Butterfly valves are frequently used in applications where flow needs to be either fully shut on and off. They are good for low pressure and low temperatures applications where the wetted part contacts the fluid directly while open. Butterfly valves get their name from the metal disc which rotates from perpendicular to the flow path (closed) to parallel to the flow path (open). In this article, you will learn about the different materials of construction used in butterfly valves, common configurations, mounting types, actuation methods, as well as repair and maintenance considerations.
A butterfly valve is comprised of the body that houses a disc. The disc is mounted on a rod and actuated through some means. When closed, the valve makes contact with a seat that blocks fluid flow.
Types of Butterfly Valves
Butterfly valves are categorized by the following attributes: material of construction, configuration, mounting type, and actuation type.
Materials of Construction
Bodies of a butterfly valve are most commonly carbon steel. Carbon steel is a good material selection because of its relatively low cost and acceptable performance in most applications. More demanding applications require stainless steel and various variants are used. Exotic materials such as titanium may be specified for seawater applications due to titanium’s extremely high corrosion resistance.
Seats are typically made of out of three different materials: rubber-lined, plastic-lined, and metal. Rubber-lined is the standard seat and is appropriate for non-demanding, non-abrasive, low-temperature applications. Plastic seats are specified for use with chemicals, abrasive applications, or where sanitation and purity are a great concern. Metal seats are the most expensive and a good fit for high-pressure or high-temperature applications.
The configuration pertains to how the disc closes and engages with the ball valves seal. There are four ways a disc can be configured: concentric, single offset, double offset, or triple offset.
The simplest design of a butterfly valve is concentric. This means the stem is centered in the middle of the valve disc and the middle of the pipe bore. The disc may contact the seat as early as 85° degrees of the way through the 0°-90° turn.
Single-offset butterfly valves have a stem located behind the disc. The offset of the stem causes later contact of the disc via the closing motion. This design provides less seat contact, thus enabling longer seat life. Single offset valves are rare as enhanced functionality at no additional cost is provided in the double-offset design.
Double-offset butterfly valves have a disc with two offsets. Like the single offset version, the double-offset butterfly valve has a stem which is located behind the disc. The double-offset geometry enables contact for only 1° to 3° of the 90° rotation. Double-offset valves are used in higher pressure applications and when disc-seat contact is of concern.
Triple-offset valves feature a stem located behind the disc and offset to a side. The third offset refers to the geometry of the disc. The disc has a cone shape that allows seating with minimal contact between surfaces during the closing process. Triple offset valves are more expensive and typically only seen in more demanding applications. As a result, these valves are typically built out of stainless steel, super duplex, or other specialized material.
Butterfly valves have four different ways to connect to the surrounding pipe: wafer, semi lug, lug, and double-flanged.
Wafer butterfly valves usually have two or four holes to secure and align the valve to the pipeline. They seat in the pipeline very securely, because they are designed to go through the mounting flange on both sides. However, maintenance is difficult on wafer valves as they require a full system shut-down to service.
Lug style butterfly valves have additional connections for attachment lugs. Flanges on one side of the valve will attach to half the lugs and the flange on the other side of the valve will connect to the other half. Lug valves can be easily removed, cleaned, and maintained. However, due to the lug connection method, the valve cannot handle as high pressures as its wafer counterpart.
Two infrequently used types of valves are semi-lug and double-flanged connections. The semi lug valve is typically used in end of line service as it offers an advantage over the semi lug by being quicker and easier to install. The double-flanged valve allows visual inspection of both sides of the bolt to determine how secure the section is everything is.
The three ways to open and close a butterfly valve are: manually, via a gearbox, or via a pneumatic/electrically actuated.
In a strict manual actuation, a quarter-turn of a lever will bring the valve from fully open to fully closed and vice versa. The gearbox reduces the amount of torque required to close the valves but ensures it will take longer to close the valve. Pneumatic or electrical actuators provide large opening torque and used for demanding and/or remote applications where manual and gearbox control is not practical.
Butterfly valves see common use in hydro-electric power stations, pneumatic systems, and a variety of process industries. These process applications may include vacuum service, slurry transport, cooling water, and other gas services. Butterfly valves operate quickly and with a low-pressure drop. When space constraint is a concern, butterfly valves are a good fit as they typically require less space than ball valves and other shut-off valves.
Butterfly valves have poor throttling capabilities and handling by an inexperienced operator may cause cavitation and choked flow. The butterfly’s disc may also be affected by flow turbulence.
Maintenance and Repair Considerations
One of the advantages of butterfly valves is they require minimum maintenance. Geared units are intrinsically lubricated and designed to be maintenance-free. The valve should be opened and closed every six months after active use.
As with any mechanical item under service, routine visual inspections will ensure the valve operates as designed. The visual inspection should include not only the valve, but associated bolts, piping, and wiring. In the instance of a leaking valve, maintenance may be possible in the field, depending on the manufacturer’s requirements. The valve seat can typically be replaced by removing the stem and disc and reinstalling per vendor’s recommendations.