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 !!!!