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The seven stages of chemical engineering technology from concept to industrialization (Issue 47/100) —— Technology finalization: PID milestone review. Dear friends: Hello everyone! The previous issue detailed the eight types of information that need to be carried by PID. The PID diagram is complete; pipes, valves, instruments, control loops, interlock logic, equipment connections, and sampling for analysis – everything that needs to be represented is included. But finishing a painting doesn’t mean it’s done right. This issue is about PID review. If the question addressed by a PFD review is “Is this process scientifically and engineering-wise sound?”, then the question addressed by a PID review is “Can this unit be operated and maintained safely and conveniently?”. The core of PFD evaluation lies in the three balances of data and process rationality, while the core of PID evaluation is operability, control rationality, material suitability, and intrinsically safe design. I. The review team must include operators and maintenance personnel; PFD reviews should focus on process and equipment-related aspects, while PID reviews require a broader scope. My experience is that a review team should include at least the following roles. The process engineering specialist is responsible for explaining the design intent and addressing questions regarding processes and controls during reviews. In the field of equipment, it is necessary to determine whether the specifications of the equipment’s connections match those listed in the data sheet, and whether the locations of the manholes and access ports are appropriate. Instrumentation specialty: assess the feasibility of implementing control loops and interlock circuits, and check the independence of DCS and SIS. In the field of safety, it is necessary to verify whether the safety valves, flame arresters, and interlock protections are properly installed. Specialization in pipeline materials: verify that the material grade is suitable for the medium. Most importantly, it is necessary to involve operators and maintenance personnel with practical experience in the process. Operators can tell you about problems that designers sitting in offices would never think of. The operating height of this valve is too high; it needs to be turned on and off several times per shift, so an operating platform is required. The handwheels of the two valves are too close to each other, causing interference during operation; it’s impossible to turn them even when wearing gloves. This sampling port is too high off the ground; one has to stand on a stool to take samples, and there’s a risk of slipping if not careful. The dashboard face is facing the column, so it’s impossible to see the readings during inspections. These details are difficult to detect on the drawings; only those who work on site every day know them best. Maintenance personnel can see another type of issue. The core-pulling direction of the heat exchanger’s tube bundle is blocked by the pipes, making it impossible to pull it out during maintenance. The manhole opens toward the wall, preventing people from entering or exiting properly once it is opened. There is not enough lifting space at the top of the filter, so the chain block cannot be used to lift it when replacing the filter element. If these issues are identified during the PID review phase, modifying the drawings takes just half a day. Only after the device was built did they realize that either significant costs would be required for modifications, or the operators would complain all day long. II. What is reviewed in the PID review? The PID review is carried out page by page, with all participants examining each item by referring to the PID diagram, equipment data sheets, and instrument data sheets. The key aspects of the review include the following. Pipeline integrity review. Have the pipe diameters, pipe numbers, material grades, and insulation grades of all process pipelines been correctly indicated? Are the auxiliary pipelines complete—start-up pipelines, shutdown pipelines, purging and replacement pipelines, discharge pipelines, and pipelines for handling defective products? It is important to verify whether the valves are set to open or closed permanently – whether the shut-off valves upstream and downstream of the safety valve are marked as open and sealed with lead, whether the drain valve is marked as closed, and whether the root valve of the startup pipeline is marked as closed. Is the outlet direction of the relief valve aimed at a safe area, to prevent harmful substances from being released near walkways or electrical equipment? Review of the rationality of valve selection. Whether the type selection of each process valve is appropriate. Gate valves are used in applications where throttling is required, while ball valves are used in situations where a tight shut-off is needed and throttling is not necessary. Check whether the flow direction arrow on the check valve is correct. Check whether the fault conditions of the control valves are consistent with the safety analysis – whether an FO is set for the coolant control valve feeding the reactor, and whether an FC is set for the reactor feed valve. Whether the operating height of the valve and the space for the handwheel are sufficient, whether the handwheels of two adjacent large valves will interfere with each other, and whether an operating platform or sprocket is needed for valves located at higher heights. Compliance review for special pipe fittings. Are the set pressure of the safety valve and the discharge direction clearly indicated, and does the design of the inlet and outlet pipes meet the requirements regarding back pressure and pressure drop? Is the flame arrester installed in the correct location, and is a fire-resistant type required? Is the filtration accuracy of the filter indicated? Is it necessary to install a bypass to facilitate cleaning without shutting down the system? Are the type and drainage capacity of the steam trap indicated, and is the outlet of the steam trap connected to a safe collection system? Review of instrument information integrity. Are the tag numbers of all instruments indicated, and are there any omissions? Are the instrument type and range consistent with the instrument data sheet? Are the location and direction of the sampling points correctly indicated? Has the insertion depth of the temperature measurement points reached the center of the pipe? Are the pressure measurement ports located on the upper side of gas pipes or on the sides of liquid pipes? Have sufficient straight sections been provided before and after the flow meter? Are local instruments and remote instruments labeled separately? For instruments that require on-site readings, is their installation location convenient for operators to view? Review of control loop correctness. Can a complete chain of detection, judgment, and execution be found for each control loop? Has the fault position of the control valve been correctly selected based on safety analysis? Special attention should be paid to the fault conditions of several key circuits: the reactor feed valve should be set to FC, the reactor refrigerant control valve should be set to FO, and the distillation column bottom