WET SEAL SYSTEM

SOLAR GAS COMPRESSOR

Mark Reynold Silitonga

1-Feb-2019
 CONTENTS
• Basic Seal Design • Buffer Gas System (cont’d)

• Conventional Wet vs Dry Gas Seals • Regulator reference point

• Balance Piston • Balance piston outboard cavity sensing

• Suction pressure reference and “modes” of

• Buffer Gas System operation
• “Throttling type” vs “By-pass type” • Balance piston return line sensing
system designs
• Old “by-passing” type without external gas
• Source of buffer gas supply

• Compressor discharge • Buffer gas flow paths

• True external supply • External vs Internal source

• Old by-passing type buffer gas system

• Balance piston outboard cavity leak
gas • Operation; Advantage & Disadvantage
 CONTENTS
• Seal Oil System • Overhead Tank
• Trap vs Suction Pressure Sensing • Degassing Tank or Flue
• Seal Oil Traps
• Seal Oil Consumption
• Quality of oil/gas separation
• Excessive trap gas flow
• Filter coalescer
• Missing buffer gas flow
• Trap float valve and oil drain orifice
• Restricted buffer gas flow
• Number of traps

• Trap gas output orifice • Flow reversal in “seal mix out”

line
• Drain line between capsules and
trap(s) • Eductor effect
 CONTENTS
• Correcting Excessive Oil Consumption • System Improvements

• Seal oil regulator reference • Existing packages

• Balance piston outboard pressure
• Additional improvements
• Nominal regulator settings
• Dry seals
• Buffer gas regulator

• Balance piston gas return line

• Old, tall seal oil trap

• Orifice size in Trap gas “out” line

• Re-referencing the buffer gas regulator

sensing line
 BASIC SEAL DESIGN
Conventional Wet Seals Dry Gas Seals

• Seal oil injected at a pressure above • Depend on face contact of axial seal plates
process gas pressure to prevent the
escape of gas • In a stationary compressor, the face seals
are physically contacting and seal near
• Buffer gas to prevent migration seal oil perfect
into compressor and to avoid any contact
• In a rotating compressor, the axial plates
between seal oil and process gas are forced slightly apart by their own
pumping action
• Seal oil and buffer gas mix then routed to
an external trap for separation and • A small percentage of seal gas continuously
recycling leaks past the seal plates and vented to
atmosphere or flare system

• Additional fine filters must be installed into

seal gas supply line when “pipeline quality”
gas is used
 BALANCE PISTON
- Purpose & Operation -
• To compensate axial thrust force (dP • BP O/C concept: Process press
across a piston counteracts the axial
thrust produced by compressor wheels) acting upon suction and discharge
capsule nearly identical
• Process:

• Some of discharge gas leaks past BP • PBP I/C - O/C x AAnnular BP Cross-Section =
labyrinth seals into the BP outboard FAxial Thrust Compensating
cavity (BP O/C)

• From BP o/c the leak-gas routed back

• Normally “over-compensates” its
to suction port via an external line (BP rotors; BP larger than necessary;
gas return line) BP pushes rotor towards the
• BP gas return line must be large
discharge capsule
enough to minimize losses
• Compressor runs on its inboard
• Press in BP O/C slightly above suction
thrust bearing
press: PBP O/C > PS
 BALANCE PISTON
- Outboard Cavity Pressure -
• Floating and varies depending on:

