I assume you are selecting butterfly valve and asking when triple offset is required. There are no mandatory requirements that I am aware of but client and/or contractors standards will normally stipulate services where concentric, double offset and triple offset can be used with typically concentric in water service only and at the other extreme triple offset in severe H2S service and/or where bubble tight shut off is required.
Double offset and concentric butterfly valves are usually soft seated containing some thermoplastic or elastomer seat. This results in temperature limitations and chemical compatibility concerns. However soft seated valves provide better seal performance. The triple offset valve design allows one to get tight shut off without the temperature/chemical limitations.
If we are talking triple offset in lue of gate valves, this could be done for several reasons: space or weight limitations, quicker actuation time, throttling application etc.
Wouldnt know about "mandatory", but some tank farm operators have a preference for these valves in failsafe safety shutoff service. In other upstream and downstream oil and gas, no operators to my knowledge have a preference for these.
The preference of butterfly valves in oil & gas is weight, if this is a oil plattform or a ship it is important and if you have lack of room or you are in a compact skid assembly.
Concentric valves are little hard to do a good elastomeric seat caused by risk of deformation during operation. High performance and triple off set are better but take care with the change of Cv. For class 150 it is something important, over #300 the Cv is so very important to evaluate.
