Before the 4th Edition, API 682 did not specify a minimum clearance between the inside diameter of a stationary seal part and the outside diameter of a rotating seal part. The 4th Edition specifies this minimum clearance – typically the clearance between the sleeve OD and the mating ring ID. The clearances specified in 4th Edition are representative of standard clearances that have been used for decades. I’ve written a historical perspective on how those sleeve to mating ring clearances came to be.
It should be emphasized that the minimum clearance specified in API 682 4th Edition applies only to equipment within the scope of the standard. Equipment outside that scope, such as non-cartridge seals, older pumps, non-API 610 pumps and certain severe services, might benefit from larger clearances.