Mechanical seals are designed to prevent leakage between a rotating shaft and its housing under conditions of extreme pressure, shaft speed and temperature.  Mechanical seals can be single acting or double acting.  Single (acting) mechanical seals have one sealing gap. The lubrication film required by the sliding seal faces is provided by the medium to be sealed.  The lubrication film required by the seal faces in double (acting) mechanical seals is provided by a higher pressure buffer medium (sealant liquid) that is compatible with the pumped product. The sealant liquid is at a higher-pressure so that any leakage across the seal faces will be the sealant liquid into the pumped product. This buffer serves to separate the product and the atmosphere.  Seal design choices include pusher, metal bellows, and elastomeric bellows.  A pusher mechanical seal utilizes a dynamic secondary seal or o-ring that is responsible for sealing the fluid path between the pump shaft and the inside diameter of the rotating seal face. The secondary seals move axially along a shaft or sleeve to maintain contact at the seal faces, compensating for seal face wear and for any seal wobble due to misalignment.  Metal bellows design is a non-pusher seal design. The secondary seal in a non-pusher design does not have to move along the shaft or sleeve to maintain seal face contact. The bellows itself provides the necessary spring loading for seal face contact. Metal bellows provide effective sealing in a wide range of temperatures and use no elastomers.  An elastomer bellows seals is a non-pusher seal design in which a single spring coil fits over the shaft and bellows.