Against this, API 6D defines a double-isolation-and-bleed valve (DIB) to be a “solitary valve with two seating surfaces, each of which, from the shut placement, gives a seal in opposition to stress from one supply, with a method of venting/bleeding the cavity among the seating surfaces.” The Observe adds http://kylerdthvi.mpeblog.com/11793170/what-does-double-block-and-bleed-valve-mean