Explosion and fire at the polyethylene wax plant of Marcus Oil & Chemical Company in the United States

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Summary of the explosion and fire incident at the polyethylene wax plant of Marcus Oil and Chemicals in the United States. This article analyzes the causes of the accident from the perspective of change management, based on the investigation conducted by the U.S. Chemical Safety and Hazard Investigation Board (CSB) into the explosion and fire that occurred at Marcus Oil and Chemicals’ polyethylene wax processing plant southwest of Houston, Texas. It also proposes measures for improvement. 1. It is described that at 5:50 p.m. on December 3, 2004, an explosion and fire broke out at the polyethylene wax processing plant of Marcus Oil and Chemical Company, located southwest of Houston, Texas, United States. Several employees heard a loud explosion; about 45 seconds later, another even more powerful explosion occurred. The fire department arrived at the scene about 5 minutes after the explosions took place, and it took 7 hours after the explosions (by midnight) to extinguish the three large fires. Three firefighters suffered minor injuries while putting out the fire. Several local residents were injured by shattered glass; the explosion broke the windows of buildings and vehicles, and it caused damage to building structures within a 400-m radius. Ceilings and lighting fixtures in nearby buildings, commercial facilities, and churches fell down as well. The 7# tank, with a diameter of 3.7 meters, a length of 15 meters, and a weight of 23 tons, was thrown 68 meters away by the shock wave of the explosion; it eventually collided with the warehouse of another company in the vicinity (Figure 1 shows the change in the position of the 7# tank as a result of the explosion). Some other larger fragments from the explosion reached nearby communities; for example, a 9-kilogram steel plate was found in a residential area 152 meters away ； 54 kg of steel plates were found on the grass at 274m away ； A 1 kg steel plate was found in a residential area 400 meters away. Figure 1: The explosion caused the position of Tank No. 7 to shift by 45 m2. Process description, physical evidence, and tests related to the explosion fire at Marcus Oil and Chemical Company. Marcus Oil and Chemical Company was established in 1987; it primarily produces high-density polyethylene wax, which is widely used in the production of coatings, adhesives, polishing agents, in rubber manufacturing, and in textiles, with an annual production volume of over 113,000 tons. 2.1 Process Description: The simplified flow diagram of the polyethylene wax production process at Marcus Oil and Chemical Company is shown in Figure 2. Paraffin from paraffin tank cars is pumped into the washing tank, where the raw material settles; impurities settle at the bottom of the tank. The paraffin in the upper part of the washing tank is extracted using hexane and other hydrocarbons, after which it solidifies into polyethylene wax particles that are then packaged. The extracted hydrocarbons are stored in storage tanks for sale. Nitrogen is used for protection throughout the entire production process to prevent the molten paraffin from coming into contact with oxygen in the air. The oxidizing action of oxygen causes the bright-colored paraffin to change color, which is unacceptable. Figure 2: Schematic diagram of the polyethylene wax production process. Operators regularly clean the residue at the bottom of the washing tank, which contains a high amount of impurities; once this residue has solidified, it is stored in a warehouse for further processing. 2.2 Operators handling the residual liquid manually load the solidified residual liquid into Tank No. 4, which is equipped with high-temperature steam coils, for melting. It is then pumped to Tanks No. 6 and 7 for temporary storage prior to further processing. Steam is circulated through the coils inside the tanks to maintain the temperature of the liquid paraffin at around 149°C. Operators use the nitrogen system to pressurize Tanks 6 and 7; the pressure difference then causes the liquid residue in Tank 4 to flow through a riser pipe to the pump inlet. The pump transports this residue to a treatment unit, where further removal of hydrocarbons takes place. After refinement, the residue solidifies into granules, which are then packaged. The solidified refined residue is shown in Figure 3. Figure 3: Paraffin spherical particles after residue treatment. 2.3 Nitrogen system – The nitrogen is produced on-site using a small nitrogen generator and stored in two large pressure vessels. The maximum pressure of the nitrogen system is 0.83 Mpa (gauge pressure). The nitrogen pressure regulator reduces the pressure to 0.28–0.48 Mpa (gauge pressure) before supplying it to tanks #6 and #7. Since the load of small nitrogen generators is not sufficient to meet the requirements for transferring materials from the tanks, the pressure of nitrogen sometimes drops to the minimum level required for such transfers. While the operator waits for the nitrogen generator to refill the nitrogen tank, the delivery of paraffin will be delayed. The operators used a temporary hose to connect the compressed air system with the nitrogen distribution system of the nitrogen generator. Since the high-capacity air compressor could quickly raise the pressure in the nitrogen system by supplying additional air, this resolved the aforementioned production delay issue. Finally, the operator replaced this temporary hose with a permanent pipeline and valves; this change eliminated the need for the unconventional procedure of connecting a temporary hose every time the pressure in the nitrogen system was too low. However, the