Single Mechanical Seal (Internal) Single Mechanical Seals are used when leakage needs to be minimized. Single seals have a viscosity limit of approximately 5,000 cps and temperature limit of 500 °F. External Mechanical Seal External Mechanical Seals are used to isolate the seal body from the liquid being pumped or when pumping liquid under vacuum conditions. Double Mechanical Seal Double Mechanical Seals require a flushing system and are typically used when pumping liquids that are abrasive, crystallize on contact with air, or are very dangerous. Packing Seal Packing is suitable and the most economical for safe, non-hazardous liquids.