Photo Gallery E1: Tank Leaks And Repairs
Most tanks are fabricated from sheets of flat or rolled mild carbon steel, and are subject to various forms of corrosion. Generally, an internal coating is applied to isolate the underlying steel, although like most forms of protection, failure of the coating at some future date will occur. Since the predictive assessment of tanks is rarely performed, the first indication of a problem is typically when a leak or failure occurs. For domestic water storage tanks, the annual requirement to physically clean and sterilize the interior should provide a regular opportunity to also inspect for other physical deficiencies, although this minor action is quite often overlooked
Leaks at the pipe to or from a water storage tank also provides a good indication to a potential problem, given that the same level of corrosion activity will impact all such metal surfaces. A failure at either the tank or pipe serving to or from the tank provides an indication to the deterioration of the other component as well. Generally, the type of failure and its location is greatly dependent upon the type of tank or pressure vessel involved. Dedicated fire sprinkler water storage tanks, since the water is more stagnant, experience less corrosion activity than domestic cold water storage or those providing combination domestic water/fire reserve service.
Old riveted pressurized fire storage tanks from the 1920s typically first show their problem at the rivets, and at the front half of the tank where water movement is greater. Compressed air tanks and oil storage tanks show their greatest wear at the bottom where condensed water collects. Domestic cold water storage tanks more heavily deteriorate between the high and low water marks, and then show a lesser amount of deterioration below the low water line. Expansion tanks, are similar, and exhibit the greatest deterioration where water level rises and falls.
The consequences of failure vary widely. A leak at a rivet is likely to take years to advance whereas corroded through internal supports and cross members can bow out the side of a 20,000 gallon tank to a point approaching catastrophic failure. Smaller compressed air storage tanks are extremely vulnerable to exploding from condensed water acting against their generally thinwall construction; proving far more dangerous than the largest tanks by killing those in the nearby vicinity when the failure occurs.
Given the large size of most water storage tanks and the fact that most seem to have had the building constructed around them, addressing a corroded tank problem can be challenging. Ultrasonic testing performed in a standard grid will not only define its condition, but also provide a 3D visualization of its inside wall profile if sufficient thickness measurements are taken. With its condition known and sufficient wall thickness remaining, the best remedial step is to remove any previous coating or rust and apply a more effective alternative. For those much older tanks badly neglected for decades, replacement is often the only choice.
