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The seven stages of chemical engineering technology from concept to industrialization (Issue 69/100) — Engineering translation: HAZOP analysis and SIL classification within the design review system. Dear colleagues: Hello! In the previous issue, we discussed energy efficiency assessment and soil and water conservation. Starting from this issue, we will dedicate two issues to discussing the design review system during the engineering translation phase. In this issue, we will first discuss the application of HAZOP analysis and SIL classification in the detailed design phase. The term HAZOP analysis was mentioned in issue 51 earlier – HAZOP during the process package phase, with an emphasis on identifying critical hazard scenarios and determining interlock protection strategies. Now, we have entered the detailed design phase. The depth of PID analysis has increased, and the equipment layout and piping routes have been specified. Many operational and maintenance issues that couldn’t be elaborated upon during the process package phase have now come to light. HAZOP during the detailed design phase is the final systematic review of the plant’s safety and operability before the construction drawings are prepared. 1. What are the differences between HAZOP during the detailed design phase and during the process package phase? During the process package phase, the PIDs are still at a preliminary design stage; many auxiliary pipelines, specific valve configurations, and details regarding instrument installation have not yet been finalized. The analysis at that stage focused on whether the protection strategies were appropriate – whether the interlock circuits were independent enough, whether the discharge capacity of the safety valves was sufficient, and whether there was adequate protection against critical hazard scenarios. During the detailed design phase, the HAZOP was conducted. The PIDs had been developed to the level of construction drawings; all auxiliary pipelines, valve configurations, instrument installation locations, and interlock reset methods had been determined. The analysis at this stage focuses on whether the protective measures can be implemented – whether there is a corresponding sensor for each trigger condition in the interlock logic diagram, whether there is a corresponding actuator for each interlock action in the PID, and whether there are corresponding auxiliary pipelines to support each operational step in the PID. The two cannot replace each other. The HAZOP conducted during the process package phase addresses whether the design direction is correct; the HAZOP carried out during the detailed design phase determines whether all design details have been adequately addressed. If only HAZOP is conducted during the process package phase and HAZOP is skipped at the detailed design stage, many detailed issues in the construction drawings can only be identified on site. II. What is reviewed in the HAZOP analysis during the detailed design phase? The HAZOP analysis in the detailed design phase covers more specific and detailed aspects compared to the process package stage. Several review focuses are unique. The first priority is the feasibility of operation and maintenance. During the process package phase, one must analyze whether “this interlock can effectively protect the plant”. In the detailed design phase, it’s necessary to ask: After the interlock is triggered, how do operators verify that all safety conditions have been met? Is the reset button on the control room operating station or on-site? If the operator is required to go to the site for verification, how long will it take to reach that location, and are there any potential hazards along the route? Is the position of the manual valve easy to operate? Is there enough space for the handwheel? Can one hold the handwheel and turn it while standing on the platform? The second focus is the in-depth analysis of specific operating conditions. Safety risks during driving and parking conditions are often analyzed rather superficially at the package design stage, as many auxiliary pipelines have not yet been designed. During the detailed design phase, the start-up pipelines, shutdown pipelines, nitrogen purging interfaces, and drain and vent interfaces have all been depicted on the P&ID. At this point, it becomes feasible to analyze the safety risks associated with both the start-up and shutdown processes one by one. Is the interface location for nitrogen purging correct? Does the purging process cover all equipment and pipelines that need to be purged? Under power outage conditions, which equipment remains operational and which has stopped after the interlocks activate; and in what order must the operator perform certain manual operations? Under conditions of steam interruption, which material pipelines are affected in terms of safe operation when the tracing system is shut down, and how long does it take to restore steam supply? The third focus is the ‘last mile’ verification from HAZOP to the interlock logic diagram. After completing the HAZOP analysis during the process package phase, the interlock logic diagram was prepared. During the detailed design phase, the interlock logic diagram is further developed and the selection of instruments is finalized. At this point, three documents—the HAZOP report, the interlock logic diagram, and the PID diagram—must be compiled together for a thorough verification: For each interlock loop recommended in the HAZOP report, it must be confirmed whether there is a corresponding logical representation in the interlock logic diagram, as well as corresponding sensors and actuators shown on the PID diagram. For each trigger condition on the interlock logic diagram, is there a corresponding sensor in the PID? Do the sensor’s tag number, range, and interlock trigger value match? For each action shown in the interlock logic diagram, does there correspond a corresponding actuator in the PID – whether it is a shut-off valve or a relief valve? Are the tag numbers identical, and are the fault conditions correct? The fourth focus is the reconfirmation of the isolation between SIS and BPCS. The process package phase has already established the requirements for the independence of SIS and DCS. During the detailed design phase, once the instrument selection and cable routing plans have been determined, it