Inspection & Repair of Ammonia Vessel

Inspection & Repair of Ammonia Vessel

Inspection of ammonia vessel is a critical job which requires carefulness and complete attention of inspectors. A consolidated inspection plan must be in place to inspect the ammonia vessels including applicable NDTs in order to identify the suspected damage mechanism. Known damage mechanisms as defined in API 571 include Stress Corrosion Cracking and Brittle Fracture. References for inspection of ammonia vessels can be made from NBIC which specifies inspection works to be carried out on ammonia vessels.

This article relates to inspection and repair works of ammonia vessel which has been in operation since 1966. Material of construction for shell is A 212 Gr. B which has been replaced by latest ASTM standards including fully killed steel A 516. There are different opinions on grain structure of A 212 Gr. B, mostly recommend to consider it a coarse grain material – not killed. The vessel in its original construction has not been post weld heat treated.  Historically, frequent cracks /fissures initiation in weld joints and pin holes of nozzles welds almost in every inspection have been noticed. The maximum depth of cracks noticed in the past was 3 mm in 2005.  The phenomenon of this cracking is attributed to Stress Corrosion Cracking due to anhydrous ammonia and carbon steel vessel. PWHT has never been performed on any of the repairs done in past.

Internal Inspection of Ammonia Reservoir was performed after 2 years of operation. General condition of vessel was found satisfactory. However, during PT of Seam welds, T-Joints & nozzles a 203mm long and 8mm deep crack (vessel thickness, 17.5 mm) was noticed alongside a T-joint in the vessel shell adjacent to manhole. Also, clusters of Pinholes were noticed on 18’’ M2 nozzle weld. Both defects were ground off till complete elimination & repair works were carried out followed by PWHT of shell repair. Hardness readings ranging between 120-140 BHN obtained on repairs confirmed effective heat treatment. RT of shell crack repair was also performed which was found OK.

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Fig. 1 Schematic representation of defects in ammonia vessel

Observations Summary:
1) - Initial visual inspection of vessel revealed oil deposits inside vessel which were cleaned for further NDT examinations. Power brush cleaning of all Weld seams & T-joints was done.
2) - PT of all T-joints & seam welds was done. A 203mm long & 8mm deep crack was noticed alongside a T-joint in the vessel shell at west side of Manhole. All other weld joints were found in healthy condition.
3) - 25 -30 pinholes were found during PT examination of 18’’ M2 nozzle-to-shell weld. All other nozzles were found in healthy condition.
4) - Magnetic Particle Inspection of all the T-joints, seam welds and nozzles was done. No abnormality other than specified above, was noticed.  
5) - Hardness readings were taken randomly along the weld seams & vessel shell course and were found normal. (125-150BHN)
Work Done:
1) - The crack appeared in the shell was ground till elimination (8mm deep) & PT was performed for verification.
2) – After grinding to sound metal, welding was done using ER-70S3 filler wire. PT was performed after 1st & final pass of welding. No abnormality was noticed during PT examination.
3) – Afterwards, MT of repaired area was also done revealing no abnormality.
4) - The Pinholes appeared in the M2 nozzle were also removed by grinding & flushing (10~12mm deep) & were verified using PT examination. The ground area was re-build using ER-70S3 filler wire. PT was performed after 1st & final pass of welding. No abnormality was noticed during PT examination. Final verification of repair was made using MT examination revealing no abnormality.
5) - PWHT of both repairs was done at 1150F followed by Hardness testing which was found within limits, indicating proper PWHT. Hardness values range between 125~150BHN for the repaired area.
6) - The shell crack repair was qualified using Radiography examination.

Shell Crack Investigation:
Investigations were done under the light of API-571, NBIC (National Board Inspection Code) & inspection history files to know the probable cause of crack. Following is the summary:
1) - This vessel is not Post weld heat treated (as per design) and therefore, prone to Ammonia Stress corrosion cracking. Cracks initiated in the history also, as already mentioned.
2) - A crack was noticed in 2005(152mm long & 3mm deep) inspection and was adjacent to the crack noticed in current outage. (July-2011).
3) - The crack (in 2005) was ground off & was locally repaired thru welding as per procedure & no clue of PWHT was observed in history files indicating that local stress relieving of the repair was not done leaving the repaired area in stressed condition.
“API-571 States: Cracking will occur at exposed non-PWHT welds and heat affected zones”.
"NBIC RB-6450 for Ammonia Vessels states: “Weld repairs regardless of the size should be post weld heat treated”
Conclusion:
From the above summary, this can be concluded that lack of PWHT of the repair being done in 2005 initiated stress corrosion cracking in Heat Affected Zone and propagated thru the shell because the HAZ area was a stress concentration point. The same is confirmed by API-571 under “Ammonia Stress Corrosion Cracking” topic (Ref- API 571 -Damage Mechanisms Affecting Fixed Equipment in the Refining Industry)
Picture Gallery

203 mm long and 8mm long crack in HAZ of 2005 weld repair
Crack disappeared after 8 mm grinding in shell wall
Clusters of pin-holes in 18" M2 nozzle to shell weld
Pin-hole in M2 nozzle to shell weld - existing even after 10 mm grinding
Pin-holes removed after 12 mm grinding	Repaired nozzle joint after pin-holes removal
Repaired M2 nozzle to shell weld and DPT found OK
Local PWHT in progress	PWHT elements installed inside vessel
PWHT in progress on M2 nozzle joint	View of external insulation
Final approved PWHT graph

Important Considerations:
1) - Wet fluorescent magnetic particle testing (WFMT) must be performed on 100% T joints and random on circumferential / longitudinal seam welds to detect stress corrosion cracking. Sometimes, Penetrant testing also helps & shall be given due consideration.
2) - Weld repair shall only be undertaken if there exists a threat to mechanical integrity of the vessel unnecessary weld repairs initiate stress concentration points for SCC.
3) - Before making any weld repair, baseline data of hardness shall be acquired in the area to be repaired.
4) - Hardness shall be taken after welding also to see any changes. Normally, carbon steels do not show up any considerable difference in hardness values but the data must be taken for reference purposes.
5) - All the defects (cracks or pin-holes) must be completely removed before welding. This is prohibited in codes (NBIC & API 510) to perform welding over existing defects.
6) - Weld repairs shall be qualified by following sequence of NDTs: DPT after first and final pass, WFMT after completion of welding followed by radiography.
7) - Radiography must be done after performing post weld heat treatment.
8) - Consider applying Zinc spray (0.2mm thick) on HAZ area adjacent to welds to inhibit SCC in future by cathodic protection technique.
9) - Ammonia vessels shall be stress relieved to inhibit stress corrosion cracking.
10) - Oxygen ingress in the vessel shall be restricted to zero ppm as SCC only occurs in the presence of oxygen , even 0.5ppm may initiate SCC.
11) - Maintain at least 0.20% water (by weight) inside vessel to avoid SCC.
12) - Whenever a vessel undergoes welding repair job, it must be post weld heat treated as per guidelines provided in API 510.