Even though metals will continue to be used throughout the valve industry for their own unique characteristics of cost and performance, it has been the many developments in plastic and other nonmetals that have greatly expanded the applications of valves. The following is a brief review of some of the nonmetallic materials being used in today's valve designs.

Fluorocarbons are used extensively in chemical process equipment because of their chemical inertness. Valve seats, liners, and in some cases entire valves may be constructed of these materials. Manufacturers choose specific fluorocarbons [tetrafluorethylene (TFE), fluorinated ethylene-propylene (FEP), or ultrahigh-molecular-weight poly- ethylene (UHMWPE)] based on the comparative resistance of these plastics to permeation and absorption of chemicals in addition to their physical and mechanical properties.

TFE
The properties of TFE which make it particularly useful in valves for chemical process are lubricity, nonpermeability, nonstick characteristic, abrasion resistance, corrosion resistance, etc., which can be realized at temperatures below 450^oF (230^oC). To make the best use of these properties, valves containing TFE materials are generally restricted to temperatures below 400^oF (200^oC).

TFE deforms under pressure or temperature or over a period of time; however, it will always tend to return to its original shape. This deformation is known as cold flow. When you remove either the pressure or the temperature, the inherent memory of TFE will tend to bring it back to its original shape. However, it does not have perfect memory as many elastomers and may return to only about 60 percent of its original shape. For this reason TFE and other fluoropolymers when used as valve seats are often backed by a resilient material or spring. When continuous pressure on the seat as on a plug valve is employed, the cold flow can cause a mechanical interlocking of the closure element and seat which increases operating torque.

TFE is here used as a seal for rotary-motion valves and for some linear valves. It also is used as a liner and an encapsulate for ball, butterfly, and plug valves. It is attacked only by molten alkali metals, or by chlorine or fluorine trifluoride under special conditions. So corrosion is not a problem. It is a plastic, not an elastomer. When deformed it will partially recover slowly. It does not resist abrasion well and should not be exposed to highly abrasive streams. The nominal temperature range of TFE is from cryogenic to 400^oF (200^oC).

When considering TFE, there are two characteristics to keep in mind. Permeation and absorption are quite similar phenomena except that permeation refers to the passage of foreign material through the polymer while absorption refers to the retention of material within the polymer. All polymers will absorb or be permeated to some degree by materials which they come in contact with, but fluorocarbons are more resistant to these effects than other plastics. The mechanism for the occurrence of this phenomenon is diffusion or movement of the gas or liquid through submicroscopic, intermolecular spaces within the polymer. In PTFE and PEP this is purely a physical process since these fluorocarbons are inert to chemical attack in virtually all chemicals. Temperature has an extremely important influence on the permeation or absorption of all polymers. As the temperature increases, the expansion of the polymer will increase the size of the intermolecular spaces and allow less restriction to the migration of the vapor or liquid being handled.

FEP
PEP is a copolymer of TFE and hexafluoropropylene, and it is a true thermoplastic. It is melt processable at a temperature of 634^oF (335^oC), making it an excellent resin for transfer-extrusion molding of parts with difficult shapes. Through precise control of the molding process, the most desirable properties of PEP-chemical inertness, low coefficient of friction, insolubility in solvents, low adhesion properties, wide service temperature range, toughness, and flexibility and be realized in the finished valve product. Because PEP is molded and machined to close tolerances, parts such as the body and plug or liner and discs can be fitted for excellent sealing.

Pewiluoroalkoxy resin
PFA is a class of perfluoropolymers that offers the processing ease of conventional thermoplastics but substantially extends its temperature limits. Like PEP, PFA is a true thermoplastic and is melt processable, allowing it to be molded to extremely difficult shapes. PFA is processed at 700^oF (370^oC). Otherwise the molding technology is the same as that used for PEP.

PFA has been found to be better in handling some monomers, such as butadiene, permitting the use of PFA-lined products on a wider range of applications to temperatures as high as 500^oF (260^oC). where lined valves are used in abrasive service many manufacturers recommend that UHMWPE be used. UHMWPE is one of the most abrasion-resistant liner materials available today: almost 5 times as abrasion resistant as TFE and more than 6 times that of carbon steel.

Tefzel
Tefzel can best be described as a rugged thermoplastic with an outstanding balance of properties. It has mechanical toughness, broad thermal capability, and the ability to meet severe environmental conditions.

Chemically, Tefzel is a copolymer of 25 percent ethylene and 75 percent TFE. Mechanically, Tefzel is tough, exhibits high tensile strength, and is more creep resistant than Teflon, TFE, PEP, and PFA fluorocarbon resins. The Tefzel used in some valves is reinforced with glass, yielding a tensile strength approaching 12,000 lb/in² (830 bar). Tefzel has outstanding resistance to attack by chemicals and solvents that often cause rapid deterioration of other plastic materials. Tefzel is inert to strong mineral acids, inorganic bases, halogens, and metal salt solutions. Carboxylic acids, anhydrides, aromatic and aliphatic hydrocarbons, and classic polymer solvents have little effect on the material.

Thermoplastic valves
Thermoplastic valves provide a dependable and economical means for fluid control and management in a variety of piping systems. These valves are made entirely of thermoplastics including the body, stem, and closure element.

