The seven stages of chemical engineering technology from concept to industrialization (Issue 46/100) -- Eight categories of PID information

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This post was last edited by xiouxingzhe on 2026-6-17 22:38. Seven stages of chemical technology from concept to industrialization (Issue 46/100) —— Technology refinement: Complete representation of the eight types of ‘information’ in PID programming. Dear friends: Hello everyone! In the previous issue, we discussed PID page planning, refinement of control loops, and fault bit labeling. This issue delves deeper still, detailing the eight types of information that need to be carried by PID. The PID is the document with the highest information density in the process package. A complete PID diagram includes eight types of information: pipeline information, process valve information, special fitting information, instrument information, control scheme information, interlock safety information, equipment port information, and sampling and analysis information. These various pieces of information are layered on top of each other, together forming a comprehensive engineering representation of the installation. Each type of information, considered individually, represents a professional input requirement; together, they constitute all the details of the entire system, from the intended process to its practical implementation. This issue goes through these eight types of information one by one, explaining what each type includes, why it is important, and what is easy to overlook when labeling them. I. Pipeline Information Pipeline information is the most fundamental and largest category of information in PID. It includes the route of all pipelines, pipe diameters, medium codes, pipeline numbers, flow direction arrows, pipe material grades, and insulation grades. Each process pipeline should have a unique pipeline number in the PID. The pipe number usually indicates the following information: medium code, pipe sequence number, diameter, material grade, and insulation grade. Taking “P-10103-50-M1B-H” as an example, it is clear at a glance that this is a pipe used for transporting process materials; the sum of the drawing number and the pipe number is 10103, the nominal diameter is 50, the material grade is M1B, and the insulation grade is H. Once this numbering rule is established, it will serve as the basis for the naming scheme, material tracking, and construction installation in the piping discipline. The diameter specification is not arbitrary; it must be derived from the flow data in the PFD. Calculate the pipe diameter based on the fluid flow rate, allowable flow velocity, and pressure drop requirements, and round it to a standard size. The flow velocity ranges vary depending on the application: for liquid transport, it is typically between 0.5 and 2 meters per second, while for gas transport it is between 5 and 20 meters per second. In gas-liquid two-phase flows, special care must be taken to avoid the formation of slug flow. In microchannels, due to the small scale and high mixing efficiency, a high flow velocity is not necessary. The selection of these flow rates is not done in isolation; pressure drop, noise, vibration, and long-term operational reliability must all be taken into account. Flow arrows are marked on the piping using arrow symbols, so that those who view the diagram can immediately see in which direction the material flows. The pipe material grade and insulation grade are indicated in or next to the pipe number. The material grade is determined by the piping materials specialist based on the corrosiveness of the medium, operating temperature, and pressure. Thermal insulation design includes heat retention, heat isolation, heat tracing, and scald protection – pipes with operating temperatures above 60 degrees that are in areas where people may come into contact with them must be equipped with scald protection measures; this is not a requirement of the process itself, but rather a necessity for ensuring personnel safety. Auxiliary pipelines also belong to the pipeline information. Startup pipelines, shutdown pipelines, emergency relief pipelines, purging and displacement pipelines, and pipelines for handling defective products – although they are not part of the normal operating process, they are essential for ensuring that the facility can start up properly, shut down safely, and cope with abnormal conditions. The root valve of the auxiliary pipeline is usually marked as normally open or normally closed, so that those who view the diagram can immediately know what state the pipeline is in under normal conditions. There is another detail in pipeline information that is easily overlooked: draining and venting. For every section of a pipe where fluid or gas accumulation is possible, it should be considered whether a drain valve or vent valve is necessary. The outlet of the drain valve must point in a safe direction, and not toward the operation area or electrical equipment. II. Information on process valves Valves are the most numerous components in pipeline systems, and they are also the key information that must be specified individually