STORAGE TANKS ARE available in many shapes: vertical and horizontal cylindrical; open top and closed top; flat bottom, cone bottom, slope bottom and dish bottom.
Large tanks tend to be vertical cylindrical, or to have rounded corners (transition from vertical side wall to bottom profile, to easier withstand hydraulic hydrostatically induced pressure of contained liquid. Most container tanks for handling liquids during transportation are designed to handle varying degrees of pressure.
Over the past few years there have been numerous catastrophic failures of storage tanks, one of the most recent being the Buncefield Oil Depot accident in Hertfordshire, UK; another notorious one being that which occurred in Boston Massachusetts USA, in, 1919. The large tank had only been filled eight times when it failed, and resulting wave of molasses killed 21 people in the vicinity. The Boston molasses disaster was caused by poor design and construction, with a wall too thin to bear repeated loads from the contents. The tank had not been tested before use by filling with water, and was also poorly riveted. The owner of the tank, US Industrial Alcohol, paid out $300,000 in compensation to the victims or their relatives.
There have been many other accidents caused by tanks since then, often caused by faulty welding or by sub-standard steel. However, storage tanks also present another problem, surprisingly, when empty. If they have been used to hold oil or oil products such as gasoline, the atmosphere in the tanks may be highly explosive as the space fills with hydrocarbons. If new welding operations are started, then sparks can easily ignite the contents, with disastrous results for the welders. The problem is similar to that of empty bunkers on cargo ships, which are now required to use an inert gas blanket to prevent explosive atmospheres building up from residues.
A hazard is a physical situation with a potential for human injury, damage to property, damage to the environment, to capital investment or some combination of these. Hazards can be identified through a review of the physical properties and product characteristics of the product to be cleaned.
Tank operators/owners can conduct enhanced follow-up inspections/audits to ensure not only that any violations that had been cited were corrected, but also to check on whether the contractor is addressing and implementing the issues raised. One way this occurs is to identify the hazards and eliminate the risk to an acceptable level. In addition there should be continued monitoring during ongoing works for potential hazards which may not be highlighted at the commencement of work.
Regulatory tank inspection is the key
For aboveground tanks, regular visual tank inspections and NDE by qualified inspectors and/or technicians is probably the best measure that can be taken to minimise the risk of tank failures. This has been the industry norm and is still the best approach to tank evaluation.
Visual inspections can catch most of the problems with tanks and tank systems. Corrosion inside the tank cannot be seen, but most internal corrosion leaks develop slowly and will be noticed as a wet spot on the tank exterior by visual inspections before there is a release of oil. Installing inexpensive leak containment trays under oil tanks will offer protection between inspections and prevent damage from most corrosion leaks.
The great value of early detection of deficiencies is to be able to recommend corrective action before there is a significant problem. Spill avoidance and containment , accidents are all preventable.
G. Mclean, SGS Inspection Services Nigeria Limited