Polyvinyl chloride (PVC)
The most frequently specified thermoplastic material, PVC, has excellent strength, rigidity, modulus of elasticity, and chemical resistance. It is used extensively in water lines, irrigation, plating, chemical drainage, and chemical processing. PVC has good physical properties and resistance to corrosive and chemical attack. It is, however, subject to attack by aromatics chlorinated organic compounds, some hydrocarbons, polar solvents, and ketones. PVC has a maximum operating temperature of 140^oF (60^oC) and has a design stress of 2000 lb/in² (140 bar) at 73^oF (23^oC). Joining is accomplished by solvent cementing, threading, or flanging.

Chlorinated polyvinyl chloride (CPVC)
The physical properties of CPVC are equal to or better than PVC, and its use is ideal for hot or cold corrosive liquids, hot or chilled water, and generally where escalated temperatures preclude the use of PVC. CPVC has a maximum operating temperature of 2000F (900C) and a design stress of 2000 lb/in² at 73^oF (140 bar at 23^oC). Joining is accomplished by solvent cementing, threading, and flanging.

Polypropylene (PP)
PP is generally lower in physical properties as compared to PVC. It is chemically resistant to acids, organic solvents, and alkalines. PP is excellent for sulfur-bearing compounds and saline solutions, and its use is ideal for drainage where mixtures of solvents, base chemicals, and acids are transported. It has a maximum operating temperature of 180^oF (80^oC) for drainage purposes and a design stress of 2000 lb/in² at 73^oF (140 bar at 23^oC). Joining is accomplished by solvent cementing, threading, and flanging.

Polyvinylidene fluoride (PVDF)
PVDF is superior in physical properties to other thermoplastic piping system components. It retains most of its strength to working temperatures as high as 260^oF (125^oC) and is chemically resistant to most acids, bases, solvents, bromines, chlorines, aliphatics, aromatics, and alcohols. PVDF is generally not recommended for ketones or esters. Joining is accomplished by thermoseal fusion, threading, or flanging.

Composite materials
Composite ball and butterfly valves are primarily selected for chemical handling services where internal and external corrosive conditions exist, particularly when metal or lined metal valves have proven un- suitable for the chemical service.

Typical materials used in molding composite valves are highly chemical and temperature resistant rigid polymers reinforced with carbon graphite or glass fibers. Polymers that have been used successfully are vinyl esters and polyphenylene sulfide (PPS). Both of these resins, which have a fiber reinforcement content in the range of 25 to 50 percent, have been used to mold valve bodies and discs for design service to withstand pressures to 275 lb/in² (19 bar) and fluid temperatures 350^oF (175^oC).

Typically, these materials offer high rigidity at both low and elevated temperatures even 50 percent higher than the rated temperature. This stability extends the valve life by providing dimensional accuracy (necessary for sealing surfaces) over extreme limits of temperature. In general, carbon graphite fiber reinforcement is used in the higher-temperature, high-chemical-concentration services because of its chemical inertness. It is also a good choice in valve components that move against sealing surfaces because of its inherent lubricity and improved surface integrity.

Valve components (e.g., valve shafts) can be molded with metal inserts to improve the physical strength of critical components. In such cases, materials within the valve body (wetted surfaces) should be fully encapsulated by the molded composite material or a metal alloy selected to give equal or better chemical corrosion resistance according to the service conditions.

Glass-reinforced polysulfone (GRP)
GRP-polysulfone composite valves are intended for extremely reactive chloride-type radical compounds that have chemically attacked other types of resins. The chloride radical compounds referred to are of the type found in unbalanced hypochlorite generation, chlorine cell effluent, and other chloride-alkali-type processes. At this point in time, laboratory tests show that the modified polysulfone composite is resistant to a wide variety of corrosive transient conditions involving chloride compounds for temperatures up to 350^oF (175^oC).

Summary
Since thermoplastic and composite valves usually weigh less than one-third the weight of equivalently rated metal or lined metal valves, there is reduced stress on a piping system; therefore, fewer pipe supports are required. One of the major benefits of composite valves is external corrosion resistance in corrosive environments.

Other seat and seal materials used in valves

Butyl. Butyl is a vulcanized rubber copolymer generated from isobutylene and isoprene. Butyl has good chemical resistance to most acids.

Ethylene propylene diene monomer (EPDM). EPDM is used for 0-ring seals and valve seats. It has excellent abrasion and tear resistance and is chemically resistant to a large variety of acids, alkalines, alcohols, and oxidants. It is recommended for use with petroleum products, strong acids, or strong alkalines.

Viton (a fluorocarbon elastomer). Viton is used for 0-ring seals and valve seats. It has resistance to a broad spectrum of chemicals with greatly 6xpanded temperature ranges. Viton is abrasion resistant and tough and resists most mineral acids, salt solutions, chlorinated hydrocarbons, and petroleum oils.

Ceramic construction of valve components Alumina ceramic has a hardness near that of a diamond and is far superior to metals and stellited materials in wear resistance. Alumina ceramics are inert to oxidation, are not corroded by chemical agents, and are not subject to radiation damage.

Ceramic materials not only provide excellent abrasion and corrosion resistance but are also addressing higher-temperature valve applications. Some of the ceramic materials such as silicon nitride are so stable at elevated temperatures that they can be used with molten metals. Ceramic materials and their use in control valves is a rapidly growing technology that will provide alternatives for the control valve requirements of the future and the solutions for many of the control valve problems of the present. Along with these advantages, fine ceramics also have some disadvantages such as brittleness, low impact resistance, low tensile strength, and susceptibility to thermal shock damage.