Principle of operation:
Pressure
Rotating ring
Stationary ring
Leak off
Seal Gap only 5 microns/ 0.2 mils
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.
Hydro-dynamic force
Gas is scooped into this tight space forcing it to overflow outwards into the gap between the faces forcing them apart
Spring tension
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
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
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•
Low pressure Low cost
Double Seal
Low pressure Hazardous gas
Tandem Seal
Most popular seal Multiple arrangements
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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.
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
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)
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
B E A R I N G
S E A L
This pressure balancing system also means the compressor seals are only ever exposed to suction pressure!
B a l a n c e d r u m
Axial thrust from high discharge pressure
H i g h p r e s s u r e d i s c h a r g e
S E A L
L o w p r e s s u r e s u c t i o n
B E A R I N G
6 centrifugal impellers
Pressure balance drum
HP rotor & casing
Flow controllers
Gas filters
Secondary gas supply
Vent system pressure monitoring
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.
RO
c s i d t s r u B
RO
g n i r a e B
RO
g n i r a e B
HP
RO
g n i r a e B
SEPARATION SEAL GAS
g n i r a e B
LP
RO PRIMARY SEAL GAS
RO FC
FC FC
SECONDARY SEAL GAS
FC
FC
SECONDARY SEAL GAS
PDT
PDC
N2 HP Fuelgas
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. P r i m a r y G a s
t n e V y r a m i r P
Compressor Suction pressure
Atmospheric pressure
Compressor drive shaft
Bearing housing
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) P r i m a r y G a s
t n e V y r a m i r P
Compressor Suction pressure
S e c o n d a r y G a s
t n e V y r a d n o c e S
Atmospheric pressure
Compressor drive shaft
Bearing housing
Separation Gas Separation gas is nitrogen used to keep lube oil from the adjacent bearing housing from leaking into the dry gas seal system P r i m a r y G a s
t n e V y r a m i r P
Compressor Suction pressure
S e c o n d a r y G a s
t n e V y r a d n o c e S
S e p a r a t i o n G a s
Before starting lube oil system ensure separation gas is on to keep the lube oil out of the seal!!
Atmospheric pressure
Compressor drive shaft
Bearing housing
Dry gas seal assembly
Labyrinth ring for separation gas
John Crane Dry gas seal assembly
Sliding out
“Keeper” pin Inserted to support Assembly while Sliding out
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 S e a l g a s p r e s s u r e
Suction pressure
P
P
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
HP Trip P
RO
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. O M P R P A A L T R A I F T I E K E C N ’ E T 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.
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
-Control room DCS
Filter Dirty dirty filteralarm alarm
Gas flow is controlled from 3 DP transmitters routed through a high selector to the valve output
Rupture disc Failure alarm
XA-032
XA-033
-Quiz
Can you answer the following questions?
1. 2. 3. 4. 5. 6.
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 t han 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
