Mark Dye

2009

Principle of operation:

Rotating ring Stationary ring

Pressure

Leak off

Seal Gap only 5 microns/ 0.2 mils

Principle of operation:

Hydro-dynamic pressureTiny 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-dynamicforce

Gas is scooped into this tight space forcing it to overflow outwards into the gap between the faces forcing them apart

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

Single Seal Double Seal Tandem Seal

•Low pressure•Low cost

•Most popular seal•Multiple arrangements

•Low pressure•Hazardous gas

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.

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.

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 into compressor

suction

Leak off to vent system

A labyrinth seals works like a maze, creating a torturous path which the fluid needs to flow through in order to escape

Labyrinth seals are used in conjunction with dry gas seals in order to restrict the leakage between chambers around the seal

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)

High pressure discharge

Pressure balance line

Low

pressure suction

Balance drum

Axial thrust from high discharge pressure

Balancing pressure created by balance drum

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.

This pressure balancing system also means the compressor seals are only ever exposed to suction pressure!

SEAL

SEAL

BEARING

BEARING

Gas that leaks into here is passed back to the suction, so the pressure here always going to be the same as suction pressure

HP rotor & casing

6 centrifugal impellers

Pressure balance drum

Pri

mar

y ga

s st

art u

p su

pply

Secondary gas supply

Gas filters

Flow controllers

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.

LPHP

PDC

PDT

PRIMARY SEAL GAS

FC FC

SECONDARY SEAL GAS

SEPARATION SEAL GAS

N2

Bur

st d

isc

FC FC

SECONDARY SEAL GAS

HP Fuelgas

RO

RO

RO RO RO ROB

eari

ng

Bea

ring

Bea

ring

Bea

ring

FC

LP compressor

HP compressor

LP seal gas skid

HP seal gas skid

Prim

ary Gas P

rim

ary

Ven

t

Compressor Suction

pressure

Atmospheric pressure

Compressor drive shaft

Bearing housing

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

Prim

ary Gas

Secon

dary G

asPri

mar

y V

ent

Sec

ond

ary

Ven

t

Compressor Suction

pressure

Atmospheric pressure

Compressor drive shaft

Bearing housing

Secondary 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)

Prim

ary Gas

Secon

dary G

as

Sep

aration G

asPri

mar

y V

ent

Sec

ond

ary

Ven

t

Compressor Suction

pressure

Atmospheric pressure

Compressor drive shaft

Bearing housing

Separation gas is nitrogen used to keep lube oil from the adjacent bearing housing from leaking into the dry gas seal system

Separation Gas

Before starting lube oil system ensure separation gas is on to keep the lube

oil out of the seal!!

Labyrinth ring for separation gas

Dry gas seal assembly

John CraneDry gas sealassembly

“Keeper” pinInserted to supportAssembly whileSliding out

Sliding out

Typical control scheme:

DP

P

P

DP

ALARM

Suction pressure

Seal gas pressure

A controller maintains a constant differential pressure above suction pressure, while the flows to each seal are also monitored to check seal integrity.

FILTERS

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.

RO

MA

RK

’SPA

TE

NT

OR

IFICE

PLA

TE

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.

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.

P

HP Trip

Vent/Flare

Filter Aisolated

Filter Bin service

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.

A B

Filter Aisolation

Filter Bisolation

Equilisationvalve

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.

-Control room DCS

Dirty filter alarmFilter dirty alarm Gas flow is controlled from 3 DP transmitters routed through a high selector to the valve output

XA-033XA-032

Rupture discFailure alarm

-Quiz Can you answer the following questions?

1. What must you do before starting the compressor’s lube oil system?2. During normal running, where is primary seal gas supplied from?3. What is the source of secondary gas?4. How are the 2 seal faces prevented from touching during normal operation?5. What is the purpose of separation gas?6. 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