on PID diagrams. Each process valve must be labeled with its type, nominal diameter, and pressure rating. The selection of valve type depends on the properties of the medium, operating conditions, and frequency of operation. Globe valves have good regulating performance but high flow resistance, and are suitable for applications that require throttling. When fully open, ball valves have low flow resistance and good sealing performance, making them suitable for applications that require tight shut-off, but they are not appropriate for throttling control. Butterfly valves are compact and lightweight, suitable for applications with large diameters and low pressure differences, but their control precision is inferior to that of globe valves. Check valves are used to prevent the backflow of fluid, and care must be taken to follow the flow direction indicator when installing them. If installed in the wrong direction, they will not perform their check function and may even cause dangerous pressure buildup. For valves with special requirements, those requirements must also be indicated on the PID. Jacketed valves that require heating, leak-proof valves with diaphragm seals, and critical valves that need locks to prevent accidental operation – if these special requirements are not clearly specified in the PID, they may be overlooked during the procurement and installation phases. The root valve of the auxiliary pipeline should be marked as normally open or normally closed; this is the simplest and most effective way to distinguish between valves that are in normal operation and auxiliary valves. The root valve of the startup pipeline is usually kept closed and is opened only during startup. The drain valve is usually kept closed and is only opened during maintenance for drainage. The upstream and downstream isolation valves of the safety valve are usually kept in the open position and sealed with lead to prevent accidental closure, which could render the safety valve ineffective. Although the operating space for the valve’s handwheel is not visible in the PID, it becomes apparent during the review of the 3D model. Whether the handwheels of two adjacent valves will interfere with each other, and whether a platform or sprocket is needed for the valve located at a higher position – these are issues that PID engineers need to take into account when determining the valve locations; they cannot wait until the 3D model is reviewed to discover them. III. Information on special fittings: Special fittings are key components directly related to process safety, and their types and tag numbers must be clearly indicated on the PID. Safety valves and rupture discs shall be marked with the set pressure, discharge volume, and discharge direction. Safety valves serve as the final protective barrier for pressure vessels. What value should be set for their set pressure, whether the relief capacity is sufficient, and whether the discharge path is safe—these are all core issues in process safety. The inlet and outlet piping of the safety valve must be designed to meet the requirements regarding back pressure and pressure drop – an excessive pressure drop in the outlet piping can prevent the safety valve from returning to its normal position after it opens, while an excessive pressure drop in the inlet piping can cause the safety valve to open and close frequently. Flame arrester, with the installation location and model indicated. The function of a flame arrester is to prevent external flames from entering equipment pipelines containing flammable and explosive gases, or to stop flames from spreading between such pipelines. Which locations require flame arrestors and what model of flame arrestor to use – whether it should be a pipeline flame arrestor or one built into the breather valve, and whether it should be a fire-resistant type or a regular type – must be clearly specified in the PID. Filter, indicating the filtration accuracy and processing capacity. Filters are used to protect downstream equipment from clogging or wear caused by impurities. Is it necessary to install a bypass to facilitate cleaning without shutting down the system? If cleaning without shutdown is essential for operation, then parallel configurations of one active and one standby filter should be depicted in the PID diagram; otherwise, the system must be shut down during cleaning. Check valve, indicate type and discharge volume. Thermostatic valves are used to automatically remove condensate from steam pipelines; improper selection can either lead to wasted energy as a result of steam escaping directly, or it can cause condensate to accumulate and trigger water hammer. Throttle orifice plate, with indicated orifice diameter and designed pressure drop, used to limit flow rate or reduce pressure. IV. Instrument Information Instrument information serves as the link between physical devices and control systems. All instruments must be labeled on the PID with their tag number, type, range, and installation location. Temperature instruments — thermometers, thermocouples, thermal resistors; indicate the material of the protective sleeve and the insertion depth. Pressure instruments – pressure gauges and