Options for Sealing (Các loại làm kín)
• Packing – Oldest method
• Bushings – atm side of seal for quenching
• O-Rings
• Lipseals – commonly found in gearbox,
bearings, atm side of seal for quenching
• Mechanical Seals
Mechanical Seals can be Classified into many different groups. Intent today is not make you a seal expert but, get you familiar with the basic seal terminology we use in our day to day activities.
- Single, Tandem, Double
- Basic Component or Complete Cartridge
- Pusher and Non-Pusher or Bellow
Single seals are the most common type of seal and the most economical to operate and maintain.
• Seal Faces are lubricated by the process fluid.
• Working envelope from vacuum to over 1000 psi and Temperatures from -40 to 550 F
• Typically less expensive compared to Tandem/Dbl seals.
• Generally limited to temperatures below 500 F
• Has potential for Dynamic oring “hang up” in dirty services.
Tandem seals generally used where leakage of process to atmosphere is unacceptable.
• Two seals work independent of each other.
• Primary seal in Process fluid and secondary seal operates in the buffer fluid.
• Tandem seal will require an auxiliary support system. E.g.. Reservoir.
• Typically more expensive than single seals and requires auxiliary support system .
• Requires higher level of monitoring
Double seals are used when zero leakage to atmosphere cannot be tolerated.
• Used in toxic, lethal, carcinogenic, dirty, viscous, abrasive fluids.
•Barrier Fluid is now at higher pressure than process. Generally recommend 25 psi differential.
• Barrier must be compatible with process and the environment.
• Requires auxiliary support system and reliable Nitrogen pressure
• Requires higher level of monitoring and detailed startup and shut down procedures.
Gas Seals
• Two sets of seal faces, inner (process side) and outer (atmospheric side).
• Gas (nitrogen, instrument air, mixed gas) is pressurized between the two sets of seal faces at a higher pressure than is acting on the stuffing box.
• A small amount goes to process and atmosphere (usually between .05 and 3 SCFH (standard cubic feet per hour). This is measured on the flow meter in the gas barrier control panel
• Never turn off the gas while the pump is in service. Gas can be turned off if the pump has been cleared and suction and discharge valves have been closed.
Single Vs Dual
• Why use a dual seal?
– Emissions
– Environmental
– Abrasives/Viscosity
– Value of product
• Heavy cross section can withstand greater degree of corrosion
• Considered less likely to clog in viscous services
• Care must be taken in seal design to permit uniform loading on faces
• Can unwind in high speed services
• Require more axial space
• Different spring required for each seal size
• Usually provide more uniform face loading than single spring design
• Same spring can be used with many seal sizes by changing the number of springs
• Cheaper compared to Single Spring
• Much shorter allowing more compact seal designs
• Resist unwinding from centrifugal forces
• Subjected to corrosion and stress corrosion cracking if proper materials are not selected
Bellows Vs Pusher
• Advantages of Bellows
– No dynamic gaskets
– Clogging
– Corrosion
– Smaller cross-section
– Higher temperature potential
– Higher Pressure capability
– Emission control
– Cheaper (initial cost and repair)
– Failure less catastrophic
– Handles some vibration, slip stick, and mis-alignment Mechanical
Materials – Choices are dependent on the following:
• Temperature and pressure
• Fluid properties such as corrosiveness and
abrasives
• Seal type
• Standardization
