What is the seal function of the outer covering?

Answer 1

There are various applicaitons of mechanical shaft seals, but the most common are centrifugal pumps, propeller shafts on ships, and industrial mixers. The common factor here is a rotating shaft, where one end is in fluid that must be sealed off from the other end. Mechanical seals typically replace packing in a chamber called a stuffing box. Packing is a resilient material. A part called a gland compresses the packing, causing it to expand against the rotating shaft and the walls of the packing chamber, sealing it off. Packing is higher maintenance than a mechanical seal, as it must be continually adjusted as it wears out. If compressed too tightly in the process of eliminating all leakage, packing can cause damage to the shaft. A mechanical seal eliminates the damage and most of the maintenance, but has the drawback of much higher cost.

Answer 2

I think, that its purpose is to protect the tank by the instable pressure inside