The seven stages of chemical technology from concept to industrialization (Issue 66/100) -- Special section on safety

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Seven stages of chemical engineering technology from concept to industrialization (Issue 66/100) — Engineering translation: A special focus on safety facility design. Dear readers: Hello! In the previous issue, we discussed pipeline stress analysis and 3D piping design. Starting from this issue, we will devote two issues to discussing the preparation of several special sections in the engineering transition phase. Let’s first discuss the safety facility design section, the only one among the four sections of current chemical projects that still needs to be submitted for review by the relevant government authorities. In accordance with the provisions of the Work Safety Law, the design of safety facilities for construction projects used in the production, storage, and handling of hazardous materials must be submitted to the relevant authorities for review. Chemical plants fall into this category. Only after the review is approved is the safety technology framework of the entire project officially recognized and accepted by all parties. If the review is not approved, the project cannot commence construction legally. The safety special review is an important checkpoint in the process of moving a project from design to construction. If this hurdle cannot be overcome, all subsequent plans will have to be postponed. In this issue, we will discuss the core logic, key points for preparation, and approaches to review in the security section. I. The core of the safety section is the “protection layer.” Preparing the safety section does not involve simply putting together the HAZOP report and the interlock logic diagrams. Its core concept is the \"layer of protection\" – from inherently safe process design, to basic process control systems, to safety instrumented systems, to physical protection devices, and to emergency response measures; each layer of protection requires that its installation standards and design parameters be specified in dedicated sections. The logic behind the protection layer is layered defense. The innermost layer features intrinsically safe design – eliminating or reducing hazards at the source of the process, such as using less toxic materials in place of highly toxic ones, operating at lower temperatures and pressures, and reducing the amount of hazardous chemicals present on-site. One layer outside is the basic process control system – DCS, which is responsible for keeping the process parameters within the normal operating range and preventing deviations from occurring. Further outside are alarms and operator intervention – when the DCS is unable to control the deviation, alarms alert the operator to intervene manually. Further outside is the Safety Instrumented System – when operator intervention cannot prevent the deviation from increasing, the SIS automatically triggers interlock actions to bring the unit into a safe state. Further outside are physical protection devices – safety valves and burst discs, which prevent the equipment from rupturing due to overpressure by releasing pressure when interlocks also fail. The outermost layer is emergency response – fire dikes, fire suppression systems, and emergency evacuation measures, designed to mitigate the consequences once an accident has occurred. What needs to be done in the security section is to clearly describe each layer of protection: what this layer of protection is, where it is located, what its design parameters are, in what scenarios it provides protection, and whether it operates independently from other layers of protection. By obtaining the dedicated document, review experts can verify layer by layer: if one layer fails, whether the next layer can take over. Before preparing the safety section, a systematic analysis of the hazard factors associated with the device is required. Fire, explosion, poisoning, chemical burns, asphyxiation – it is necessary to identify individually which equipment, which operational positions, and which working conditions correspond to each type of hazard factor. For each identified major hazard scenario, a comprehensive analysis is conducted following the logical sequence of \"cause → consequence → existing protective measures → risk level → recommended measures\". A lot of work has already been done on HAZOP analysis; the safety section needs to systematically integrate the findings of HAZOP, rather than simply attaching the HAZOP report at the end of the section. II. How to implement the conclusions of HAZOP and LOPA: For each high-risk scenario identified through HAZOP analysis, corresponding protective measures must be specified in the safety section. If an additional SIS interlock circuit is required for a certain scenario, and the LOPA analysis determines the required SIL level, the safety documentation must specify the sensor configuration, actuating elements, reset methods, and testing schedule for that SIS circuit. This is the part where the most problems arise in compiling the security section. The HAZOP report suggests adding interlocks, but no corresponding interlock circuits can be found in the safety section – this discrepancy is immediately apparent during expert reviews, and corrections are required. I have encountered this more than once in actual projects: the HAZOP analysis and the dedicated report are prepared by separate teams, each working on its own. During the review meetings, experts would ask repeatedly: Which recommendation on page X of the HAZOP report corresponds to which item in the dedicated report? Neither group could answer. The