Accelerated Corrosion in Benfield Absorber
A complete description of the problem and solution. From Inspection, identification of the problem, monitoring, root cau...
Accelerated Corrosion in Benfield Absorber Upper half of Benfield absorber at FFC-MM has had a problematic history from corrosion point of view. The paper discusses the problem identification, root cause determination, results of progressive inspections carried out, remaining life assessment, solutions considered and tried, and factors behind the decision to change the damaged portion of the Vessel The new vessel was made available by rehabilitation of an old redundant shell by FFC-MM using local facilities. An account of various issues faced during this phase and solutions adopted with costs, activity durations and lessons learned is also presented. Information given will be of interest to all those operators facing similar situation and wish to embark on a comparable project. Suhail Khalid Khawaja (Unit Manager Process), Fakhar Ul Hasan (Unit Manager Maintenance), Mehmood Raza Gillani (Unit Manager operations, Ali Abrar (Section Head Inspection) Fauji Fertilizer Company Limited Mechanical Details OEM is M/S Hitachi Zosen Corporation, Osaka Japan, Benfield Solution Absorber Column C-302 stands tall almost 54.4 meters high at FFC Plant III. The shell material is Carbon Steel ASTM A 516 Gr. 70 and the thickness varies from 34.5 mm to 57 mm along the length as indicated in Fig.1.
CS Paul Rings
The whole column is stress relieved to prevent stress corrosion cracking (SCC) which is a well known issue with these columns in this service. History: This equipment was commissioned in 1980. The leakage through the reinforcement pad of one of the manhole (1MD), between bed no.1 and 2 was observed in 1992. Major areas affected by the corrosion are indicated as C1 & C2 in figure-1. Various measures were adopted to control the problem including application of SS lining inside the vessel on affected regions. The SS lining welds failed and eventually the column upper half was replaced, above (Circumferential Weld) CW12 in 1994. In turnaround (TA)-1998 severe corrosion was observed at CW-14. This was followed by repairs, post weld heat treatment (PWHT) and radiography.
CS Paul Rings
CS Paul Rings
CW-3 CW-2 CW-1
In TA-2002 the observed condition of the vessel found further deteriorated and weld build up was carried out on corroded portion of the shell in the turnaround in area C2 without any PWHT. This plant was then taken over by FFC in year 2002. A detailed study was carried out to identify the root cause of the problem and inspection plan was developed to finalize details of the repair/ remedial plan for this chronic issue.
As an interim solution, Belzona epoxy coating was applied in area C2 to control corrosion.
Figure 3 Close up view of the non passivated corroded portion of the shell (area C1).
Figure: Internal view of the failed lining of 1994 removed upper half TA-2003 Maintenance/ Inspection: Active corrosion was noted in area C2. Packing bed no.1 was also removed and deep corrosion and channeling was also observed at area C1. Minimum thickness noted was 24.5 mm against design value of 34.5 mm.
Figure 2 A view of of Bed. No.1 after unloading. Non passivated corroded areas are visible.
Figure 4 Close up view of the corroded weld build up (area C2) which had been repaired in TA-2002
Figure 5 Belzona coated surface of the shell in area C2.
Areas of Concerns: Online Thickness Mapping
Belzona epoxy coating This was our first experience with this type of coating for this application. So it was to be kept under observation as pinholes or other damages to the coating could lead to active corrosion of carbon steel shell beneath, due to non passivated condition. Continual thickness loss As up till now the root cause of the problem was not addressed, therefore continual thickness loss in these areas of the column was an area of serious concern for the integrity of pressure vessel. Based on these concerns it was decided to proceed with the following line of actions: 1.
Thickness mapping of the corroded portions for integrity assessment. 2. On line thickness trending for corrosion rates assessment 3. Selection of options available for permanent solution of this problem 4. Determination of corrosion root cause and remedial measures It is evident from the history of this column that the previous plant operators were not able to identify the root cause of the corrosion in these two packing bed areas and only reactive maintenance was utilized to keep it in operation.
