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S‐502 scaffold for external inspection 

S‐502 after job completion

Fitness for service evaluations and repair of Ammonia storage sphere after 25 years of service

Syed Naveed Haider; Unit Manager Project; FFC- Pakistan

Ali Abrar; Unit Manger Inspection; FFC- Pakistan

1. Abstract Fauji Fertilizer Company Ltd (FFCL) is the largest urea producers in Pakistan with a urea market participation of around 60%. In June, 2002, FFC acquired a government owned plant, Pak Saudi Fertilizer Ltd (now FFC Plant-3) at Mirpur Mathelo. This plant was commissioned in early 1980 with design capacities of 1,000 metric tons of ammonia and 1,740 metric tons of urea per day.

At the time of acquisition in 2002, Plant-3 was in extremely poor condition with respect to product quality, capacity and maintenance practices. Besides initiating immediate actions on all these important issues, FFC chalked out a phased program to enhance reliability and integrity of the plant equipment. Several equipment were either replaced or revamped based on detailed inspection and evaluations. Two pressurized and partially refrigerated type ammonia spheres were among the most critical equipment at plant. One of the spheres, S-502 was originally designed and manufactured in 1975 in ordinary carbon steel BS-1501-224-32A LT 50 without stress relieving by M/s Mother Well Bridge Engineering Ltd-UK. The sphere was designed against BS-1515 Part-1 having considerably less design wall thickness as compared to the second sphere manufactured against ASME Section-VIII Division-1. As per available records, the sphere remained in service for more than 25 years without any major inspection till 2006 in which a major internal and external inspection was carried out by FFC. Being in the vicinity of main

cooing tower and poor condition of its external insulation, water/moisture ingress caused severe pitting corrosion in the form of clusters. Internal inspection revealed SCC in HAZ areas of welds. After detailed inspection and Fitness for Service Evaluations by M/S Force Technology Denmark and followed by essential repairs, the sphere was taken into service again and a possible disaster was saved. M/S Force Technology also published the results in European Project FITNET, a net work for preparation a

guide line for fitness for service evaluations.

This paper describes the inspection results of the sphere, after 25 years of service containing observations of severe corrosion under insulation, SCC and low design thickness according to BS-1515 Part-1. Remaining life assessment including repair methodology and re-commissioning are also part of this paper.

2. Key words SCC due to ammonia service, Corrosion under insulation, Pressurized ammonia spheres, Fitness for service evaluations, Inspection of ammonia sphere after 25 years of service, Less design thickness according to design Code used.

3. Introduction Fauji Fertilizer Company Ltd was incorporated in 1978 as a joint venture between Fauji Foundation and Haldor Topsoe A/S of Denmark. The company is operating three world scale urea plants with an aggregate design capacity of over 2 million metric tons per annum.

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The first plant (Pant-1) was commissioned in early 1982 with name plate capacity of 1,725 metric tons of urea per day and later it was successfully revamped to 122 % of its design capacity in 1990. The second plant (Plant-2) having design capacity of 1,925 metric tons of urea per day was commissioned in March, 1993. In June 2002, FFC acquired a government owned plant, Pak Saudi Fertilizer Ltd (PSFL) at Mirpur Mathelo (Plant-3). This plant was revamped to 125% of name plate capacity in 2008. FFC is amongst the top 25 companies of Pakistan for the last 14 years. Since commencement of commercial operation in June 1982, till December, 2008 the company has sold nearly 50 million tones of fertilizer achieving urea import substitution of US $ 7.31 billion and company has contributed Rs. 106 billion to the national exchequer. All the three ammonia plants employ Haldor Topsoe Denmark design of late 1970’s and Sipem S.P.A Italy process for urea plants.

4. Description of Ammonia Storage System Liquid ammonia from plant is transferred through a 6" NPS line and can be diverted to any of the two spheres. Nominal capacity of S-501 is 4000 m3 where as S-502 is 3200 m3. Separate refrigeration system has been provided for each sphere to recycle ammonia vapors generated in these spheres. Average liquid level remains approximately 33% of the nominal capacity. Both spheres are designed against different design Codes and design conditions; a brief comparison is given in table-1below. Table 1- Comparison between two Ammonia Spheres

S-501 S-502 Manufacturer ISHI-IRON WORKS

JAPAN MOTHERWELL BRIDGE ENGINEERING LTD. UK

DESIGN CONDITIONS Design Code ASME-SEC. VIII-DIV.1 BS 1515 PART-1 Nominal Capacity 4000 m3 3200 m3 Design Pressure 3.9 Kg/cm2 4.92 Kg/cm2 Design Temperature -4 °C -35 °C Corrosion Allowance 1.0 mm 1.6 mm Joint Efficiency 1.0 -

