Double Block and Bleed (DBB) is provided when there are two isolation methods with a vent valve between them.
"Positive Energy Isolation" is required for certain kinds of work such as hot work, confined space entry, or hot work in a confined space. Typical methods of providing positive energy isolation are blinds, disconnecting or misaligning piping, or DBB. There are specific categories of work where DBB is excluded from consideration as a viable method of positive energy isolation in certain jurisdictions, but most of the time it is acceptable.
Typically DBB is provided by shutting two block valves and opening a vent valve between them. The theory is that any leakage past the upstream valve will go up the vent instead of leaking through the downstream valve. Several companies sell DBB valves that have two valves in a single valve body which includes a void space that has a vent valve on it. These valves work well for many situations, but they can be expensive, often introduce a measurable pressure drop, and are generally not piggable.
Trunnion Ball Valves have two independent seating surfaces (with spring loaded seats), and a body cavity vent. People frequently want to use a single Trunnion Ball Valve for DBB. The problem that other people find with this is that the body cavity is self-relieving (i.e., if pressure builds up in the cavity then it will relieve to the downstream piping--this can happen if the small ports in the body bleed plug up). Some governments have explicitly excluded Trunnion Ball Valves from consideration as a DBB device. Other governments are silent on this. Some companies exclude Trunnion Ball Valves from consideration as a DBB device, other companies allow Trunnion Ball Valves to be considered as DBB devices, others are silent. If your company is silent, and the applicable government agencies are silent, then it is up to the Engineer to decide if he feels that a Trunnion Ball Valve provides adequate protection for workers in a Positive Energy Isolation scenario. This is not a trivial decision, and Engineers acting in good faith have come down on both sides of the issue. I've always come down on the side that my risk of a body bleed plugging is acceptably low that I regularly use Trunnion Ball Valves for DBB and have never gotten anyone hurt or had a near miss. Others will say that I've just been lucky for 20 years.