people associate with plastics.


In corrosive applications, plastic valves are not prone to "stick" or fail due to rust, scaling, or other corrosive build up. Similarly, plastic piping provides the benefit of remaining smooth and free of build up, which means that flow rates and pressure drop will be unaffected after years of use. Externally, plastic valves resist attack by airborne corrosives, which eliminates the need for painting or special coatings.

In ultra-pure applications, certain plastics such as Teflon, Kynar, and natural unpigmented polypropylene are preferable for their non-leaching properties. They also are highly resistant to adherence and subsequent growth of organic impurities, which is a paramount concern in processes such as semiconductor fabrication.

Beyond corrosion and purity advantages, plastic valves and piping have the added benefit of being generally lighter in weight and therefore less costly to ship. The polyvinyls also install easily, either with simple hand-threading or solvent cementing.

Plastic ABCs

The "alphabet soup" of different plastic materials may seem daunting, but in reality the industry uses only a few common resins. The most prevalent plastic valve material, Grade 1 Type 1 PVC (polyvinylchloride), has been used successfully for over 35 years in such areas as chemical processing, wastewater treatment, industrial plating and electronics manufacturing. It carries a pressure rating of approximately 150 psi -- at 75 F -- depending on valve design. As temperature rises, pressure ratings fall. Maximum safe temperature rating of PVC is 140F; minimum safe temperature is 40F. Chlorinated PVC (CPVC) is rated to 180. These ratings can change drastically depending on the properties of the process media.

As applications requiring the benefits of plastic began demanding higher pressure and temperature ratings, higher performance resins such as Kynar PVDF and Teflon PTFE became popular. Certain Kynar valve designs are rated to 230 psi. Teflon is capable of withstanding temperatures to 500F, although it is not recommended in valve use beyond 300F. Some materials, such as PEEK (Polyetheretherketone) are rated even higher, but tend to be cost prohibitive and thus defer to metal or lined metal as the material of choice.

Other popular materials used in better quality plastic valving are Teflon PFA and polypropylene. In less demanding applications such as irrigation, nylon and polyethylene are popular resins due to their lower costs. Each of these plastics fills a niche in the market.

Notable limitations of plastic valves are high pressure and extreme temperatures. Many system designers are simply unaware that plastic valves are not suited for temperatures below freezing, or may soften at elevated temperatures when used with certain chemicals. Furthermore, plastic valves are not as forgiving as metal valves in terms of abuse such as errant hammer blows. Plastics are also restricted to certain types of media.

Most plastic valves are designed for liquids, and many are suitable for slurries. Powders tend to scour the valve body, and most gas applications are simply not suited to plastic. ABS (Acrylonitrile-butadiene-styrene) is a popular plastic material for compressed air piping, but has reduced capability versus metals, and tends to be used only where atmospheric corrosion impacts the life and safety of metal piping.

Plastic Valves vs. Metal Valves

Overall valve design is similar between materials. The plastic counterpart to cast metal valves is injection molding, done when quantities warrant. Like a cast valve body, some finishing machining is needed prior to final assembly. More specialized valves have machined bodies, performed with the same CNC machining centers and lathes used in a metal machine shop. Plastic valve bodies are generally threaded or cemented together, or assembled with fasteners. In addition, elastomers perform generally the same function in plastic valves as they do in the metal versions. One of the few basic differences is that virtually no high quality plastic valve design has a plastic plug or stem seal against a plastic orifice.

Service tends to be easier with plastic valves. No unusual tools or equipment are needed to disassemble the typical plastic valve; with proper design considerations the seals and key parts can be replaced in the field with minimal downtime.

The overall ease of service and installation does have its drawbacks, however, as most mechanical contractors unfamiliar with plastic valves try to install them in the same manner as metal valves: pipe wrench, channel locks, cheater bars, and plenty of force. A well-designed threaded plastic valve should be installed hand tight only, with an additional quarter-turn using a strap wrench. Most plastic valve "defects" are the result of too much installation muscle, which stresses the plastic body and can eventually lead to cracking under prolonged stress conditions.

Summary

As the plastic valve industry approaches the half-century mark, it is clear that while not a threat to the mainstream metal business, it has found a highly successful niche. Although it may never overcome the stigma of its cheap white cousins, engineered plastic valves will certainly continue to make inroads wherever corrosive or high purity liquids are involved.

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Endnote

The valve pioneer mentioned above was Plast-O-Matic founder Bruce L. DeLorenzo, who virtually launched an industry from one laundry valve application. Throughout his 30+ years as President of Plast-O-Matic, Mr. DeLorenzo developed a number of valve innovations that have since been adopted as industry standards.

The Author

Rick Bolger is Marketing Manager for Plast-O-Matic Valves, Inc., Cedar Grove, NJ.


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