Simply, a butterfly valve consists of a
circular disc with its pivot axis at right angles to the direction material is
flowing. The valve is made up of a body, seat, disc, stem, and actuator. Each
component part is available in a variety of materials called “trim features.”
Properly combining trim features to address material handled and environment is
important in selecting the correct model valve for its intended service.
Body
A butterfly valve body can be made of cast
iron, ductile iron, aluminum, carbon steel, stainless steel and exotic metals.
Butterfly valves are available in wafer and
lug style. Wafer style valves are installed “sandwiched” between pipe flanges.
This style of valve is easier to replace or install. However, replacing a wafer
valve requires the conveying line to be drained. Once the valve is removed,
there is nothing to seal material either upstream or downstream from the
removal point.
Lug style valves contain tapped holes that
allow them to be bolted directly to a mating flange. In the closed position,
the valve independently seals material to the upstream and downstream side.
Because of this independent sealing, lug valves may be used to isolate
downstream equipment that may require replacement or maintenance.
Seat
Resilient seated valves are the most commonly
used types. The interior of the valve body is lined with an elastomer seat.
Seats may be made of EPDM, buna, viton, Teflon, natural rubber, carbox,
chlorbutyl, white buna, or white neoprene as well as other materials. Choice of
seat material depends on temperatures, pressures and material handled.
The seats of some inexpensive butterfly valves
are typically molded into the body and cannot be repaired or replaced.
Precision butterfly valves typically contain removable seats that are
repairable or replaceable.
Disc
The valve disc (controlled by the actuator)
regulates the flow of material within the conveying line. Disc materials are
available to meet a variety of application demands: stainless steel, aluminum/bronze,
ductile iron, ductile/epoxy coated, ductile/nickel plated, ductile/nylon II
coated as well as others. As the disc is directly in the material flow stream,
care must be taken in specifying the proper material of construction and disc
shape. Some discs are designed to allow increased flow patterns through the
piping.
Stem
The stem passes through the center of the
valve, attaches to the actuator, and positions the disc for material flow
control and shut off. Depending on the application and valve size, stems may be
one or two-piece construction. Typical materials of construction include carbon
steel and different grades of stainless steel.
Actuator
A variety of actuators are available for
butterfly valves: manual handle, gear, pneumatic, electric and
electro-hydraulic. Also, actuators that may be enclosed in special housings and
buried are available for certain underground applications.
Other considerations
Depending on the application, additional valve
features that may need to be considered. Some of these features include: the
manner in which the stem and disc are attached, the way the stem and disc
assembly mount inside the valve body, undercutting of disc, type of material
used in stem bushings and packing, and availability of a ring seal between the
seat and valve body.
Working with a company that can offer and
properly apply a variety of trim features will enhance the success of the
butterfly valve in a specific application.
Applications
As stated earlier, Butterfly Valves are
generally specified for most air, gas, steam, and liquid applications. They
offer an excellent, economically priced, positive shut-off valve for handling
materials (e.g. gases and liquids) that are easily displaced by the valve disc
as it closes. When handling this type of material, butterfly valves provide a
reliable, bubble-tight, bi-directional shut off.
Care must be taken when applying a butterfly
valve in semi-abrasive or abrasive dry material applications (this includes
slurry applications that contain suspended particles because:
1) A valve disc closing on dry bulk material
will create premature wear to the rubber seat.
2) The obstructed orifice created by the disc
may cause bridging of material on the inlet side of the valve.
3) A disc opening or closing on a standing
column of dry material may cause the material to jam / pack.
4) Particles of dry material or suspended
particles in slurries may become trapped between the disc and seat, causing
conveying line inefficiencies.
Other Types of Butterfly Valves
As a person becomes familiar with butterfly
valves, they will undoubtedly discover other types of butterfly valves on the
market. Listed below is a brief introduction to some of these other types.
Inflatable Seated Butterfly Valves
Inflatable seated butterfly valves utilize an
inflatable seat or bladder to create a seal around the disc. Once the disc is
brought to the closed position, the bladder is inflated pneumatically.
Pros:
1) Since the seal is created after the disc
closes, less torque is required to move the disc into the closed position. A
smaller sized actuator costing less money is can be used.
2) Again, because the seal is created after
the disc closes, minimal abrasion exists between the disc and the seat.
Cons:
1) In addition to the controls needed to
verify the position of the disc, additional controls are needed to identify
whether the bladder is inflated or not. These controls add cost to the valve.
2) The bladder may be damaged and not creating
a seal, even though controls identify that a seal has been established.
3) The valve must be located near an air
supply to pneumatically inflate the bladder.
4) The inflatable bladder is subject to damage
and abrasion, requiring periodic replacement.
High Performance -- Double Offset Butterfly
Valve
This type of butterfly valve is ideal for
applications involving higher temperatures (up to 1200º F) and higher pressures
(up to 1440 psig). The disc and stem are double offset, causing the disc to
“cam” into place upon closure.
Pros:
1) A reliable butterfly valve for handling
higher temperature and pressure applications.
Cons:
1) Cost -- considerably more expensive than
standard butterfly valves.
High Performance -- Triple Offset Butterfly
Valve
This type of butterfly valve is often used in refinery
and off shore applications where more extreme conditions exist. Most triple
offsets utilize a metal on metal seat and disc closure, are rated up to 600#
class, and are fire safe.
Pros:
1) A reliable valve for extreme applications.
Cons:
1) Cost -- more expensive than the
double-offset butterfly valve.
The double and triple offset butterfly valves
are specialized valves for extreme applications.
Salina Vortex Corporation manufacturers a
variety of slide gates, diverter valves and iris valves for handling dry bulk
material. Vortex butterfly valves are available for air, steam, liquid and
certain dry material applications.
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