Suction press; Press ratio; BP labyrinth condition; Size of ext. BP

return line

• BP O/C press = 5 - 30 PSID above suction press

• Small compressors in high ratio application (e.g., the final C160 in

a tandem application) are prone to high BP O/C press

• Large press drop across BP produces high flow rates; increased

flow generates considerable press losses in return line (due to
limited size of capsule ports and BP gas return line); increased
press losses generate increased back pressure in BP O/C
 BUFFER GAS SYSTEM
- General Information -
• Prevents contact of seal Flow Paths and Gas Velocity
oil with process gas • Most of BG flows underneath “Inboard Labyrinth Seal”
and mixes with process gas. Direction flow into
• Regulated above compressor especially important when process gas is
process gas suction heavy or toxic
press
• Small part of BG flows in an outward direction across
“Outboard Labyrinth Seal”. Prevents entry of seal oil,
• In newer packages, an mixed, separated and recycled.
external supply of gas
fed to a throttling type • BG flows across outboard labyrinth seals must be
controlled between tight limits:
BG regulator
• Minimum gas velocity of 10 ft/sec (3 m/s) must be
• Senses the process gas maintained across labyrinth seals. Small press drop of
ref. press (Suction or BP 1 or 2 PSID only is sufficient to achieve this velocity
O/C press) and reduces • To ensure full separation of mixture in trap(s), BG
to a value of 15 to 20 volume must be limited and flow velocity cannot
PSID above ref. press exceed a certain threshold value
 BUFFER GAS SYSTEM
- General Information -
System Design Buffer Gas Source
• dP of 1 or 2 PSID enough to achieve the correct BG flow and velocity
• Bottled nitrogen
across outboard labyrinth seals

• The design above difficult to regulated and requires specialized • External sweet gas
equipment
• Filtered process gas
• To avoid potential control problem, SOLAR uses simple regulators
set at dP of 15 - 20 PSID
• Raw process gas
• The excess pressure is reduced in additional orifice(s) installed in taken off compressor
series with labyrinth seals. discharge line
(requires filtration)
• Newer packages; Flow limiting orifice installed in the BG return line
between trap(s) and comp. suction. Elevates trap and capsule
drain press to 6 - 12 PSID above suction press Notes:

• Older SOLAR packages; Using small hand valves (needle valves) Wet seals system
or check valves as flow limiting devices in trap gas output line relatively insensitive to
(check valves have a fixed press drop regardless of flow)
BG quality
 BUFFER GAS SYSTEM
- Basic System Design -
“Throttling Type” “By-pass Type”

• Consists of throttling type dP • Without use of external BG

regulator and supplement gas supply supply
• Mostly called “external BG system”
• Internal compressor leak-gas
since source of BG is external to
compressor capsules
taken from BP O/C used as BG

• Variations of this system: • This system can be used only

on handling sweet process gas
• Source of BG may be external or
internal

• Different dP regulator ref. point

used
 BUFFER GAS SYSTEM
- Source of Buffer Gas -
Compressor Discharge True External Buffer Gas Supply
(Sweet Gas System)
• Common source of BG
• Requires a throttling type BG system
• Eliminates additional valves and no gas supply
lines external to package required • Requires additional on-skid equipment
(shut-off valve and controls)
• Only be used if process gas is clean and sweet
• Requires an external supply of BG
Notes: (sweet gas)

• Gas buffering system does not work during • Applications for this system:
turbine start up and during low speed operation
• Used if sour process gas. For full
• Compressor by-pass valve open during this time protection, BG regulator must be
and disch press is low (not enough for complete referenced to BP output pressure
buffering)
• “Pressurized hold” condition
• Consumes oil
 BUFFER GAS SYSTEM
- Source of Buffer Gas -
Balance Piston Outboard Cavity Leak Gas

• Applied in old style by-pass type BG system

• Being applied incidentally on “Throttling type regulator with suction

press sensing - Mode 4”

Notes:

- Small part of total BP leak flow passes under inboard BG labyrinth

seal from BP O/C into BG cavity

- Most of BP leak gas still returns from BP O/C directly to suction

via external BP leak gas return line
 BUFFER GAS SYSTEM
- Regulator Reference Point -
Balance Piston Outboard Cavity Sensing Balance Piston Return Line
• Most conservative ref. point for BG regulator and used on all
Sensing
newer units
• Old compressors have no static
• Highest process gas press seen by either suction or discharge BP outboard press - sensing trap
capsule seals

• To avoid the escape of process gas all the time due to BG press • Therefore not possible to utilize
always above BP O/C press (effective when combined with a true true sweet gas system with
external BG system and an external sweet gas supply)
throttling regulator and static BP
• Sensing port equipped on comp built since 80s (1/8” NPT tap on O/C sensing
discharge side of comp)
• If compensation of BP O/C press
• Disadvantage:
important, ext. return line can be
• High compressor ratios or worn BP seals increase BP O/C used as ref. for BG regulator
pressure; Raise BG regulator ref. press; Regulated BG press
raises • Sensing tap should be near as
• Increases dP between BG and suction press; Increases BG possible to BP gas exit port
flow; At certain flow rates, trap(s) cease to separate mixtures
and starts using oil (carry-over to BG return line)
 BUFFER GAS SYSTEM
- Regulator Reference Point -
Suction Pressure Reference

