True Meaning of Double Block and Bleed

True Meaning of Double Block and Bleed

Linked from ValveMagazine.com

It's time to do maintenance on a section of process. You don't want to shut down the entire facility, so you decide to block off and depressurize just the section you're working on. Just upstream is a double block and bleed valve—a trunnion-mounted ball valve with self-relieving seals and a bleed valve to vent the cavity. You close the ball valve and open the bleeder. Now you can de-pressurize the line downstream and open it up to work on it.
Not so fast, says Rudy Garza. You may think that valve gives you double isolation, but it doesn't—and that could be dangerous.

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