Procedures to prevent inadvertent mixing must be followed
There is no room for complacency when delivering and receiving large amounts of chemicals. Case in point: In October 2016, a delivery of a chemical from a tank truck into a storage tank—what most would view as a simple operation—went horribly wrong at MGPI Processing Inc. in the small town of Atchinson, Kansas (pop. 11,000). MGPI produces food-grade alcohol, corn oil, wheat starches, and proteins for food and nonfood applications. After an MGPI operator showed the driver of the tank truck the fill line for the storage tank, the operator left the area, and the driver mistakenly connected his cargo of sulfuric acid to an outdoor storage tank containing sodium hypochlorite (concentrated bleach). The incompatible mixture produced a cloud of toxic chlorine gas so powerful the driver could not safely break the connection and had to flee the area. Eventually, 4,000 gallons (gal) of sulfuric acid mixed with 5,850 gal of sodium hypochlorite spread in a greenish-yellow cloud over the facility and then headed northeast over Atchinson. About an hour after the improper connection was made, emergency responders arrived and closed the discharge valve on the truck. The accident caused more than 140 people to seek medical attention; six were hospitalized. Approximately 11,000 people were forced to evacuate the area or shelter in place.
The accident was investigated by the U.S. Chemical Safety and Hazard Investigation Board (CSB). Established under the Clean Air Act, the CSB is a federally funded, independent, nonregulatory agency that looks into all aspects of chemical accidents. In this case, no super sleuthing was needed to determine the immediate cause of the event—the truck driver connected the discharge valve to the sodium hypochlorite storage tank’s fill line valve when he should have made the connection to the sulfuric acid fill line valve, which was 18 inches away. According to the CSB, both intake lines were physically identical, and neither was properly labeled with the name of the chemical intended for that line. But that simple error and the catastrophe that followed was the consequence of a collection of deeper problems about how chemical transfers are managed, the adequacy of transfer equipment, and personnel responsibility and training.
Following closure of the investigation, the CSB produced a case study report and an 11-minute video of the incident. The video is professionally produced, concise, informative, and highly recommended. CSB’s major message is that whenever large amounts of chemicals are involved, no operation, even a “simple” delivery, should be conducted without strict adherence to standard operating procedures that ensure safety.
Related incidents
As part of its investigation, the CSB found that from 2014 through 2017, unloading incidents involving hose connections to incorrect tanks occurred frequently, but most commonly involved compatible materials overflowing from tanks. Less common are incidents similar to the MGPI incident where two incompatible materials are inadvertently mixed due to incorrect tank connections. However, the CSB found that since January 1, 2014, there have been eight incidents where inadvertent mixing of incompatible materials resulted in a dangerous chemical reaction. These occurrences resulted in 44 injuries and the evacuation of 846 people. One incident virtually identical to the MGPI event occurred in Holly Hill, Florida, in 2015. There, the driver of a tank truck filled with sodium hypochlorite connected the delivery hose to a fill line to two storage tanks containing sulfuric acid. The mixture of the two materials caused a release of gas that overcame the operator and several local residents.
Human factors
CSB’s case study indicates that a combination of human, equipment, and operational factors contributed to the MGPI incident. Specifically:
- The proximity of the sulfuric acid fill line to the sodium hypochlorite fill line increased the likelihood of an incorrect connection during chemical unloading. Physically isolating or using distance to separate fill lines can lower the risk of incorrect connections. Physical separation can be especially important when receiving various classes and types of chemicals. At the MGPI facility, five fill lines, each for a different chemical, were in close proximity.
- Dust caps for the sodium hypochlorite and acetic anhydride fill lines were not locked when delivery occurred; the cap for the sodium hypochlorite line could not be locked because it was missing a part. MGPI started using dust caps to prevent product contamination or tampering issues with all receiving lines in response to a 2010 food safety inspection. Though the practice of locking dust caps was not required for process safety or environmental reasons, when executed properly, it was the only physical barrier preventing drivers from incorrectly connecting to the wrong fill line.
- Of the five fill lines in the area, only propylene oxide had a pipe marker at its connection point. Had MGPI placed pipe markers or identification tags on all the fill line connection points (or, at the very least, on the sodium hypochlorite fill line connection point), it might have been immediately obvious to the driver that he was connecting the discharge hose to the incorrect fill line.
- The MGPI operator said that he pointed out the fill line to the driver and that the driver acknowledged the location. But the driver said the operator did not point out the location. MGPI’s operating procedures indicate that the operator must observe the connection between the discharge hose and fill line. The CSB found that some operators were unfamiliar with procedures indicating that a correct connection must be verified. The CSB concluded that operating procedures and work practices at the facility were not aligned.
