Chapter 3: Manual Valves: Section 3.4 - Manual Butterfly Valves

3.4.1 Introduction to Manual Butterfly Valves

The manual butterfly valve is a quarter-turn (0° to 90°) rotary-motion
valve that uses a round disk as the closure element. When in the fullopen
position, the disk is parallel to the piping and extends into the
pipe itself.

Manual butterfly valves are classified into two groups. Concentric
butterfly valves are used in on–off block applications, with a simple
disk in line with the center of the valve body. Generally, concentric
valves are made from cast iron or another inexpensive metal and are
lined with rubber or a polymer. For throttling services, eccentric butterfly
valves are designed with a disk that is offset from the center of the
valve body. When butterfly valves are automated, eccentric butterfly
valves are preferred since the disk does not make contact with the seat
until closing, which prevents premature wear of the seat with the continual
positioning associated with automated throttling. In most
designs, simple concentric butterfly valves are used for strict on–off
service and even when used in throttling applications do not lend
themselves as well to automatic control as those butterfly designs
designed specifically for throttling control. Because the initial development
was for blocking service, concentric butterfly valves have poor
rangeability and inadequate control close to the seat, while throttling
butterfly valves have design modifications to allow for better flow
control through the entire stroke.

Butterfly valves have a naturally high pressure-recovery factor,
which is used to predict the pressure recovery occurring between the
vena contracta and the outlet of the valve. The butterfly valve’s ability
to recover from the pressure drop is influenced by the geometry of the
wafer-style body, the maximum flow capacity of the valve, and the service’s
ability to cavitate or choke. Overall, because of the high-pressure
recovery, butterfly valves work exceptionally well with low-pressure-drop
applications.

The largest drawback to using a butterfly valve is that its service is
limited to low pressure drops because of its high-pressure recovery.
Although flashing is not associated with butterfly valves, cavitation
and choked flow easily occur with high pressure drops. While some
special anticavitation devices have been engineered to deal with cavitation,
users normally prefer to deal with cavitation with other valve
styles that allow the introduction of internal anticavitation devices.

Butterfly valves are used for on–off and flow-control applications.
Common service applications include both common liquids and gases,
as well as vacuum, granular and powder, slurry, food-processing, and
pharmaceutical services.

The sizes of butterfly valves are limited to 2 in (DN 50) and larger
because of the limitations of the rotary design. Because of the side
loads applied to the disk, the maximum size that a high-performance
butterfly can reach is 36 in (DN 900). Manual designs are limited to
ANSI Class 150 (PN 16), although some manufacturers offer ANSI
Classes 300 and 600.

3.4.1 Introduction to Manual Butterfly Valves

The manual butterfly valve is a quarter-turn (0° to 90°) rotary-motion
valve that uses a round disk as the closure element. When in the fullopen
position, the disk is parallel to the piping and extends into the
pipe itself.

Manual butterfly valves are classified into two groups. Concentric
butterfly valves are used in on–off block applications, with a simple
disk in line with the center of the valve body. Generally, concentric
valves are made from cast iron or another inexpensive metal and are
lined with rubber or a polymer. For throttling services, eccentric butterfly
valves are designed with a disk that is offset from the center of the
valve body. When butterfly valves are automated, eccentric butterfly
valves are preferred since the disk does not make contact with the seat
until closing, which prevents premature wear of the seat with the continual
positioning associated with automated throttling. In most
designs, simple concentric butterfly valves are used for strict on–off
service and even when used in throttling applications do not lend
themselves as well to automatic control as those butterfly designs
designed specifically for throttling control. Because the initial development
was for blocking...
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