Intemally-mounted, multi-spring, balanced, large shaftdiameter and double cartridge mechanical seals foragitators. The seals have an optimized design instructure and its torque is transmitted by set screwsand pins. The self- aligning bearing can effectivelyrestrain swing of shaft. The position limit of stationaryring in process fluid side can effectively prevent fromtaking off due to medium pressure variation. The sealhas self-adaption ability. Its installation is easy. Theseal performance is stable and reliable. This type ofseal is typical structure of vertical and large shaftdiameter reaction agitator seals at the pressure below2.5MPa.