HAZ MAT: ON THE LINE
BY STEPHEN L. HERMANN
ACID CARGO TANK OVERTURN ACCIDENT
A recent southern Arizona accident damaged a highway cargo tank transporting sulfuric acid so seriously that the product had to be off-loaded before the vehicle was uprighted. The incident lasted more than 28 hours, and one wrecker company worker was slightly injured when an air line connector failed and his arm and leg were sprayed with the corrosive material.
The tank was transporting several thousand gallons of 93-97 percent sulfuric acid when it apparently entered a sharp curve too fast and overturned. Although it was traveling fairly slowly, the impact of the overturn significantly damaged the tank. Emergency responders and officials of the company that operated the tank were concerned that it could not safely be uprighted while loaded and made arrangements to off-load the contents.
Although the carrier did have nearby an empty trailer into which the load could be transferred, a pump or compressor was not readily available. The accident was almost directly over a dry stream bed, where the transfer vehicle could be located about 50 vertical feet lower than the overturned tank. The decision was made to commence gravity off-loading from the damaged tank.
A regular large-diameter off-loading hose of sufficient length was not available, but a quantity of much smaller-diameter hose was obtained. It was attached to a fitting on the tank that was normally used to pump in air for off-loading the vehicle. Since the tank was on its side on a slight uphill grade, the air fitting was now located in an ideal position to allow the transfer of almost the entire tank contents.
TRANSFERRING THE SULFURIC ACID
The assessment and planning took a number of hours, and it was just before midnight when the transfer of the sulfuric acid began. Since the normal product off-loading plumbing was not being used, responders had to field-improvise by using fittings on the tank for other than their normal functions. Also, gravity-unloading and the use of a much smaller-than-normal off-loading hose are unusual procedures and there had to be a consensus on which methods to employ.
The scene was secured for the balance of the night while the product started to slowly flow into the off-loading trailer. The carrier guarded for the night the scene, which was just off the edge of the lightly traveled highway.
At sunrise the next day, the tank was closely examined. Several more indications of serious damage were found. It appeared that the entire tank, which did not have a separate frame or subframe, was bent slightly from the accident. Additionally, several welds on the tank-strengthening “ring stiffeners” were broken. This damage reconfirmed the determination that the tank might fail if it were lifted with the approximately 15-pound-per-gallon sulfuric acid still onboard.
The small-diameter product transfer hose had the acid flowing extremely slowly. It was calculated that at the current rate, it would take an additional 24 hours or more to off-load the overturned tank. Following the arrival on the scene of the tank owner`s terminal manager, along with the owner of the heavy-duty towing and wrecking company, plans to speed up the off-loading and the method for uprighting were discussed.
The towing firm had suggested using air bags to upright the tank once the sulfuric acid was transferred, but they weren`t needed. It was agreed that air from the air bag compressor could be used to pressurize the tank and greatly speed the unloading rate of the acid. Since the normal product off-loading valve was now located above the liquid level of the remaining product, it was decided to continue to off-load the acid through the normal air inlet line.
Compressed air to speed the transfer rate was pumped in through the safety relief rupture disc fitting on top of the tank, since the pressure relief disc itself had been blown by the hydrodynamic surge of the acid during the overturn. No proper size direct-attachment metal couplings were available, so a field adapter was fashioned for the compressor air line to the rupture disc fitting hose.
At this point, a safety error was made. The exclusion zone around the tank itself up until this time had been fairly small, since the spilled sulfuric acid had completely soaked into the ground and had been neutralized. The spill area was checked with Draeger tubes and found not to be releasing any sulfuric acid vapors. It was not taken into consideration that once air pressure was introduced into the tank, a failure of either the product transfer line or air coupling could allow sulfuric acid to be sprayed rather than just gravity flow.
After several hours of operation, one of the towing company workers at the scene was standing within several feet of the air line field coupling when it failed. Since the rupture disc fitting being used to introduce the air was still below the product surface, sulfuric acid was now spraying from the broken fitting as a result of the internal air pressure inside the tank. Several ounces of the acid hit his arm and leg.
The worker was immediately helped out of his clothes, and large quantities of water were used to dilute the sulfuric acid splash on his skin. Following brief emergency treatment at the scene, he was taken by ambulance to the closest medical clinic, where the sulfuric acid burns were found not to be serious. He was treated and released within several hours.
