Determining Inspection Strategies for Anhydrous Ammonia Storage Tanks – A Joint RBI and FFS Approach

Introduction

When developing and implementing an inspection strategy for any equipment, it is crucial to identify and evaluate all credible damage mechanisms thoroughly. When using RBI for ammonia storage, the inspection strategy strikes a balance between assessing the tank’s condition and managing the associated risks, including those related to decommissioning and recommissioning the equipment. Ammonia stress corrosion cracking (SCC) can develop in carbon steel material when exposed to anhydrous ammonia in the presence of tensile stress and oxygen. Ammonia SCC has led to failures of pressurized, slightly refrigerated, and ambient ammonia storage vessels, particularly in areas with significant residual stress, such as cold-formed heads and welds. Several factors affect susceptibility to ammonia SCC, with oxygen contamination playing a key role in both initiating and accelerating damage.

In determining the optimal inspection strategy for an ammonia storage tank, it is important to consider the potential for oxygen ingress during preparations for an internal inspection. Industry experience suggests that the risks of SCC and other safety hazards inherent in emptying and the subsequent warming of a refrigerated ammonia storage tank for out-of-service inspection may outweigh the risks of continued service with less frequent internal inspections. A formal risk assessment can help address these challenges, enabling the development of optimal inspection strategies and intervals, often focused on nonintrusive inspection methods.

Combining risk-based inspection (RBI) with fitness-for-service (FFS) can be a powerful solution for optimizing inspection planning for ammonia storage tanks. Fertilizers Europe’s RBI guidelines provide a qualitative risk assessment developed specifically for anhydrous ammonia storage tanks [1]. A proactive FFS analysis plays a crucial role in assessing the tolerance levels for crack-like flaws and predicting the potential growth of ammonia SCC using the design and operating parameters unique to the subject tank. In addition, the FFS analysis provides valuable information for the inspection strategy including inspection location guidance and acceptance criteria when indications are found. The definition of acceptable versus unacceptable flaw sizes affects the selection of appropriate NDE methods and their sensitivities. By integrating the results from both the RBI and FFS assessments, a well-rounded, data-driven recommendation can be developed to optimize safety, reliability, and costs.

Methodology

Risk-based Inspection Assessment

Fertilizers Europe’s Guidance for Inspection of Atmospheric, Refrigerated Ammonia Storage Tanks provides a qualitative RBI assessment that was developed through a Fertilizers Europe member survey of 48 tanks and focused on factors that affect both the probability and consequence of failure. The Fertilizers Europe approach involves a series of questions regarding operating history, commissioning procedures, tank secondary containment design, fracture susceptibility, and oxygen and water content in the ammonia to determine a POF and COF for the tank. The resulting POF and COF can then be used with the Fertilizers Europe risk matrix to determine the tank’s risk level and corresponding recommended inspection interval. The risk matrix categorizes risk from very low to high, with recommended inspection intervals ranging from 20-25 years to immediate action recommended, respectively.

For ammonia storage tanks, a thorough review of operating procedures and historical data is crucial to assess the tank’s vulnerability to ammonia SCC. The review should consider historical water content measurements, potential for oxygen ingress, and systems designed to keep these parameters within guidelines.