recommended approach is to have the safety section prepared under the leadership of the process engineering team, rather than leaving it to the safety team to handle on its own. Because only the process engineering team understands best the background of each scenario in the HAZOP analysis, as well as the technical rationale behind each recommended action. The responsibility of the safety professionals is to review and verify that the specific documents meet the requirements of regulatory standards. Process takes precedence over oversight; safety has the authority to audit – each specialty performs its own role. For each SIS interlock circuit, at least several things need to be clearly stated in the safety section. What are the trigger conditions, which sensor detects them, and what is the trigger value? How are the sensors configured—single-channel or redundant? If redundant, is it majority voting or two-out-of-two voting? What is the actuator – a shut-off valve or a relief valve, and is the fault condition FC or FO? What is the SIL level, and what is the basis for determining this level—the LOPA analysis results. What is the reset method—manual reset or automatic reset? Is manual reset performed at the control room operator station or must it be done on-site? What is the testing cycle length – and can it meet the availability requirements of the SIL level? With all this information clearly written down, the complete picture of the SIS circuit becomes clear. III. Design of the relief system: The safety section must provide detailed information on the design of the safety valves and emergency relief systems. The installation of each safety valve requires answering several basic questions. What is the object to be protected—reactors, towers, storage tanks, or heat exchangers? Under what conditions will overpressure occur—fire, refrigerant interruption, power outage, fully open control valves, or mistakenly closed outlet. What are the discharge amounts under each operating condition? The maximum value shall be used as the design discharge amount for the safety valve. Whether the discharge destination is safe – whether it is discharged into a flare system or directly into the atmosphere; whether the discharged material is toxic or harmful, and whether it has been separated or treated. When calculating the discharge volume under fire conditions, the surface area of the container exposed to the flames must be taken into account. In the case of a refrigerant interruption, it is necessary to take into account the total amount of vapor generated by the reboiler at its maximum heating capacity. In the case of a power outage, steam generated by residual heat must be taken into account. In the fully open condition of the control valve, it is necessary to take into account the additional amount of steam generated by the difference between the maximum flow rate when the valve is fully open and the normal flow rate. What should be the setting pressure of the safety valve? It is usually set at the design pressure of the equipment or slightly below that pressure, as determined by relevant standards. The relationship between the set pressure and the release volume must be appropriate – if the set pressure is too low, even slight fluctuations in the normal operating pressure can trigger release, resulting in an unplanned shutdown ； If the set pressure is too high, the safety valve cannot activate in time when overpressure occurs, and thus fails to provide protection. The design of the discharge pipeline should also be described in a separate section. Does the diameter of the discharge pipe meet the requirements for the maximum discharge flow rate? Is the back pressure in the discharge pipeline within the range permitted by the safety valve? Excessively high back pressure can prevent the safety valve from returning to its normal position after it opens. Does the drainage pipe have sufficient support and expansion compensation—thermal expansion also occurs in the pipe itself during high-temperature drainage. Is the location of the discharge pipe outlet safe? The outlet must not face the operation area or electrical equipment, and its height must meet the requirements for proper dispersion of emissions. The safety section also requires a list of safety valves, detailing the tag number of each valve, its installation location, the equipment it protects, the set pressure, the discharge volume under various operating conditions, the designed discharge volume, the medium discharged, and the discharge destination. IV. Major Hazard Sources and Safety Distances: The safety section requires the identification and classification of the major hazard sources associated with the facility. In accordance with the provisions of \"Identification of Major Hazard Sources for Hazardous Chemicals\", all hazardous chemicals involved in the project are identified, their inventory levels are calculated to determine whether they exceed the critical threshold, and it is determined whether they constitute major hazard sources along with their classification level. Hazardous sources are classified into four levels, with the classification criteria depending on the ratio of the stock of hazardous chemicals to their critical quantity. Level 1 is the highest grade, with the strictest regulatory requirements ； Level 4 is a general level. The grading results directly affect the configuration requirements of the security monitoring system, the requirements for developing emergency response plans, and the scale of investment in security measures. Compliance with safety distances is also a key focus of the dedicated review. Whether the distances between the devices within the facility, as