4.3.1 Introduction to Butterfly Control Valves
Although the butterfly valve has been in existence since the 1930s, it
was used mainly as an on–off block valve until the past two decades,
when it began to be used for throttling services. In the late 1970s,
design advancements were made to the butterfly valve that not only
made it more applicable for throttling service, but also made it preferred
over globe valves in some applications. Such butterfly control
valves are differentiated from their on–off block cousins by the name
high-performance butterfly valves. In simple terms, the high-performance
butterfly control valve is a quarter-turn (0° to 90°) rotary-motion valve
that uses a rotating round disk as a regulating element. Typically, butterfly
control valves are available in sizes 2 through 8 in (DN 50
through DN 200) from ANSI Classes 150 to 600 (PN 16 through PN
100); 10 and 12 in (DN 250 and DN 300) in ANSI Classes 150 and 300
(PN 16 and PN 40); and 14 through 36 in (DN 350–900) in ANSI Class
150 (PN 16).
When fully open, the disk actually extends into the pipe itself, which
makes butterfly valves distinct from other valve designs. Butterflyvalve
bodies have very narrow face-to-face dimensions compared to
other types of valves, allowing the body to be installed between two
pipe flanges without any special end connections. This type of
arrangement is called a through-bolt connection and is only permissible
with certain bolt lengths. If the bolt length is too long, the bolting may
be subject to thermal expansion of the process or during an external
fire, causing leakage.
Initially, butterfly control valves were designed as automatic on–off
block valves. However, with recent improvements to rotary-valve
actuators and body subassemblies, they can now be used in throttling
services with the addition of an actuator or an actuation system. As
detailed in Sec. 3.4, the family of butterfly valves is classified into two
groups. Concentric butterfly valves are normally 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 polymer. Because of
their lower performance, they are normally equipped with manual
operators. In some applications, the manual operators are replaced
with an actuation system for throttling service. In most applications,
however, simple concentric butterfly valves are used strictly for on–off
service. Even when used in throttling applications, they do not lend
themselves as well to automatic control as other butterfly designs
specifically designed for throttling control. This is because the initial
development was for blocking service. Concentric butterfly valves
have poor rangeability, while throttling-specific butterfly valves have
design modifications to allow for better flow control through the entire
stroke.
Eccentric butterfly valves are valves designed specifically for high-
performance throttling services, using a disk that is offset from the
center of the valve body. The majority of butterfly valves used as control
valves feature the eccentric design. For the most part, eccentric
butterfly valves are specified in common valve materials, such as carbon,
stainless, or alloy steels. When equipped with actuators and positioners,
they are much more precise than concentric butterfly valves
that have been automated.
Compared to other types of throttling valves, eccentric butterfly
valves are one of the fastest growing types of control valves today for
a number of reasons. Because of the increased dead band associated
with the mechanical conversion of linear motion to rotary motion,
globe valves are more precise in high-pressure-drop applications than
butterfly valves. However, the control provided by today’s butterfly
valves is more than adequate for many low-pressure-drop applications
and other standard services.
When compared to globe control valves, butterfly control valves are
much smaller and lighter in weight because the butterfly valve’s body
subassembly weight can be anywhere from 40 to 80 percent of a comparable
valve and less than half the mass of the globe body subassembly.
In addition, smaller actuators can often be used with butterfly
valves since the weight of the regulating element is not a critical factor
in factoring the necessary actuator force. The difference in regulating-
element weight between butterfly and globe control valves becomes
much more evident as sizes become larger, as shown in Table 4.1. This
means that butterfly valves are preferred in applications where limited
space or weight is a consideration.
Another major benefit of using a butterfly control valve is that, size
for size, it has a larger flow coefficient, producing a greater flow than
comparable globe valves. Because the shaft of the butterfly valve
moves in a rotary motion instead of a linear motion, the frictional
forces are far less than a linear-motion valve, requiring less thrust and
permitting a smaller actuator. A butterfly valve has a naturally high
pressure-recovery factor (Sec. 7.2.9). This factor 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, a butterfly valve
works exceptionally well with low-pressure-drop applications.
The largest drawback to using a butterfly valve is that its service is
usually limited to low-pressure drops because of its high pressure
recovery. Although flashing is normally not associated with a butterfly
valve design, cavitation and choked flow occur easily with a butterfly
valve installed in an application with a high-pressure drop.
Although some special anticavitation devices have been engineered to
deal with cavitation, users prefer to deal with cavitation in a globe
valve because of its design versatility in allowing the inclusion of an
anticavitation device. Another disadvantage is that a butterfly valve
has a poor-to-fair rangeability of 20 to 1 because of the difficulty the
disk has in holding a position close to the seat. The process pressure
applied to the butterfly disk creates a significant side load, which can
only be remedied by using a larger-diameter shaft. Another drawback
to the butterfly control valve is the increased hysteresis and dead band
associated with the mechanical transfer of linear action from the actuator
to the rotary motion needed for the regulating element. Valve manufacturers
have utilized splined shafts or other secure linkages to minimize
this problem, although a globe valve avoids this problem
altogether with its direct linear motion. The sizes of butterfly valves
are also limited to 2 in (DN 50) and larger because of the limitations of
the rotary regulating element. Because of the side loads applied to the
disk, the maximum size that a high-performance butterfly can reach is
36 in (DN 900).
4.3.1 Introduction to Butterfly Control Valves
Although the butterfly valve has been in existence since the 1930s, it
was used mainly as an on–off block valve until the past two decades,
when it began to be used for throttling services. In the late 1970s,
design advancements were made to the butterfly valve that not only
made it more applicable for throttling service, but also made it preferred
over globe valves in some applications. Such butterfly control
valves are differentiated from their on–off block cousins by the name
high-performance butterfly valves. In simple terms, the high-performance
butterfly control valve is a quarter-turn (0° to 90°) rotary-motion valve
that uses a rotating round disk as a regulating element. Typically, butterfly
control valves are available in sizes 2 through 8 in (DN 50
through DN 200) from ANSI Classes 150 to 600 (PN 16 through PN
100); 10 and 12 in (DN 250 and DN 300) in ANSI Classes 150 and 300
(PN 16 and PN 40); and 14 through 36 in (DN 350–900) in ANSI Class
150...
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