Figure Scaffolding being arranged for online thickness monitoring of the shell in area C1. 7.9 m 1.7 m
Figure 6 A complete map of thickness profile at area C2 Utilizing ultrasonic thickness measurement, an extensive thickness monitoring was carried out up to 4.4 m length of the shell at C2. Data was collected at more than 7000 points. The profile in Fig.6, indicated in red (24 to 30.9 mm) and blue (31 to 34 mm) shows low thicknesses along the circumference of the measured length. Corrosion rate trending 0.30 A v g C orros ion R ate in m m /m onth
TA 2002 weld build repair in area C2 Although ASME pressure vessel code does not require PWHT for welding in this thickness range, however due to susceptibility of SCC, the manufacturing standard for this service necessitates providing PWHT to reduce stresses. This repair was carried out without PWHT and the vessel remained in service for one year with this condition till TA-2003 when Belzona coating was applied. Thus possibility of cracks existed in these repairs.
0.24 0.22 0.20
Figure 7 corrosion rate trend in the year 2004 & 2005 at C1 At area C1, points of low thicknesses were selected in North, South, East and West sides for precision thickness monitoring on monthly basis. Based on this average corrosion rate was determined. Note significant drop in corrosion rate after improvement in Hot and Cold solution ratio and replacement of distributor in Nov 2004.
Condition assessment Based on the thickness mapping, the vessel was analyzed on API-579 for the fitness for service evaluations. The results on Level- I and Level –II assessments were negative. This meant Level-III assessment was necessary to further analyze the integrity of equipment against the thickness loss. Options for restoring vessel integrity To restore the mechanical integrity of the vessel following options were considered Application of epoxy coating system (Belzona -1311/1591/1321) Replacement of damaged window (rectangular section) Application external banding Provision of SS lining Replacement of upper half
Figure Precision thickness monitoring to evaluate corrosion rate Coating inspection in Nov 2004 During an emergency shutdown, after 20 months of operation, the applied belzona coating was inspected again. This time cracks and detachment was noted on the coating surface. This was considered a serious condition as it could result in aggravated corrosion and more likely development of SCC.
Figure Cracks and detachment of the Belzona epoxy coating.
All the options were deliberated upon. Following were the considerations and reasons for acceptance/rejection. Solution 1 to 4 required level 3 remaining life assessment for which in house expertise was not available. Cost of external engineering assistance though not very high in absolute terms was not feasible to incur in view of low cost of solution No. 5. APPLICATION OF EPOXY COATING: Epoxy coating had been applied in TA-2003. It was an attractive option as it did not require extended down time. However its durability remained a cause for concern. Lining was inspected in August 2003 (5 months operation) and was found in average condition with slight wrinkles. It was therefore concluded that the lining needed continuous monitoring and inspections, sometimes by removing the lining. This was found impractical. Later in November 2004 (after 20 months operation) it was found in deteriorated condition. By that time decision to replace the vessel had been taken. The
observation confirmed the apprehension that Epoxy coating is not a permanent/long lasting solution. In itself the coating is expensive. (See the cost chart)
engineering and material ordering were undertaken. In November 2004 final go ahead was given. The rehabilitation project Cost Comaprison of Different Options 92,000
90,000 90,000 88,600 88,000
Cost in US$
REPLACEMENT OF DAMAGED PORTION: This option was considered in 1991 for the original shell. The solution was not supported by repair contractor and SP on that basis that it will induce uneven stresses and may cause ovality of the vessel.
