Dry Gas Seals
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This manual discusses the principle of operation of dry
gas seal. It also covers the different seal arrangements and control related
aspects.
1 Introduction
2 Principle of operation
3 sealing interface
4 Self aligning mechanism for radial film
stability
5 Arrangements
6 control and monitoring
7 key operational consideration
8 Benefits of dry gas seals
9 cause and effect on dry gas seals
1 Introduction
Twenty years ago sealing of
centrifugal compressors was revolutionized by the introduction of dry gas
seals. During the mid 70’s a survey was
carried out into all compressor failures that had occurred during the previous
years. The finding showed that
approximately 80 percent of compressor failures were due to seal oil system
faults. The development and introduction
of dry gas seals solved several seal problems of compressor users. Dry gas seals are now accepted world wide as
a mature product handling gases on a very wide variation of plants and
compressors.
2 Principle of operation
The majority of compressors
sealing applications use a tandem seal configuration as showed in the Figure 1a & 1b
Figure 1a
Figure 2b
This type of seal is a cartridge
design where all components are held within a meter retainer. The stationary section of the seal comprises
a spring loaded carbon primary ring with an O-ring sealing between the back
face of the carbon primary ring and the metal sleeve.
The rotation section of the seal
comprises a mating ring with grooves in the running face. This ring is normally manufactured from
tungsten carbide or silicon carbide. The
component is contained within a metal shroud and driven through drive lugs or
flats machined on the outer diameter of the mating ring. The O-ring sealed rotating seat is profiled
with a series of spiral grooves having a depth of between 0.0023 to 0.010 mm as
illustrated in the following Figure 2.
Figure-2
The design of the grooves is a
logarithmic spiral. The primary ring
floats on a thin film of gas generated by the logarithmic spiral grooves (see Figure 3) in the surface of the
mating ring. As the mating ring rotates,
this creates a hydrodynamic effect which, draws gas toward the root of the
grooves and forces the two faces apart from the dynamic seal
Figure
3
The principle of operation of the
spiral groove gas seal is the balancing of aerostatic and aerodynamic forces to
provide a stable, minimal running clearance.
When pressure is applied, the force exerted on the seal is aerostatic
and is present both when the seal is stationary or rotating. Aerodynamic forces are generated only upon
rotation. During rotation the spiral
grooves play a vital role by generating a separating force which helps provide
the means of achieving an acceptable sealing gap. Figure
4 represents one of the spiral grooves.
The rotation of the seal scoops gas into the spiral grooves where it is
induced toward the center until it meets the sealing dam.
Figure 4
This effect compresses the gas at
the root of the grooves creating a pressure increase causing the flexibly mounted
face to “lift off” thus establishing an operating clearance. The size and number of grooves and the
diameter of the sealing dam all contribute to the force balance of the
seal. Adjustment of which will determine
whether “lift off” occurs under static pressure or at slow speed, a
consideration necessary if the seal is to survive the crucial period of start
up or shut down.
There is an optimum groove angle
that, during operation, will generate maximum lift. The constructor of the seals patents the optimized
groove angle. A radial slot will
generate lift, but this may be insufficient to ensure that sealing gap is
maintained at all times, thereby increasing the risk of contact particularly
during transient conditions.
The pressure will also reduce
when the groove angle is more acute. A
groove profile uses the key features of the optimized spiral to maximize lift
and separation of the seal faces. The
optimized groove angle is also a critical design feature of the seal.
3 sealing interface
Under
design conditions the forces acting upon the seal in operation can be
graphically represented by those shown in Figure
5 producing an operating sealing gap of approximately 0.003mm.
Figure 5
The
closing force is a result of the system pressure acting behind the force plus a
small force form the springs. The
opening force is a result of the system pressure plus the increase of force
generated by the spiral groove.
Equilibrium in operation, with the designed sealing gap, is achieved
when the opening force equals the closing force.
