An estimated 3,500 gallons of sulfuric acid was safely unloaded from a highway acid cargo tank, with no injuries, after it overturned on a central Arizona highway enroute to a copper mine. Company officials from the trucking company that owned the tank determined the damage was sufficient to make it unsafe to attempt to upright it loaded.
They did not have immediate access to a product transfer pump, so they brought in another empty tank and began to transfer product by gravity unloading. During this process, a leak developed around the gasket of the dome cover, discharging the 93% sulfuric acid onto the ground. The leak could be eliminated only by stopping the product transfer and allowing a partial vacuum to be re-established inside the tank. Once that was accomplished, the carrier determined that the closest product transfer truck was a vacuum truck out of Phoenix.
On this type of highway acid cargo tank, when it is equipped with a bottom unloading discharge valve, it will be on the bottom dead center of the tank. The liquid level was now below this valve, so it could not be used by the vacuum truck. Since the tank was lying about 45 degrees below horizontal over on its side, this meant that the dome cover, at top dead center, was below the liquid level of the acid. Thus, they could not open the dome cover to insert a pump-off hose.
The only other option in this instance is to undo all of the bolts holding the bottom unloading discharge valve onto the tank and remove it completely. This then allows you to insert a pump-off hose into the valve fitting hole and transfer the product. While awaiting the arrival of the vacuum truck, an engine from the Bagdad Fire Department arrived on the scene, even though the incident was over eight miles outside its town limits. The crew agreed to stand by during the product transfer operation for first aid and rescue, if necessary.
A mechanic from the carrier unbolted the bottom discharge valve and its connected plumbing, then set it aside to clear the way for product transfer. Upon arrival, the vacuum truck crew simply inserted a hose into the opening in the bottom of the tank and began to off-load the sulfuric acid. The mechanic was able to re-tighten the locking dogs around the overturned tank's dome cover to stop the leak, which resumed when the partial vacuum was broken by removing the bottom discharge valve.
A representative of the Arizona Department of Environmental Quality (DEQ) had arrived in the meantime, and discussed cleanup options with the carrier. The sulfuric acid had spilled onto a public right-of-way, and it was possible that motorists, hikers, or other innocent by-standers could unknowingly walk through the area in the future.
Due to the possibility of accidental exposure to the spilled acid, the DEQ representative required that the carrier remove all of the dirt containing any spilled acid. Arrangements were made by the carrier with the copper mine where the acid was going to accept the acid contaminated dirt, and the entire operation was accomplished before sunset that same day. The 3,500 gallons of sulfuric acid was pumped into the vacuum truck without incident, and the scene was turned over to the heavy-duty wrecker operators to upright the now-empty acid cargo tank.
Lessons Learned
- Hazardous materials leaks can develop in otherwise stable packages if valves or fittings are opened.
- Needed product-transfer equipment may not be available for hours at a remote scene.
- Allowing the carrier to make decisions that may result in additional spills and costs avoids subsequent "finger pointing" and arguments.
- In the absence of a hazmat response team, almost complete reliance needs to be put on carrier personnel for technical decisions and operations.
Stephen L. Hermann is hazardous materials coordinator for the Arizona Department of Public Safety.