operator did not realize that introducing air directly into the nitrogen system would result in the presence of oxygen in the nitrogen storage tank. Following the explosion accident, the U.S. Chemical Safety and Hazard Investigation Board (CSB) analyzed samples of the gas in the nitrogen generators, nitrogen pressure regulators, and nitrogen storage tanks, paraffin, residues in the processing equipment, and metal samples from Tank 7. It is determined whether the nitrogen generator is operating under design conditions by testing the gas exiting the undamaged nitrogen generator. The test results confirmed that the gas sample contained 92%-96% nitrogen and 4%-8% oxygen, meeting the design specifications of the device. Gas samples taken from the nitrogen storage tanks were analyzed to determine the purity of the nitrogen supplied to tanks #6 and #7. This sample contains 82% nitrogen and 18% oxygen. Therefore, CSB concluded that the compressed air connection lines installed by Marcus Oil and Chemical Company to pressurize the nitrogen system caused the nitrogen in the nitrogen storage tank to become contaminated with oxygen. CSB’s further conclusion is that the oxygen content in nitrogen is sufficient to support combustion. CSB tested the nitrogen pressure regulator using various values for inlet pressure and flow rate, in order to determine the possible operating pressure of Tank 7 at the time of the accident. The originally set pressure was 0.46 Mpa (gauge pressure). Further tests confirmed that the pressure relief valve was functioning properly, and the accuracy of the pressure gauge on the valve was less than 0.007 Mpa. 2.4 Physical evidence and inspection: CSB visually inspected Tank No. 7, the recovered tank body, and the head; no equipment nameplates were found, and Marcus Oil & Chemical Company was unable to provide documents regarding the design, manufacture, and safe operating pressure of Tank No. 7 or the other tanks (No. 5, No. 6, and No. 8). In accordance with the ASME Boiler and Pressure Vessel Code, CSB calculated the safe operating pressure for Tank No. 7 to be 0.55 Mpa (gauge pressure). CSB found that Marcus Oil and Chemical Company had modified tanks #5, #6, #7, and #8. To ensure that the temperature of the paraffin inside the tank remains above its melting point, a hole with a diameter of 609 mm was made in the head of each tank. A steam heating coil was installed in this hole, after which it was welded shut. The welding repair plate for Tank No. 8 is shown in Figure 4. Figure 4: Welding patches for Tank No. 8. Although the patch from Tank No. 7 could not be found, CSB investigators concluded that it was similar to the patches on the other 3 tanks. Since the weld of the patch plate failed to meet the industrial quality standards for general pressure vessel manufacturing, this weld failed during the accident. Marcus Oil and Chemical Company did not use qualified welders or appropriate welding methods to re-weld the patch plates on the container heads, and installed a nozzle for a steam pipeline on the tank. The company’s staff also admitted that no hydrostatic test was conducted to inspect the container after the welding work was completed. CSB tested the flammability and reactivity of the waste paraffin samples. The flash point of paraffin is 110°C. The paraffin-like samples did not exhibit exothermic behavior; therefore, CSB concluded that uncontrolled chemical reactions were not among the factors contributing to this accident. 3. Analysis of the causes of the explosion at Marcus Oil and Chemical Company. The CSB conducted an investigation into the explosion at Marcus Oil and Chemical Company; based on the relevant physical evidence and test results, the CSB determined that the most likely scenario for the accident was as follows: 1) The operators used nitrogen containing 18% oxygen to fill tanks #6 and #7, instead of the nitrogen with an oxygen concentration of no more than 8% as originally planned ； 2) The internal pressure of Tank No. 7 (0.46 Mpa (gauge pressure)) may exceed the strength of the welds on the patch plates of the tank. This weld has completely failed; the patch plate has broken away from the tank ； 3) The pressure inside the tank is rapidly released through a 609 mm diameter opening. Hydrocarbon vapor, compressed air, and hot liquid paraffin are all ejected from the opening together ； 4) Sparks were generated when the patch plate struck the concrete base, which may have ignited the paraffin and hydrocarbon vapors ； 5) The oxygen concentration inside Tank #7 was sufficient to cause the flame to propagate back into the tank; an internal explosion resulted in the tank’s head breaking into several pieces ； 6) The 2,268 kg tank was blown apart and struck a nearby idle piece of equipment, eventually coming to rest in a warehouse 137 meters away ； 7) Burning polyethylene wax scattered onto the warehouse and other equipment, igniting combustible materials. The resulting fire burned for nearly 7 hours. Among them, CSB pointed out that three modifications made by Marcus Oil and Chemical Company led to this accident. Modification 1: An opening was made in Tank No. 7 to install a steam heating coil inside the tank. The weld thickness of the patch was less than 20% of the overall thickness of the patch, and it contained numerous cracks. The effects of these defective welds were twofold: on one hand, they reduced the strength of that portion of the tank by 75% ； Meanwhile, a lack of weld metal and poor weld quality may lead to the development of fatigue cracking, further reducing the strength of the weld ； Change 2: Based on the test results from the nitrogen pressure regulator, the internal pressure of tank No. 7 is most likely 0.46 MPa (gauge pressure). The operator stated that this nitrogen