INSPECTION AND TEST Radiography Full (100 %) Full (100 %) PWHT Column & Crown plates Column & Crown plates Hydro-test Pressure 5.85 Kg/cm2 9.45 Kg/cm2 (At bottom) Pneumatic Test 3.9 Kg/cm2 NA Magnetic Particle Before & after Hydro-test NA 

MATERIAL Shell Plate A-516 Gr. 60 BS-1501-244-32A-LT50 Column A-516 Gr. 60 + SS 41 BS-1501-244-32A-LT50 Electrode JIS D 5016 (LB52N) LOWHEES 35

OTHER TECHNICAL DATA Sphere Weight (Empty) : 320, 000 Kg 187,970 Kg

Number of Columns 12 08

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5. Decommissioning of Ammonia Sphere for inspection Based on detailed HAZOP study, a procedure was developed to transfer liquid ammonia from S-502 to S-501 and operate plant with one ammonia sphere during inspection activities. After shifting of liquid ammonia, S-502 was depressurized to 0.15 kg/cm2 carefully at the rate of 0.9 kg/cm2 /hr within 6 days time. During depressurization skin temperatures and vapor temperatures were monitored regularly. Pressure was further reduced to 0.1 kg/cm2 before introducing N2 in to the sphere. Pressurization with N2 was done up to 0.5 kg/cm2 while maintaining minimum pressure of 1.1 kg/cm2 in the N2 supply header to avoid reverse flow. S-502 was pressurized with N2 and depressurized up to pressure slightly less than atmospheric pressure for 6 consecutive cycles and after last cycle ammonia concentration was reduced to 5.5%. Afterwards purging with air was started and sphere was pressurized (up to 1kg/cm2) and depressurized with air for 10 consecutive cycles. Ammonia concentration was reduced to 160 PPM after air purging. The next step was the washing of sphere with utility water to remove any mud, oil and ammonia present at the bottom of sphere. Special safety precautions / arrangements were made at site before opening of top and bottom manholes for vessel entry. Proper ventilation, oxygen concentration and temperature inside sphere were checked at different elevations of sphere before issuance of vessel entry permit.

6. Inspection Plan Internal and external inspection scope is given in Table-2 below. Inspection was carried out in light of API Recommended practice 571 December, 2003 (Abridged). As a matter of fact, pure anhydrous ammonia does not cause cracking in carbon steel. Experience to date indicates that the probability of ammonia SCC is the highest for carbon and low allow steel vessels that meet one or more of the following criteria:

1. Oxygen content greater then 1PPM or the vessel is frequently exposed to air internally 2. Cyclic pressure operation 3. Fabrication with higher strength steel (minimum specified tensile strength higher than 480 MPa)

especially if not stress relived.

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Table-2 Inspection job scope

External inspection

Internal inspection

7. Inspection Techniques Since it was the first thorough inspection of this sphere we were expecting evidence of ammonia induced SCC. Typically this is investigated by using magnetic particle inspection of the welds. Wet fluorescent magnetic particle inspection (WFMT) with an AC yoke on a properly prepared surface has been reported to be the most sensitive inspection technique for detecting ammonia SCC. This technique also finds linear indications that are not SCC. Detection of cracks by WFMT followed by removal of crack indications is an approach that has been successfully used to restore the integrity of existing anhydrous ammonia vessels. Detection of small ammonia SCC has not always been successful using visual dye penetrant or radiographic techniques. Ultra sonic examination may detect some cracks, but is limited in sensitivity. Acoustic emission is used

Activity Description Observations/ scope Visual Insulation vapor barrier inspection Cracks, Leakages Pressure plates at damaged locations of

insulation vapor barrier Cracks, Leakages

Condition of top nozzles Corrosion. Metal loss Anchor bolts Corrosion, Deformation Foundation settling (by Theodolite) Uneven settling Nozzle welding with shell Corrosion, Metal loss Stair supports with shell Support leg joints with shell Magnetic Particle Suspected Locations / Indications Sub surface cracks Dye Penetrant Nozzle Welds (if any) and Suspected locations