Whilst
conditions remain steady and the forces remain in the same parallel
relationship, the seal will continue to operate in the mode indicated. Should
however there be some disturbances that results in a decrease in the sealing
gap, the pressure generated by the spiral grooves considerably increases (see Figure 6).
Figure 6
Similarly
should the upset cause the gap to increase, a reduction in the pressure
generated by the grooves will occur (see Figure
7). In each case, the closing force
remains constant and so whichever situation is apparent, equilibrium is quickly
established and the designed sealing gap restored. This restoring mechanism is known as film
stiffness.
Figure 7
This
significant increase in film stiffness with small sealing gap changes ensures
the seal becomes insensitive to pressure or mechanical disturbances, and there
is no direct contact between the face and seal, regardless of system and
mechanical upsets.
The spiral
groove seal has both aerostatic and aerodynamic influences when in
operation. On rotation the aerostatic
and aerdynamic effects are combined in the compression zone of the face, i.e.
the area covered by the spiral grooves, whereas in the expansion zone across
the sealing dam, the pressure distribution is governed only by the aerostatic
effect.
4 Self aligning mechanism for radial film stability
Under ideal
conditions the hard rotating ring should be perfectly flat and normal to the
axis of rotation, in practice this is impossible to achieve. There will always be some angular
misalignment, whether from manufacturing tolerances or movements of the shaft
in operation.
The
mechanisms within the seal that produce such high levels of film stiffness
compensate for these conditions and quickly re-establish equilibrium and film
stability.
Ideally,
there should always be a parallel presentation between the face and seat but in
practice angular variations occur.
Generally, pressure deformations tend to close the faces at the outside
diameter producing a different gap (see
Figure 8).
The angular
variation brings the outer half of the primary ring closer to the mating ring
and the aerodynamic pressure generation rises.
As the gap widens in the inner half the pressure profile reduces. The changes in pressure distribution between
a parallel and a divergent film result in a returning moment, restoring
parallel presentation to the operating gap.
Figure 8
Thermal
deformation tends to close the faces at the inside diameter producing a
convergent gap (see Figure 9). The resultant changes in pressure profile
again combine to restore equilibrium.
Figure 9
The net
result of a high stiff fluid film is that the spiral groove seal can maintain a
minimum running clearance without risk of face contact whilst compensating for
a wide range of shaft displacements.
5 Arrangements
Different
sealing arrangements can be formed with any of the seal types. Selections for each arrangement are based on
the type of gas, the equipment, the operating conditions and safety. For non-hazardous gases where a small amount
of leakage to the atmosphere does not present a problem, a single used. An inboard labyrinth is usually includes in
the arrangement to allow a small injection of clean gas to the seal
environment, usually filtered gas from the compressor discharge (see Figure 10).
In cases
where absolutely no process gas leakage to the atmosphere can be tolerated, a
double face to face seal is used. Buffer
gas at a higher pressure than the sealed process gas is supplied between the
sealing faces, thus ensuring that no process gas can leak to the atmosphere
(see Figure 11).
Figure 11
Tandem seal
arrangements are a much-favored solution to most gas sealing problems and
ideally suited for flammable, hazardous and low toxicity gases. A tandem seal arrangement may have two or
more seal modules oriented in the same direction behind each other (see Figure 12).
Figure 12
The tandem
arrangement may be used to share the sealing load or more commonly one seal
handles the full system pressure while the outer seal runs as a standby or
back-up seal while functioning as an additional barrier between the process gas
and the atmosphere. Very high pressures
may require triple tandems for ultimate safety where the two inner seals share
the sealing load, while the outer seal is a backup and barrier seal.
A variation
of the tandem seal includes a labyrinth in the inter space between the seals
(see Figure 13). Utilized on more toxic applications, the
intermediate labyrinth is purged with inert gas directing all the gas leakage
to the primary vent, ultimately to be flared.
Figure 13
6 Control
and Monitoring
The dry gas
seal does not require complicated ancillary equipment. In most cases, all that is required is a
simple control and monitoring system comprising filters, flow metering devices
and pressure instrumentation. The role
of the control and monitoring system is to control the environment of the gas
seal, monitor performance and initiate alarms or shutdown. A single control system will usually
supervise several seals in operation.
