A Tank Truck Catastrophically Fails

This case history of the tank truck collapse has been reproduced with the permission of the American Institute of Chemical Engineers. Copyright 1993. It first appeared in Don t Become Another Victim of Vacuum, Chemical Engineering Progress, 1993. All rights reserved. [1] 

The plant operator opened the nitrogen valves that were both upstream and downstream of the plants nitrogen pressure regulator to pad the truck. The truck manifold that was in front the right rear wheels contained a pressure gauge that read the expected 20 psig (1.36 bar). The operator opened the proper liquid delivery valves, started the unloading pump, and left to handle other chores. The driver stayed with the truck, as was the normal procedure. 

Engineers determined that a nitrogen valve was opened to the hose connected to the tank trailer manifold. The nitrogen gas never entered because there was a closed valve atop the tank. It seems the truck driver misunderstood the recent nitrogen supply piping modification the open and closed positions on this oval handle ball valve. 

Investigators observed a device that appeared to be a combination safety relief valve and a vacuum breaker. The information on this chrome-topped instrument read, Press 26,700 

CFM 32.5 psig, and VAC Relief 1,750 1 psig. Despite the nameplate, the trailer was not equipped to deal with vacuum. [1] 

About 10 05 a.m., people in the area reported hearing a loud rumble noise and a number of employees gaMered to witness the catastrophic collapse of Me tank truck (see Fig-ure.s 2—i and 2-5). There were no injuries, no leaks of hazardous materials, and no damage to the plant receiving the feed stock. 

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