discharge valve should be set to FC. Whether there are conflicts or couplings between control loops — whether two control loops are competing for the same process variable. Review of interlock circuit independence. Whether the sensors and actuators of the SIS interlock and DCS control remain independent, and if there are any shared components. If the SIS and DCS share a sensor, the loss of function of that sensor results in the simultaneous loss of control and interlock functions – a situation that is not permitted in layer of protection analysis. Whether the causal relationships shown in the interlock logic diagram are correct, and whether the reset methods and reset conditions are clearly indicated. Review of equipment port consistency. Are the numbers and specifications of each device’s ports completely consistent with those in the device data sheet? The port numbers indicated on the PID must be the same as those on the manufacturer’s drawings, using the identical numbering system. Are the manholes, access holes, and spare pipe openings properly labeled? Have the port details of the auxiliary pipelines been added to the equipment data sheet? Review of the accessibility of sampling and analysis. Are the tag numbers and types of the sampling points indicated? Are the samplers for high-temperature or toxic media equipped with a cooling or sealed sampling system? Are the utility interfaces of online analytical instruments—cooling water, carrier gas, calibration gas—shown in the diagram? Is the height of the sampling port convenient for the operator to work with, and is there enough space below the sampling port to place the sampling container? III. Graded handling of review comments: The issues identified during the review meeting are recorded in grades according to their severity, just as in the PFD review. Grade A issues, which affect safety, compliance, or design accuracy, are issues that must be resolved; the PID cannot be frozen until they have been addressed. Such issues include: shared sensors or actuators in interlock circuits, incorrect selection of fault states for control valves, a set pressure for safety valves that exceeds the design pressure of the equipment, and blind spots in the coverage area of combustible gas detectors. Category B issues, which affect operability or ease of maintenance, are those for which modification is recommended. For example, the valve operation height is too high, the sampling port lacks an operating platform, and the numbering of pipe ends is inconsistent. Category C issues: optimization suggestions for reference by the development team; modification is not mandatory. All review comments are recorded item by item, responded to item by item, and corrections are confirmed item by item. It’s not an oral response at the meeting; it’s a written response – clearly stating for each suggestion whether the outcome was \"accepted, revised\" or \"rejected, with reasons given\". After confirming that all Class A issues had been resolved, the review team leader announced that the PID had passed the review. IV. What does PID freezing mean? Once the PID review is approved, the PID is frozen. This is the second milestone following the PFD freeze. If PFD freezing locks in the framework of the device’s process flow, then PID freezing locks in the engineering implementation plan for the device. Two freezes – one to lock in the macro-level process logic and the other to lock in the micro-level engineering details – together constitute the two pillars of the project’s technical baseline. Freezing means that from this moment on, the pipes, valves, instruments, and interlock systems of the entire installation are finalized. The subsequent detailed design — pipeline stress analysis, 3D piping layout, material statistics, instrument wiring, and DCS configuration — is all based on the finalized PID, and no major changes are permitted in principle. Any change, regardless of size, must go through the formal change management process, with the impact assessed and updates made across relevant specialties to ensure that the drawings always correspond to the actual conditions. Freezing doesn’t mean preventing changes; it means preventing “secret changes”. If issues are indeed identified during project execution, they are corrected through the change management process, and all affected downstream documents are updated accordingly. This mechanism ensures that the drawings of a device always match its actual physical form—this is the foundation for the safe operation of chemical processing units throughout their entire lifecycle. V. Some experiences in PID review: Having worked on projects for many years, I have a few insights to share regarding PID review. First, what reviewers fear the most is not finding problems, but seeing that everything seems fine. It’s a complex device, with dozens of pages of PID settings, each page containing hundreds of labels. If someone says “I took a look and there seems to be no problem,” it’s likely because they didn’t examine it carefully. A good review is filled with questions and inquiries—\"Why was this type of valve chosen?\", \"Why is this interlock set at this value?\", \"Why is this sampling port located in this position?\" The more detailed the questions, the greater the probability of identifying problems. Second, the opinions of operators and maintenance personnel are extremely valuable. Many young process engineers feel that operators do not understand design and cannot provide valuable suggestions. On the contrary, the questions raised by operators are often the most practical — because it is these issues they deal with on a daily basis. They don’t need to understand the drawing standards for PID; they just need to tell you that “we can’t reach this valve” or that “it’s inconvenient to take samples from this location”. These are the best review comments. Third, the review is not done by sitting in a meeting room and looking at the drawings behind closed doors. If possible, during the PID review phase, one should use the preliminary equipment layout to visualize the three-dimensional spatial relationships in mind. Is there enough space on the valve operating platform? Is the space required for removing the tubes from the heat exchanger bundle blocked by pipes? Is there enough space for people to stand once the manhole is opened? These spatial issues cannot be seen in a 2D PID diagram, but they need to be considered at this stage; it’s not acceptable to wait until the 3D model is reviewed to discover them. Preview for the next issue: Issue 48 – Auxiliary pipeline design: startup, shutdown, accidents, purging, and non-conforming products. All the main process pipelines for normal operations under PID have been drawn. But the device cannot operate only under normal operating conditions. How to raise the temperature and build up the liquid level while driving; where to discharge the material when stopping; how to carry out emergency release in case of an accident; how to isolate, purge, and clean the system before maintenance; and where to send defective products—all these auxiliary pipelines, if not properly designed, will prevent the plant from being started up, stopped, or maintained, even if it has been built. The auxiliary pipeline design will be carried out in the next issue.