- MGPI’s training program requires operators to annually review all procedures, including those for unloading. In addition, MGPI said it conducts talks where operators discuss their observations about chemical deliveries and any changes necessary. Operators interviewed told the CSB that some procedures did not call for verification of the correct fill line, and some operators were unaware that the procedure required operators, not truck drivers, to open the fill line valve. The CSB said these gaps were indicative of the inadequacy of MGPI’s training program.
- After reviewing the unloading procedures for the delivery truck company, the CSB found that the driver did not meet several requirements. For example, drivers must “carefully check to make certain that the material to be transferred will be going into the correct vessel.” Second, drivers must “continuously monitor transfer.” In this case, the driver made the connection and then returned to the truck’s cab. The CSB said the driver was “likely unfamiliar” with these two steps in the tank truck unloading procedure.
- The major gap in equipment was the absence of instrumentation in MGPI’s process control system that would have automatically shut down the transfer of chemicals in the event of a process deviation after detecting a temperature, pressure, or level exceedance in the sodium hypochlorite tank.
Benefits of collaboration
According to the CSB, the incident could have been avoided in any number of ways. However, looking to the future, the CSB believes that some form of collaboration that clearly defines the roles of deliverers and receivers of chemicals would be effective in avoiding incidents. The CSB states:
“For example, on the day of the incident, the truck driver opened the valve to the fill line, although, according to [the delivery company’s] procedures, the MGPI operator was supposed to. This action conflicted with [the delivery company’s] procedure. Had the procedures been developed together, or agreed upon by both parties, the roles might not have been switched. Procedures should also establish a process that requires facility personnel to be physically present during deliveries because they are more familiar with their equipment. Having both facility personnel and drivers monitor the chemical unloading process allows either party to identify concerns and increases the likelihood for safe execution.”
Companies respond
Immediately following the incident, MGPI made temporary changes to its transfer equipment and unloading area to reduce the potential for a similar incident. These included placing dedicated locks with separate keys on the different fill line caps, replacing the fill line caps with caps that have different locking mechanisms, placing new (or more securely affixing existing) pipe markers closer to fill line connection points, placing new color-coded tags on the fill lines, and updating the chemical unloading procedures. Additionally, the delivery company worked with MGPI to select and install new couplings on the sulfuric acid fill line and sulfuric acid delivery hoses. These couplings share the same unusual size and shape so that only the correct delivery hose can be connected to the sulfuric acid fill line. The couplings are also colored differently from other couplings.
In addition, both MGPI and the delivery company revised their chemical unloading practices and procedures. MGPI now prohibits the start of unloading within 20 minutes of shift change and requires that a safety observer be present when connecting and disconnecting a cargo tank hose to a chemical fill line. Procedures also require operators to inspect fill lines to ensure all dust caps are locked before and following deliveries. The delivery company created a new sulfuric acid unloading procedure specific to MGPI and now requires its drivers to complete a tanker pre-unloading checklist with customers.
Recommendations
Based on the investigation, the CSB made the following general recommendations that could be applied anywhere large quantities of hazardous chemicals are delivered.
- For receiving facilities:
- For all chemical unloading activities requiring human interaction, by either facility or delivery personnel, identify and address human factors that may increase the potential for an incorrect connection.
- Evaluate chemical transfer equipment and processes (e.g., fill lines, transfer valves, piping, and receiving tanks) and, where feasible, install and configure safeguards such as interlocks and mitigation measures in the process control system to maintain safe operations during chemical unloading. The control system should monitor and respond to hazardous process conditions (e.g., temperature, level, pressure, or airborne concentrations) and automatically shut down the transfer of chemicals and other processes in the event of an unintended reaction or release during chemical unloading.
- Design or modify chemical transfer equipment to ensure fill lines for incompatible materials are adequately separated.
- Work with chemical distributors to select hose couplings and fill line connections with uniquely shaped and color-coded fittings for each chemical.
- Ensure that pipe marking and identification of transfer equipment (e.g., fill lines, valves, transfer piping, and tanks) are clearly identified and accurately represent the material contained in the equipment.
- Work with chemical distributors to conduct a risk assessment, and based on the results, develop agreed upon procedures for chemical unloading and emergency operations to ensure responsibilities are clearly defined.
- For chemical distribution companies:
- Ensure drivers are fully aware of the location and use of all emergency shutoff mechanisms.
- Evaluate the need to train drivers to don appropriate personal protective equipment and to respond to chemical spills or releases during unloading operations.
Routine and risky
The CSB points out that in 2016, more than 39.9 million tons of product were delivered to customers every 8.4 seconds. It’s difficult to view such a common occurrence as anything but routine, a tendency the CSB views as dangerous.
“Because chemical deliveries are so common at fixed facilities, the CSB urges facilities and chemical distributors to adopt the Key Lessons and Recommended Practices from this Case Study and work collaboratively to implement controls and practices that prevent or reduce the opportunity for inadvertent mixing incidents,” the CSB concludes.