Butterfly Valves working principle and types

Butterfly valve working principle – Butterfly valves consist of a disc attached to a shaft with bearings used to facilitate rotation. These are considered high recovery valves, since only the disc obstructs the valve flow path. The flow capacity is relatively high and the pressure drop across the valve is relatively low. The butterfly valve offers a simple  design that is lightweight, compact, and inexpensive, particularly in larger valve sizes. Because of the advances in seating material, butterfly valves have found general acceptance in the oil, gas, chemical, water, and process fields. The valve is often used in place of a gate valve, but has the added advantage of flow regulation. 

Butterfly valves are available in two basic body types:

  • Wafer-type butterfly valve
  • Double flanged-type butterfly valve

Butterfly Valve Seat Construction :-

Stoppage of flow is accomplished by the valve disk sealing against a seat that is on the inside diameter periphery of the valve body. Many butterfly valves have an elastomeric seat against which the disk seals. Other butterfly valves have a seal ring arrangement that uses a clamp-ring and backing-ring on a serrated edged rubber ring. This design prevents extrusion of the O-rings. In early designs, a metal disk was used to seal against a metal seat. This arrangement did not provide a leak-tight closure, but did provide sufficient closure in some applications (i.e., water distribution lines).

Butterfly Valve Body Construction :-

Butterfly valve body construction varies. The most economical is the wafer type that fits between two pipeline flanges. Another type, the lug wafer design, is held in place between two pipe flanges by bolts that join the two flanges and pass through holes in the valve’s outer casing. Butterfly valves are available with conventional flanged ends for bolting to pipe flanges, and in a threaded end construction.

Butterfly Valve Disk and Stem Assemblies :-

The stem and disk for a butterfly valve are separate pieces. The disk is bored to receive the stem. Two methods are used to secure the disk to the stem so that the disk rotates as the stem is turned. In the first method, the disk is bored through and secured to the stem with bolts or pins. The alternate 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. Uniform sealing is accomplished and external stem fasteners are eliminated. This method of assembly is advantageous in the case of covered disks and in corrosive applications.

 

In order for the disk to be held in the proper position, the stem must extend beyond the bottom of the disk and fit into a bushing in the bottom of the valve body. One or two similar bushings are along the upper portion of the stem as well. These bushings must be either resistant to the media being handled or sealed so that the corrosive media cannot come into contact with them.

 

Stem seals are accomplished either with packing in a conventional stuffing box or by means of O-ring seals. Some valve manufacturers, particularly those specializing in the handling of corrosive materials, place a stem seal on the inside of the valve so that no material being handled by the valve can come into contact with the valve stem. If a stuffing box or external O-ring is employed, the fluid passing through the valve will come into contact with the valve stem

Wafer-type butterfly valve

The wafer-type butterfly valve is mounted between two flanges and is held in place by flange bolts.In situations where dismantling of equipment or lines may require disconnecting of one of the holding flanges, a lug-wafer butterfly valve can be used. The lug-wafer butterfly valve  has tapped lugs through which the flange bolts are screwed and hold the valve in place, even when one flange is removed. In most wafer butterfly valves, the elastomer seal also acts as its own flange gasket.

Double flanged-type butterfly valve

The double-flanged butterfly incorporates two flange ends which are bolted individually into the pipework or equipment flanges. Gaskets are used between the valve ends and connecting flanges.

Recommended Uses:


• Fully open/closed or throttling services
• Frequent operation
• Minimal fluid trapping in line
• Applications where small pressure drop is desired.

 

Applications:


• Most economical for large lines in chemical services, water treatment, and fire
protection systems. General sizes available are 2″ to 48″, although sizes up to 96″ are
available from certain manufacturers.
• Due to the valve design, incorporating a small face-to-face dimension and lower
weight than most valve types, the butterfly valve is an economical choice for larger
line sizes (i.e. 8″ and above).
• The butterfly valve complies with ASME face-to-face dimensions and pressure
ratings. This enables the valve to be easily retrofitted in line regardless of the
manufacturer
• The ASME pressure classes adhered to by most manufacturers include 150, 300 and
600# allowing a maximum pressure of 1500 psi.

 

Butterfly valve Advantages:


• Low cost and maintenance
• High capacity
• Good flow control
• Low pressure drop


Butterfly valve Disadvantages:


• High torque required for control
• Prone to cavitation at lower flows
• Lack of cleanliness and inability to handle slurry applications.
• Generally not rated as bubble tight, and the cavities and leak paths around the disc
stem are potential entrapments for fluids and slurries. Some high performance
butterfly valves meeting ASME class VI leakage ratings are however available on
demand.

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