APPLICATION OF EXTERNAL BANDING: External Banding to reinforce the vessel was proposed by engineering consultant. Detailed analysis was required which was cost intensive. In any case the solution would not have catered for the internal uneven surface resulting in uneven wetting / passivation. Epoxy coating could have been applied but would require frequent inspections. PROVISION OF S.S. LINING: This solution was tried in 1991. Could not be implemented as the base metal was uneven and required extensive stress relieving REPLACEMENT OF UPPER HALF: The shell removed in 1992 was available. Its detailed inspection was carried out. It was determined that 11 meters length including Cap were in good condition. The vessel could be salvaged at a fraction of cost of new vessel. Eventually this solution was chosen, as the cost was almost comparable to other solutions but was a permanent solution. Therefore it was decided to replace the vessel up to CW-13. Replacement Planning Management decided to explore replacement option in May 2004. From that time on till October 2004, inspection/detailed
was to be completed before March 2005 (the scheduled Turnaround) To ensure on time implementation all pre TA activities were identified and planned. See attached bar chart. The job was completed two and half months after receipt of metal sheets in Karachi even though it involved two trips to HMC-3 once for fabrication and then for Heat treatment. FABRICATION ASSESSMENT OF REMOVED HALF A portion 19.9 meter length removed in 1992 was available. Condition was assessed. A portion of 8.9 meter was found in poor condition rest was usable. REHABILITATION PLAN The size of damaged portion was odd since standard size sheets introduced an additional circumferential weld joint. Various combination and permutations were tried and costs compared. Finally 4m & 3.5 m width sheet of 35 mm thickness were ordered. Another 2.7 m portion was obtained by cutting the shell from CW-13 instead of CW-12.
LEGEND New Reclaimed CW (HMC) CW (FFC-III
Figure: Welding in progress
MANUFACTURING Only two agencies in Pakistan had the facility to roll 4m wide and 35 mm thick sheet. The order was placed on HMC-3 who not only rolled but also welded the shell. The tolerances were specified as per SP specifications. New shell was finally joined with existing shell courses without significant difficulty. LOGISTICAL ISSUES Owing to short time available and location of sheet rolling / heat treatment facility, complex logistical issues were involved. These were controlled through bar charts and consistent follow up with vendors. CIRCUMFERENTIAL WELDS A total of four circumferential welds were made during prefabrication phase. One was machine welded at HMC-3. Three were manually welded in FFC workshop. The joints were preheated with natural gas. Internal and external ring headers with holes were fabricated to evenly preheat the joint. Monitoring was carried out with laser gun by inspection unit. The completed joints were radiographed after root pass and then at joint completion. Stress relieving was skipped as whole vessel was to be furnace heat treated.
Figure: Pre-heating arrangement of manhole nozzle welding with shell SIMULATION To simulate final field joint one circumferential joint was welded with vessel in vertical position. This helped in training of fabricators and welders.
Figure Circumferential weld being made in vertical position to simulate field conditions. HEAT TREATMENT
The whole vessel (20m length) was transported to HMC-3 who performed the heat treatment. FFC provided the heat treatment procedure and reviewed the data before dispatch of the vessel.
INSTALLATION RIGGING The weight and height combination made the rigging difficult. The weight of vessel consisted of its own weight and weight of packing. To optimize size of crane it was decided to unload the packing from 1st bed. Final weight to be lifted was 50 tons.
Figure: Newly fabricated shell being shipped to HMC for furnace heat treatment
Figure Old half being removed.
HYDRAULIC TEST A special flat plate blind was manufactured to perform the hydraulic test. A two stage grid of 10” H beams were used to provide reinforcement. Hydraulic test was carried out at 39 KG/Cm2. A deflection of 50 mm was noted in the pressurized condition.
Figure Preheating arrangement for welding of end plate for hydrostatic testing.
Initially construction of pavement was foreseen. However after careful review of site conditions it was observed that site is well compacted and filled with crush. Two 80 mm C. S. sheets of 8’ x 4’ size were placed under each jack. The arrangement remained satisfactory. Coles 310 mobile crane was chosen as it offered considerable flexibility in comparison with ringer crane and was previously used and found OK.The crane was load tested at vendor premises and again after assembly at FFC-worksite.
Figure New half, ready for installation.
UNLOADING OF PACKING Packing had to be unloaded to reduce the load to be lifted. To speed up this activity an additional davit was bolted on the 2nd bed packing loading flange. Pre TA simulation were carried out to ensure safety of operation and reliability of winch. Figure Lower half end cut and beveled with automatic beveling machine. POSITIONING & ALIGNMENT To ensure correct positioning, external tapering lugs were welded on the bottom portion. Four jack bolts were provided to vertically align the vessel. Alignment was carried out with the help of theodolite at two locations separated by 90°. WELDING
CUTTING OF VESSEL Fast, clean and accurate cutting of vessel at the required point was a critical activity. This was accomplished with a track mounted gas cutter available from a previous project. The tool not only cut the vessel but also beveled it at the required angle. This single operation was completed within couple of hours.
