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COMPRESSOR DRY GAS SEALING SYSTEMS

COMPRESSOR DRY GAS SEALING SYSTEMS. Mark Dye 2009. Pressure. Rotating ring. Stationary ring. Leak off. Seal Gap only 5 microns/ 0.2 mils. Dry gas seal systems. Principle of operation:. Hydro-dynamic force. Spring tension. Dry gas seal systems. Principle of operation:.

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COMPRESSOR DRY GAS SEALING SYSTEMS

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  1. COMPRESSOR DRY GAS SEALING SYSTEMS Mark Dye 2009

  2. Pressure Rotating ring Stationary ring Leak off Seal Gap only 5 microns/ 0.2 mils Dry gas seal systems Principle of operation:

  3. Hydro-dynamic force Spring tension Dry gas seal systems Principle of operation: Hydro-dynamic pressure Tiny grooves cut into the face of the rotating ring scoops gas in between the seal faces creating enough pressure to over come the tension of the spring holding the faces together. Gas is scooped into this tight space forcing it to overflow outwards into the gap between the faces forcing them apart

  4. Dry gas seal systems The seals are kept from touching by the generation of hydro-dynamic pressure, created by small grooves cut into the face of the rotating ring which draw gas into the seal, forcing the two surfaces apart. Because the space between the seals is less than a human hair, the sealing gas used must be completely dry and free from grit, dust or moisture. An external source of sealing gas is therefore used to ensure cleanliness

  5. Dry gas seal systems The tandem seal system is designed so that after failure of the primary seal, the machine can be safely shutdown using the containment provided by the secondary seal without hazardous release of gas to the atmosphere. Single Seal Double Seal Tandem Seal • Low pressure • Low cost • Low pressure • Hazardous gas • Most popular seal • Multiple arrangements Although these dry gas seals are able to handle high levels of vibration without damage, reverse rotation of the shaft at high speeds will damage the seals as they are not able to develop the hydro-dynamic pressure required to push the sealing faces apart.

  6. Seal gas Leak off to vent system Leak off into compressor suction Seal gas A supply of filtered buffer gas is injected into the cavity between the primary gas seal and inboard labyrinth. This supply of gas will leak past the inboard labyrinth back into the compressor which will ensure that the cavity is free from liquids or particles that could damage the gas seal. The supply gas will also leak past the primary gas seal, into the cavity between the primary and secondary gas seals which is directed to an approved vent or flare system.

  7. Labyrinth Seals Labyrinth seals are used in conjunction with dry gas seals in order to restrict the leakage between chambers around the seal A labyrinth seals works like a maze, creating a torturous path which the fluid needs to flow through in order to escape Lubricating oil from the compressor bearings is prevented from entering the dry gas seal by the use of a simple labyrinth seal supplied with separation gas to create a barrier to the migration of lube oil. The labyrinth seals is a “non contact” type of seal with very fine clearances. Using a system of notches and grooves, the pressure is broken down little, by little so that leakage is minimised (not stopped)

  8. Gas that leaks into here is passed back to the suction, so the pressure here always going to be the same as suction pressure Balancing pressure created by balance drum Axial thrust from high discharge pressure Pressure balancing The thrust created by the high pressure discharge pushing the compressor rotor back towards the suction is cancelled by creating opposing thrust using a balance drum. Pressure balance line SEAL SEAL BEARING BEARING Balance drum This pressure balancing system also means the compressor seals are only ever exposed to suction pressure! High pressure discharge Low pressure suction

  9. K3401 Boil off gas compressor

  10. K3401 Boil off gas compressor Pressure balance drum 6 centrifugal impellers HP rotor & casing

  11. Flow controllers Gas filters Secondary gas supply K3401 Boil off gas compressor seal skid Primary gas start up supply

  12. Vent system pressure monitoring K3401 Boil off gas compressor seal skid

  13. K3401 Boil off gas compressor seal skid Primary seal gas is supplied from HP fuelgas for start up, then automatically changed over from compressor discharge. It is vented back to the low pressure flare system. (Backup supply for process compressors is defrost gas) Secondary and separation gas is supplied from the nitrogen system and vented to a safe location on the roof of the building.