Figure 14 shows a typically flow
diagram for a tandem seal with intermediate labyrinth. Process gas is taken from the compressor
discharge, cleaned through one of two filters and injected ahead of the seal
cartridge. The buffer gas is usually
controlled at approximately ten times the seal leakage rate, thus ensuring that
the seal environment remains clean.
Figure 14
A leakage
outlet port is provided between the seals and piped via a pressure switch, flow
restriction orifice and flow meter to a safe area. A further leakage outlet port is provided on the
atmospheric side of the seal, however, under normal conditions the outboard
seal operates under very low differential pressure and thus leakage is
minimal. Continuous monitoring of seal
leakage will immediately detect a malfunction in either the inboard or the
outboard seals and initiate alarms while the process gas is still safety
contained.
It is
essential that the dry gas seal operates in a clean environment. This is normally achieved by circulating the
process gas from compressor discharge via one or two 5 micron filters and
injected inboard of the seal cartridge at a rate of flow greater than the
normal leakage rate of the inboard seal.
Flow of the filtered gas may be controlled either with a simple restricting
orifice or flow control valve.
During
static pressurization of the compressor, the filteration system is flooded with
gas. As soon as the compressor has
developed a head of pressure at its discharge, the flow of process gas through
the selected filter will commence. A
differential pressure gauge monitoring upstream and downstream pressures across
the selected filter will determine filter condition. A differential pressue high signal will alert
the operator of the need to change the filter.
Leakage:
Gas seal
integrity is confirmed by the systems leakage monitoring instrumentation (see Figure 15). While statically pressurized, gas seal
leakage is usually slight. Under
conditions of normal dynamic leakage, a flow will be registered in the primary
vent. A reduced primary leakage rate is
indicative of an outboard seal malfunction.
An inboard seal malfunction will cause an increase in primary
leakage. A flow meter with high and low
leakage alarm signal will give the operator warning of the malfunction.
Figure
15
Should a
serious breach of the inboard seal occur, a restricting orifice would restrict
leakage through the primary vent. A trip
signal is generated by the pressure increase upstream of the orifice.
Buffer Gas:
Tandem
seals with intermediate labyrinth are often purged with an inert gas. This ensures that process gas leakage from
the inboard gas seal is directed to the primary vent. The outboard seal operates on the inert gas
and process gas is prevented from entering the bearing area of the compressor
inert gas flow is controlled by a simple control valve and monitored by a flow
meter. A low flow alarm provided warning
of inert gas header failure.
7 key operational consideration
Experience
has shown that the majority of seal malfunctions are caused by contamination of
the seals by solids and liquids. It is
particularly important for reliable operation to keep the dry gas seal clean
and dry. This can be normally achieved
by circulating the warm gas from compressor discharge via the filter to the
seal chamber.
Dirty Gas:
In some
instances, the process gas may be considered particularly dirty and during
periods of standstill there may be a risk of the dirty gas entering the seal
cavity. In such cases, it may be
appropriate to utilize a clean buffer gas.
This must be compatible with the process gas and be available at a
pressure higher than system pressure.
The buffer gas is circulated to the seal chamber via the filter gas
supply system.
Gas Condensate Liquids:
Condensing
of the compressed gas occurs mainly when there are significant quantities of
heavy hydrocarbons present. They will
condensate out when the temperature is below the dew point of the gas. Gas condensate liquid can present themselves
as an oily, sticky substance. When
present in the seal area, this can coat all seal components.
The liquid
will congeal and clog, preventing free movement of the seal components.
Condensing
of the compressor gas is most likely to occur:
·
When the filtered gas
stream pressure reduces through a throttling device such as a restriction
orifice or pressure regulator. As the
gas expands the “Joule Thomson” effect causes it to cool and the heavy
hydrocarbons condense out as liquid.
