2 Dry gas seal systems Principle of operation: Pressure Rotating ring Stationary ringLeak offSeal Gap only 5 microns/ 0.2 mils
3 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.Spring tensionHydro-dynamicforceGas is scooped into this tight space forcing it to overflow outwards into the gap between the faces forcing them apart
4 Dry gas seal systemsThe 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 systemsThe 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 SealDouble SealTandem SealLow pressureLow costLow pressureHazardous gasMost popular sealMultiple arrangementsAlthough 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 gasA 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.Seal gasLeak off to vent systemLeak off into compressor suction
7 Labyrinth SealsLabyrinth seals are used in conjunction with dry gas seals in order to restrict the leakage between chambers around the sealA labyrinth seals works like a maze, creating a torturous path which the fluid needs to flow through in order to escapeLubricating 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 Balancing pressure created by balance drum Pressure balancingThe 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 lineGas that leaks into here is passed back to the suction, so the pressure here always going to be the same as suction pressureBalancing pressure created by balance drumAxial thrust from high discharge pressureBEARINGSEALBalance drumSEALBEARINGThis pressure balancing system also means the compressor seals are only ever exposed to suction pressure!High pressure dischargeLow pressure suction
10 K3401 Boil off gas compressor 6 centrifugal impellersPressure balance drumHP rotor & casing
11 K3401 Boil off gas compressor seal skid Primary gas start up supplyFlow controllersGas filtersSecondary gas supply
12 K3401 Boil off gas compressor seal skid Vent system pressure monitoring
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 K3401 Seal gas system HP LP N2 HP Fuelgas RO RO RO RO Burst disc BearingHPBearingBearingLPBearingSEPARATION SEAL GASROPRIMARY SEAL GASROFCFCFCFCFCSECONDARY SEAL GASSECONDARY SEAL GASPDTPDCN2HP Fuelgas
15 K1450 END FLASH COMPRESSOR LP seal gas skid HP seal gas skid LP compressorHP compressor
16 Seal gasPrimary GasPrimary 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 GasPrimary VentCompressor SuctionpressureAtmospheric pressureCompressor drive shaftBearing housing
17 Seal gasSecondary GasSecondary 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 GasSecondary VentPrimary GasPrimary VentCompressor SuctionpressureAtmospheric pressureCompressor drive shaftBearing housing
18 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 systemBefore starting lube oil system ensure separation gas is on to keep the lube oil out of the seal!!Secondary GasSecondary VentSeparation GasPrimary GasPrimary VentCompressor SuctionpressureAtmospheric pressureCompressor drive shaftBearing housing
19 K3401 Boil off gas compressor Dry gas seal assemblyLabyrinth ring for separation gas
20 K3401 Boil off gas compressor “Keeper” pinInserted to supportAssembly whileSliding outJohn CraneDry gas sealassemblySliding out
21 Seal gas Typical control scheme: DPALARMTypical control scheme:FILTERSA controller maintains a constant differential pressure above suction pressure, while the flows to each seal are also monitored to check seal integrity.DPSuction pressureSeal gas pressurePP
22 Seal gas ventThe 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/FlareA 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 TripPROPLATEORIFICEPATENTMARK’SIn 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 Seal gas filtersIt 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 AisolationEquilisationvalveFilter BisolationTurning 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 AisolatedFilter Bin serviceAB
24 Dry gas seal systems -Control room DCS Filter dirty alarm Gas flow is controlled from 3 DP transmitters routed through a high selector to the valve outputDirty filter alarmXA-033XA-032Rupture discFailure alarm
25 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?MANHELPER MOTORCENTRIFUGAL COMPAXIAL COMPGAS TURBINE