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Experience from inspection of a double wall Ammonia tank at Yara Porsgrunn Kjetil Bakli – Yara Porsgrunn SAFETY SEMINAR 4-6 May 2011 Hotel SkjaerGarden, Langesund NORWAY

Ammonia tank V-07101 Yara Porsgrunn       

Double integrity tank Volume 60.000m3 Inner tank diameter: 58,0m Inner tank height 22,77m

The tank is used for import of ammonia from ships Designed and erected by CBI Industristahlbau GmbH according to API 620 appendix R in 1976. It was inspected in 1979. No defects was found

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Preparing the tank for inspection 

Emptying all liquid ammonia, 3.- 13. December 2008 : 

Emptying down to 150 mm by use of existing product pumps.

Evacuation NH3, by nitrogen purging,15. December – 16. January :

Evacuating nitrogen. 16.January – 1. February 2009 :    

Below 8% NH3 16. February. Purging nitrogen is stopped and air purging is started. The tank is opened and entered by the emergency force, wearing gas diving suits and compressed air. Tank cleaning starts. Just a thin layer of oil was found on the bottom. February 1st the tank atmosphere is stabilized at 20-25 ppm NH3. Inspection can start.

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Internal inspection

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Internal inspection was carried out by a Dutch company. The magnetic particle method was used (wet fluorescent MPI, Magnetic Particle Inspection). The extent was from the very beginning heavily expanded relative to the recommendations from EFMA 1). Some waiting time was used to extend the inspection further.


European Fertilizer Manufacturers Association er gjødselindustriens interesseorganisasjon i Europa)

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Scaffolding and basket

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Internal inspection, cont. 

No cracks were found! 

No cracks or fissures were found in any of the welds.

6 fissures were found in the base material of the tank wall. 

Theses were found in the heat affected zone below the welds.

The deepest fissure was found to be ca 1mm deep. The rest were less than 0,3mm and with varying length. All fissures were removed by careful grinding by a small angel grinder with flaps disk. A grinding wheel was not used. The fissures were caused by the welding method that was used, SAW (Submerged Arc Welding).

 

No welding repairs were performed.

No welding was carried out on any of the elements essential to the structural integrity of the tank.

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Bringing the tank back into operation 

Evacuating oxygen, 10. – 23. March 2009 :  

Filling of NH3 -gas, 27. – 30. March 2009 :  

Tank is closed and nitrogen purging is started. Nitrogen purging is closed when O2 content is below 0,5 %. Tank is filled with NH3 -gas. Compressors are started as well as transfer to the NA.

Filling of liquid NH3, March 30th – April 2nd 2009 :    

The ammonia plant starts production of NH3 with 0,1% water 17. March. Liquid ammonia is filled in batches as the oxygen level is lower than 0,5% Liquid NH3 is filled at a rate that decreases the temperature with slightly less than 1deg C/hour. The tank was readily cooled down April 2nd.

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Inspection of T-welds

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Status now   

The tank has been and is in very good condition. It is in operation and everything works as it should. Important modifications.   

We have installed 2 deepwell pumps to pump out the ammonia over the tank wall. No pipes are penetrating both inner and outer tank walls. We have installed state-of-the-art electronic safety system. 2 additional manholes were installed for future robot inspections

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No serious incidents – 11attempts were recorded We have recorded 1 personal injury – A contractor cut his leg with a knife during the external cleaning of the tank.


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We have had a 0-emission philosophy for the project. We have not had any serious emissions. We have had some few complains about ammonia smell from neighbour companies.

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New pumps

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The tank seen from inside

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Robot inspection, 1st generation Yara has been working to develop a method for inspection of atmospheric ammonia tanks for several years. •Inspection can be carried out during operation. •No ingress of oxygen, reduced risk for future ammonia SSC.

As the tank in Porsgrunn was to be opened anyway, it gave us a good opportunity to review the results from the remotely controlled robot inspection.  

Access to the inner ammonia tank was made by connecting a purpose made lock to the outer manhole. A remotely controlled robot was placed manually in the annular space, on the outside of the inner tank.

– Between 50 and 60m of weld were checked by the robot. – 9 indications were found 

Force Technology, our collaborating partner, claimed that none of the indications found looked like cracks.

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Lock used for getting access to annular space.

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FORCE was right when they claimed non of the indications were cracks. The indications found were basically surface roughnesses on the weld caps. The robot unmasked the fissure that was passed. During the internal inspection, the robot was able to unmask all fissures that was passed. TWI (The Welding Institute) in UK has verified the method Results from 1st generation robot inspection formed foundation for the development of 2nd generation.

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Robot inspection, 2nd generation 

Extensive work has been carried out in close cooperation with Force Technology to develop the robot for service and limit the need for manual work in the ammonia atmosphere The shortcomings that was recognized during 1st generation have been taken into account. Extensive testing will be carried out as well as training of the emergency force/gas divers/mechanics and operators at Force’ premises in Copenhagen. The concept will be employed for inspecting 1 ammonia tank at our Rostock plant in September 2011.

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Deployment concept. 2. generation robot.

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Deployment arm manoeuvring the robot past the inner manhole.

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Deployment arm positioning the robot on inner tank.

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Thanks for your attention!

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