• Used in earlier compressor sets for throttling regulator

• Several high ratio compressors have been also converted to this system to
prevent oil consumption

• Operation:

• BG regulator output press = 20 PSID above suction press

• When press in BP O/C increases (high pressure ratio or labyrinth seal

wear), BG press regulator does not sense the change and press
supplied does not increase

• BG flow does not increase and mixture separation in trap not affected
 BUFFER GAS SYSTEM
- Regulator Reference Point -
Four modes operation depending on BP O/C pressure

Mode 1: BP O/C press < BG press Mode 3: BP O/C press ~> BG press

• Direction of flow in inboard BG labyrinth seal reverses (at disch

• Identical to the one with BP sensing capsule only)

Mode 2: BP O/C press = BG press • This flow reversal is harmless and may not detected

• The correct direction of flow across outboard BG seal still

• All gas flow across inboard labyrinth maintained
seal stops at discharge capsule
• BG regulator instability might be observed (due to operates near
• In theory, labyrinth seal could its fully closed position)
overheat/clog due to missing self- Mode 4: BP O/C press >> BG press
cleaning effect
• Throttling type BG regulator shuts off completely (fully closed)
• No actual problem has ever been • All BG now supplied by balance piston leak gas (including BG
observed due to this theoretical used in the suction capsule)
shortcoming
• Now identical to the “old” bypassing type BG system
 BUFFER GAS SYSTEM
- Regulator Reference Point -
Old “By-passing” Type Buffer Gas System Without
External Gas Supply

• No ext. BG supply line or throttling regulator

• BG supplied internally as leak gas from BP O/C

• Using bypassing regulator (“Rivett" valve)

• Elevates BP O/C press by 20 PSI

 BUFFER GAS SYSTEM
- Buffer Gas Flow Paths -
External Source Internal Balance Piston Leak Gas

• Discharge Capsule Inboard Seal • Discharge Capsule Inboard Seal

BG regulator outlet - Disch capsule BG cavity - Inboard Comp disch - BP labyrinth seals - BP O/C (back cavity) - Inboard BG
BG labyrinth seal - BP O/C - BP gas return line - Comp labyrinth seal - Disch capsule BG cavity
suction
• Suction Capsule Inboard Seal
• Suction Capsule Inboard Seal
Comp disch - BP labyrinth seals - BP O/C - Disch capsule inboard BG
BG regulator outlet - Suction capsule BG cavity - labyrinth seal - Disch capsule BG cavity - External BG supply line - Suction
Inboard BG labyrinth seal - Comp suction capsule BG cavity - Suction capsule inboard BG labyrinth seal - Comp suction

• Outboard Labyrinth Seal (identical suction and disch • Outboard Labyrinth Seal (identical for suction and disch capsule)
capsule)
Capsules buffer gas cavity - Outboard BG labyrinth seal - Seal mix out line
BG regulator outlet - Capsules BG cavity - Outboard BG (together with some seal oil) - Seal oil trap(s) for oil/gas separation - Flow
labyrinth seal - Seal mix out line (together with some seal limiting orifice(s) - Comp suction
oil) - Seal oil trap(s) for oil/gas separation - flow limiting
orifice(s) - Comp suction Notes:

Notes: - Using discs gas leaking underneath BP seal as BG in both capsules

- Throttling type dP regulator with source of gas: a true - Due to line losses in external lines from disch to suction, BG press in suction
external source (Sweet gas supply) or taken directly from capsule may considerably lower than in the disch capsule
compressor discharge
- Application: Throttling regulator with suction press sensing and worn BP
- Exclusively for regulators with BP O/C sensing or with labyrinth seal (Mode 3 and 4) or old BG system without external gas supply
suction (Mode 1)
 BUFFER GAS SYSTEM
- Old Bypassing Type Buffer Gas System -
Operation Advantage

• Bypassing regulator (“Rivett” valve) installed in the external • Well suited for smaller compressor with relatively low
BP leak gas return line flows and/or power

