Keywords: ammonia storage tanks, risk based inspection, non-intrusive inspection, stress corrosion cracking, SCC
Several hundred atmospheric ammonia storage tanks are in operation in the world. Because of the large individual volumes of these tanks (>10,000 m3) and with the associated safety and health issues of the stored product, these tanks need to be properly operated, inspected and maintained.
The European Fertilizer Manufacturers Association (EFMA) has updated their 2002 guidance document on the safe and reliable inspection of atmospheric, refrigerated ammonia storage tanks. The most important parts of this EFMA document, together with other information on the subject of ammonia storage tank inspection and incidents, are included in this IFS publication.
In considering the tank inspection frequency, the Risk Based Inspection (RBI) approach is described. With this Risk Based Inspection approach a good understanding on how the relevant degradation mechanisms affect the probability of failure of the tank is achieved. Ammonia Stress Corrosion Cracking (SCC) is especially addressed. Also the other axis of the RBI Inspection Frequency Diagram, dealing with the consequence of failure, is discussed in more detail. For the latter the importance of the type of tank is explained.
Historically ammonia tank inspections were always internal inspections. A new development, on which much information is given, is the use of non-intrusive inspection methods. This non-intrusive in-service inspection method from the outside of the tank is an emerging technology that has major advantages over an internal tank inspection because it is easier, it does not affect the integrity of the tank (no need to warm up and open the tank) and it is less hazardous to carry out. Also the risk of introducing SCC during the opening of the tank is eliminated.
Furthermore the procedure on how to empty and refill an ammonia tank in a safe way is addressed in detail. This is especially relevant in the case of an internal inspection. The re-commissioning and decommissioning has to follow procedures which ensure the efficient removal of oxygen and a careful and uniform cooling and warm-up of the tank. This is important in order to keep the thermal stress to a minimum level and to reduce the risk of initiating SCC.
Finally information is also given on auxiliary tank equipment that needs regular monitoring between the major tank inspections.
Harrie Duisters, DSM Agro, Geleen, The Netherlands.
26 pages, 8 figures, 2 plates, 5 tables, 18 references.