There are no standards that define the permissibility of flange connectors.
Because only one weld is required to connect two sections of pipe in a freshly constructed plant, it is typical to minimize flange connections. This eliminates the need for two flanges, a gasket, Stud Bolts, a second weld, the cost of nondestructive testing for the second weld, etc.
There are other disadvantages of flange connectors.
Each flange joint can potentially leak (some people claim that a flange connection is never 100 percent leak proof).
Flanged pipe systems require considerably more room (just think of a pipe rack).
Insulation of flanged pipe systems costs more (special flange caps).
Obviously, flange connectors have numerous advantages; some instances.
Manufactured at a workshop, a new line may have many pipe spools.
These pipe spools can be built without the requirement for welding in the plant.
NDO (X-ray, Hydro test, etc.) at the plant is not required, as this has already been performed in the workshop.
Blasting and painting at the plant are unnecessary, as these processes have already been completed in the workshop (only paint damages during installation should be repaired).
As with many things, everything has advantages and disadvantages.
