Low lift Dry Gas Seals in high vapor pressure,...
Transcript of Low lift Dry Gas Seals in high vapor pressure,...
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Low lift Dry Gas Seals in high
vapor pressure, flashing
hydrocarbon pump applications
Authors: Patrick Shaw, EagleBurgmann
Kent Prichard, EagleBurgmann
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Problem
Ethane pumps with process lubricated; ‘liquid (wet)’ 2CW-CS,
plan 11, 76, seals suffered a high frequency of seal failures
from initial commissioning in November 2009
• 2 fractionator units each with 2 ethane export pumps
• 12 stage, BB3 axially split
• 5.7MW absorbed at BEP
• suction pressure 40.7-50.3 barg
• suction temperature 12.8-18.9 °C
• 3570 rpm, 89mm shaft size
• Shaft speed 16.6 m/s, p.v 677-837 bar.m/s
• 81-96% of API682 scope
• Typical flow range 277–345 m3/h
• 136 / 331 / 408 m3/h (MCSF / BEP / EOC)
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BB3 axially split, 12 stage pump, 6 + 6 opposed impellers
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‘Generic’ Type A, Arrangement 2 seal, 2CW-CS
Process Side
Flush
(plan 11)
Vent
(plan 76)
Quench
(optional, not used)
Drain (normally closed)
Primary, contacting
‘wet’ (CW) seal
Secondary, containment
‘dry’ (CS) seal
Atmosphere side
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Flush - API Plan 11
recirculation from a high pressure region of the pump through a
flow control orifice to the seal
• Primarily: to remove seal generated heat
• Optionally: increase seal chamber pressure (special measure)
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Vent - API Plan 76
Key
1 to vapour collection system
2 tube
3 pipe
4 flush (F)
5 containment seal vent (CSV)
6 containment seal drain (CSD), closed
7 gas buffer inlet (GBI)
8 seal chamber
PIT pressure transmitter with local indicator
Vent to vapour collection
system via leakage
detection / monitoring
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Engineering phase
The process condition at suction did not satisfy API682
requirement of a minimum 30% pressure or 20K
temperature vapour margin for process at seal:
Vapour margin at suction conditions:
• Pressure margin approx. 17% (40,7 / 34 barg)
• Temperature margin approx. 5K
Mitigation:
• Seal design featured a special throat bushing to permit
plan 11 flow to ensure pressure margin in seal chamber
• Discussion with oem to ensure API requirement would be
met
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Commissioning phase
During commissioning there were numerous process
incidents resulting in loss of suction vapour margin
• Process flow
• Process heat exchanger
Once resolved problems continued to occur
• Cavitation
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Post commissioning
Plan 11 - three machine build / installation issues were
discovered:
1. special floating throat bushing had not been installed
• no increase in vapour pressure margin at seal
2. minimum size 3mm dia flow orifice installed in plan 11
• flow rate marginal for removing seal heat
• insufficient to significantly improve seal vapour
margin if throat bushing were installed
3. connection in seal chamber incorrectly positioned
• misaligned to seal faces; seal cooling not optimal
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Plan 11 connection: incorrectly positioned – flush flow (cooling)
not directly to seal faces
Initially not
installed
3mm FO limits flow rate
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Typical operating case
July 30th 2011
36.0
38.0
40.0
42.0
44.0
46.0
48.0
-10.0 0.0 10.0 20.0 30.0
Pre
ssure
(bara
)
Temperature (°C)
Operating points
Vapour curve
API682 temperaturevapour margin
Vapour margin at
suction (seal)
conditions
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Seal rotor redesigned
to align faces with
flush connection
Throat bushings
revised to improve
flow control, wear, etc.
Larger FO installed;
increase flow /heat
removal
Source relocated from 1st
stage to 6th stage; increase
vapour margin at seal
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Example: pump operating conditions
July 30th 2011
-200.0
-100.0
0.0
100.0
200.0
300.0
400.0
500.0
-20.0
-10.0
0.0
10.0
20.0
30.0
40.0
50.0
9:00 10:40 12:20 14:00 15:40 17:20 19:00
Flo
w (
m3/h
)
Su
cti
on
Tem
pera
ture
(°C
) S
uc
tio
n p
ress
ure
(b
arg
)
Time
Suction, C
Suction, barg
flow, m3/h
Process issues: EOC operation
(cavitation)
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Further measures taken in attempts to improve vapour margin
included:
• Alternative floating throat bush; carbon and PEEK materials
• Moving plan 11 source from 1st stage discharge (7.5bar dP)
to crossover; 6th stage discharge (45bar dP)
Result:
• Seal chamber pressure 63 barg (+18 bar); API minimum
vapour margin ensured
MTBF remained unacceptable
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Results
Seal life: increased from as little as 1 day to < 4 weeks
Result; greater consistency in seal life, but no significant
improvement
Symptoms remained similar:
• Plan 76 pressure pulsations from first start
• Increasingly frequent till in permanent alarm
• Icing of plan 76 lines
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Process side silicon carbide seat, highly polished,
worn and wavy
Typical ‘dry running’ symptom
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Result: increased wear rate of carbon stationary seal face
Left: new face, 3mm
face height
Right: 16 days, 0mm
face height
Extreme ‘dry running’ symptom
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Investigation
Similar 2CW-CS, plan 11, 76, engineered seals were giving
satisfactory performance in similar, related, pipeline services
• Smaller machines; lower shaft power, lower p.v. factor
• API minimum vapour margins suitable for high power
applications?