pressure reducing valve is normally set at 0.31 MPa, but the pressure was increased artificially in order to allow paraffin to flow into the processing unit. The higher pressure in tank 7# may have caused the weld at the failure point to deform at the time of the accident. Change 3: Marcus Oil & Chemicals linked the nitrogen system and the compressed air system, enabling rapid pressurization of the nitrogen system. During the treatment of waste paraffin, the decolorization that occurs as the product is exposed to air did not draw attention. However, the management failed to assess the risks arising from process changes. Compressed air is used to pressurize the nitrogen system, resulting in nitrogen containing up to 18% nitrogen. This nitrogen concentration is sufficient to support the combustion of paraffin and hydrocarbon vapors inside the tank. 4. Lessons learned from the explosion at Marcus Oil and Chemical Company: Through the CSB’s analysis of the causes of the accident, it is clear that it was the three changes that led to the incident. Chemical production is constantly subject to various changes in order to meet market demands, improve production efficiency, optimize operating conditions, enhance safety, or achieve other objectives. However, these changes are a double-edged sword; while they bring benefits, they can also increase risks. Many catastrophic accidents in the chemical and petrochemical industries have been caused by improper modifications to processes or facilities. Statistics show that approximately 80% of process safety incidents can be traced back to “inappropriate changes”; therefore, managing changes essentially means preventing and controlling potential accidents. An important cause of the accident at Marcus Oil and Chemical Company was the lack of a system for managing changes in the plant; no proper safety reviews were conducted on the process systems that underwent changes, and there was no one to oversee and approve such changes. The lack of a change management system allowed unreviewed changes to proceed smoothly through design, installation, and commissioning. The personnel making the changes lack training and sufficient experience; they fail to realize the serious consequences that such alterations to the process system could have. 4.1 What is Change Management? Change Management (MOC) is an important element of the Process Safety Management System. It refers to the planned control of permanent or temporary changes in chemicals, process technologies, equipment, procedures, and operational processes, involving the determination of the type, severity, and implementation steps of such changes. Changes related to process safety in chemical plants and petrochemical plants can generally be divided into four categories: process technology changes ； Change in operating procedures ； Factory facility changes ； Organizational change. Among these, changes in process technology, changes in operating procedures, and changes in plant facilities generally have a direct impact on process safety, while organizational changes affect process safety indirectly. However, in China, changes are generally classified into: same-type replacement, minor changes, and changes to process equipment and management (technical changes). The management processes for changes include: change categorization ； Establish a change management team, conduct process hazard analysis, handle applications and approvals, implement changes, carry out tracking and verification, prepare closure reports, update and archive documents, and provide training and communication regarding the changes made. The general steps are shown in Figure 5. Figure 5: Change management flowchart 4.2 Risk management during the change process. The focus of change management is to control the risks that arise during the change process; risk control in this context is divided into the phase of proposing change plans, the phase of implementing the changes, and the phase of restarting operations after the changes have been made. 1) Risk control during the proposal stage of changes: After a personnel member proposes a change, a corresponding plan for that change is developed. Before such a plan can be approved by higher-level management, it is necessary to conduct a proper hazard analysis on it. This may include carrying out Hazard and Operability Studies (HAZOP), Layer of Protection Analysis (LOPA), What-If analyses, checklists, Event Tree Analysis (ETA), Fault Tree Analysis (FTA), Bow-tie analysis, quantitative risk assessment (QRA), Failure Mode and Effects Analysis (FMEA), as well as analyses related to reliability, availability, and maintainability (RAM). Since the existing processes and equipment have all been designed through calculations and evaluations, when making new changes, we cannot simply consider the hazards related to the process systems and equipment involved in those changes; we also need to take into account the impact of these changes on the entire system, adopting a holistic approach. 2) Risk control during the implementation phase of changes: When implementing a change plan that has been assessed for risks, tasks such as hot work, work in confined spaces, work at heights, and isolation/locking are involved. At this stage, risks need to be identified through methods such as Job Safety Analysis (JSA), Task Risk Analysis (TRA), and Preliminary Hazard Analysis (PHA); operation safety is managed in accordance with the procedures within the HSE system. 