is necessary to verify again whether the sensors of the SIS and those of the DCS are installed in separate mounting bases, whether the cables belonging to the SIS and those belonging to the DCS run along different tray layers or follow different routes, and whether the power supply circuit for the SIS is independent from that of the DCS – the DCS can lose power during a blackout, but the SIS must continue to operate. These physical isolation measures may not be represented very intuitively on PID and instrument loop diagrams; however, during the detailed design phase, they can be verified more clearly through 3D models and cable routing diagrams. III. Organization of HAZOP in the detailed design phase: The team composition for HAZOP during the detailed design phase is roughly the same as that in the process package phase, but there are several roles that are particularly important at this stage. The importance of operation representatives becomes even more evident during the detailed design phase. The operators at the process package stage are primarily responsible for ensuring compliance from an operational logic perspective ； During the detailed design phase, the operation representative can refer to the already refined PID to point out the actual issues encountered in operation one by one: Is the position of this valve convenient for operation? Is the height of this sampling port appropriate? To which location must the interlock be reset, and is that location safe? These details are something that designers can never fully replace from the perspective of frontline operators. The role of instrument engineers in the detailed design phase is more significant than in the process package phase. The refinement of the interlock logic diagrams, the selection of instruments, and the specific implementation of SIS and DCS isolation solutions all require in-depth analysis by instrument engineers. 3D model reviewers also play an important role in the HAZOP during the detailed design phase. Many operability issues—such as space for valve operation, maintenance access routes, and escape paths—are not visible on a PID, but can be clearly seen in a 3D model. If a 3D model can be displayed simultaneously during a HAZOP analysis, allowing participants to examine the feasibility of operational interventions for each deviation scenario in relation to that model, the outcome is much better than relying solely on PID diagrams. IV. Review and refinement of SIL classification: A preliminary SIL classification was completed during the process package phase, determining the SIL level and basic configuration requirements for each SIS interlock circuit. During the detailed design phase, once the instruments have been selected and specific failure data for the instruments to be purchased are available, it is necessary to reevaluate the SIL level. Based on the data of the selected instruments, the actual safety integrity level of each SIS loop is calculated to determine whether it meets the SIL level required by the LOPA analysis. If the review reveals that the actual SIL does not meet the requirements, the configuration needs to be adjusted: the sensors should be upgraded from single-channel to redundant configurations; the actuators should be equipped with certain stroke testing functions; and the testing interval should be reduced from one year to six months—until the actual SIL meets the requirements. During SIL review, special attention should be paid to the assessment of common-cause failures. Readers asked this question specifically in issue 52 earlier. During the detailed design phase, the specific layouts of instruments and cables are determined. At this point, a more substantive analysis of common-cause failures can be conducted: whether the two redundant sensors are installed in different nozzle blocks, whether the two redundant signal cables run through different cable tray levels or routes, and whether the redundant actuators are powered by different power circuits. The degree to which these physical isolation measures are implemented directly affects the actual reliability of the redundant configuration. V. Integration between HAZOP and 3D model review: The HAZOP analysis and 3D model review during the detailed design phase are not two separate tasks; their interrelationship deserves special mention. The focus of HAZOP analysis is on “process safety”—that is, whether protective measures can effectively prevent accidents when process parameters deviate from their normal values. The focus of the 3D model review is on \"space safety\" – whether there are any collisions between equipment and pipelines in terms of space, whether there is sufficient space for operation and maintenance, and whether fire exits and escape routes are unobstructed. When analyzing certain deviation scenarios, the two naturally intersect. For instance, when conducting a HAZOP analysis for the scenario of “leakage in the heat exchanger tube bundle”, it is necessary to verify whether there is sufficient space for maintenance when the tube bundle is removed, and whether the direction in which the manhole opens is safe. These aspects cannot be determined from the PID diagram alone; however, they become immediately apparent when referring to the 3D model. For instance, when conducting a HAZOP analysis for the scenario of “flammable gas leakage”, it is necessary to verify whether the installation location of the flammable gas detector is near potential leak points and whether its coverage area is sufficient. The installation height and angle of the detector can be intuitively verified in a 3D model. By combining HAZOP analysis with 3D model review, both the spatial and process dimensions of safety analysis can be covered. The next issue will focus specifically on the practical steps involved in reviewing 3D models; for now, we’ll just touch on this briefly. Preview of the next issue, Issue 70: 3D model review – the three stages of 30%, 60%, and 90%. We’ve covered the HAZOP analysis during the detailed design phase. Next time, we’ll discuss another important component of the design review system—the review of 3D models. What is reviewed at the 30% model review stage, what at the 60% stage, and what at the 90% stage? Who are the participants at each stage? At which stage is it most effective for operators and maintenance personnel to get involved? To be continued in the next issue.