pressure transmitters, indicating the location of the pressure sampling port and whether an isolation fluid is required. Flow meters – orifice plate flow meters, vortex flow meters, Coriolis mass flow meters; specify the requirements for the upstream straight pipe section and the methods for pressure tapping. Level gauges – glass plate level gauges, differential pressure transmitters, radar level gauges; indicate the measurement range and whether heating is required. Remote instruments and local instruments must be clearly distinguished in the PID. Remote instrument signals are sent to the DCS or SIS for remote monitoring and automatic control. Local instruments are only for reading by on-site inspectors and are not involved in automatic control. In some situations, both are needed – for example, for the liquid level at the bottom of a tower; it is necessary to have a remote liquid level transmitter to feed the data to the DCS for control, as well as an on-site glass level gauge for site inspectors to compare and verify. The location and direction of the instrument source point must be clearly indicated. Has the temperature measurement probe been inserted deep enough to reach the center of the pipe? If it is not deep enough, then the temperature measured is that of the pipe wall rather than that of the fluid. Has the correct direction been chosen for the pressure sampling port? For gas pipelines, the port should face upward to prevent liquid accumulation; for liquid pipelines, it should face sideways to prevent air accumulation. Is the length of the straight pipe sections before and after the flow meter specified in the diagram? Different types of flow meters have varying requirements regarding straight pipe sections; vortex flow meters typically require a straight pipe section that is 20 times the diameter of the pipe ahead and 5 times the diameter behind, while orifice flow meters require even longer sections. V. Control scheme information: The control loop is the “brain” of PID, and the complete description of all control loops is condensed in the diagram. Every control loop should have a complete “sense → decide → act” chain shown in the diagram. Where is the sensor, which controller does the signal go to, and which actuator does the controller drive? As an actuator, the control valve’s tag number, fault status, diameter, and Cv value are all clearly indicated on the PID. Marking the fault positions of control valves is particularly crucial. FC is fault off, FO is fault on, FL is fault hold. The control valve for the refrigerant entering the reactor is usually selected as FO – in case of a failure, the valve opens fully to ensure a continuous supply of refrigerant, thereby preventing the reaction from overheating and getting out of control. The reactor feed valve is usually selected as FC – it cuts off the feed in case of a failure, preventing the reaction from getting out of control. The bottom discharge valve of a distillation tower is usually selected as an FC type – it cuts off discharge in the event of a failure, thereby preventing the liquid level in the tower bottom from dropping to such an extent that vaporization in the pump occurs. The selection of fault locations is not arbitrary; it is based on the results of safety analysis. The interlocking relationships of the control loop and the alarm values must also be clearly specified in the PID. High high alarm value, high alarm value, low alarm value, and low low alarm value: which alarms require interlock activation and which merely serve to alert the operator – these details constitute the various layers of protection for the safe operation of the device. VI. Interlock safety information: The SIS interlock circuits and DCS control circuits differ fundamentally in terms of safety levels and maintenance requirements; it is necessary to clearly distinguish between the two at the PID level. The sensor tag numbers, interlock logic relationships, actuator tag numbers, and operating status of all SIS interlock circuits should be clearly shown on the diagram. The sensors and actuators of SIS and DCS must remain independent – if a single sensor or actuator is shared, then in the event that this shared component fails, both control and interlock functions are lost, leading to uncontrolled risks. The reset method for interlocking should also be indicated on the PID. Manual reset or automatic reset? Is manual reset done on the DCS operator station, or must a person go to the site to press the reset button? High-risk interlocks—such as reactor over-temperature interlocks—usually require manual reset, and the operator must do so after confirming that safety conditions are met. Low-risk interlocks can be designed to reset automatically. The causal relationships, reset methods, and reset conditions on the interlock logic diagram must be consistent with those indicated on the PID; there should be no discrepancies between the two. VII. Equipment Port Information The equipment port information represents the most important data linkage between the PID and the equipment data table. The port numbers, nominal diameters, flange standards, and pressure ratings of all equipment must be exactly consistent