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Butterfly valves are quarter-turn rotary motion valves used as throttling valves to control flow through a system. They can be used with many different media. Butterfly valves offer several advantages including quarter-turn, openness for less plugging, and good control capabilities. They may be used in a wide variety of chemical services, are available with small dimensions allowing for use in areas where space is limited, and allow a high coefficient of flow. Disadvantages include difficulty cleaning internal parts; therefore they should be avoided in situations that call for sterile, medical or food processing applications. Additionally, some styles may have difficulty dispensing slurries.
|Quarter turn||Difficult to clean|
|Open port||Difficulty with slurries|
|Used with chemical or corrosive media||Throttling limited to low differential pressure|
|Compact, lightweight design||Potential for cavitation and choke|
|Available in large sizes||Unguided disc movement is affected by flow turbulence|
|Low pressure drop and high-pressure recovery|
|High coefficient of flow|
Industrial Valves can be classified in a number of different ways including the method of control and function. Butterfly valves are quarter turn valves. They function to open/close systems as well as controlling flow.
The closure element of a butterfly valve consists of a metal circular disc or vane that pivots on an axis at right angles to the direction of flow in the pipe. When rotated on a shaft, the disc seals against seats in the valve body. The thin disc is always in the passageway but it offers little resistance to flow. These valves offer a rotary stem movement of 90 degrees or less, in a compact design. Unlike ball valves, butterfly valves do not have any pockets in which fluids may become trapped when the valve is closed. The valve operation time is short because the valving element is rotated a quarter turn to open or close the passageway.
Butterfly valves can be used for on/off service or throttling. When a valve throttles or modulates the flow it is controlling the speed and capacity of media through the valve.
The type of butterfly valve is noted by the design of the seat and disc.
Concentric designed valves have the center of rotation moved back from the centerline of the valve disc. The design relies on a frictional interference seal between the seal and seat which are designed conically and on center. It is best applied to soft seated valves.
Double eccentric designed valves are also known as high performance valves. The sealing plane of the disc is offset from the axis of rotation, this leads to uninterrupted circular sealing surface on the disc that makes it possible for a circular sealing element to be placed in the valve. The axis of rotation of the disc is laterally displaced from the center so it will move away from the seat in order to prevent jamming as the valve opens and closes. Double eccentric valves eliminate wear points around the disc at the top and bottom of the seat as well as extending the life of the valve’s leak-free performance. Seats are available in metal or plastic. Metal seats are long lasting but do not provide as good a seal as soft plastic seats such as polytetrafluoroethylene (PTFE) and filled PTFE.
Triple eccentric designed valves have a metal sheet which ensures a strong conical sealing principle. The centerline of the cone is rotated away from the valve centerline resulting in an ellipsoidal profile and providing the third offset. There are three offets to the design; the center of rotation is offset from the tightness surface to allow for a total contact around the complete seal, the center of rotation of the disc is offset from the pipe centerline to allow a seal opening valve, and the seal cone tilting cancels jamming and friction. This allows for complete tightness without seal deformation and the seat-seal interface is completely eliminated ensuring long-sealing life. The design is durable even under extreme temperature fluctuations and pressures drops.
Media is a term used to describe the material in system. The media plays an important role when selecting the type of valve and the material of construction for the disc. There is a wide variety of materials that could be in the valve system including:
Gas: Valves for gas systems seal tightly up to a minimum specified leakage rate at rated operating temperatures and pressures. When there is a small volume, the use of the equal percentage characteristic* is recommended. For large volumes the linear characteristic* is preferred if more than 25% of the system pressure drop is available to the valve.
Liquid: Valves for liquid systems require tight seals to prevent leakage. If greater than 25% of system pressure drop is possible at maximum flow conditions, use the linear characteristic. If less than 25% of system pressure drop is available to the valve at maximum flow conditions, the equal percentage characteristic provides the best result.
Solids: When using a valve for semi-abrasive or abrasive material applications (including slurry applications) there are several things that should be considered. A disc closing on dry bulk material will create premature wear on the rubber seat and the obstructed orifice created by the disc may cause bridging of material on the inlet side of the valve. Other considerations include the potential of the disc jamming on dry materials or the material becoming trapped between the disc and seat causing conveying line inefficiencies.
Butterfly valves have a unique body construction and motion when compared to other types of valves.
Butterfly valves get their name from the shape of the body and closure elements. They have a simple design that consists of fewer parts, making repair and maintenance easy.
Valve body- Butterfly valves have bodies that fit between two pipe flanges. There are two types of valve bodies, the lug type and the wafer type.
The lug body has protruding legs that provide bolt holes matching those in the pipe flange. This style has metal inserts installed in the valve’s bolt holes. The valve is installed between two flanges using a separate set of bolts for each flange. The advantage of the lug body style is it allows for dead-end service or removal of downstream piping.
