Double Block and Bleed Valves, are intended for specific applications. Their DBB properties are sometimes associated with trunnion mounted ball valves but this is incorrect. The reason is that API 6D specifies a DBB test. However, this stipulates that the pressure on both upstream and downstream seats are identical. In the field, there will be differential pressure across the seats, a ball valve will then relieve to the low-pressure side.  A true DBB valve will not, by design, leak downstream. As a direct result, this type of valve must be fitted with a bleed system that allows overpressure from within the valve to be diverted in a controlled manner. These valves will have their application where zero leakage is paramount. This can be on offloading jetties or for instance airport fuelling terminals where leaking of volatile aircraft fuel could be extremely dangerous.