What's mechanical seal?
Mechanical seal is a device used to seal the interface between a rotating shaft and a stationary housing of an equipment, in order to improve safety and reliability, reduce emissions and costs. It is commonly used in centrifugal pumps as well as other rotating equipments such as agitators, turbines, compressors, blowers.
Mechanical seal consists of four basic components: Seal face, Secondary sealing elements, Flexible elements and Metallic components, each of these components plays an important role in the operation of the seal. The seal faces are the elements that define the sealing interface. One seal face will be mounted onto and rotating with the shaft. The other one will be mounted into and stationary with the gland.
The sealing performance is achieved by the contact of the rotating face and stationary face, which are lapped within 1 - 3 helium light bands (11.6 millionths of an inch) of being extremely flat. If the seal is out of flat by 4 light bands, especially if two hard faces are being used, it will leak. To check seal faces for flatness, we use an optical flat. (Click here to see more about Optical Flat.)
Since there is some level of contact results in wear and heat generation, the rotating and stationary sealing interface’s been designed to be separated by a very small film in balancing the low leakage and the low wear when a seal is working, therefore it is important not only to the seal faces but also to all of the components in a seal that contribute to its ability to maintain an acceptable fluid film. Normally this film is on the order of 20 millionths of an inch(or a half micron).
Mechanical seals are precision engineered components. In order to get better seal performance and expected seal life, it must be installed properly and fitted in good conditions. (Click here to see the proper Seal Installation Procedures.)
Other Terms and Definitions
balanced seal
mechanical seal in which the seal balance ratio is less than 1
barrier fluid
fluid, at a higher pressure than the process pressure being sealed, introduced between pressurized dual (double) mechanical seals to completely isolate the pump process liquid from the environment
buffer fluid
fluid, at a lower pressure than the process pressure being sealed, used as a lubricant or buffer between unpressurized dual (tandem) mechanical seals
cartridge seal
completely self-contained unit (including seal faces, flexible elements, seal gland plate, sleeve and mating ring) which is pre-assembled and preset before installation
contacting seal
seal design in which the mating faces are not designed to intentionally create aerodynamic or hydrodynamic forces to sustain a specific separation gap
non-contacting seal
seal design in which the mating faces are designed to intentionally create aerodynamic or hydrodynamic separating forces to sustain a specific separation gap between the seal ring and the mating ring
flexible element
combination of components which move axially relative to the shaft/sleeve or seal chamber
flush plan
configuration of pipe, instruments and controls designed to route the fluid concerned to the seals
non-pusher seal
seal in which the secondary seal is not required to slide axially to compensate for wear and misalignment
pusher seal
seal in which the secondary seal is mounted between the seal ring on the flexible element and the sleeve or seal gland plate in which this secondary seal slides axially to compensate for wear and misalignment
quench
neutral fluid, usually water or steam, introduced on the atmospheric side of the seal to retard formation of solids that may interfere with seal movement, or for other purposes
seal balance ratio
ratio of seal face area exposed to closing force by hydraulic pressure in the seal chamber, to the total seal face area
throat bushing
device that forms a restrictively close clearance around the sleeve (or shaft) between the inner seal and the impeller
throttle bushing
device that forms a restrictively close clearance around the sleeve (or shaft) at the outboard end of a mechanical seal gland plate
Mechanical seal is a device used to seal the interface between a rotating shaft and a stationary housing of an equipment, in order to improve safety and reliability, reduce emissions and costs. It is commonly used in centrifugal pumps as well as other rotating equipments such as agitators, turbines, compressors, blowers.
Mechanical seal consists of four basic components: Seal face, Secondary sealing elements, Flexible elements and Metallic components, each of these components plays an important role in the operation of the seal. The seal faces are the elements that define the sealing interface. One seal face will be mounted onto and rotating with the shaft. The other one will be mounted into and stationary with the gland.
The sealing performance is achieved by the contact of the rotating face and stationary face, which are lapped within 1 - 3 helium light bands (11.6 millionths of an inch) of being extremely flat. If the seal is out of flat by 4 light bands, especially if two hard faces are being used, it will leak. To check seal faces for flatness, we use an optical flat. (Click here to see more about Optical Flat.)
Since there is some level of contact results in wear and heat generation, the rotating and stationary sealing interface’s been designed to be separated by a very small film in balancing the low leakage and the low wear when a seal is working, therefore it is important not only to the seal faces but also to all of the components in a seal that contribute to its ability to maintain an acceptable fluid film. Normally this film is on the order of 20 millionths of an inch(or a half micron).
Mechanical seals are precision engineered components. In order to get better seal performance and expected seal life, it must be installed properly and fitted in good conditions. (Click here to see the proper Seal Installation Procedures.)
Other Terms and Definitions
balanced seal
mechanical seal in which the seal balance ratio is less than 1
barrier fluid
fluid, at a higher pressure than the process pressure being sealed, introduced between pressurized dual (double) mechanical seals to completely isolate the pump process liquid from the environment
buffer fluid
fluid, at a lower pressure than the process pressure being sealed, used as a lubricant or buffer between unpressurized dual (tandem) mechanical seals
cartridge seal
completely self-contained unit (including seal faces, flexible elements, seal gland plate, sleeve and mating ring) which is pre-assembled and preset before installation
contacting seal
seal design in which the mating faces are not designed to intentionally create aerodynamic or hydrodynamic forces to sustain a specific separation gap
non-contacting seal
seal design in which the mating faces are designed to intentionally create aerodynamic or hydrodynamic separating forces to sustain a specific separation gap between the seal ring and the mating ring
flexible element
combination of components which move axially relative to the shaft/sleeve or seal chamber
flush plan
configuration of pipe, instruments and controls designed to route the fluid concerned to the seals
non-pusher seal
seal in which the secondary seal is not required to slide axially to compensate for wear and misalignment
pusher seal
seal in which the secondary seal is mounted between the seal ring on the flexible element and the sleeve or seal gland plate in which this secondary seal slides axially to compensate for wear and misalignment
quench
neutral fluid, usually water or steam, introduced on the atmospheric side of the seal to retard formation of solids that may interfere with seal movement, or for other purposes
seal balance ratio
ratio of seal face area exposed to closing force by hydraulic pressure in the seal chamber, to the total seal face area
throat bushing
device that forms a restrictively close clearance around the sleeve (or shaft) between the inner seal and the impeller
throttle bushing
device that forms a restrictively close clearance around the sleeve (or shaft) at the outboard end of a mechanical seal gland plate
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