well as between the facility and surrounding structures – such as residential areas, public facilities, and critical infrastructure – meet the requirements of fire safety regulations. The safety distance is the first line of defense in design, used to prevent accidents from spreading and to protect external safety. If the safety distance is insufficient, it is necessary to explain in a separate section what compensatory measures have been taken—whether it is the addition of firewalls or other equivalent security measures. Fire protection design is also an important part of the safety section. Firewater volume, capacity of fire pump rooms, layout of fire protection pipelines, coverage area of fire hydrants and fire monitors, as well as the installation of foam extinguishing systems and dry powder extinguishing systems – whether the design of these fire protection facilities meets the regulatory requirements, covers all areas of the facility, and can ensure a continuous supply of water in the event of an accident. V. Procedures for reviewing the safety section: After the preparation of the safety section is completed, the production and operation entity shall submit an application for review to the emergency management department, along with materials such as the design documents for safety facilities, proof of the design unit’s qualifications, and the safety pre-assessment report. After accepting the case, the emergency management department organizes an expert review panel; these experts are selected from provincial or municipal databases of safety experts and typically include specialists in areas such as process safety, equipment safety, instrument control, and fire protection. During the review meeting, the experts will refer to the safety section, and in conjunction with the HAZOP report and LOPA analysis report, verify one by one whether there are reliable protective measures for each major hazard scenario, and whether each SIS interlock circuit has a complete design plan. Review experts usually focus their attention on the following aspects. Whether the protection layer is independent and effective — whether the sensors and actuators of the SIS are separate from the DCS, and if there are any shared components. Whether the SIL level matches the interlock configuration — whether circuits at SIL2 are equipped with redundant sensors and independent actuators, and whether the testing intervals meet the availability requirements. Are the calculations for the safety valves and relief systems correct? Has the amount of discharge under various operating conditions been properly calculated, and is the destination of the discharged fluid safe? Are the control measures for major hazard sources sufficient? Do primary and secondary major hazard sources be equipped with independent safety instrument systems, and are the monitoring signals transmitted to the control room? Whether the fire protection design complies with the regulations – whether the calculation of the fire water volume is correct, and whether the fire pump room meets the power supply requirements for Class 1 loads. The comments from the review experts are mandatory; responses and corrections must be provided for each item. Upon approval of the review, the emergency management department issues a review approval document. This approval document is an important legal basis for the lawful construction of the project; construction shall not commence without such approval. To deal with censorship, there are several practical experiences that can be shared. First, before submitting it for review, conduct an internal preliminary review first, checking each item from the perspective of experts. Problems identified during internal reviews can be corrected before submission, preventing delays in the review process when pointed out by experts during the evaluation session. Second, it is necessary to clarify the relationship among the HAZOP report, LOPA report, and the special report. Given the limited time available to the review experts, if the special report indicates clearly the correspondence between each HAZOP recommendation and its implementation status – that is, which page and which item in the special report corresponds to the recommendation made on which page of the HAZOP report – the review process will be much more efficient. Third, the qualifications of the design firm and its personnel must be prepared in advance. The organizations responsible for preparing and designing the safety section must possess the appropriate qualifications, and the design personnel must have valid certificates as registered safety engineers or hold relevant professional qualifications. At the review meeting, supplementary materials were requested due to qualification issues; this affected not the technical review itself, but rather the overall approval timeline for the project. Preview for the next issue: Issue 67 – Special features on fire safety, environmental protection, and occupational health. The section on safety has been covered. In the next issue, we will discuss three other special topics: fire protection design, environmental protection facility design, and occupational health and safety facility design. Although these three special sections are no longer submitted separately for review by the relevant government authorities, they remain essential components for the compliant development of the project. The core calculations in fire protection design, how to implement the requirements set out in environmental impact assessment approvals for environmental protection facilities, and what the priorities should be for occupational health protection – these will be covered in the next issue.