with those in the equipment data sheet. It is not uncommon at project sites for the pipes to fail to connect properly during installation, once piping prefabrication is complete, when the port numbers on the PID do not match those on the manufacturer’s drawings; the root cause is usually the inconsistency between the port numbers on the PID and those in the equipment data sheet. The equipment port information also includes the location and specifications of manholes, handholes, and spare ports. Auxiliary pipelines — startup pipelines, purging pipelines, and pipelines for defective products — need to be connected to specific pipe ports; it is important to check whether these ports have been reserved in the equipment data sheet prior to preparing the PID for the auxiliary pipelines. If not, it is necessary to return to the device data table to add auxiliary ports. It is recommended to reserve at least two spare nozzles on towers and tanks; their sizes should be between DN25 and DN80. VIII. Sampling and analysis information: Sampling and analysis serve as the \"eyes\" of process control, but many PID controllers do not provide detailed information in this regard. The tag numbers of all sampling points, the types of samplers, and the configurations of sampling valves must be indicated on the PID. A conventional liquid sampler, using a needle valve and sampling tube, is simple and reliable. Sampling of high-temperature or toxic media requires a cooled sampler or a closed sampling system; doublevalve isolation may be necessary before the sampling valve. The gas sampling line should be as short as possible; when necessary, trace heating is required to prevent condensation from distorting the analytical results. When sampling solid-containing media, the diameter of the sampling port should be large enough to prevent clogging. The height of the sampling valve should be between 1.0 and 1.3 meters to facilitate operation by the staff. Space should be reserved below the sampling port for placing the sampling container. It is best not to weld the sampling valve directly to the main pipeline; instead, it should be connected via a short branch pipe. This way, sampling does not affect the flow pattern in the main pipeline, and it is also easier to replace the sampling valve in case it gets damaged. Online analytical instruments need to indicate the analysis item, range, and signal output type. If the data from the online analyzer is used for interlock control, its tag number must also be consistent with the interlock logic diagram, and the same requirements as those for other SIS sensors regarding SIL level must be met. The utility interfaces of the online analyzer—cooling water, carrier gas, and calibration gas—also need to be represented in the PID; they must not be omitted. IX. Association and cross-verification of Category 9 and Category 8 information: The eight categories of information are not isolated from one another; there are close connections between them. The pipe number in the pipeline information and the instrument tag number in the instrument information are independent of each other in terms of numbering rules, but they must correspond one-to-one in a piping and instrumentation diagram. The tag number of an instrument installed on a pipeline should be logically linked to the pipeline’s identifier. The actuator specified in the control scheme information is also the control valve mentioned in the process valve information; the designations on both sides—address, diameter, fault status—must be identical. The actuator in the interlock safety information may be a standalone emergency shut-off valve, which is also a safety-related fitting in the special fitting information. A good PID has all eight types of information in their proper places, with each having a clear role; they corroborate one another, without any contradictions or omissions. To achieve this, the careful attention of one person is not enough; a systematic verification mechanism needs to be established. When examining PID, each specialty has its own areas of focus and blind spots. The process engineering team focuses on the completeness of the processes and the rationality of the control schemes; the equipment engineering team pays attention to the numbering and specifications of pipe connections; the piping engineering team is concerned with pipe diameters and material grades; the instrumentation team focuses on tag numbers and measurement ranges; the safety engineering team deals with interlocks and relief systems. The purpose of cross-checking is to use the insights of multiple teams to cover up any blind spots that might exist. Next issue preview: Issue 47 – PID milestone review: a comprehensive assessment centered on operability. The PID has been prepared, and now the review of it follows. If the question addressed in a PFD review is “Is this process scientifically and engineering-wise sound?”, then the question addressed in a PID review is “Can this unit be operated and maintained safely and conveniently?”. Who must participate in the PID review, what are the eight core dimensions of the review, and how are the review comments classified? To be continued in the next issue.