Pumps: one operating, one standby usually on open
discharge relying on NRV to prevent spillback
• Low speed turning due to valve passing
• Contributing factor but not primary
• Instances of pump cavitation
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Conclusions
• Pump and seal duty points (shaft power, suction vapour
margin, seal p.v.) are too high to permit creation of an
appropriate degree of seal chamber vapour margin for a
‘wet’ primary seal
• Cooling process to seal to increase vapour margin is not
viable; process temperature, CAPEX required, time, etc.
Recommendations
1. Allow process to vapourise in seal chamber and utilise a
‘gas’, non-contacting seal; 2CW-CS to 2NC-CS
2. To minimise the normal static leakage of a gas seal
solution use low friction face materials to permit seal start
in full contact mode
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Solution
Early 2011 programme to develop 2NC-CS ‘Gas’, vapour
phase seals
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Solution
1. Compressor DGS taken as basis for design
2. Plan 02 for primary seal (plan 11 plugged) to minimise
face cooling / retain seal generated heat
3. Throat bushing opened / removed
4. Process labyrinth to retain seal heat at seal
X 2
4
3
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Seal face features:
Minimise Joule Thomson cooling effects = minimise leakage
rate, particularly static; target nil
Hybrid design required; contacting (static) / lift off (dynamic),
requires low friction faces
Diamond face (DF) coating providing wear resistance, low
friction, high heat conductivity
face friction coefficients
SiC / SiC SiC-DF/SiC-DF
Dry running 0,70 0,15
Liquid lubricated 0,08 0,01
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Diamond properties
Chemical bonding => Superior diamond film adhesion
Cross section prepared by
Focused Ion Beam (FIB) cutting
SiC
Diamond
Extreme hardness, excellent wear resistance
High thermal conductivity
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2NC-CS with:
• Process labyrinth
• Inter-stage labyrinth
Plan 02, 76 (optional 72)
Expected leak rates DF
DGS (conventional DGS)
Dynamic: 2.7 (19) Nl/min
Static: nil
Primary seal dynamic leak rate on test: 2.5-2.8 Nl/min
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Results
First pump converted: July 2011
Second pump: November 2011
Observations
• Plan 76 pressure fluctuations no longer evident
• on switch over or plant start
• icing on plan 76 lines eliminated
Plant transients had no adverse effect on seal performance
• suction temperature to -8.3 °C
• EOC operation (cavitation)
Approx. 93% reduction in ethane emissions to flare
First pump de-staged in March 2012, seals inspected and
returned to service in same machine, in service to date
Life: >40 months
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Example: pump operating conditions
July 30th 2011
-200.0
-100.0
0.0
100.0
200.0
300.0
400.0
500.0
-20.0
-10.0
0.0
10.0
20.0
30.0
40.0
50.0
9:00 10:40 12:20 14:00 15:40 17:20 19:00
Flo
w (
m3/h
)
Su
cti
on
Tem
pera
ture
(°C
) S
uc
tio
n p
ress
ure
(b
arg
)
Time
Suction, C
Suction, barg
flow, m3/h
Process issues: EOC operation
(cavitation)
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March 2012 inspection (+8 months)
Seals from first machine inspected whilst machine was de-
staged from 12 to 11 stages
Secondary sealing elements replaced, all other parts reused
without any reconditioning. Test dynamic leakage rate: < 2
Nl/min
Seal life:
2CW-CS (wet) approx. 4wks
2NC-CS (dry) > 40 months in service
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Lessons learnt
For high shaft power, high seal p.v. factor applications
• Machine waste energy becomes significant
• Seal generated heat is significant
Impractical to attempt to reliably operate ‘wet’ seals (liquid
phase) even at vapour margins in excess of API682
Permitting / promoting process medium to change to vapour
and operating ‘gas’ seals is realistic and such solutions are
tolerant of process transients
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Experience
36 machines at this and related operator sites since equipped
with diamond face 2NC-CS gas seals
1 installation related failure
For process plants: plans 02, 72 (nitrogen buffer), 76
Plan 02 usually with a process bushing
Plan 72 as it is available (1 SCFM / seal)
For pipelines: plans 12 (flush via filter), 76
Plan 12 for pipeline debris; products of pipeline corrosion,
liquid pooling, atmospheric transients, pigging, etc.
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Questions?