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The operating environment provides the basis for selecting the proper body material and trim Butterfly valves are the most specified valve type for moderate-duty conditions in pipe sizes of two inches and larger. Their reduced weight, reliable bubble-tight shut-off, ease of operation and automation allowed them to displace gate valves in traditional applications. Bi-directional shut-off is one of the hallmarks of butterfly valves. Continuing development of the butterfly valve is making it possible to use them in aggressive applications that previously were the exclusive domain of gate and globe valves. Initially, the limited capabilities of elastomers restricted the applications for butterfly valves. However, developments in synthetic elastomer technology vastly increased the scope of applications. For example, new compounds and formulations feature increased cycle life, expanded chemical compatibility and extended temperature limitations at both extremes. PTFE and metal-seated butterfly valves are available for those applications that are limited by the elastomer seating. Design elements The basic butterfly valve design elements are the seat, disc, body, shaft and the actuator. Within each of these elements are the variations of material and designs that adapt the butterfly valve to the intended service. Understanding the importance and features of the design elements is key to matching the butterfly valve to the intended service. The discussion of the seat will be limited to valves with a resilient seat in the body. Discs with resilient seats--grooved-end butterfly valves--will be reviewed below. The most common way to form a resilient seat is by lining the interior of the valve body with an elastomer. The lining serves three purposes:
The lining may be removable and repairable, in which case it is integrally molded to a phenolic backing ring that imparts structural integrity to the elastomer. This molded unit is then assembled into the valve body. Alternatively, the elastomeric seat can be molded and vulcanized directly into the body. These valves are typically called rubber-lined valves. This type of seat cannot be repaired; one replaces the entire valve body. Typical liner elastomers are EPDM (ethylene-propylene-diene monomer) and nitrile. EPDM is widely used in HVAC applications. It has a operating range of -20 to +250 degrees Fahrenheit. It resists attack by water, glycols, chlorinated water, 20 percent sodium hypochlorite (bleach), phosphate esters, alcohols and acids. EPDM is not resistant to hydrocarbon solvents and oils, gasoline, chlorinated hydrocarbons, turpentine or petroleum-based oils. Nitrile is another commonly used elastomer. It has a temperature range of 0 to 180 degrees Fahrenheit and is well suited for use with most petroleum oils and greases, most automotive-grade gasoline, fuel oils, glycol, alcohol and other petroleum products. Other less commonly available liners are fluorocarbon for use in chemically aggressive environments, and food grade versions of nitrile and EPDM. For more demanding applications, the elastomeric liner is, in turn, lined with a PTFE layer. This layer isolates the elastomeric seat from the medium flowing in the pipeline. The PTFE liner also seals against the faces of the mating flanges. An adhesive attaches the liner to the elastomer backing. The thickness of the PTFE liner is important. If it is too thick, the PTFE unduly increases the torque required to close the valve. If it is too thin, the fluid medium may be able to permeate the liner and attack the underlying elastomer. For the most demanding services, the seat is made of a pressure responsive contour. The seat material can be solid PTFE, solid PTFE with a secondary metal seat, or a pressure responsive all-metal seat. The secondary metal seat provides a fire-safe seal if a fire damages the soft PTFE. A metal seat is used for high temperature or abrasive applications. An all-metal seat allows the valve to meet the pressure temperature ratings of ANSI B16.5; a general design referred to as a high-performance butterfly. The butterfly disc provides the structural integrity to resist the hydrostatic shut-off loads and operating torque. The disc also acts as the moveable sealing member. In the elastomer-lined butterfly valve, the axis of the valve stem passes through the center of the disc and the axis lies in the plane of the seating edge. This means that the stem passes through the seating edge of the disc. Consequently, special contours machined into the disc provide a sealing surface. The care given to machining and dressing this area is one of the determining factors in the longevity of a valve. Valve discs are made of ductile iron, stainless steel, or aluminum-bronze. Ductile iron discs are nickel plated to provide a corrosion resistant edge. Care must be taken to match the disc to the fluid and the piping. Galvanic corrosion can become a problem if the disc is less noble than the pipe. The proper selection of disc material or disc coating mitigates the problem of galvanic corrosion. The stem axis on discs for use with solid PTFE and metal seats is shifted away from the seat so the stem axis does not penetrate the disc and body seat. This configuration is called an offset disc. Hydrodynamic forces and the forces that offsetting the stem axis introduce increase the torque needed during seating and unseating. Discs for installation in grooved piping systems have the rubber seat molded into the disc to increase the flow area when the disc is open. In addition, the rubber covering provides excellent abrasion resistance. A rubber-coated disc in a rubber-seated butterfly valve achieves the same degree of abrasion resistance. Installing the valve The method of mating with the pipeline characterizes the butterfly valve body. Valves that are trapped between mating flanges and long studs are called wafer valves. Neither the wafer valve nor the mating flanges can be loosened before draining the system. This can be a problem if the valve is intended to isolate a piece of equipment that requires servicing. Lug valves have bosses with tapped holes. Bolts are inserted through the flanges and threaded into the bosses. Either