Many different types of valves are used in flow control. They are used for a variety of reasons, such as phase (liquid or gases), pressure, piping restrictions and solids content. Other valves are chosen for their capability to open and close in a quarter turn. Of all the valve types, the butterfly valve is used as a control device for many reasons including some or all of the above.
This article explains the workings of a butterfly valve and its operation. The butterfly valve offers many advantages that include quarter-turn, openness for less plugging and good control capabilities. Both manual and control versions are used.
Rotating diskA butterfly valve is a flow control device that incorporates a rotational disk to control the flowing media in a process. The disk is always in the passageway, but because it is relatively thin, it offers little resistance to flow.
Butterfly valve technology has evolved dramatically over the past half century, as has its industry popularity. This popularity can be partly attributed to the quarter-turn operation, tight shutoff and its availability in a variety of materials of construction.
Early use of butterfly valves focused on water applications, but new designs and component materials have allowed them to be utilized in growing industrial fluid applications. Presently, butterfly valves can be found in almost every chemical plant handling a variety of diverse fluids.
Butterfly valves range in size from 1 in to more than 200 in and most have a pressure capability of 150-psi to 740-psi cold working pressure. The general temperature rating for a resilient seated valve is 25 Degrees F to 300 Degrees F and 400 Degrees F to 450 Degrees F for a high-performance butterfly valve.
The butterfly valve can be used for on-off service or modulating service. Actuation is typically achieved either manually (handle, wrench, gear operator) or through an external power source to cycle the valve automatically.
Automatic actuators include electric, pneumatic and hydraulic operators.
There are many advantages offered by butterfly valves compared to other types of valves including an inherently simple, economic design that consists of fewer parts, which makes butterfly valves easy to repair and maintain. The wafer-shaped body and relatively light weight offer a savings in the initial cost of the valve and installation costs -- in person-hours, equipment and piping support.
Basic componentsThe butterfly valve consists of only four main components: body, disk, stem and seat.
Body. Butterfly valves generally have bodies that fit between two pipe flanges. The most common body designs are lug and wafer. The lug body has protruding lugs that provide bolt holes matching those in the pipe flange. A wafer body does not have protruding lugs. The wafer valve is sandwiched between the pipe flanges, and the flange bolts surround the body.
Each type of body has advantages, some of which are listed:
The wafer style is less expensive than a lug style.
Wafer designs do not transfer the weight of the piping system directly through the valve body.
A lug body allows dead-end service or removal of downstream piping.
Disk. The flow closure member of a butterfly valve is the disk. Many variations of the disk design have evolved relative to the orientation of the disk and stem in an attempt to improve flow, sealing and/or operating torque.
The disk is the equivalent of a plug in a plug valve, gate in a gate valve or a ball in a ball valve. Rotating the disk one-quarter turn or 90 Degrees opens and closes the butterfly valve.
Stem. The stem of the butterfly valve may be a one-piece shaft or a two-piece (split-stem) design.
The stem in most resilient seated designs is protected from the media, thus allowing an efficient selection of material with respect to cost and mechanical properties.
In high-performance designs, the stems are in contact with the media and, therefore, must be compatible, as well as provide the required strength for seating and unseating the disk from the seat.
Seat. The seat of a resilient-seat butterfly valve utilizes an interference fit between the disk edge and the seat to provide shutoff. The material of the seat can be made from many different elastomers or polymers. The seat may be bonded to the body or it may be pressed or locked in.
In high-performance butterfly valves, the shutoff may be provided by an interference-fit seat design or a line-energized seat design, where the pressure in the pipeline is used to increase the interference between the seat and disk edge. The most common seat material is polytetrafluoroethylene (PTFE) or reinforced PTFE (RTFE) because of the wider range of compatibility and temperature range.
Metal seats are also offered in high-performance butterfly valves. These metal seats allow a butterfly valve to be used in even higher temperatures to 1,000 Degrees F. Fire-safe designs are offered that provide the shutoff of a polymer seat valve before a fire, and the metal seal backup provides shutoff during and after a fire.
"Non-wetted" and "wetted"Lined butterfly valves rely on elastomers (rubber) and/or polymers (PTFE) to completely isolate the valve body and stem journal area from the corrosive and/or erosive effects of the line media. When the body and stem journal area are isolated from the line media, the valve is considered a "non-wetted" design. By isolating the valve body and stem with rubber or PTFE, it is not necessary for the valve body to be made of expensive corrosion-resistant materials such as stainless steel, Alloy 20 and C-276.