Even though the worker was a hazardous materials-trained member of his hometown`s volunteer fire department, he was not wearing proper personal protective equipment (PPE). The safety officer and incident commander failed to take into account the potential for acid to be released into the environment if the air-line coupling being used to speed the off-loading failed. Normally, compressed air is introduced into an upright tank at the top, where there would be no product to leak if the connection failed. Also, the safety officer failed to ensure that personnel walking into the potential hot zone were wearing the proper PPE, consisting of Level B suits and gloves or splash aprons, jackets and gloves, along with face shields. Personnel making or undoing hose and fitting connections within the hot zone must wear this equipment. No respiratory protection was deemed necessary due to a lack of readings on the colorimetric detection tubes used to check the atmosphere above the spilled acid.
UPRIGHTING THE TANK
The broken fitting was repaired and the transfer operation completed. Due to the orientation of the tank, it still contained several hundred gallons of product, but it was determined that the vehicle could be safely uprighted with this product still inside. As the tank was slowly lifted and came up off the ground, the damage on the down side was inspected to ensure that the lifting could safely continue.
Once the tank was back on its wheels, it was closely examined. It was determined that it could be safely towed the approximate one mile to its normal off-loading location and the remaining several hundred gallons of acid would not have to be off-loaded at the scene. Following uprighting of the tank, a front-end loader belonging to the mining firm that was destined to received the acid was used to spread dirt over the immediate accident scene area to completely cover any residue of the spilled acid. The calcium carbonate-equivalency of the soil in this area was high, and the soil itself acted as a neutralizing agent so that additional neutralizing chemicals did not have to be employed.
The incident had taken more than 28 hours to resolve and, as a result of an improper relaxation of safety procedures at the scene, one worker had been slightly injured. The tank itself was so seriously damaged in the accident that it has not yet been determined if would be economically feasible to repair it.
1. When employing nonstandard procedures at a hazardous materials incident, all personnel must take extra time to think through the implications of possible equipment failures. The normal reasoning process failed to protect the injured worker.
2. When compressed air is used to facilitate product transfer, the hot zone must be significantly expanded to protect against product transfer line failures that may result in the area`s being sprayed with the chemical.
3. The safety officer must not allow himself to become complacent and ignore the use of proper PPE in or near the incident hot zone.
4. Sector or division commanders or supervisors must closely monitor their personnel and not allow them to take unsafe actions.
5. Thorough safety briefings need to be conducted throughout prolonged incidents, to ensure all personnel understand and comply with proper safety practices. n
Emergency responders may face unsuspected dangers from propane tanks damaged in accidents. An ultrasonic thickness detector was used in this incident to discover a crack in a 10,200-gallon tank that could have failed catastrophically with major loss of life.
About 3 a.m., during a rainstorm, the propane highway cargo tank ran off the road and overturned in the median on Interstate 8 north of Yuma, Arizona. I arrived at the scene about 5:30 a.m. and, along with the fire chief from a nearby department, visually examined the tank. A flammable gas detector determined no leaks of the 9,100 gallons of propane.
However, the tank had received a major dent on its front head, in the heat-affected zone (HAZ) of the head-to-shell weld. This HAZ is typically the weakest portion of the tank, due to unrelieved stresses from the welding process. For this reason, it was determined to pump off as much of the propane as possible before attempting to upright the tank. The vehicle owner didn`t have a pump-off truck close by, so he contracted with two Yuma propane companies to actually handle the product transfer. The carrier also dispatched a safety officer to the scene. He arrived several hours later. The Yuma companies sent two trucks to the scene and began off-loading propane.
REEVALUATION OF THE DAMAGE
A close inspection of the dented head raised additional concern about the tank`s structural integrity. With an ultrasonic thickness detector, the dented area was examined for internal cracks. The hand-held detector bounces an ultrasonic sound wave off the tank`s inside wall and can detect cracking or defects in the tank metal. After 30 minutes of checking, we detected a crack.
The head was one-quarter-inch-thick steel, and the instrument indicated the crack was .027 inch deep and about 38 inch long. We phoned a pressure vessel expert at the state`s largest cargo tank repair facility. It was agreed that since this was an older tank and was constructed of quenched and tempered steel, the crack had probably penetrated the hardened outer surface of the steel plate and stopped.
The crack was checked every five minutes for the next 20 minutes and did not grow. Due to heating from the sun, the tank`s internal pressure increased from 115 pounds per square inch (psi) to 120 psi during the two hours it took to off-load the first 2,100 gallons from the tank. Since stress on the crack increased as the internal pressure increased, immediate steps had to be taken.
One of the two companies conducting the off-loading had the equipment to “flare” the tank, and the company`s manager was transported in a highway patrol vehicle, with red lights and siren, to bring the items to the scene. When a propane tank is flared, a pipe or flare stand is hooked up to a hose from the tank at a safe distance and propane is burned off at the end of the pipe. This simultaneously accomplishes two things:
1. It reduces the internal pressure. As a liquefied compressed gas converts to a vapor during the flaring, the remaining liquid cools. This “autorefrigeration” reduces the internal pressure, since there is a direct relationship between product temperature and pressure.
