Urea Stripper prototype repairs using micro TIG welding

Keeping up with highly corrosive Urea environment

Contents

Urea ACES Loop Stripper Specification Stripper Construction Stripper History Corrosion & repair criteria Micro TIG Welding Technique Repaired Tubes Conclusion

Urea ACES Loop

Construction year 1997 Designer Toyo Engineering Corporation

Constructor Mecnica de la Pea Equipment T-1101

Tube side Service Urea solution Shell side Service Steam Number of Tubes 3100

External Dia. 31.8mm Thickness of Tubes 2.9mm MWT Tube/TS Material Duplex SS Sumitomo DP12

Shell Material CS Internal Lining Sumitomo DP12 Duplex SS Swirls Material Sumitomo DP28W Duplex SS

Design Tube Side Pressure 184kg/cm2

Design Tube Side Temperature 220C Design Shell Side Pressure 25kg/cm2

Design Shell Side Temperature 240C

Urea Stripper Specs

Swirls

Top Tubesheet

Bottom Tubesheet

Urea Stripper Construction

6

1998: Installed & Commissioned

TA 2002: 30% ECT done

TA 2005: 45% old swirls replaced with DP28W

FO 2006: 3% ECT done Leakage from seal-welds repaired 05 tubes plugged due to burn-thu corrosion

TA 2012: 100% ECT done 16% seal-welds & tube-ends severely corroded 72% seal-welds & tube-ends had minor corrosion

2008: Leakage in Stripper established Outage postponed

TA 2009: 2008 Leakage repaired. 180 seal-welds repaired 05 tubes plugged due to burn-thru 3% seal-welds & tube-ends had minor corrosion Remaining swirls replaced with DP28W

Urea Stripper History

Corrosion at Heat Affected Zones Corrosion

31.8 mm

26 mm

2.9 mm MWT Defects Observed: Internal Grooving External Grooving / weld

toe-detachment Corroded Tube-ends

Defects Suspected: Incubated defects

8

Priority A Severe Burn Thru > 1.5mm

in Tubes near edge

Priority B Minor Burn Thru < 1.5mm

in Tubes near edge

Internal Grooving at Stripper bottom

9

Priority A Severe Corroded / Eroded Tube ends and T-TS welds

Tube-end corrosion at Stripper Bottom

10

Priority C Severely Corroded T-TS weld Joint

External grooving at Stripper Bottom Priority D

Thinned out / weld toe detached T-TS weld Joint

Criteria 1: If Groove depth > 1.5mm then check criteria 2. If depth < 1.5 mm no need for repair

6 mm

14 mm

2.4 mm

1.5 mm

Criteria 2: If distance from tube end is > 14mm, then repair / plug If it is < 14mm then no repair

Internal Groove Corrosion Top Tube sheet

12-13 mm

Criteria 1: If Groove depth > 1.5mm then check criteria 2 If depth < 1.5 mm no need for repair

4 mm 8 mm

2.4 mm

1.5 mm

Criteria 2: If distance from tube end is > 8mm, then repair / plug If it is < 8mm then no repair

Internal Groove Corrosion Bottom Tube sheet

8 mm

1.5 mm

8 mm

1.5 mm

Criteria: If depth is > 1.5mm, Repair using rotating cutting machine If depth is < 1.5mm then no repair

External Groove Corrosion Both Tube sheets

2.4 mm

3 mm

8 mm

1.5 mm

Criteria: If depth is > 8mm, Plug tube. If depth is < 8mm then no repair

Tube end corrosion Both Tube sheets

2.4 mm

Micro TIG Welding Mock-ups Internal Groove repair

3 OD 31.8 32.10 0.07 hole

Min. 2.5t

40

200

200

60

40

SA516 Gr70 DP28W Tube

:Welding area

Min 7.9

2

20

35

329J4L

Micro TIG Welding Mock-ups Internal Groove repair

Overview Arrangement of welding torch

Welding Bead appearance

Micro TIG Welding Mock-ups Internal Groove repair

18

Bead flushing

Flushing tools

Micro TIG Welding Mock-ups Internal Groove repair

19

After flushing tube inside of 2nd row

PT on tube inside of 2nd row

Micro TIG Welding Mock-ups Internal Groove repair

20 Cutting for cross-section specimen

Micro TIG Welding Mock-ups Internal Groove repair

21

Cross-section specimens

Micro TIG Welding Mock-ups Internal Groove repair

22

Freshly welded Tubes on Top Tube-sheet

Internal groove repaired Micro

TIG weld External groove

repaired

23

Dye Penetrant tested tubes

24

Soap film test of repaired tubes

Conclusion

Urea strippers have experienced similar corrosion on tube-ends & T-TS welds after 12-14yrs life on other Toyo plants Causes of these defects include High heat input during original fabrication or

during repair-work

Other causes include inefficient operation of stripper especially, when liquid falling film is disturbed. It becomes crucial that its cause be corrected immediately, e.g., re-installation of swirl in Stripper

Lowered corrosion resistance in HAZ of repaired tubes deteriorate the surrounding conditions leaving neighbouring tubes susceptible to early corrosion

Few owners have replaced their vessels after 12-14yrs of operation after facing similar deterioration

An alternate to this repair was plugging of all affected tubes, however, besides loss in efficiency, since this option posed the same heat input risk with same consequences expected in the repair-work, plugging of tubes was ruled out

Conclusion

Subsequent inspection was carried out in April 2014 after around 01 year of continuous operation and all the repairs were found in healthy condition with no signs of aggravation in surrounding region

Micro-Tig Welding technique helped Engro defer an imminent Capital Investment for a few yeas more

The Stripper at Engro has outlived all other Toyo Strippers in the world by being in operation for 16yrs now

Had this technique been developed/applied before 2008, Engros Urea Stripper could have given an better life

With a similar technology now developed for even lower diameter (19mm) Carbamate Condenser tubes, it is most likely that the entire ACES loop can be run for more years to come

26

Questions?

27

Urea Stripper prototype repairs using micro TIG weldingContentsUrea ACES LoopUrea Stripper SpecsUrea Stripper ConstructionUrea Stripper HistorySlide Number 7Internal Grooving at Stripper bottomSlide Number 9Slide Number 10Slide Number 11Slide Number 12Slide Number 13Slide Number 14Micro TIG Welding Mock-ups Internal Groove repairMicro TIG Welding Mock-ups Internal Groove repairMicro TIG Welding Mock-ups Internal Groove repairMicro TIG Welding Mock-ups Internal Groove repairMicro TIG Welding Mock-ups Internal Groove repairMicro TIG Welding Mock-ups Internal Groove repairMicro TIG Welding Mock-ups Internal Groove repairSlide Number 22Slide Number 23Slide Number 24ConclusionConclusionQuestions?