When the valve body and journals are exposed to the line media such as in gate valves, globe valves and lubricated plug valves, the valve is considered to have "wetted" parts.
Characteristics and system requirementsThe following are some general control valve terms and characteristics for butterfly valves when used for modulating service. A valve having a stated inherent characteristic may provide a different installed characteristic due to interaction with the system.
Linear. The flowrate is directly proportional to the amount of disk travel. For example, at 50% open, the flowrate is 50% of maximum flow.
Equal percentage. Equal percentage characteristic means that equal increments of valve travel produce equal percentage changes in flowrate as related to the flowrate that existed at the previous travel position.
For example, if a valve travel change from 20% open to 30% open produced a 70% change in flowrate, then a valve travel change from 30% open to 40% open would produce another 70% change in flowrate. If the flowrate at 20% open was 100 gpm, then flowrate at 30% open would be 170 gpm and the flowrate at 40% open would be 70% greater than at 30% travel or 289 gpm. The same would be true for each additional incremental travel position.
Quick opening. A quick-opening valve means exactly that. Flowrate through the valve increases very rapidly for incremental changes in valve travel when valve position is near closed. As valve position becomes more open, flowrate changes diminish with incremental changes in valve travel approaching zero change as the valve position nears full open.
Traditionally, most butterfly valves have exhibited equal percentage inherent characteristics at angles of opening from 20 Degrees to 70 Degrees . Advances in disk design have allowed the extension of the equal percent characteristic through to the 90 Degrees , full-open position.
Designs employed for the extension of the equal percentage characteristic have varied from special contouring with planned flow disturbance to wafer-thin types with almost no flow disturbance. The former causes flow restriction at intermediate travel characteristics.
There are other types of valve disks offered that exhibit inherent flow characteristics approaching linear. This deviation from the traditional characteristic is the result of very heavy disk cross sections. As these valves are used in control applications, the user must ensure suitability of the linear characteristic.
Still another type of disk exhibits a characteristic midway between linear and equal percentage. Typically, this would be a disk design for high-pressure service but with minimal available capacity.
The selection of the appropriate control valve characteristic is dependent on the needs of the system. Because there are several factors to be considered, a complete system analysis is required to determine precisely which is the optimum characteristic. Often, it is not practical to perform a system analysis; therefore, certain rules of thumb are offered:
If in doubt about the preferred characteristic, choose equal percentage. Such a choice may result in a perfect match. If the match is not perfect, it will not be as detrimental as the selection of a linear characteristic when it is not a perfect match.
Except for pressure-relief applications, the quick-opening characteristic is seldom employed for control applications.
Other rules of thumb for the selection of characteristic for liquid applications are:
If greater than 25% of system pressure drop is available to the valve at maximum flow conditions, the use of the linear characteristic provides the best results.
If less than 25% of system pressure drop is available to the valve at maximum flow conditions, the use of the equal percentage characteristic provides the best results.
If the system is for control of pressure, the use of equal percentage characteristic is preferred.
Rules of thumb for selection of characteristic for gas applications include:
For small-volume systems, the use of the equal percentage characteristic is preferred.
For large-volume systems, the use of the linear characteristic is preferred if more than 25% of the system pressure drop is available to the valve.
In the course of selecting the desired control valve characteristic, do not be misled into choosing a linear one on the basis that it will provide an overall linear system.
While it may be desirable to have a linear system, the linear valve characteristic may not provide system linearity. This is due to a difference between the inherent valve characteristic and the installed characteristic.
The inherent characteristics are what the valves provide under constant pressure drops that are typically found in test situations. Installed characteristics occur when the valve is installed in a system where pressure drops vary with changes in valve position.
Economy and reliabilityObviously, butterfly valves cannot perform in every application. Variables dependent on the service, temperature and pressure also determine which valve type to choose. The service conditions must first be determined in order to properly apply any valve.
Technological innovations combined with its simple design have made the butterfly valve a dependable, economical and flexible solution to a variety of industrial flow control needs.
By Hugh Konigsmark, product manager for Resilient Seated Products, Tyco Valves and Controls, Houston, Texas. Konigsmark is a graduate of the University of Texas.