• No external sensing line • Minimize parasitic losses since no external buffer gas
supply required
• “Rivett” valve increases press drop in gas return line by 15 -
20 PSID and elevates press in BP O/C by same amount • Found mainly on older, smaller compressors driven by
Saturn or Centaur turbines
• System works in exactly same way with Mode 4
Disadvantage
• Since most of BP leak gas flows • If BP O/C pressure elevated, it reduces internal thrust compensation
through bypassing regulator, if the flow
exceed the regulator flow capacity (on • Press increases even further when comp ratio increases or when BP seal wears
larger compressors) then can lead to
• High BP O/C press can lead following problems:
high BG dP
• Reduced thrust load compensation and possibility of overloading thrust bearing
• BG regulator set-point is not adjustable
• Increased BG flow and risk of oil consumption due to increased trap gas flow
• Process gas is used as BG
• Risk of flow reversal in seal mix out line from suction capsule and possible oil
• Insufficient BG press is available in low consumption
ratio machines due to low pressure of
BG source
 SEAL OIL SYSTEM
• Prevents escape of gas (process or BG) into LO system

• Two carbon seal rings installed into each capsule as oil seals

• Located on compressor rotor outboard of BG labyrinth seals

• Seal oil at pressure above the highest gas pressure seen inside the capsules

• Seal oil flow paths:

• SO leaking underneath the outboard carbon seal ring towards the bearing and compressor exterior
enters an atmospheric drain where it mixes with LO coming from bearing. This mixture of used SO
and LO drains directly back to main LO tank

• SO flowing past the inboard carbon seal ring (in direction of compressor interior) enters capsule
“seal mix drain cavity”. This drain cavity remains at a pressure slightly above suction pressure. In
this cavity, SO meets and mixes with BG flowing in outboard direction. SO/BG mixture is collected
and routed to external seal oil trap(s) for separation and recycling

• To prevent collection of seal oil or other liquids in the regulator or sensing line, regulator should be
installed above pressure sensing port and sensing line should have a continuos downwards
slope away from regulator
 SEAL OIL SYSTEM
- Trap Pressure Sensing -
• To contain process and BG inside • Newer packages use BG return line after SO trap (trap gas
comp under all circumstances, out line before throttling orifice) as ref. point. Ref. gas is free
of oil, preventing sensing line contamination
SO press supplied to capsules
must be above the adjacent drain If two traps used, trap serving disch capsule has normally a
cavity pressure at all the times slightly higher press and preferred ref. point. All newer
packages use a pneumatic “high pressure selector” (shuttle
valve) in the SO regulator sensing line and automatically select
• Ideally seal mix drain cavities are
the higher of two trap press as ref. point
the reference point for SO
regulator, but not directly • SB 13.2/109 handles packages with one trap only.
accessible Recommends use trap input press as SO regulator
reference point. Though basically correct, this
recommendation must be used carefully
• However, the trap(s) are directly
connected to those cavities and dP SO regulator instability may be experienced if the sensing line
between two is minimal fills with oil. The line should branch off from trap input line in
an upward direction and should avoid low points
• Therefore the trap(s) used as seal
If instability problems persists, sensing line connection should
oil pressure regulator reference be relocated to gas output side of the trap (before orifice or
point hand-valve)
 SEAL OIL SYSTEM
- Suction Pressure Sensing -
• Used on early packages in combination with bypassing BG regulators

• Works reliably as long as gas pressure and pressure ratios remain

modest

• No built in compensation for increasing BG pressures

• Any increase pressure in BP O/C will increase BG pressure. It in turn

will elevate trap and seal-mix-out cavity press. Elevated seal-mix-out
cavity press might eventually exceed SO pressure, blowing gas
through oil seals and into LO drain system

• Follow SB 13.2/109 recommendation for the modification of system to

trap pressure sensing
 SEAL OIL TRAP(S)
General Quality of Oil/Gas Separation

• Velocity of incoming mixture is lowered • Any increase in BG flow increases gas velocity inside the trap
inside the trap(s) to such an extent • Gas velocity will finally reach a threshold, above which
(large volume and cross section), that separation no longer effective
tiny liquid oil droplets cannot stay in
suspension any longer and fall out • Oil droplets in suspension carried back to compressor suction
(starts “using oil”)
• Additionally, mixture separated by
• Most of oil droplets are so fine, that cannot be seen in the
centrifugal action. Heavier oil particles
sight glass(es) installed in trap gas return line(s)
thrown against the wall by centrifugal
force and cannot reach BG exit at top Filter Coalescer
center of trap
• Internal filters and coalescers used to assist in formation of
• Stripped BG returns to compressor droplets and to increase the liquid fall out rate inside trap
suction line via gas flow limiting
• Experience indicates that the intended effect is minimal.
orifice an joins process gas stream Filter/coalescer assembly requires frequent maintenance,
increase risk of blockage (dirty gas) and can be removed from
• Separated SO collects at bottom of most traps without noticeable increase in oil consumption
trap and periodically drained to
degassing flue/tank by float • Blocked filter element can actually increase oil consumption
mechanism as it increases the pressure drop across trap and restricts the
outboard buffer gas flow
 SEAL OIL TRAP(S)
Trap Float Valve and Oil Drain Orifice

