Butterfly Valve

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Butterfly Valve

What is a Butterfly Valve ?

A butterfly valve is a valve which can be used for isolating or regulating flow. The closing mechanism takes the form of a disk. Operation is similar to that of a ball valve, which allows for quick shut off. Butterfly valves are generally favored because they are lower in cost to other valve designs as well as being lighter in weight, meaning less support is required. The disc is positioned in the center of the pipe, passing through the disc is a rod connected to an actuator on the outside of the valve. Rotating the actuator turns the disc either parallel or perpendicular to the flow. Unlike a ball valve, the disc is always present within the flow, therefore a pressure drop is always induced in the flow, regardless of valve position.

A butterfly valve is from a family of valves called quarter-turn valves. In operation, the valve is fully open or closed when the disc is rotated a quarter turn. The "butterfly" is a metal disc mounted on a rod. When the valve is closed, the disc is turned so that it completely blocks off the passageway. When the valve is fully open, the disc is rotated a quarter turn so that it allows an almost unrestricted passage of the fluid. The valve may also be opened incrementally to throttle flow.

 

Types of Butterfly Valve

There are different kinds of butterfly valves, each adapted for different pressures and different usage. The zero offset butterfly valve, which uses the flexibility of rubber, has the lowest pressure rating. The high performance double offset butterfly valve, used in slightly higher-pressure systems, is offset from the centre line of the disc seat and body seal (offset one), and the centre line of the bore (offset two). This creates a cam action during operation to lift the seat out of the seal resulting in less friction than is created in the zero offset design and decreases its tendency to wear. The valve best suited for high-pressure systems is the triple offset butterfly valve. In this valve the disc seat contact axis is offset, which acts to virtually eliminate sliding contact between disc and seat. In the case of triple offset valves the seat is made of metal so that it can be machined such as to achieve a bubble tight shut-off when in contact with the disc.

Types of Butterfly Valves

  1. Concentric butterfly valves - This type of valve has a resilient rubber seat with a metal disc.

  2. Doubly-eccentric butterfly valves (high-performance butterfly valves or double-offset butterfly valves) - Different type of materials is used for seat and disc.

  3. Triply-eccentric butterfly valves (triple-offset butterfly valves) - The seats are either laminated or solid metal seat design.

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Application of Butterfly Valves

In the pharmaceutical, chemical and food industry a butterfly valve is used to interrupt product flow (solid, liquid, gas) within the process. The valves used in these industries are usually manufactured according cGMP guidelines (current good manufacturing practise).

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:

 

  • Cooling water, air, gases, fire protection, etc.

  • Slurry and similar services

  • Vacuum service

  • High-pressure and high-temperature water and steam services.

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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.

Hand operated actuator. 

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. 

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Valve Parts

 

Butterfly valves have a unique body construction and motion when compared to other types of valves.

 

Body Construction 

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. 

Butterfly valve body types.

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.

 

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.

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