What is a double block & bleed fitting?

A "Double Block & Bleed" fitting is made of two shut-off fittings ("Double Block") with a fitting in between for emptying the space ("Bleed"). This type of fitting is usually used where the separation of two systems has to be particularly reliable. Whether the process media must not be allowed to mix, for maintenance work, to avoid contamination, etc., it is important to use a double block.