Plan 12 is similar to Plan 11 except that a strainer is added to the flush line.  This plan has limited usage, but can be successfully applied if adequate precautions are taken.  Note that Plan 12 uses a strainer, not a filter.  API 682 specifies that the strainer mesh be stainless steel material with a minimum mesh size of 0.0049 inch (125 μm). Plan 12 is often combined with other piping plans, especially with dual seals, for example Plan 12/52.

Plan 12 should not be used without some form of indication that the strainer is plugging.  This can be a differential pressure indicator or alarm, or it can be a flow indicator or alarm.

Using Plan 12 is discouraged; in fact, except for historical reference and cautions, it might not have been included in API 682.  On the other hand, there are flush systems that do use filters.  Unfortunately, some filtered systems are often labeled as Plan 12 but they should be labeled Plan 99.