TRINIDAD LEVEL BRIDLE FAILURE TRINGEN 2 Upgrade bridle chamber assembly to stainless steel material....

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Transcript of TRINIDAD LEVEL BRIDLE FAILURE TRINGEN 2 Upgrade bridle chamber assembly to stainless steel material....

  • TRINIDAD - TRINGEN 2

    LEVEL BRIDLE FAILURE

    2015

    R. BABWAH

    K. BALGOBIN

  • Presentation deliverables

    • Event description

    • Incident Investigation Findings

    • Recommendations

    2

  • • 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!!!!

    3

    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 !!!!

    4

  • 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

    5

  • 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

    6

  • FAILURE

    7

  • 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.

    8

    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

    9

    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

    10

    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

    11

    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

    12

    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.

    13

    Sample 2a

  • Sample (2a) was cut into smaller pieces for Fractographic analysis

    14

    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.

    15

    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 !

    16

    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.

    17

    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

    18

    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