• Float valve controls (mechanically operated) the level of separated oil

• When oil level increases, float valve opens and drains part of accumulated seal oil into degassing
tank

• An orifice inside the float valve seat restricted the drain flow

• Float valve must be able to drain under all existing pressure conditions

• Different drain orifice sizes are used to cover whole suction pressure range:

• Packages with high suction pressure (> 900 PSI) produce large differential pressure across trap
drain orifice. The drain orifice is kept relatively small (3/32” dia.) to ensure the limited upward force
of float (buoyancy) can open valve at all pressure

• Lower suction pressure (900 PSI max.) mean lower differential pressure across trap drain orifice.
Drain orifice is larger (3/16” dia.)

• Trap drain valve must seat tightly to maintain a minimum oil level inside trap and the level must
remain between two sight glasses on trap

• Oil remaining inside trap acts as seal and prevents escape of process gas into degassing tank
 SEAL OIL TRAP(S)
Number of Traps

• Newer packages equipped with individual traps for suction and discharge capsule

• Two traps give lower gas velocities inside traps and make system more versatile

• Some packages with two traps per compressor use discharge trap pressure as reference point for SO
regulator

• In most applications, a shuttle valve automatically selects trap with higher pressure as reference point

Trap Gas Output Orifice

• Orifice in trap gas output line restricts the gas flow and elevates trap pressure

• This lowers available pressure drop across capsule outboard buffer gas labyrinth seals and limits buffer gas
flow

• Older packages with SO consoles use a hand valve instead of a fixed orifice

Drain Line Between Capsules and Trap(s)

• The lines should be short (pressure loss) and should have a continuous downward slope towards trap(s)

• To ensure smooth two-phase flow of mixture into trap(s) and prevent collection of oil in low point (P-trap)
 OVERHEAD TANK

• (Skipped)
 DEGASSING TANK OR FLUE

• (Skipped)

• Current System

• Older SOLAR compressor Sets

 SEAL OIL CONSUMPTION
• Increased SO consumption is always a result of either missing or of
excessive BG flow

• Velocity of gas flowing through trap(s) is critical in avoiding oil carry

over

• Limiting orifice(s) in gas return line(s) from trap(s) to compressor suction

control this flow

• Refer to SO schematic supplied with package to determine optimal

orifice size

• The orifice size(s) might have to be changed if seal clearances or

operating conditions of compressor change drastically

• Older packages with SO consoles use a 1/4” needle valve in trap gas
out line to limit the buffer gas flow instead of a fixed orifice
 SEAL OIL CONSUMPTION
Excessive Trap Gas Flow

• Newer packages with throttling BG regulators and BP O/C press sensing (ref. point) are sensitive to
following conditions:

• Elevated BP outboard press (due to BP labyrinth seal wear or small, restricting BP gas return line)
also elevates ref. press used by BG dP regulator; BG regulator increases its output press
(increased BG supply press)

• The increased BG supply press increases the dP and flow across inboard labyrinth seal in
suction capsule, wasting BG. While dP and flow across inboard seal of disch capsule is kept
constant by BG dP regulator

• BG flow across outboard labyrinth seals increases, raising gas velocity in labyrinth seals, seal mix
out lines and in the SO trap(s). Flow increase in the SO trap(s) prevents complete separation of
mixture. Oil carried back to comp suction with returning BG

• To correct root problem:

• Balance piston O/C press (back-pressure) must be lowered by replacing BP seal

• By increasing BP gas return line

• Short-term fix; decreasing size of orifice in trap output

 SEAL OIL CONSUMPTION
Missing Buffer Gas Flow

• Exists during start up on packages using disch gas as supply to

throttling regulator (and also in old units using a bypassing type BG
system)