mating flange of a lug valve can be removed independently. Lug valves are preferred for connection to flanged equipment. Grooved valves also allow equipment to be removed for servicing. The primary caveat when specifying valves for attachment to equipment that will need servicing is that the valve must be able to retain its liner when a mating flange is disconnected. Grooved valves, by definition, provide a bi-directional installation capability. Valves with slip-in liners need to have an uncomplicated method for liner retention. Environmental factors enter the selection External corrosion can be controlled through selection of corrosion resistant body materials or coatings. Another external factor is the pipeline insulation. The neck on the valve needs to be long enough for the operator to clear the insulation on the mating flanges. The shaft or stem in a butterfly is the structural element that unites the disc and body. It also provides the means for applying torque to position the disc. The connections between the stem and the operator and between the stem and the disc are crucial elements in the selection of a valve. A build-up of tolerances in these connections causes the disc movement to lag the operator movement. This prevents precise positioning of the disc or it may allow the disc to "flutter" into premature failure. Precisely machined fits between the stem and disc are important. Valve operators run the gamut from simple levers for manually operating the valve to powered and remotely controlled actuators used for flow modulation. General design considerations need to focus on the application, the potential for future upgrades, and the external environment. Selection of a valve manufacturer that provides solutions for the needs of today and tomorrow is an important part of the selection process. Applications Butterfly valves are used in every facet of fluid control. Their light-weight, positive shut-off and ease of operation make them the valve of choice for applications that involve low- to medium-pressure and temperature. Fire protection systems have thoroughly embraced the use of wafer, lug and grooved-end butterfly valves, with approvals conferred by Underwriters Laboratories, Factory Mutual or Underwriters Laboratories of Canada. The main advantages are bubble-tight shut-off for system testing and the ability to incorporate electrically monitored tamper switches to ensure the integrity of the system. Butterfly valves are not favored in the underground portion of the system or if wall or upright indicator posts are required. Larger, in-water distribution systems frequently use butterfly valves in the hot and cold water distribution lines. EPDM liners give excellent performance in both systems. Exercise care in the selection of disc materials for hot water systems piped in copper. The large mass of copper pipe can cause galvanic corrosion. CF8M stainless steel discs provide the greatest assurance of long, trouble-free service. Aluminum-bronze may also provide excellent service if the alloy has sufficient nickel to prevent corrosion. Pinholes in the plating on ductile iron discs may lead to premature failure. Rubber coated discs, used in grooved valves, generally have a sufficient thickness of rubber coating to isolate the disc from the fluid, thereby preventing galvanic corrosion. Avoid rubber discs that don't provide 100 percent coating coverage. Use past experience with your system or the technical expertise of the supplier to determine the appropriate material. Butterfly valves provide trouble-free service in hot and chilled water HVAC systems, as well. A butterfly valve, equipped with a memory stop, provides balancing and isolation functions. EPDM liners perform well in these applications. However, temperatures below 20 degrees Fahrenheit or above 200 degrees Fahrenheit will test the technical sophistication of any manufacturer of liners. Resistance to compression set and the resistance to elastomer hardening over time are essential factors for performance. Reliable suppliers have test data, so remember to ask for it. Remote control of butterfly valves is also possible. A mechanical linkage that opens one valve and closes another produces a three-way valve assembly. Specify lug or grooved-end valves for equipment isolation. Butterfly valves can also handle "dirty" water applications. EPDM liners are satisfactory if the waste stream contains no petroleum products. Otherwise, nitrile is an excellent choice. Since these valves tend to be of larger diameter, they require either manual or powered actuators. A comfortable safety factor needs to be used in determining the torque output of the actuator and the torque carrying capability of the disc and stem assembly. For electric actuators, the valve needs to be capable of handling the maximum torque output of the motor safely, typically at stall. Specify a feedback signal on remote valves to ensure that the valve disc is in the desired position. Within the process systems of the plant, chemical compatibility, pressure and temperature are considerations. Match elastomer liners to the process fluid. Particular attention needs to be given to the presence of proprietary additives. The Material Safety Data Sheet is an excellent place to determine the constituents in proprietary formulations. It is in this area that the valves lined and seated with PTFE demonstrate their versatility in resisting chemical attack and the effects of elevated temperatures. Disc and stem material selection need to be matched because stems may now be exposed to the line medium. Processes involving flammable fluids benefit from the use of "fire-safe" valves designed to retain their internal and external integrity. Valves that meet the requirements of API Standard 607 4th Edition--Fire Test for Soft-Seated Quarter-Turn Valves--provide a good initial basis for selection. Butterfly valves provide solutions to a wide range of fluid control situations. The operating environment provides the basis for selecting the proper valve. Of equal importance is selecting a valve manufacturer with the appropriate technical and manufacturing expertise for meeting your valve needs and long-term satisfaction.
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