TRINIDAD LEVEL BRIDLE FAILURE TRINGEN 2€¢Upgrade bridle chamber assembly to stainless steel...
Transcript of TRINIDAD LEVEL BRIDLE FAILURE TRINGEN 2€¢Upgrade bridle chamber assembly to stainless steel...
TRINIDAD - TRINGEN 2
LEVEL BRIDLE FAILURE
2015
R. BABWAH
K. BALGOBIN
Presentation deliverables
• Event description
• Incident Investigation Findings
• Recommendations
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• Loud explosive noise
• Benfield liquid release
• Gas release which ignited
• Fire contained - Synloop
and CO2 system
immediately stopped -
complications with trip
valves passing
• Many helpers!!!!
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EVENT
INITIAL ASSESSMENT
• On inspection - Level Bridle piping had ruptured.
• No prior warning signs
• Fire damage to insulation and electrical wiring in vicinity
• Loss of Benfield solution to environment – quickly contained !!!
• Possible Causes:
• Metal fatigue,
• Internal corrosion,
• External corrosion - ruled out as inspection was done in 2013 turnaround without concerns.
• CO2 Stress Corrosion Cracking – Most likely culprit !!!!
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PROCESS OPERATING CONDITIONS OF TOWER
• Process gas enters the bottom of
absorption tower T3401 at 218 °F
(103 °C) where it flows upwards
through 4 packed beds of Raschig
rings (SS).
• Lean Benfield solution (400
psig/27.6 bar), which is tangentially
introduced at the top at 145 °F (63
°C), flows in counter current.
• The solution absorbs the CO2 and
leaves the bottom of the absorber
as a “rich” Benfield solution. A
relatively CO2-free gas leaves the
absorber at 136 °F (58 °C) at the
top
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DESIGN OF BRIDLE
• Two nozzles attached to the tower (B1
and B2)
• Four nipples attached to two separate
level glasses (LG1, LG2, LG3 and LG4)
• And two further nozzles attached to a
level transmitter LT3038B, all of which
provides the level of Benfield solution
within the absorption tower
Design parameters
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FAILURE
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Involved:
• Visual,
• UT inspection
• Review of design data.
Findings:
• Failure was in the form of a sudden, total loss of
containment
• The failure took place directly in line with nozzle
B1, approximately 1’ 3” below it.
• Vertical grooves emanating from the nozzle
running downwards were noted - appeared to
have been some corrosive action due to
condensation of the Benfield vapour at nozzle
B1 which continuously ran down the sides of the
bridle wall.
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PRELIMINARY REVIEW - onsite
Bridle in 2013 – CUI done
• Detailed inspection on all areas affected by the
fire damage - insulation and instrumentation
• New bridle was rebuilt with CS Sch 80 and all
welds PWHT for service
• New bridle assembly was pressure tested prior
to installation
• UT thickness measurements were done on
bridles on other vessels in CO2 service (no wall
loss found)
• Samples of failed bridle chamber sent to Yara
Materials Engineering Dept for analysis
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IMMEDIATE ACTIONS TAKEN
There were two investigations in Yara related to this Bridle failure.
1. Failure analysis via Yara’s Materials Department
2. Process safety and incident investigation by YARA Trinidad
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FAILURE ANALYSIS
FAILURE ANALYSIS – cont’d
Sample 1a
Consists of a section of the upper corroded
part of the level bridle, which has been split
longitudinally, and includes nozzles B1 and
LG1
Sample 1b
Consists of the opposite longitudinal half of
tube Sample 1a
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4 Samples were cut from the assembly to perform the failure analysis. All four samples were
individually analyzed both Macroscopically and using Fractography
FAILURE ANALYSIS – cont’d
Sample 2a - focus was placed on sample 2a
Consists of the lower corroded part of the level
bridle, which has been split longitudinally, and
includes the ruptured part.
Sample 2b
Consists of the opposite longitudinal half of
tube Sample 2a
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Samples were cut from the assembly and used to perform the failure analysis
FAILURE ANALYSIS – cont’d
Sample (2a) exhibited internal corrosion attacks and the rupture follows two of the
deepest longitudinal corrosion grooves that were present on the internal surface (image
below right). The fracture surfaces along these grooves were very narrow, less than 1
mm in some areas and hard to identify with the naked eye – indicating wall metal loss
had almost been through-wall.
