Wednesday, September 25, 2013

General Intro to Ball Valves

Ball Valves
Variety of ball valve types
Ball valves are grouped by how the valve body is made/assembled. The groups are uni-body, 3-piece, split-body, top-entry, and welded body.

Ball valves are further defined by the machined hole in the ball, called the port.

A full port valve has a port the same size as the pipeline resulting in better flow characteristics. Flow coefficients are much better, but at a higher price. There are many cases where a reduction in diameter, or the resulting change in flow is detrimental.

In a reduced port valve, flow is normally one pipe size smaller than the pipe connected to the valve, size resulting in restricted flow through the valve, which results in increased velocity.

V-port ball valves incorporate a machined V in the seat around the outlet side of the valve. The "V" provides a more controllable flow pattern and is desirable when ball valves are used as control valves. The V-port has a much more linear flow coefficient. Going from fully closed to fully open, the small end of the "V" provides controllable, more stable flow. As the valve is opened, the gradual opening of the "V" allows more resolution in control. Velocity has to be considered in the valve body construction, so these ball valves tend to be more robust in design.

A cavity-filled ball valve is used in application where cleanliness or sanitary conditions exist. Applications where any voids, gaps or spaces between the ball, seat and stem - where bacteria or contaminates can accumulate - normally require cavity fillers.  Cavity filler material is selected consistent with the process media and application. Cavity fillers eliminate the places where contaminants can accumulate and allows for easy "flushing" (cleaning) of the valve.

Trunnion mounted ball valves have a valve stem which is mechanical attached to the ball and are used in applications on large pipelines and at higher pressures.

Most ball valves are designed with a floating ball not held in place by a trunnion. This allows the ball to be "pushed" slightly downstream and seal itself better against the seat. This design also allows valve with metal seats, to be used in "fire-safe" applications. If the valve is subject to high temperatures, such as those presented in a fire, the "soft" part of the seat will melt away, and allow the ball to secure itself agains the metal seat, and thus not allow material to pass and potentially feed the fire.

All ball valves can be equipped with electric, pneumatic or electro-hydraulic actuators for automatic control or failsafe operation.

The following video provides and excellent visual representation of the major components of the ball valve.