Brad Bertrem, Dover Corporation/Norris Division
Early applications for butterfly valves were restricted to throttling control in many types of fluid flow systems. These early throttling-type butterfly valves were very similar to the disc in an automobile carburetor and did not provide positive shutoff. In the early part of this century, however, semipositive low-pressure shutoff butterfly valves were developed, using natural rubber for seating materials. These valves received only limited acceptance because of natural rubber's limited resistance to media and its undesirable characteristic of deteriorating physical properties when exposed to temperature over a period of time. This water-works type valve was unsuitable for petroleum services due to the use of natural rubber seats. However, the development of these semipositive low-pressure valves coupled with the emergence of many synthetic rubbers during World War II led to the leak-free high-pressure butterfly valves we know today. The new elastomers also allowed the valve designer to obtain improved resistance to media, while providing positive shutoff. The primary function of a butterfly valve today is to achieve positive shutoff. They can be fully opened and closed in a quarter turn. Their ease of operation permits them to be used for throttling and on-off automatic applications in various fluid flow systems. Butterflies will handle many different types of media, including vapor (steam), gases, liquid, slurry and solids. Butterfly valves provide positive shutoff up to 250 psi and have far greater life than the early natural rubber-seated valves. A variety of unique design advantages are offered in butterfly valves, including weight, economy, simplicity of design, ease of installation and maintenance, compactness, simple quick operation, reliability, and versatility. The light weight of butterflies allows installation without the necessity of a hoist up to the 10-in. or 12-in. sizes. Weight economy is also a cost advantage because a minimum amount of materials are used in manufacturing and the valve design is simple, providing economical prices. A minimum amount of premium material is required in handling corrosive media since many butterfly valve designs prohibit the medium from reaching the valve body. This, of course, is not true in gate, ball, plug, or globe valves since they must have high alloy bodies as well as high alloy trims (such as titanium, hastelloy, stainless steel) if they are to be compatible with highly corrosive flow streams. In addition to economy, the simplicity of design of the butterfly allows on-site repair without special tools or equipment. Elastomers can be replaced right at the job site. Compactness of the butterfly valves, particularly the wafer design, minimizes the amount of wasted space necessary for piping systems. Little or no maintenance is required on butterfly valves. If properly selected, butterflies will provide leak-free service with a minimum of maintenance. Most butterfly valves are permanently lubricated and require no special attention once installed. Due to the many trims available in butterfly valves they are one of the most versatile valve types available today. Depending upon the proper selection of metals and elastomers, butterflies are capable of operating from -65" to +450_F and can handle media from low vacuum pressures up to 251 and including 250 psig working pressures. Among the limitations of butterfly valves is temperature. The range previously mentioned for butterflies is determined by the elastomer seals. Resilient or rubber-lined butterflies are generally available from moderate vacuum up to and including 250 psig with valves containing a limited elastomer in their seals capable of handling up to 720 psig in special designs. Butterfly valves should be closed slowly to prevent "water hammer". When butterfly valves are closed too rapidly, hydraulic shock will occur. When a valve disc is closed quickly the disc must absorb the energy that is stored in the movement of liquid it is suddenly stopping. This hydraulic shock can be avoided by the use of a gear operator or other device on top of the valve which prevents quick closing. A major criterion for maximum valve life is selection of the proper valve. Various media, valve use, cycle rate, temperatures and pressures affect the proper selection of valve style and trim. Prior to installation, valves should be carefully handled to prevent disc edge damage. Valve flanges should be welded to pipe prior to inserting the valve body. Piping must be properly supported and flanges accurately aligned to prevent unnecessary loads on valve bodies. Valve assemblies, pipeline and mating flanges should be cleaned prior to valve installation. These basic steps before installation combined with periodic checks and cycling during operation will prolong the life of any valve. Butterfly valves have emerged as a strong competitor to other types of valves in a variety of applications. Specific petroleum industry applications are listed later, but generally butterfly valves are found in all areas of the petroleum industry.