2. It reduces the amount of flammable gas left in the tank so that if it does fail catastrophically, the fireball won`t be as large.
The tank internal pressure dropped, reducing stress on the crack, as we burned the propane off. We consulted with the safety officer from the tank owner and agreed that the safest way to lift the tank once the internal pressure was reduced was with a heavy construction crane. By using a crane, stresses in the tank are better equalized by lifting straight up from two points, thus not concentrating stresses on the crack.
After flaring for four hours, tank pressure was reduced to below 50 psi, and it was deemed safe to lift the tank. However, due to the possibility that additional unseen damage might have occurred on the downside of the tank or that the crack might have grown and the tank might fail, the number of people in the danger area was reduced. The owner`s safety officer was told he could pick three persons to visually examine the tank as it was lifted. All other persons other than the crane operator would be evacuated a safe distance during the lifting operation. After all but the skeleton crew at the tank were safely positioned, the crane operator slowly uprighted the tank.
The propane pump-off trucks were then repositioned and the balance of the product was safely transferred. The detection of the cracked tank enabled us to avoid a potential calamitous tank failure and loss of life. The unanswered question for future operations, however, remains: How do you know if a propane tank has suffered a crack and could fail at any moment?
1. Hazardous materials emergency response teams need some field method of assessing the residual structural integrity of pressure vessels to detect possible cracks that could lead to catastrophic failures.
2. Responders must continuously reassess the incident for the worst-case scenario and have contingency plans ready for any eventuality.
3. Knowledge of the autorefrigeration effect in reducing the internal pressure of a propane cargo tank by flaring product can be critical in a technically complex incident.
4. The importance of seeking assistance from, and working with, industry cannot be overemphasized in hazardous materials emergency responses.
5. If highway cargo tanks suffer severe damage in an accident, they may have to be lifted with a crane instead of heavy-duty wreckers, to prevent overstressing them and possibly causing a catastrophic tank failure. n
Less than 100 gallons of 93-97 percent sulfuric acid spilled in the initial overturn incident, which caused serious damage to the highway cargo tank. (Photos by author.)
n STEPHEN L. HERMANN is hazardous materials coordinator for the Arizona Department of Public Safety and Arizona`s senior state-on-scene coordinator for hazardous materials emergency response. He is past national chairman of COHMED, the national organization of state and local hazardous materials enforcement officers, and past chairman of the Commercial Vehicle Safety Alliance Hazardous Materials Committee. Hermann has a bachelor of science degree in explosive technology and is a graduate of the U.S. Naval School Explosive Ordnance Disposal, U. S. Army Command; General Staff College; and the U.S. Army War College and is a hazardous materials specialist for his agency and a state Division of Emergency Services Hazardous Materials technical course graduate.
(Left) The 15-pound-per-gallon sulfuric acid load would have placed a severe stress on the tank. It therefore had to be unloaded by gravity to another tank since no transfer pump or compressor was available. (Right) The sulfuric acid flowed by gravity from the damaged tank to the empty vehicle, placed downhill. The unloading went very slowly, since the carrier had to use small-diameter hose due to the distance.
After determining that the tank could fail if it were lifted while loaded, crews decided that it could be unloaded by using plumbing features in a nonstandard fashion.
The structural integrity of the tank had been severely weakened during the overturn accident, and it probably would have failed catastrophically if an attempt had been made to lift it while loaded.
In the unloading process, an air line coupling failed, spraying a towing company worker with acid. He was quickly decontaminated and suffered no permanent injury.
(Left) The 10,200-gallon propane cargo tank overturned during a heavy rainstorm on Interstate 8 between Phoenix and Yuma, Arizona, with no initial leaks. (Right) While rolling over in the single-vehicle accident, the front head of the cargo tank forcibly hit the ground, buckling the steel in the head-to-shell weld area. (Photos by author.)
(Top left) Using an ultrasonic thickness detector to check for any cracking in the “cold-worked” dent area revealed an interior crack of the pressure vessel. (Top right) While waiting for the arrival of equipment to reduce the tank`s internal pressure, propane was pumped out to smaller cargo tanks to reduce the amount of flammable gas remaining in the overturned tank. (Middle) Hazardous materials technicians light the “flare,” to burn off propane, reduce the tank`s internal pressure, reduce stress on the crack, and keep the tank from totally failing. (Bottom) The internal pressure in the tank prior to flaring was 127 psi. After several hours of burning off propane, the internal pressure had dropped to less than 50 psi, radically reducing stress on the internal crack.