/ Indications Welding defects and surface cracks

Thickness All nozzle pipes Thickness loss Hardness Plates at random locations, covering base

metal. HAZ and weld areasHard areas

Activity Description Observations/ scope Visual Shell plates SCC, Corrosion & Erosion Shell weld joints Condition of vertical ladder Condition of level and other internals Nozzle welding with Shell Manhole cover sealing surfaces Magnetic Particle All T-Joints (1 ft. to each side) SCC / Corrosion Dye Penetrant Suspected locations / indications Surface cracks Thickness All Plates at random locations Thickness loss Hardness Plates at random locations, covering base

metal, HAZ and weld areas Hard area

Hydrostatic Pressure Test

After nozzle replacement or major repairs (if any)

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SCC crack indication in MPI Star like crack indication in MPI

10. Findings of internal inspection

As per initial scope of inspection MPI was performed on all Tee-joints and nozzle welds with shell plates. Symptoms of stress corrosion cracking (SCC) were noted at few locations in HAZ of Tee-joints at circumferential weld seam # 3 (CW3) and top manhole nozzle weld (please see Fig below). 15 cracks or crack like indications were also found on weld repairs done at the time of fabrication of this sphere. Small defects like porosity, slag inclusion and craters were also observed in these weld repairs.

Above observations confirmed presence of (SCC) in the vapor region. Therefore, MPI was extended for complete circumferential weld CW3 for further investigation. Results were amazing; more than 70 cracks in HAZ, generally above weld seam were noted. Average length of indications was 4-5 mm where as maximum length was 20 mm.  Two 15 mm long cracks were grinded off carefully; they completely vanished at a depth of 1.6 mm. Hardness at random was also measured. It was 140 BHN at plates where as it was 150-186 BHN around HAZ area. Based on recommendation of M/s Force Technology, scope of MPI was increased to all welds above the liquid level and 65 more indications were noted in this region. Almost all the indications were found in HAZ. Maximum depth of any crack was 2.3 mm and length 50 mm. After MPI of all vapor region welds some portion of liquid region was also checked by MPI. Several cracks were found even in liquid region. Therefore, it was decided to carry out MPI of all welds in liquid region as well.

SCC indications

SCC indications data for the ammonia sphere S‐502

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In liquid region total 26 indications were noted where as no indication was found in bottom crown plate. One indication in liquid region was a combination of cracks in ‘star’ like shape as shown in figure above and it was removed by grinding. This completed MPI of 100% weld joints of entire sphere. One strange phenomenon noted was the observation of ‘new’ cracks or crack like indications in areas where there was no such indication few days back in MPI. It was thought that these new indications might have appeared due to the exposure of vessel to atmospheric air. But as per M/s Force expert’s opinion, the observation of ‘new’ cracks was not an unknown phenomena and it was definitely not related to exposure of vessel to the atmospheric air. M/s Force had experience of similar problems in ammonia spherical vessels at some other plant due to which inspection was done every year, and each year new cracks were appearing resulting in closure of all spherical vessels of ammonia.

11. Repairs

11.1 External repairs The entire sphere was shot / sand blasted (SA 2-1/2) and primer was applied immediately. As no sharp edges were found on even heavily corroded surface, no grinding or any repair work was done. However, full pressure sleeves were provided on severely corroded nozzles of sphere without welding at sphere plates. Similarly severally corroded support plates were rehabilitated, again without welding at sphere plates. After completion of all repairs and thorough inspection, polyurethane spray insulation was applied of total thickness 70mm. Cladding was provided after insulation.

11.2 Internal repairs All indications including even minor indications were removed by grinding carefully without making stress concentration points. All indications vanished up to maximum depth of 2.3 mm. As explained earlier one ‘star’ like indication in the bottom portion of sphere completely disappeared after 6.1 mm grinding. As per M/s Force evaluations this ‘star’ like defect was due to some temporary attachment weld during fabrication of sphere. As thickness of plate in this portion was 15.5 mm, therefore, grinding up to 6mm in a localized area was acceptable without weld overlay. Weld repair was not done at any location due to its serious consequence in future.

12. Fitness for service evaluations Fitness for service evaluations were done by M/s Force Technology Denmark based on all the field data provided by FFC. It was a classic combination of two professional companies in which the data was precisely collected by FFC according to exact requirement of M/s Force Technology for evaluation. Inspection scope was adjusted gradually according to the result of evaluations and advice of M/s Force Technology. However, a potential disaster was saved through combined efforts.

