Double Block and Bleed Valve. A valve with two seating surfaces which in the closed position blocks flow from both directions, and having a cavity between the sealing surfaces capable of being vented. Note: This definition is similar to that in ISO 14313 (API Spec 6D) and relates to blocking of two opposing sources of flow/pressure and the ability to vent the space (i.e. the valve cavity) between them. This function is frequently confused with 13.3 above, which is a more frequent requirement. (Note that any trunnion mounted ball valve or expanding type gate valve can offer double block and bleed capability in standard form.)