A ball valve consists of a floating ball with a ho...MORE >>
Butterfly valves are among the family of quarter-turn v […]
Butterfly valves are among the family of quarter-turn valves and work very similar to ball valves. The “butterfly” is a disk connected to a rod. It closes when the rod rotates the disc by a quarter turn to a position perpendicular to the flow direction. When the valve opens, the disk is rotated back to allow the flow.
Butterfly valves are used for on-off or modulating services and are popular due to their light weight, small installation footprint, lower costs, quick operation and availability in very large sizes. These valves can be operated by handles, gears or automatic actuators.
Butterfly valves have a relatively simple construction. The main components of a butterfly valve are the body, seal, disc and stem. A typical butterfly valve has the disc positioned in the center of the connected pipe and a stem that is connected to an actuator or handle on the outside of the valve. In the closed position, the disc is perpendicular to the flow, as shown in Figure 2, and is sealed by the valve seat. The stem is also sealed by the use of an o-ring. When the actuator or handle rotates the stem back 90 degrees, the disc moves away from the valve seat and positions itself parallel to the flow. Partial rotation allows for the flow to be throttled or proportional.
Butterfly valves used for modulating services can be designed to have a linear or an equal percentage characteristic.
Linear: When the flow rate is in a linear relationship with the amount the disk travels, it means that at X% of opening of the disc the flow rate will be at the same X% of maximum flow rate. Example being that if the disc is opened 1/3 of a turn (30 degrees), then the flow rate will be 33.3% of maximum.
Equal: If a butterfly valve has an equal percentage characteristic, that means equal increments of valve travel produces equal percentage changes in the flow rate. As an example, if travelling from 30 to 40 degrees opening increased the flow rate from 100 to 170 m3/h (by 70%), then a travel from 40 to 50 degrees will increase the flow rate from 170 to 289 m3/h (by 70%). This results in a logarithmic relationship between disc travel and flow rate. Advances in butterfly valve design has made the equal percentage characteristic possible for angles of opening from 20 to 90 degrees.