Types of (Quarter-turn) Rotary Valves

Ball Valves

Quarter-turn 2-way ball valves are by far the most common type of process control valve. They are 2-way (inlet and outlet), 2-position (open and closed) valves that are used for shut-off or isolation of a system, or a loop or component within a system. The basic construction of a ball valve includes a ball as the obstructer which is sandwiched between two cup shaped seals referred to as “seats”. Typically the ball has a bore straight through it. Media flows through this bore when the valve is open. When the ball is rotated 90˚, the flow of media is stopped by the sides of the ball which now completely fill the opening in the seats.

Plug Valves

The basic construction of a plug valve is practically identical to that of a ball valve with the exception of the shape of the obstructer used. In this case, the ball is replaced by a slightly tapered cylinder. This cylinder has a bore through it just like a ball valve, and it operates the same way where it is open in one position, and closed when rotated 90˚.

There are advantages to plug valves that make them the preferred choice in certain applications. One is that they contain no voids or cavities in which media can get trapped. Another is that they can be adjusted to fix leaks that may develop over time, where as a ball valve would need to be repaired or replaced.

Like ball valves, plug valves are typically used for on/off applications. Plug valves are often used in extreme service applications such as refineries and chemical plants, where the environment of use is corrosive.

Butterfly Valves

In contrast to ball and plug valves, butterfly valves use a disc shaped obstructer. Basic, general duty butterfly valves are called "resilient seated". They have a single seal ring that acts a a seal between the valve body and disc, as well as the seal between the pipe flanges and valve body. When the disc is perpendicular to the direction of flow, the flow is stopped. When rotated 90˚ the flow goes through the valve by traveling around the slim profile of the disc.

Butterfly valves are commonly used on larger pipe sizes where a ball or plug valve would get very large, expensive, and heavy. Butterfly valves also function rather well as a flow control valve or an on/off valve. If a manual actuator is used in a flow control application, it must lock into place, so that the force of the fluid does not rotate the disc in either direction. Most manual butterfly valves do incorporate this feature.