1. COMPRESSOR DRY GAS SEALING SYSTEMS
Mark Dye
2009

2. Dry gas seal systems Principle of operation: Pressure Rotating ring
Stationary ring
Leak off
Seal 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 tension
Hydro-dynamic
force
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
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.
Seal gas
Leak off to vent system
Leak off into compressor suction

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. Balancing pressure created by balance drum
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
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
BEARING
SEAL
Balance drum
SEAL
BEARING
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
6 centrifugal impellers
Pressure balance drum
HP rotor & casing

11. K3401 Boil off gas compressor seal skid
Primary gas start up supply
Flow controllers
Gas filters
Secondary 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
Bearing HP
Bearing
Bearing LP
Bearing
SEPARATION SEAL GAS RO
PRIMARY SEAL GAS RO FC FC FC FC FC
SECONDARY SEAL GAS
SECONDARY SEAL GAS
PDT
PDC N2
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 Gas
Primary Vent
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 Gas
Secondary Vent
Primary Gas
Primary Vent
Compressor Suction
pressure
Atmospheric pressure
Compressor drive shaft
Bearing 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 system
Before starting lube oil system ensure separation gas is on to keep the lube oil out of the seal!!
Secondary Gas
Secondary Vent
Separation Gas
Primary Gas
Primary Vent
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
John Crane
Dry gas seal
assembly
Sliding out

21. Seal gas Typical control scheme:DP
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. DP
Suction pressure
Seal gas pressure P P

22. 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 P RO
PLATE
ORIFICE
PATENT
MARK’S
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. 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
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 A B

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 output
Dirty filter alarm
XA-033
XA-032
Rupture disc
Failure 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?
MAN
HELPER MOTOR
CENTRIFUGAL COMP
AXIAL COMP
GAS TURBINE

26. Thanks for your attention!