• Due to recirculation valve still open and therefore not enough

pressure available to supply BG regulator

• BG dP is low and insufficient gas flow exists across BG labyrinth

seals

• SO can migrate across BG seals (against normal direction of gas

flow) into inside of compressor

• Normally less than one gallon of oil is lost during start-up

 SEAL OIL CONSUMPTION
Restricted Buffer Gas Flow due to Elevated Trap Pressure or Blocked Buffer
Gas Return Line

• If trap output orifice is too restrictive, it may prevent sufficient BG flow

across outboard labyrinth seals

• In older tandem packages the needle valve might be too far closed

• A plugged coalescer or filter element has same effect

• Trap input pressure and pressure in seal mix out cavities are elevated to
such a point, that gas flow across outboard BG labyrinth seals drop below
minimum value or stops completely

• Seal oil can migrate against normal gas flow path into compressor interior

• Increase trap output orifice size and/or completely remove coalescer and
filter element
 SEAL OIL CONSUMPTION
Flow Reversal in Suction Capsule “Seal Mix Out” Line

• In compressor with older BG systems and one trap only, this might happen in suction
capsule only

• At risk are compressors with by-passing type BG systems or throttling type regulators
with suction reference, single traps and high balance piston outboard pressures
(Mode 4)

• Due to line losses in the connectors and interconnecting buffer gas line, BG pressure
in suction capsule can be considerably lower than pressure in discharge capsule

• The orifice in the “trap gas out” line may restrict the gas flow through trap to such an
extent that trap pressure exceeds the BG pressure in the suction capsule (discharge
capsule still fed from BP o/c). This prevents the drainage of suction “seal mix out”
cavity and forces seal oil into compressor interior

• Flow in “seal mix out” line from suction capsule to “tee” upstream trap reverses. Part
of oil/gas mixture flowing from discharge capsule to trap bypasses the trap and flows
from “tee” directly to suction capsule and into compressor interior
 SEAL OIL CONSUMPTION
Eductor Effect

• Rare cases

• Condition of excessive labyrinth seal clearance of inboard BG

seal

• BG flow to compressor increases greatly when this seal wears

• The increased gas flow can act like an eductor, decreasing the
pressure in BG cavity and pulling oil from SO system into BG
stream

• Increasing BG pressure to correct oil consumption problem will

only worsen the situation in this case, since even more gas is
flowing through the educator, increasing its effect
 CORRECTING EXCESSIVE OIL
CONSUMPTION
Before proceeding further, all equipment and instrumentation must be
properly maintained an in fully operational condition (compressor labyrinth
and carbon seals, differential pressure regulators, flow regulators, seal oil
trap(s), gauges and transmitters)

Eliminating compressor oil consumption always means restoring correct

flows and directions of flow in different buffer gas and seal oil paths

• Seal Oil Regulator Reference

• Before any corrections/adjustments are attempted on BG system, SO

regulator must be referenced to a pressure upstream of orifice in
the trap gas out line

• Trap or trap input-line are acceptable reference points as long as

sensing line to SO regulator has required upward slope
 CORRECTING EXCESSIVE OIL
CONSUMPTION
Balance Piston Outboard Pressure

• Measure pressure drop in BP leak gas return line!

• Condition BP labyrinth seal is of major importance for proper operation of gas compressor

• Increased BP leakage not only drastically increases internal recirculation and decreases the compressor
efficiency, it also affect thrust load and performance BG/seal system

• Monitoring static pressure in BP outboard cavity gives valuable information about health of labyrinth seals.
Typically about 5 - 30 PSID above suction pressure, but can climb considerably higher on gas compressors
with worn BP seals and/or high suction pressures or pressures ratios.

• SB 13.2/106B limits BP o/c pressure to 50 PSID (C28 = 30 PSID only) in order to protect thrust bearing

• Certain high ratio/low flow machines (e.g. some C160 compressors) can run successfully with higher dP, but
thrust load has to be checked individually in this case (depends on compressor staging)

• All newer compressors equipped with static BP pressure sensing tap, even when the connection is not used
as BG pressure reference point

• Any seal oil system troubleshooting should begin by measuring this pressure as a dP to suction. The same
gauge can afterwards be used to measure dP between trap(s) and suction (normally around 6 - 12 PSID).
Based on these measurements, the operation of SO/BG system can be evaluated and if required correcting
action initiated
 CORRECTING EXCESSIVE OIL
CONSUMPTION
Nominal Regulator Settings

