Siphoning Destroys a Tender Tank

A chemical plant complex used large volumes of brackish water from an adjacent river for once-through cooling. Occasionally, trace emissions of caustic soda were present in the effluent river water, and the company was concerned with pH excursions. An engineer was assigned to make modifications to reduce the opportunity of having environmental insults. 

The intentions to improve the environment were noble, but the simple vent system design possessed an unrecognized flaw that allowed a minor overfill situation to suddenly and completely destroy the vessel. The acid tank project was rather simple, so the proposal was accepted and no one asked pointed questions like, What happens if the acid storage tank is overfilled by a tank truck (As we shall see, many unforeseen problems have occurred on equipment so simple that a detailed examination of the proposal may have been considered unnecessary in years past.) 

About a year after the system was put in service, the tank was filled via a tank truck, instead of being supplied by the usual pipeline. As the delivery tank truck was unloaded, the acid level rose in the small storage tank. The company representative wanted to top off the tank. 

Many atmospheric and low-pressure tanks appear big and strong. But don t believe it Think about a can of baked beans and compare the pressure resisting strength of the can s shell and lid to that of a low-pressure storage tank. If a baked bean can was assigned a strength of one, then  

Obviously, a multidisciplined safety review committee may have detected the problems of the ill-advised use of a combination vent/overflow line, but this type of collapse is viewed as somewhat of an oddity. Several variations of this type of collapse have been reported on low-pressure and atmospheric tanks. Another incident occurred when a company requested the operating team to fill a tank as high as possible for storage needs, so they ignored the high-level alarm. After the tank level reached the overflow, the liquid started pouring out of the overflow faster than it was being pumped in and the tank collapsed. [4] 

Sfmrcr Trevor Kletz, Module 001, 1987. Used with permission. 

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