Mechanical Seal

Description

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.

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 r

esponsible 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 resists 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.

 

 Applications

Common applications for mechanical seals include pump, agitators or mixers, marine stern tube (propeller shaft), gas seal (spiral groove seal), and cryogenic seal.

 

Features

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. 

WHAT IS A SINGLE MECHANICAL SEAL?

A single mechanical seal consists of two very flat surfaces that are pressed together by a spring and slide against each other. Between these two surfaces is a fluid film generated by the product. This fluid film prevents the mechanical seal from touching the stationary ring. An absence of this fluid film (dry running of the pump) results in frictional heat and ultimate destruction of the mechanical seal. 
Mechanical seals tend to leak a vapor from the high pressure side to the low pressure side. This fluid lubricates the seal faces and absorbs the heat generated from the associated friction, which crosses the seal faces as a liquid and vaporizes into the atmosphere. So, it's common practice to use a single mechanical seal if the product poses little to no risk to the environment.

WHAT IS A DOUBLE MECHANICAL SEAL?

A double mechanical seal consists of two seals arranged in a series. The inboard, or “primary seal” keeps the product contained within the pump housing. The outboard, or “secondary seal” prevents the flush liquid from leaking into the atmosphere.
Double mechanical seals are offered in two arrangements:

• Back to back
• Two rotating seal rings are arranged facing away from each other. The lubricating film is generated by the barrier fluid. This arrangement is commonly found in the chemical industry. In case of leakage, the barrier liquid penetrates the product.
• Face to face
• The spring loaded rotary seal faces are arranged face to face and slide from the opposite direction to one or two the barrier liquid penetrates the product. If the product is considered “hot”, the barrier liquid acts as a cooling agent for the mechanical seal.

Double mechanical seals are commonly used in the following circumstances:

• If the fluid and its vapors are hazardous to the operator or environment, and MUST be contained
• When aggressive media are used at high pressures or temperatures
• For many polymerizing, sticky media 

• Stationary seal parts. This is a popular choice for the food industry, particularly for products which tend to stick. In case of leakage,

Thermosyphon:

This is used for double mechanical seals in back to back or tandem seal arrangement to provide necessary lubrication and cooling to the seal faces to achieve recommended seal life. This is equipped with cooling coil inside the shell to bring down the temperature of barrier fluid coming from
seal to Thermosyphon.

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