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12.1 Evaluation of external pitting corrosion It was simply not possible to perform calculation for an individual pit due to very high concentration of pits. Therefore, the calculations were done based on general minimum thickness for different plates of sphere according to the maximum depth of pits in that patch. Determination of critical size of possible cracks in the external pits for different depths of pits was not done mainly due to two reasons: • No evidence of cracking was found • No sharp edges were found

Therefore, no external grinding was done. The calculation of thickness was based on the design Code BS5500 (BS1515) which was given as the original design Code. Minimum thickness at the original design pressure of 4.83 bar gauge was calculated for different elevations of the sphere and it was compared with minimum local thickness reduced due to external pitting / corrosion as observed. Due to this reduced thickness maximum allowable service pressure was calculated as 3.63 bar gauge. 12.2 Evaluation of internal SCC

The evaluation of defects was performed on the basis of BS7910 and the results were published in the European Project FITNET a network for preparation of guidance for fitness for service evaluations.

The allowable crack size at the service conditions was determined to be less then 1mm in depth due to which it was recommended to remove all defect indications.

The local grinding of the observed defects given in MPI scheme were accepted without weld repair and

without further reduction of service pressure beyond 3.63 bar gauge. The calculated allowable crack depth was 0.8 mm at length 10 mm, where as critical crack depth was 5.6 mm for a crack length of 56 mm. The local grinding was performed at a “star like indication” in a small local area (40mm x 35mm) and it was accepted as such without any weld repair.

13. Box-up and re-commissioning of ammonia Sphere After thorough inspection and subsequent repairs / rehabilitation of all the mechanical and instrumentation items, box-up of S-502 was started. Seal test was conducted with utility air at pressure of 2.9 kg/cm2 maintained for about 4hrs. Blinds of all process nozzles were removal after seal test and purging with N2 was done for the 6 consecutive cycles. Next step was the pressurization of sphere with ammonia vapors up to 1.0 kg/cm2. Special attention was given to avoid vacuum inside sphere. N2 was kept connected to deal with such emergency. After 02

ammonia cycles, ammonia and oxygen plus Argon contents were 92% and 0.08% respectively. After two more cycles of pressurization with ammonia cooling was started by opening showering valve. Liquid ammonia was introduced in sphere when bottom skin temperature reached between -2.5°C to 3.5°C and vapor temperature came down to 2°C. It was noted that average vapor temperature of S-502 after commissioning and re-insulation was around 17°C against the previous value of 30°C due to better quality of insulation.

14. Suggestions and recommendations for further inspection and use It is very difficult to give an exact estimation of the frequency of internal inspection without knowing the actual working conditions and the results from only one inspection long time after taken in use give only a little indication of the risk for stress corrosion cracking (SSC). The inspection program given below is a recommendation from U.S National Board Bulletin and it can give an idea of an inspection plan. A vessel to store ammonia has been built of ordinary carbon steel without stress relief. Since a large volume of volatile hazardous material is being stored under pressure, a major rupture must be avoided. Therefore, this would be considered a critical vessel. For such a vessel an acceptable inspection program could be:

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1. Conduct acoustic emission monitoring during initial hydrostatic water test of the fabricated vessel. Any significant indication should be investigated by other techniques and repaired if appropriate.

2. After 12 to 24 months of service, conduct an on-line acoustic emission test followed (regardless of the results of the AE test) by a complete internal inspection. This inspection includes magnetic particle inspection of at least 20 percent of the welds. Areas of significant noise generation detected during the on-line AE testing should be explored thoroughly, using appropriate techniques.

3. If no significant problems are found at the 12-24 month inspection, on-line AE testing should be made at two year intervals. Effective on-line AE testing can be done frequently by increasing operating pressure somewhat (about 10 percent) above the normal level. An internal inspection should be made at five-to-eight-year intervals. This inspection includes magnetic particle inspection of at least 20 percent of the welds. Areas of significant noise generation detected during the on- line AE testing should be explored thoroughly, using appropriate techniques.

If deficiencies are found during inspection, these should be analyzed and appropriate action taken. Appropriate actions might involve repairs or decreased time intervals between inspections. All such, decisions must be made on a conservative basis to insure that the rate of deterioration can be monitored closely enough to reduce the probability of a major failure to an extremely low value. For the actual vessel step 1 has passed and the inspection just finished may correspond to step 2 above. It was the first inspection after taken in use. The next inspection should correspond to step 3 above. If no AE is performed the interval for internal inspection should be more than 4-5 years depending of risk of SCC described later. After the vessel has been aerated for maintenance (periodic inspection) it is recommended to purge the vessel with nitrogen to remove air (oxygen) or other impurities from the tank prior to use. It is advised to purge to an oxygen level of 0.5% oxygen or less. Further the risk of stress corrosion cracking depends on the contents of oxygen and water. Therefore it is recommended to add at least 0.1% wt water to the liquid ammonia.