Double (or dual) mechanical seals are designed to ensure maximum sealing safety. These seals virtually eliminate leakage of the fluid or gas being handled in pumps or mixers. Double seals provide a level of safety/zero emissions compliance not possible with single seals. This is essential when pumping or mixing a dangerous or toxic substance. Double seals are a must-have for certain applications in many countries http://www.egerseal.com/mechanical-seal/double-seals/