The wafer body style is installed between two flanges using bolts or nuts and studs. It does not have protruding legs. The shape is light-weight and has a lower initial and installation cost. However, some wafer body styles will not form a proper seal so care should be taken to avoid placing it between slip-on or screwed flange types. Wafer style valves are easier to replace and install.
Design Tip: When replacing a wafer body style, the conveying lines need to be drained because there is nothing to seal material either upstream or downstream from the removal point.
Valve seat– Most butterfly valves use an elastomeric seat and the disc seals against it. The seat utilizes an interference fit between the disc edge and the seat to provide shutoff. The flow is stopped when the valve disc seals against a seat on the inside diameter of the valve body. It may be bonded to the body or pressed or locked in. Other seal arrangements use a clamp-ring and backing-ring on a serrated edge rubber ring to block extrusion of the O-ring. In high-performance designs, the seal maybe provided by an interference-fit seat design of a line-energizes seat design. The seal is created by the pressure in the pipeline increasing the interference between the seat and disc edge. The seats of inexpensive valves may be molded into the body and cannot be repaired or replaced but in most precision valves the seats are repairable and replaceable.
Valve disc and stem assemblies– Butterfly valves have separate stem and disc pieces that are fastened together by one of two methods. In the first method, the stem is secured with bolts or pins that go through the disc. The second method allows the disc to “float” and find its center in the seat by shaping the upper stem bore to fit a squared or hex-shaped stem. The second method of assembly can be used for corrosive applications because external stem fasteners are eliminated and covered discs can be used. The disc is held in position by the stem which must stand beyond the bottom of the disc to the bottom of the valve body. The seal is accomplished with an O-ring or standard stuffing box. The fluid in the system will come into contact with the seal so it is important to pick a steal durable enough for the media used in the system. Since the stem in most butterfly valves is protected from the media, the material can be selected with respect to cost and mechanical properties. However, in high performance types the stem is in contact with the media so the stem material must be compatible. The stem must also provide the required strength to seat and unseat the disc from the seat.
Design Tip: If working with a corrosive material, put the stem seal on the inside of the valve to prevent the material in the system from coming into contact that with the valve stem.
The valve actuator operates the stem and disc to open and close the valve. There are several types of actuators to consider depending on the needs of the system such as the torque necessary to operate the valve, speed and the need for automatic actuation.
Manual/ hand operated actuators use a hand-wheel or crank to open or close the valve. They are not automatic but offer the user the ability to position the valve as needed. Manual actuators are used in remote systems that may not have access to power, however they are not practical for applications that involve large valves. The hand-wheel can be fixed to a stem or hammer which allows for the valve to be pounded open or closed if necessary. Gear-heads can be added for additional mechanical advantage and open/close speed.
Solenoid operated valves use hydraulic fluid for automatic control of valve opening or closing. Manual valves can also be used, with a solenoid valve, for controlling the hydraulic fluid; thus providing semi-automatic operation. A solenoid is a designed electromagnet. When an electric current is applied, a magnetic field is generated around the wire. An iron “T” or plunger is put in the center of the coil to concentrate the magnetism. Since iron is a strong magnetic conductor and air is not, the “T” is drawn by the magnetic field into a position where the magnetism can travel 100% through the metal conductor. The moveable “T” acts as the actuator of the valve. Solenoid valves can be arranged such that power to the solenoid either opens or closes the valve. One application of solenoid valves is to supply the air to systems like pneumatic valve actuators. These valves are not practical for large systems because their size and power requirements would be excessive.
Electric motor actuators permit manual, semi-automatic, and automatic operation of the valve. Electric actuators are the most common actuator type for butterfly valves because the valve can be operated remotely, and the actuator is reliable and maintenance-free. The high speed motor is usually reversible and used for open and close functions. The actuator is connected through a gear train to reduce the motor speed and thereby increase the torque. The actuator is operated either by the position of the valve or by the torque of the motor. A limit switch can be included to automatically stop the motor at fully open and fully closed.
Pneumatic operated valves can be automatic or semi-automatic. They function by translating an air signal into valve stem motion by air pressure acting on a diaphragm or piston connected to the stem. Pneumatic actuators are fast-acting for use in throttle valves and for open-close positioning.
Hydraulic actuators provide for semi-automatic or automatic positioning of the valve. They are used when a large force is required to open the valve, such as a main steam valve. With no fluid pressure, the spring force holds the valve in the closed position. Fluid enters the chamber, changing the pressure. When the force of the hydraulic fluid is greater than the spring force, the piston moves upward and valve opens. To close the valve, hydraulic fluid (such as water or oil) is fed to either side of the piston while the other side is drained or bled. Hydraulic actuators are available in a wide range of sizes and are economical to use in a valve system as well as with a single valve.
Self-actuated valves use the system fluid to position the valve. These are commonly found in relief valves, safety valves, check valves, and steam traps. Because these actuators use the fluid in the system, no external power is required.
