Senin, 03 Oktober 2011

AN INTRODUCTION TO VALVES - BUTTERFLY VALVE

BUTTERFLY VALVE

A butterfly valve is a quarter-turn rotational motion valve, that is used to stop, regulate, and start flow. Butterfly valves are easy and fast to open. A 90° rotation of the handle provides a complete closure or opening of the valve. Large Butterfly valves are usually equipped with a so-called gearbox, where the handwheel by gears is connected to the stem. This simplifies the operation of the valve, but at the expense of speed.

Advantages of butterfly valves
Compact design requires considerably less space, compared to other valves
Light in weight
Quick operation requires less time to open or close
Available in very large sizes
Low-pressure drop and high-pressure recovery

Disadvantages of butterfly valves
Throttling service is limited to low differential pressure
Cavitation and choked flow are two potential concerns
Disc movement is unguided and affected by flow turbulence

Image of a typical Butterfly valve, Lug Wafer type design.

TYPES OF BUTTERFLY VALVES

Butterfly valves has a short circular body, a round disc, metal-to-metal or soft seats, top and bottom shaft bearings, and a stuffing box.

The construction of a butterfly valve body varies. A commonly used design is the wafer type that fits between two flanges. Another type, the lug wafer design, is held in place between two flanges by bolts that join the two flanges and pass through holes in the valve's outer casing. Butterfly valves are even available with flanged, threaded and butt welding ends, but they are not often applied.

SEAT DISK AND STEM OF A BUTTERFLY VALVE

Stopping flow is achieved by the valve disk sealing against a seat that is on the inside diameter periphery of the valve body. Often an elastomeric seat material will be used. Disk and stem of a butterfly valve consists of two parts, and there a two methods to be fastened together.

In the first method, the disk is bored through and secured to the stem with bolts or pins. The second method involves boring the disk as before, then shaping the upper stem bore to fit a squared or hex-shaped stem. This method allows the disk to "float" and seek its center in the seat.

TYPICAL APPLICATIONS OF BUTTERFLY VALVES

A butterfly valve can be used in many different fluid services and they perform well in slurry applications. The following are some typical applications of butterfly valves:
Cooling water, air, gases, fire protection et cetera
Slurry and similar services
Vacuum service
High-pressure and high-temperature water and steam services

from www.wermag.org

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