Tales of Storage Tank Failures, Damage and Inspections of Storage Tanks

Introduction

Tanks are ubiquitous and essential in industrial societies. They store hydrocarbons, industrial goods such as sulfuric acid, agricultural products, and other products including waste. When they fail, the consequences can be dire if products/volumes they store are explosive and/or toxic. 

The sulfuric acid storage tank failure in 2001 in Delaware City^[1] make the need for inspections clear – one person was killed, eight others were injured, and significant damage ensued to aquatic life. For tanks to perform properly, design and service knowledge, along with a great deal of know-how, are needed. Some examples of the know-how needed and the lessons learned while investigating tanks are presented in this article. Though the damage and failures discussed here have had less impact, they underline that a great deal of practical knowledge and attention to detail are needed to operate tanks reliably. The tank tales presented are:

1. Roof challenges. 
2. Welding the drain nozzle in large steel tanks.
3. Concentrated sulfuric acid tanks require internal inspections.
4. Corrosion Under Insulation (CUI) of Tanks.
5. Cleanliness is next to godliness when repairing tank bottoms.
6. Non-intrusive Non-Destructive Tests.

1. Roof Challenges

Fixed-Roof Tanks Are Designed to Fail at the Shell-to-Roof Weld

A fixed-roof tank exploded while personnel were refurbishing equipment upstream of the tank. The roof was torn off and one of the persons working upstream was severely injured.

The work upstream had resulted in a pressure surge. The vents were supposed to allow the pressure to blow off, but the sudden pressurization was such that the undersized vent could not transfer gases quickly enough. The failed tank roof to shell weld had a torn/overloaded appearance. It appeared to be free of preexisting flaws. The pressure at which the roof separated was estimated to be 0.6 psi. This value appeared to be low, but many tanks are constructed according to API 650 “Welded Tanks for Oil Storage.” They intentionally have a weak roof-to shell seam so that if an internal overpressure develops, the design allows the roof to separate from the vertical shell to prevent failure of the bottom seams and the tank’s “rocketing” or propelling upward.^[2]

Fixed-Roof Condition after Removing Insulation

After removing the insulation from the roof, inspection personnel identified numerous through thickness openings in the roof (see Figure 1). This type of damage has been observed in refineries, chemical plants, and petrochemical sites.