• Check settings of SO and BG regulator

• Please observe that packages with seal oil dP regulator referenced to trap pressure are allowed to
operate with lower SO regulator dP settings compared to packages with suction pressure reference
(e.g. 12 PSID instead of 20 PSID, if alarm and shut-down switches permit)

• A throttling type BG dP regulator should be set to about 20 PSID, if suction pressure referencing is
used. If the regulator referenced to BP outboard pressure, a differential setting of 15 PSID is sufficient

• There is no mandatory relationship between BG dP and SO dP. It cannot be generally stated that BG
dP must be higher than SO dP (or vice versa)

Buffer Gas Regulator

On system with throttling type BG regulators, vary the settings of external BG regulator and observe the
results. Does differential pressure react? How is oil consumption being affected? If suction pressure
sensing is used, in which mode is unit operating?

Balance Piston Gas Return Line

Increase size external BP leak gas line if necessary. The standard 3/4” tube used in may older packages
is just not large enough!
 CORRECTING EXCESSIVE OIL CONSUMPTION

Old, Tall Seal Oil Trap

Remove the filter and filter/coalescer from old, tall trap and observe the results. It is possible, that
these filters plug up, increase dP and elevate trap pressure

Orifice Size in Trap Gas “Out” Line

Buffer gas referenced to BP O/C Buffer gas referenced to suction pressure
• If a compressor is equipped with throttling type BG • If using throttling type BG regulator referenced to
regulator and BP o/c pressure sensing, try first to suction pressure, or using bypassing type BG
decrease the size of trap output orifice system, first increase size trap output orifice
• A smaller orifice elevates the trap pressure and • Trap pressure might be elevated and might restrict
reduces the available dP between BG regulator the BG flow in capsules
output and SO trap(s)

• This limits gas flow through labyrinth seals and

• Alternatively, a flow reversal could exist in suction
trap(s) and decrease gas velocity capsule

• New orifice size is not critical, but obviously orifice • Since increasing size of trap output orifice
size reduction should be done in reasonable steps increases gas flow (and velocity) through trap,
only which in can be reason for increased oil
consumption, the increase in orifice size should be
• Never fully close BG return line from trap(s) to done in small increments only
suction
 CORRECTING EXCESSIVE OIL
CONSUMPTION
Re-referencing the Buffer Gas Regulator Sensing Line

• This modification is a last chance modification and acceptable only on compressors processing sweet
pipeline gas

• To solve a chronic seal oil consumption problem, the reference point for a throttling type BG regulator may be
moved from BP outboard pressure to suction pressure

• The regulator set-point has to be increased from 15 PSID to about 20 PSID if referenced to suction pressure

• This modification lowers the buffer gas supply pressure by lowering reference point

• A lower supply pressure lowers gas flow (and velocity) across BG system labyrinth seals and trap(s),
increasing chance for proper separation of mixture in trap

• After re-referencing, all or part of the buffer gas might be supplied by internal leak gas rather than by external
regulator. The modification can therefore only be considered on compressors using discharge gas as buffer
gas or when process gas is sweet, light, clean and dry

• This modification works best on high ratio machines or on compressors with an elevated pressure in BP
outboard cavity

• Increased BP o/c pressure always indicates reduced compressor efficiency and also affects thrust load. After
first decreasing, thrust load reverses and starts to load the normally unloaded axial bearing
 SYSTEM IMPROVEMENT
Main goal for the system improvements is always to enforce the correct flow of SO and BG under all
circumstances and to improve the SO/BG separation in trap

• Existing Packages

• Trap exit sensing for SO regulator (before orifice)

• SO regulator sensing line branches off in an upward direction

• Throttling type buffer gas system

• Use BP back pressure as reference for BG regulator to increase safety

• Install two traps to allow each capsule / trap system to find its own optimum pressure

• Install larger traps to decrease gas velocity

• Shuttle valve to select higher of two trap pressure as SO reference pressure

• Increase size of BP gas return line and internal ports to reduce the line losses and lower BP o/c pressure

• Additional Wet Seal Improvements

• Individual and completely separated BG and SO systems for suction and discharge capsules

• Dry Gas Seals

 References

• Technical Letter # 13.2/103

• Service Bulletin # 13.2/109