Actuators can vary in operating speed. The speed should be selected based on the speed and power requirements of the system and availability of energy to the actuator.
Fast acting actuators are best used when a system must be quickly isolated or opened. Fast action is provided by hydraulic, pneumatic, and solenoid actuators. The speed of actuation is set by installing the correct orifice in the lines and the valve is closed by spring pressure, which is opposed by hydraulic or pneumatic pressure to keep the valve open. Electrical motors can also provide fast actuation when the speed is set through the motor speed and gear ratio. Fast acting valves quickly increase the flow rate in increments as it travels through the valve when the valve position is near closed. Except for pressure-relief applications, the fast acting characteristic is rarely used for control applications.
Slow acting actuators are best used when cold water is injected into a hot system or slower opening is needed.
Due to the wide variety and variations in valves, the actuator must be sized to the specific valve in the system. If the actuator is undersized, it will be unable to overcome the forces against it. This will cause slow and erratic stroking. If the actuator is not stiff enough to hold the closed position, the closure element will slam into the seat, causing a pressure surge. If the actuator is oversized, it will cost more, weigh more, and be more sluggish in terms of speed and response. Larger actuators may also provide a higher thrust that will damage internal valve parts. Actuators tend to be oversized because of safety factors but smaller sizes function just as well when built-in safety factors are considered.
Valves are made of a wide variety of materials including metallic and nonmetallic options. When selecting a material, the operating environment (i.e. ambient heat), lifespan (i.e. maintenance), and media (i.e. gas or corrosive liquid) should be considered. The most common material is carbon steel because it responds very well to high heat, is easily available and inexpensive. However, it is not suited for corrosive materials. Stainless steel is strong and exhibits resistance to both corrosion and high temperatures, but costs more than carbon steel. Special alloys are used for severe applications such as high pressure or extremely corrosive materials.
Choice of seat material depends on the temperature, pressure, and media handled. The most common seat material is PTFE or reinforced PTFE (RTFE) because of the wider range of compatibility and temperature range. Metal seats are also available for use in high temperatures and some elastomeric seats have metal backups in case of fire.
A butterfly valve body can be made of cast iron, ductile iron, aluminum, carbon steel, stainless steel and exotic metals.
Disc materials are available to meet a variety of application demands: stainless steel, aluminum/bronze, ductile iron, ductile/epoxy coated, ductile/nickel plated, ductile/nylon II coated as well as others. As the disc is directly in the material flow stream, care must be taken in specifying the proper material of construction and disc shape.
Selection Tip: Will the valve be mostly open or mostly close? Some materials show different characteristics in stagnant verses continuous-flow conditions.
There are several key terms and characteristics for butterfly valves that effect their operation and performance. For more information on valve performance specifications please refer to the Industrial Valves page on IHS Engineering360.
Non-wetted and wetted are terms used to describe the body and stem design.
A valve can be used to stop and start as well as throttle or regulate the flow of media through a system. The given and desired properties of the flow can be used when selecting a valve.
The valve flow coefficient is the number of U.S. gallons per minute of 60°F water that will flow through a valve at a specified opening with a pressure drop of 1 psi across the valve. The coefficient is used to determine the size that will best allow the valve to pass the desired flow rate, while providing stable control of the process fluid. For a control valve, the flow rate is related to the opening of the valve. There are two relationships available to determine flow rate.
Pressure drop is the change in pressure that occurs between the inlet and outlet of the valve. It’s an important specification to understand when selecting the size of the butterfly valve needed. If the pressure drop across the fully opened valve is not a large enough percentage of the total system drop, there will be little change in the fluid flow until the valve closes. In this case, a fast acting valve would be appropriate.
Selection Tip: When selecting a butterfly valve for a control system, consider that 25% to 50% of the total system pressure drop should be absorbed by the valve.
Sizing is very important when selecting a butterfly valve as a throttle device. Since there is no pressure drop across an open/close system, the inlet and outlet ports are generally the same size. In this case, the size of the valve is determined by the volume of media going through the system and the flow coefficient. There are several variables to consider when determining sizing for a valve. First is what type of media the valve will be controlling. The specific gravity and viscosity of the media will affect flow rate. Second is the maximum inlet pressure and temperature, along with the outlet pressure (pressure drop) at maximum load. Third is the maximum capacity and last is the maximum pressure drop the valve must close against.
Since butterfly valves are high capacity, a very small pressure drop is required to control the flow, which allows for reduction from the line size when sizing a valve. This pipe reduction affects the flow characteristics and will reduce the effective flow rate of the valve. This is known as the piping geometry factor. The pipe geometry factor can be adjusted for using the chart below.
Piping- geometry factor for valves with reducers and increasers on both ends. Table Credit: Valtek International
Butterfly valves can be used in many applications because they can be used with many different media types including water applications, corrosive materials and some slurries. Typical applications include:
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