3) Risk control during the restart phase after changes: After the implementation of the changes, a Pre-Startup Safety Review (PSSR) must be conducted prior to restarting operations. It is necessary to inspect the construction site where the changes were made, verify that the installation of the changed items has been completed, and ensure that the construction, installation, and equipment specifications are in accordance with the previously approved change documents and drawings. Additionally, relevant procedural documents pertaining to safety, operation, maintenance, and emergency response must be prepared or revised. A checklist can be prepared for inspections prior to the commissioning of the modified project; before starting operation, each item listed in the checklist should be checked one by one. Only when all items have been completed as required and the conditions for operation are met can production begin. The several changes at Marcus Oil and Chemical Company failed to identify risks at various stages, nor did they take appropriate actions based on those identified risks, which ultimately led to the accident. 4.3 Updating documents after the change is completed: Even after the modified process system is put into operation, the work related to the change is not yet complete; it is necessary to update and save all drawings and documents that are affected by the change. Typically, the documents that need to be updated include the plant layout plan, equipment layout diagrams, piping and instrumentation diagrams, operating procedures, maintenance procedures, emergency response plans, and training materials. The factory also needs to keep documents related to the changes themselves, such as change orders, drawings, records of pre-production safety inspections, written records of training or notifications to relevant personnel, etc. In accordance with OSHA’s requirements for PSM, facilities must revalidate the effectiveness of the process hazard analyses conducted in previous years every five years. An important part of this process is reviewing the changes that have taken place over the past five years, and for this review, relevant documentation related to those changes is necessary. Currently, many enterprises are facing issues where various documents do not correspond to the actual on-site conditions—for instance, electronic PID diagrams not matching reality, or equipment operation manuals not aligning with the actual equipment present. These discrepancies pose challenges to subsequent production management, conducting HAZOP analyses, and training employees who work with newly installed equipment. In fact, this kind of problem is caused by a vicious cycle wherein the data isn’t updated in a timely manner after each **minor change** is implemented. 4.4 After a change is made, it cannot be simply assumed that the change is complete once the relevant documentation is archived; rather, it is necessary to track the effects of the change to determine whether the intended objectives of that change have been achieved Have there been any other secondary effects on the process system? Has it affected other devices in the system? Only after tracking confirms that the purpose of the change has been achieved, there are no side effects, the process safety information related to the change has been updated, relevant personnel have been trained, and relevant documents have been archived, can it be considered that the change is complete. 4.5 Management of temporary changes Temporary changes can also lead to catastrophic accidents. Although temporary changes generally do not require the updating of drawings and documents as is the case with permanent changes, they still need to go through the usual procedures for changes, including review, approval, and safety inspections prior to implementation. Temporary changes include: temporarily adding or removing equipment from the process system, using temporary pipes or valves in the process system, and temporarily bypassing critical alarms or interlocks, etc. Temporary changes need to be reverted to their original state after a certain period of time, so there is a time limit; the factory can set its own deadline for such temporary changes, which can be several days or weeks. If this deadline is exceeded, an extension can be granted, but written approval is required for such an extension. Even if there is a delay, the period usually should not exceed 6 months; otherwise, a new temporary change application will need to be submitted when the currently approved temporary change expires. 5. Conclusion: Any change has the potential to cause the process system to deviate from its original design intent. Even seemingly minor changes, if not managed properly, can lead to catastrophic consequences. This is why it is important for companies to establish change management procedures and standardize the change management process. Companies need to assign dedicated personnel to manage changes, establish detailed change management procedures, clearly classify changes, and implement risk management at each stage of the change process in order to minimize risks. 6. References Center for Chemical Process Safety (CCPS), 1992. Plant Guidelines for Technical Management of Chemical Process Safety (Revised Edition), American Institute of Chemical Engineers, (AIChE) ； Center for Chemical Process Safety/ (CCPS), 1993. Guidelines for Engineering Design for Process Safety, (pp. 539) American Institute of Chemical Engineers, (AIChE) ； Center for Chemical Process Safety/ (CCPS), 1993. Process Safety Management and Accident Prevention ； CASE STUDY Explosion and Fire at Polyethylene Wax Processing Facility, No. 2005-02-I-TX June 2006 ； AQ-T3034-2010 Guidelines for the Implementation of Process Safety Management in Chemical Enterprises ； “Guidelines for the Implementation of Process Safety Management in Chemical Enterprises” (AQ-T3034-2010) ； “Guiding Opinions on Strengthening the Safety Management of Chemical Processes”, General Administration of Work Safety, Document No. San [2013] 88.