Trunnion Design is simply the use of upper and lower supports to retain the ball under pressure. Named for the "trunnion" historically used to support a cannon, a ball valve trunnion essentially doubles the safety and usability of a ball valve.

The trunnion feature is common to many better quality metal ball valves, but in plastic, is found only on Plast-O-Matic True Blue brand ball valves. It permits the valve to be installed in either direction, since the trunnion prevents the ball from shifting -- or worse, blowing out -- under pressure.

This feature, and that the valve can be piped in either direction, leads to the concept that the True Blue Ball Valve can indeed be used in applications with reversing flow.

Another important benefit of the trunnion design is that it allows the True Blue Ball Valve to act as a true union: The downstream piping can be disconnected under full upstream pressure (user is responsible to ensure that downstream piping is drained of liquid and that valve is indeed closed and secured to upstream piping). Many plastic ball valves claim to be "true union" because of their union fittings, but cannot indeed be disconnected in this manner.

The trunnion on a two-way ball valve supports the ball in much the same way as the stem does at the top. The trunnion on a three-way ball valve is a much different design, being more like a supporting ring, but provides the exact same function and inherent stability.

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