Ball valve terminology can be misleading, you need to understand which specific valve standard is being referenced when defining the isolation characteristics. API 6D is generally considered to be the main reference standard for trunnion mounted ball valves, however, there are many other standards so great care and understanding are required.
The most common “double block & bleed” valve arrangement is a trunnion mounted ball valve with a single ball and two independent seats. However, depending on the isolation and safety requirements it’s also possible to specify a “tandem” valve with two balls in one body or two completely separate valves used in series with a bleed or vent in between.
Because of these sealing characteristics, one of the reasons trunnion mounted ball valves are used is to enable the body cavity to be vented as a leakage or safety check.
Following the API 6D standard, we can carefully consider the isolation characteristics for a single valve and the definition of the piston effect of the seats in the valve. Therefore, If we follow API 6D there are three basic seat arrangements in a trunnion mounted ball valve.
Double Block & Bleed (DBB) is the most common arrangement with two (Uni-directional), or Single Piston Effect (SPE) seats. These seats are self-relieving and will seal if the upstream pressure is greater than the cavity pressure but will automatically relieve if the cavity pressure is greater than the upstream pressure. Note that if the upstream seat leaks, the downstream seat will not seal in the same direction.
When more critical isolation is needed, often in Gas applications, double isolation can also be used.
Double Isolation & Bleed (DIB) means that if the upstream seat fails the downstream seat will still provide isolation.
DIB Valves are available in two configurations DIB-1 & DIB-2.
DIB-1 (API 6D definition) valves have Double Piston Effect (DPE) seats both upstream and downstream providing a seal in both directions. If the upstream seat fails, the cavity pressure will cause a double piston effect on the downstream seat making a second seal on the seat. Consideration may be needed for external cavity relief.
DIB-2 (API 6D definition) valves have one SPE seat and one DPE seat. If the upstream seat leaks there is automatic cavity pressure relief on the upstream side and the downstream seat will isolate because of the double piston effect.
As you can see, the use of terminology, different standards, and specifications can be confusing, so we urge you to consult with our technical teams in order to ensure the right selection.