·
As a result of cooling when
the compressor is circulated through pipework from compressor discharge via
filters to the seal area
·
During static settle-out
conditions when the compressor casing is pressurized and the temperature drops
below the dew point of the gas.
Small quantities of condensate
can be tolerated by the dry gas seal due to the heating effect of the small
gap. The seal generates a small
temperature rise (typically 20 C) which is normally sufficient to “boil away”
condensate. For applications involving
substantial amounts of heavy hydrocarbons, various solutions to prevent the
formation of gas condensate liquid include the following:
·
Maintain the temperature
above the dew point of the compressor gas.
Heat trace filters gas pipework if necessary or source clean gas supply
horn a warmer area.
·
Keep length of filter gas
pipework between compressor discharge and seal chamber to a minimum. Lag pipework to reduce heat loss.
·
Minimize pressure
differential across throttling device I filter gas line, which will limit the
cooling effect of the gas. Consider
receiving the compressed gas from an intermediate stage of the compressor.
·
Install coalescing filters
in the filter gas line, preferably downstream of the throttling device to
maximize removal of condensate. This may
result in a requirement for large filter size due to a resuction in the filter
capacity at lower pressure.
·
Control the seal
environment b circulating a filtered dry external buffer gas to the inboard dry
gas seal.
·
Avoid or minimize duration
seals are subjected to high-pressure settle out condition.
·
Install a double seal
design with a pressurized nitrogen barrier gas between the seals.
Avoidance of gas condensate liquids may require any one or
a combination of the above solutions dependent on risk or severity. A full analysis of the process gas should be
made to assess the potential for liquids to condense out.
Sour Gas:
The application of dry gas seals to sour gases has been
extensive many of the natural gas applications in offshore environments and
hydrocarbon rich applications on refineries include quantities of hydrogen
sulphide of sulphur.
Critical considerations are material selection, leakage
hazards and environmental limitations.
Materials are selected in compliance with NACE specification MR0175-96
which specifies acceptable materials, the requisite heat treatment and the
maximum permitted hardness, for avoidance of sulphide stress cracking.
Generally, for natural gas and hydrocarbon recycle
applications, tandem seals are preferred.
To prevent sour gas leakage to the secondary vent an intermediate
labyrinth may be installed between the seals and purged with nitrogen to ensure
that under normal operating conditions sour gas leakage through the inboard
seal is directed to the primary vent to be flared.
In extremely sour gas application, it is normally required
that the compressor gas is fully contained with no venting to atmosphere. In such cases, a double seal design is
selected with a pressurized nitrogen barrier gas between the seals.
Hydrate and ice
formation:
Under certain conditions hydrates may form in hydrocarbon
gases. This occurs when molecules of
water attach elements of the hydrocarbon gas to form crystals. The conditions promoting hydrate formation
are shown as follows:
·
Gas is at or below its
water dew point with “free” water present
·
Low temperature
·
High pressure
Secondary considerations include:
·
High gas velocities
·
Pressure pulsations
·
Any type of agitation
·
Introduction of hydrate
crystals
Cold ambient shutdown conditions
are those more likely to cause hydrate formation in the gas seal cartridge,
when the inboard seal is pressurized to compressor casing settle-out pressure.
There is also potential for icing if there is a release of
water from the gas during high-pressure cold ambient shutdown conditions. As the gas containing the “free” water or
vapor expands across the inboard seal faces, the Joule Thomson expansion
cooling can potentially form ice.
Application where there is a risk if ice or hydrate
formation during prolonged periods of pressurized shutdown should incorporate a
purge of the gas seal interspace with “warm” buffer gas flow prior to
compressor start-up or some method of gas seal cartridge warm through.
Wet chlorides:
Wet chloride is an aggressive contaminant. Material distress takes the form of pitting
and stress corrosion cracking. Tungsten
carbide and stainless steels can degrade in the presence of wet chloride
contamination so alternative materials such as duplex stainless steel, hastelloy and silicone
carbide are normally selected.
