Just upstream is a double block and bleed Valve - a trunnion-mounted Ball Valve with self-relieving seals plus a bleed Valve to vent the cavity. You close the Ball Valve and open the bleeder. Now you'll be able to de-pressurize the road downstream and open it up to operate http://waylonpywrk.review-blogger.com/11369011/5-simple-statements-about-double-block-and-bleed-valve-explained