Monday, November 11, 2013

Butterfly Valves Types, Construction, Applications and Advantages

Butterfly valves are used to control and regulate or throttle the flow. They are characterized by fast operation and low-pressure drop. They require only a quarter turn from closed to full-open position. A typical flanged butterfly valve is illustrated in Fig. A. Butterfly valves are produced in sizes ranging from NPS 1¹⁄₂ (DN 40) to over NPS 200 (DN 5000). They are usually manufactured in flanged, wafer, and lug, or single-flange-type designs. The welding-end style is a specially engineered valve for a specific application. Threaded-end, grooved-end, and shouldered-end butterfly valves are also available to satisfy the joint type selected for the piping system. Butterfly valves are produced with metal-to-metal seats, soft seats, and with fully lined body and disc. The soft seats permit bubble-tight shutoff and the full lining enhances erosion and corrosion resistance.

Typical flanged-end butterfly valve
Fig. A: Typical flanged-end butterfly valve
Butterfly valves are suitable for low-pressure and low-temperature applications as well as high-pressure and high-temperature applications. The term high-performance butterfly valve is intended to signify their suitability for moderate- to high pressure and temperature services.

The butterfly valves have a low-pressure drop and high-pressure recovery factor. They are suitable for low-pressure drop applications. These valves are extensively used in large water transmission, distribution, and cooling water lines.

Construction of a Butterfly Valve

A butterfly valve has a short circular body, a round disc, shaft, metal-to-metal or soft seats, top and bottom shaft bearings, and the stuffing box. The valve body may have flanged ends, lugs, or wafer style (Fig. B) configurations to be installed between pipe flanges. The welding-end (Fig. C) butterfly valves are usually large and have butt-welding ends. Sometimes butterfly valves are manufactured in rectangular or square configurations.
Wafer butterfly valve
Fig. B: Wafer butterfly valve
Butterfly valve - welding end on one end
Fig. C: Butterfly valve - welding end on one end
The wafer-style butterfly valves are usually available in sizes NPS 12 (DN 300) or smaller. The limitation on size is essentially imposed by the difficulty of holding the larger weight valve in place between the flanges. The lug and flanged-end butterfly valves are available in all sizes and pose no problem in installation between flanges except for the normal problems associated with warped-flanged surfaces and uneven torquing of bolts.

Types of Butterfly Valves

Low Pressure or Concentric Butterfly Valves

In low pressure and low temperature designs of a butterfly valves, the disc and shaft axes are concentric. In open position, the disc divides the flow in two equal halves, with the disc in the middle and parallel to the flow. These valves are provided with resilient seats. These valves are available as lined or unlined. Most commonly used lining and seating materials include: Buna N, Neoprene, Fluorcel, Hypalon, EPDM, TFE, Viton, among others. The application temperature is limited by the temperature capability of the resilient material. These valves are generally produced in Classes 150 and 300.

High-Performance or Eccentric Butterfly Valves

The disc in high performance butterfly valves is offset from the center of the valve, and the shaft is also offset from the center of the disc. The offsets provided allow the disc to move eccentrically uninterrupted away from or toward the valve seat. Thus, the uninterrupted motion of the disc until it seats against the valve seat prevents unwanted wear and tear of the valve seat and disc due to friction and rubbing of the seating surfaces. The high-performance butterfly valves are used for on-off and throttling services. Some butterfly valve manufacturers produce high performance butterfly valves with triple offset, which enhances their actuation and leak tightness.

Advantages of Butterfly Valves

The following summarizes the advantages of butterfly valves:

1. The compact design requires considerably less space, compared to gate, globe, or other valves.
2. Light in weight.
3. Quick acting; as a quarter-turn valve, it requires less time to open or close.
4. It is available in large sizes, ranging from NPS 1¹⁄₂ (DN 40) to over NPS 200 (DN 5000).
5. They have low-pressure drop and high-pressure recovery.
6. Provide bubble-tight service.

Disadvantages of a Butterfly Valve

1. Throttling service is limited to low differential pressure.
2. Throttling is restricted to a 30- to 80-degree disc opening. Location of valve, pipe routing, free, and closed discharge are to be considered while using a butterfly valve in a throttling application.
3. Cavitation and choked flow are two potential concerns.
4. The disc movement is unguided and affected by flow turbulence.


Typical Applications of Butterfly Valves

Concentric or low-pressure and low-temperature butterfly valves, the high-performance butterfly valves, cover a wide range of applications. These applications include:
1. Cooling water, air, gases, and other similar applications, such as fire protection, circulating water, etc.
2. Corrosive services requiring lined valves
3. Food processing, chemical, and pharmaceutical services
4. Slurry and similar services
5. High-pressure and high-temperature water and steam services
6. Throttling service involving low differential pressures, as in cooling water or air supply systems
7. Vacuum service

Application Considerations for Butterfly Valves

1. It is noted that the disc of butterfly valve is unguided; therefore, operability of the valve is affected by the flow characteristics. A butterfly valve should not be located just downstream of a source of flow turbulence, such as pump-discharge nozzle, elbow, control valve, or a tee-branch. To minimize the effects of flow turbulence on the valve, attempts should be made to
● Locate the valve 4 to 6 diameters downstream of the source of flow turbulence.

● Orient the valve shaft in the same plane as the elbow or the pump outlet configuration. When there is more than one component on the upstream side of the valve, then the component adjacent to the valve is to be considered in determining the valve-shaft orientation.
2. When used in throttling applications, an attempt must be made to provide an adequate straight length of pipe downstream of the valve to allow the flow turbulence to subsidize prior to diverting the flow.

3. Butterfly valves are essentially bidirectional. In the case of high-performance butterfly valves, and sometimes in the case of low-pressure and low-temperature butterfly valves, the valve design may require more actuating torque to open or close the valve when the flow is reversed. In such cases, the valve manufacturer must be contacted to ensure that the valve is designed and the actuator is sized for flow in both directions.

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