It can be seen that where gas conditions exceed the design
criteria of the seal (as in very dirt gases) the seal environment is adjusted
to assure long and trouble free life.
8 Benefits of dry gas seals
The spiral groove gas seal
offers several benefits over conventional oil lubricated seals:
·
Low pressure consumption
·
No wear in operation
·
No seal oil system required
·
No pressure/velocity limit
Low power
consumption:
The parasitic power consumption of a sealing device is
very often a hidden cost that is not fully appreciated. To make a comparison between a conventional
seal and a spiral groove gas seal, consider a 125 mm liquid lubricated
mechanical contact seal handling gas at 50 bar, 10000 rpm, the power absorbed
would be 20 to 25 kW. Dry lubricated
seals offer virtually no resistance reducing frictional losses by up to 98
percent, leading to significant power saving.
No wear in
operation:
The spiral groove gas seal is non-contacting; hence there
are no wearing parts. While the shaft is
rotating, a thin film of gas separates the seal surface. Therefore seal wear is avoided.
No seal oil
system required:
In dry gas seals, the sometimes complex and heavy oil
systems are replaced by a clean and compact control and monitoring system. An oil system must contain a reservoir,
pumps, coolers, filters, pipe work, separators and controls, some of which must
be duplicated for overall separators and controls, some of which must be
duplicated for overall integrity. The
spiral groove gas seal avoids this complication. It requires a simple gas backup system and
can be procured and installed at lower costs.
The dry gas seal system cuts maintenance and removes the need for
lubricating oil. As a result,
contamination of the process gas is effectively eliminated. Elimination of the wet seal gas seal system
operational safety by eradicating any dangerous builds up of hydrocarbon gas in
the seal oil.
No
pressure/velocity limit:
Although a general speed limit of 100 m/s has been imposed
on the gas seal, the ultimate speed that can be achieved is limited by the
material strength. In the case of liquid
with higher power consumption and hence higher interface temperatures, the
limit is not only one of strength but also the ability to conduct heat the
spiral groove gas seal is independent of any pressure/velocity limit.
Improved rotor
stability:
Traditional oil ring seals can be unpredictable to
excitement of the shaft and rotor stability.
Dry gas seal are very predictable and will not effect rotor stability.
9 Cause and effect on dry gas seals
Cause: lack of buffer gas
while the tube oil pump is running
Effects:
·
The lube oil can migrate
from the oil separation seal via the compressor shaft toward the DGS
·
This oil will fill up the
clearance volume in secondary leakage and slowly penetrate through DGS
·
The stator ring and the
rotor ring of DGS will stick together and will not lift off while the
compressor is rotating
·
The rotor ring of the DGS
will burst immediately
Cause: compressor casing
pressurized during long period of standstill
Effect:
·
In that case, no clean gas
will be supplied via the clean gas filters to the DGS from the process side
·
A small leakage rate
through the seals in always present. As
there is no clean gas supplied to the seal for protection, dirt from process
side can migrate through the seals.
·
When the compressor rotor is
started, this dirt can cause damage to the rotor ring as well as to the stator
ring.
Cause: no maintenance on the clean
gas filters
Effects:
·
In case of excessively high
differential pressure on the filters, only a small amount of clean gas can be
supplied to the DGS for protection. This
amount will be smaller than the normal leakage rate for the DGS.
·
Due to this effect, a small
amount of dirty process gas will migrate through the seals, and this will act
like sand blasting.
·
Over a long period of
compressor operation, an increasing leakage rate on the primary leakage can be
recognized.
Cause: wet process gas
Effects:
·
The volatile constituents
of the leaking process gas will disappear through the primary leakage of DGS,
while crystals will be precipitated out of the process gas (knocked out). This process precipitation will be enforced
by the temperature loss of the process gas expanding through the seals and
labyrinth.
·
Crystal will block the
installed springs and labyrinth. To
protect the seal from wet and dirty gas, the clean gas system can be adapted
with trace heating as well as with knockout containers installed in the line
behind the normal clean gas filters.
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