Welding was performed by previously trained welder. Welding was 1st performed from inside, back gouged from outside Xrayed and then welded from outs the x-rayed and stress relieved joint completion time (from tacking to completion of heat treatment was 88 hrs. 3 to 4 welders worked simultaneously. Close coordination with Q. C. ensured on time x-ray results and stress relieving.
PRECAUTIONS/LESSONS LEARNED / TIPS FOR MECHANICAL FABRICATION
Figure Pre-heating elements being fixed on the main joint.
Figure Welding of field joint in progress. OVERALL PROJECT SCHEDULE The planned mechanical activity time was 305 hrs. Job was completed in 299 hrs. TIME SAVING To three main following activities were planned to reduce execution time: Parallel unloading/loading of Pall rings from bed no. 1 and 2. Cutting of shell with track mounted shell cutting machine. Pre installation of platforms on new shell to cut scaffolding time.
Transportation • Know the highway rules for transportation. • Length, height, width diameter and weight of all objects to be transported should be carefully reviewed and communicated to the transportation contractor. • Consider prime mover engine power while reviewing the quotations. • Discuss the transportation route with the contractor. • Run pilot vehicle provided with communication equipment on all critical transportations including crane. Crane and Rigging • The crane should be accompanied by specialist technicians to provide instant services in case of a break down. The crane contractor should have a system of dispatching any required spare part at a short notice. • In addition to drawings carefully measure actual height of lifting lug from ground. • In cases where the crane is operating at the limits, size of slings should be carefully calculated. • Fabrication • Cooperation from HMC-3 was of high level. Job has handled by them in a professional manner. • Before dispatch of vessel for furnace heat treatment verify that all internal and external attachments have been welded • Size of new vessel should be kept 1 to 1.5 m extra to cater for welding of hydraulic test blind and final adjustments. • For field joint Vessel should be cut at a distance from the weld joint. Cutting exactly from the weld joint would result in problems in rewelding and subsequent inspections.
Review position of jack bolts lugs etc. with agency carrying out heat treatment to avoid any later difficulties.
Figure Internal view of the removed shell, all surfaces found well passivated. Conclusion
Figure Corrosion pattern on the removed shell.
Root cause analysis is a prime factor for elimination of problems. Reactive maintenance does not address the chronic nature of the problems and adds to the maintenance costs and efforts. Systematic team based efforts help in achieving challenging targets.
Rolling of s heets
Welding of longitudinal seams
RT and Repairs
Welding of circumferential seam
Shipment of Shell
Alignment and tacking
Welding of circumferential seams
Welding of Ring
Alignment and tacking
Welding of circumferential seam
Marking and cutting of nozzle
Nozzles tacking and welding
Ring marking (03 Nos )
Ring welding (3 Nos )
Pads m arking and welding
Ins ulation ring and Platform attachm
Transportation Back to MM
Fabrication of end plate
Welding with ves s el
Transportation to HT Shop
Vertical leveling & Beveling
Receipt of s hell from HMC-3
Manufacturing at MM phase 2
Cutting of s hell to remove damaged
Manufacturing at MM Phase1
Cutting and beveling of straight s he
Receipt of Shell Plates at HMC-3
Tas k Name Manufacturing at HMC-3
Duration 28 days
Start Wed 05-01-05
ember 01 January 21 January 11 February 01 March 21 Ma Finis h 19-12 26-12 02-01 09-01 16-01 23-01 30-01 06-02 13-02 20-02 27-02 06-03 13-03 20-03 Tue 01-02-05 05-01 Wed 05-01-05
Cost Chart Application of Epoxy Coating (Belzona 1311/1591/1321) •
=$ 110 / Square Feet
= 760 Square Feet
= $ 83,600
Replacement of Damaged Window / Application of external Banding •
= $ 30,000
Crane Rental = $ 35,000 (To hold vessel during Stress relieving)
Heat Treatment (4 seams)
= $ 25,000
= $ 90,000
Cost of Replacement with rehabilitated shell •
Cost of material
= $ 38,300
= $ 12,800
= $ 37,500
= $ 88,600