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.
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Design Choices for Mechanical Seals
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 seal is a non-pusher seal design in which a single spring coil fits over the shaft and bellows.
Specifications for Mechanical Seals
Other important design parameters to consider for mechanical seals include spring configuration, shaft mounting, and seal configuration.
Spring configuration can be single or multi. Single springs are sometimes called "monocoil" or "single coil" design. This type of seal uses a large spring cross section that resist corrosion. Its chief limitations are its tendency to distort at high surface speeds, the large axial and radial space it requires and the need to stock a different size spring for each seal size. Multiple small springs are not as susceptible to distortion at high speeds as are single coil springs and they consequently exert an even closing pressure on the seal ring at all times.
Shaft mounting choices include cartridge unit, noncartridge, split seal (fully split), and semi-split seal.
The seal can be tandem, face-to-face, back-to-back, or concentric.
Common applications for mechanical seals include pump, agitators or mixers, marine stern tube (propeller shaft), gas seal (spiral groove seal), and cryogenic seal.
The seal can be internally or externally mounted. Important shaft size and service limits to consider when searching for mechanical seals include nominal shaft diameter, shaft speed, alternate shaft or rubbing speed, operating pressure, and operating temperature.
Common features for mechanical seals include balanced or unbalanced construction, dependent on direction or rotation or independent of direction of rotation, capability to handle slurries, and encased spring element.
The direction of the shaft rotation is important to consider. This is the direction of a shaft's rotation as seen from the drive. Mechanical seals that are dependant on the direction of rotation are those that transmit torque using a conical spring or those that are equipped with a pumping screw. The direction can be clockwise or counter-clockwise.
BS EN 12756 - Principal dimensions, designation and material codes
ASTM F1511 - Standard specification for mechanical seals for shipboard pump applications
BS ISO 17712 - Freight containers - mechanical seals
DIN 24960 - Mounting dimensions, principal dimensions, designation and material codes
Additional standards can be found in the IHS Standards Store.
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