The ball is mounted on trunnions supported in body bearings above and below the ball. Sealing is achieved by spring loaded piston type seats, which shut off flow when the line pressure acts on the upstream seat. This design provides automatic relief of cavity overpressure. The standard design cannot be used for double block and bleed isolation duty, but “twin seal” design available, despite they would seldom be the first choice for such a service.
In this case, automatic cavity relief is forfeited and an external relief valve must be provided if specifically requested.
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