  14. N2 PDC PDT FC FC FC FC FC K3401 Seal gas system RO RO RO RO Burst disc LP HP Bearing Bearing Bearing Bearing SEPARATION SEAL GAS RO PRIMARY SEAL GAS RO SECONDARY SEAL GAS SECONDARY SEAL GAS HP Fuelgas

  15. K1450 END FLASH COMPRESSOR LP seal gas skid HP seal gas skid LP compressor HP compressor

  16. Seal gas Primary Gas Primary gas is injected in front of the seal to create a positive flow. It is used to ensure the gas entering the seal is completely dry and clean, completely free from dirt, dust and moisture. Primary Vent Primary Gas Compressor Suction pressure Atmospheric pressure Compressor drive shaft Bearing housing

  17. Seal gas Secondary Gas Secondary gas is nitrogen used to provide a clean source of gas to the secondary seal faces. The 2 seals are separated by another labyrinth and the leakage gas is lead to an atmospheric vent in a safe location (above the building) Secondary Vent Secondary Gas Primary Vent Primary Gas Compressor Suction pressure Atmospheric pressure Compressor drive shaft Bearing housing

  18. Before starting lube oil system ensure separation gas is on to keep the lube oil out of the seal!! Seal gas Separation Gas Separation gas is nitrogen used to keep lube oil from the adjacent bearing housing from leaking into the dry gas seal system Secondary Vent Secondary Gas Separation Gas Primary Vent Primary Gas Compressor Suction pressure Atmospheric pressure Compressor drive shaft Bearing housing

  19. K3401 Boil off gas compressor Dry gas seal assembly Labyrinth ring for separation gas

  20. K3401 Boil off gas compressor “Keeper” pin Inserted to support Assembly while Sliding out Sliding out John Crane Dry gas seal assembly

  21. DP DP P P Seal gas ALARM Typical control scheme: FILTERS A controller maintains a constant differential pressure above suction pressure, while the flows to each seal are also monitored to check seal integrity. Suction pressure Seal gas pressure

  22. MARK’S PATENT ORIFICE PLATE P Seal gas vent The vent system is kept under a back pressure by passing the flow through a restriction orifice. A secondary path is opened by bursting a rupture disc to safely vent the higher flows caused by seal failure. Vent/Flare A differential pressure transmitter monitors the pressure between seal supply and the low pressure vent. When the seal is healthy, this dP is high (Primary Seal Gas supply pressure on one side and Flare pressure on the other side). This high dP is used as a start permissive for the compressor. When the seal fails, the dP becomes low, initiating a compressor trip. HP Trip RO In addition we measure the pressure in the vent line, upstream of the orifice. In the event of seal failure this pressure becomes high due to the increased flow to flare. This high pressure alarm is also used to trip the compressor.

  23. Turning the valves in the correct sequence will firstly de-isolate A filter, then equalise so both filters are in service, and the third valve will isolate B filter for element replacement. Filter A isolated Filter B in service Seal gas filters It is extremely important the seal gas is clean and dry. A 2 micron fine filter is fitted with a standby element ready for change over. A “smart ring” sequencing system is used to prevent mal-operation of filter valves. Filter A isolation Equilisation valve Filter B isolation B A

  24. Filter dirty alarm Gas flow is controlled from 3 DP transmitters routed through a high selector to the valve output Rupture disc Failure alarm XA-033 XA-032 Dry gas seal systems -Control room DCS Dirty filter alarm

  25. MAN HELPER MOTOR CENTRIFUGAL COMP AXIAL COMP GAS TURBINE Dry gas seal systems -Quiz Can you answer the following questions? • What must you do before starting the compressor’s lube oil system? • During normal running, where is primary seal gas supplied from? • What is the source of secondary gas? • How are the 2 seal faces prevented from touching during normal operation? • What is the purpose of separation gas? • True or False? • The seal at the low pressure end of the compressor casing has an easier job than the seal on the high pressure end. • 7. If the compressor is accidentally turned backwards by the reversal of gas flow, the seals will be damaged. Explain why? • 8. True of False? • Some of the seal gas will leak back into the compressor casing. • 9. Where is secondary gas vented to? • 10. When a high Pressure is detected in the vent system, what is the most likely cause?

  26. Thanks for your attention!

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