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Sample 2a
Sample (2a) was cut into smaller pieces for Fractographic analysis
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FAILURE ANALYSIS – cont’d
Findings of analysis:
• The longitudinal fracture surface exhibited a dimple rupture morphology, which confirmed that this
part of the rupture was ductile. The longitudinal fracture surfaces were very narrow; the analysis
suggests a wall thickness less than 0.5 mm in some areas.
• Several smaller areas of undamaged fracture surface revealed that the transverse part also had
ruptured in a ductile manner
• There were no indications of brittle crack propagation, presence of secondary cracks or corrosion on
any of the fracture surfaces.
• The results of the analysis and observations made during the examination of the tube implies that
the tube has ruptured due to ductile overload as a result of localised thinning in form of
longitudinal corrosion (grooving) on the internal tube surface.
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FAILURE ANALYSIS – cont’d
CONCLUSION/DISCUSSION by Materials Department & YTL
✓ The tube ruptured due to ductile overload as a result of localised thinning from corrosion on the
internal surface.
✓ The source of corrosive liquid appears to be condensation of Benfield vapours at nozzle B1, which
had continuously run down the bridle wall. The depth of the corrosion grooves tapered off as the
general Benfield liquid level was reached.
✓ The appearance of the rupture indicated that the tube first ruptured along two deepest longitudinal
corrosion grooves.
✓ The very narrow fracture surfaces on the longitudinal parts of the rupture showed that the bridle had
suffered from severe local wall metal loss. The bridle wall thickness was less than 0.5 mm along
parts of the deeper corrosion grooves. This thickness was below minimum required thickness to
withhold the internal pressure of 400 psig (~28 bar), and suggests that the rupture was initiated due
to this reason (mechanical overload).
✓ The failure was not due to CO2 Stress Corrosion Cracking as initially thought !
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FAILURE ANALYSIS – cont’d
POINTS:
• The characteristics (shape and localization) of the internal
thinning observed supported the theory of a corrosive liquid
phase condensing at the upper nozzle, going along the
internal wall of the equipment down to the liquid level.
• The vapor phase is composed of syngas rich in CO2 and
water is present from the Benfield solution – possible that
“mixture” temp is not maintained above the dew point, acid
carbonic formation and condensation are inevitable.
• Signs of long term exposure to acidic condensation can be
associated to a weakness in the insulation/tracing system
allowing this process condition (low temperature) to happen
at the nozzle.
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PROCESS CONDITIONS AND PASSIVATION - Review
POINTS:
• At the location of failure - low turnover of the solution in the
bottom part of the bridle, the upper part was most of the time
in contact with a vapor phase. Conditions required to build
up a proper passivation layer require constant renewal of the
passivation solution at the surface of the metal.
• Passivation is well performed for the Tower and piping in the
loop but it appears that little or no passivation are done on
areas with low to no circulation of passivation fluid and in
this case the bridle chamber
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PROCESS CONDITIONS AND PASSIVATION - Review
Steam Tracing - Designing/operating/maintaining tracing systems including nozzles remains a
challenge for older plants. If this phenomena was identified during original design or plant operation,
the decision to upgrade this level bridle in SS would have been taken.
Plant Operating procedures - in transient conditions or during passivation process, procedure failed
to include these type of bridles and also with such a complex design, it may not even be practical.
RBMI Program - analysis at a loop level to define a detailed inspection scope must be down to a
component level (the level bridle) should have been treated as a specific component. In this way, the
bridle would have had defined degradation mechanisms. For this loop the damage mechanisms
known were erosion/corrosion, CUI and CO2 SCC. CUI was part of the general Inspection scope for
small diameter connections to pressure equipment, however UT measurements were not done for the
bridle.
Industry Improvements – Newer plants have installed SS bridles. Sharing and obtaining knowledge
is important in this industry to prevent accidents.
Handling incidents- Emergency response review to be done. Many helpers!!! STOP WORK Policy
to be adopted – if cause is unknown then response cannot be accurate. Stop and investigate.
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LESSONS LEARNT
• Upgrade bridle chamber assembly to stainless steel material.
• Revise/review corrosion study - chambers and attachments to pressure equipment
such as this should be classified as as separate component for a better definition of the
potential damage mechanisms according to the corrosion loop. As such it should also
have its own detailed inspection plans.
• Periodic inspection of carbon steel bridle chamber installed until the next turnaround
cycle.
• SHARE THE KNOWLEDGE !!!!!
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RECOMMENDATIONS
Thank you for the opportunity !!!!