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I. Within a process unit: 1. The numbering sequence of the process pipelines within a process unit should be determined based on actual conditions. Generally, starting from the first piping and instrumentation diagram, each pipeline carrying a specific process medium is numbered in accordance with the diagram sequence and the flow order. On each flowchart, the process numbers that indicate the forward flow from device to device as shown in the flowchart. For example, in the numbering sequence of a distillation column reflux system, it should go from the top of the column to the condenser, then to the collection tank (reflux tank), followed by the reflux pump, and back to the column. 2. The pipes are numbered in sequence from the starting device to the subsequent devices. The process number unit in the pipe number (such as “12” in Figure 5-1) matches the process number “12” in the device identifier of the device where the pipe begins. For the pipes between two devices, regardless of whether their specifications or dimensions change, only one pipe number is assigned. If there are pipes that branch off to other devices or pipes in between, then separate pipe numbers are assigned, as shown in Figure 5-1. (As an example, the pipe numbers in the following figures all represent only the basic pipe numbers.) 5-1 Numbering and labeling of process pipelines (I) 3. The pipeline sequence number should be continued until it reaches a piece of equipment or a connection point with another pipeline, as shown in Figure 5-2. 5-2 Numbering and labeling of process pipelines (II) 4. Equipment vents (including the inlet pipes of safety valves and the outlet vent pipes), as well as pipes used for draining liquid – any pipe must be assigned a pipeline number. 5. Connection pipes from equipment or pipelines to multiple devices with the same tag number but distinguished by subscripts (non-series), as well as individual branches from multiple devices with the same tag number to other equipment or collection pipelines, must be assigned pipe numbers. 1) When there is a main pipe (i.e., the pipe end is equipped with a head), a pipe number is assigned to the main pipe; separate numbers are then assigned to the branch pipes leading to each device, as required in Figure 5-3. 5-3 Numbering and labeling of process pipelines (III) 2) In the absence of a main pipe, a pipeline number is assigned to the pipeline leading to the farthest equipment with the same tag number, while the remaining pipelines are given separate numbers. As shown in Figure 5-4. 5-4 Numbering and labeling of process pipelines (IV) 6. For pipelines that run from one device (or pipeline) to another, there are several scenarios: 1) From different pipe outlets on one device to different pipe outlets (or a single pipeline) on another device, each pipe must be assigned a pipeline number, as shown in Figure 5-5. 5-5 Numbering and labeling of pipes at the connection ports on equipment (I) 2) For different ports (or pipes) on one piece of equipment connecting to the same port on another piece of equipment, each pipe must be assigned a pipe number, as shown in Figure 5-6. 5-6 Numbering and labeling of pipes at the connection ports on equipment (II) 3) When one port on one piece of equipment (or pipe) connects to multiple ports on another piece of equipment, there are two possible scenarios: (1) If the multiple connection ports on the receiving equipment serve the same purpose, and only one port is used under normal conditions while the others serve as backup or switching ports, then only one pipe number is assigned, as in the case of the feed port of a tower, as shown in Figure 5-7. 5-7 Numbering and labeling of pipes at the connections on equipment (III) (2) When there are multiple connection ports on a receiving device that serve the same purpose (used simultaneously), as well as multiple connection ports with different purposes, each pipe must be assigned a pipe number. As shown in Figures 5-8, the pipe numbers and markings for the connections on devices PL-0408 and PL-0409.5-8 (section 7); the bypass pipes that connect two pipes (not those related to control valves); the bypass pipes at the inlet and outlet of safety valves; as well as the return pipes for devices and pipes, all require pipe numbering, such as the return pipe of the pump in Figure 5-8. 8. The numbering rules for process pipelines with series numbers are similar; within each identical series, the remaining parts of the pipeline number as well as its components remain exactly the same, with only the series number changing. For the main pipes assigned before this series and the collective main pipes of the various series’ branch pipes, regardless of whether there are end caps (such as flange covers) on the main pipes, a pipe number that does not belong to any series must be assigned to those main pipes; thereafter, the branch pipes and connection pipes are numbered according to their respective series, as shown in Figure 5-9. 5-9 Numbering and labeling of process pipes with serial numbers II. Between processes: 1. Process pipes that run from the equipment or pipes of one process to those of another process (or several processes) are numbered based on the starting process, and this numbering remains valid until the connection to the equipment or pipes of the other process is completed, as shown in Figure 5-10. 5-10 Numbering and labeling of process pipes between processes (without inlet and outlet main pipes at the plant boundary); 2. Numbering of process pipes that come from within the plant boundary or go outside it (under normal circumstances), as shown in Figure 5-11. 5-11 Numbers and labeling of the process main pipes entering and leaving the equipment area – Process interconnections 1) The main pipe coming from the area, as well as the process pipes through which the main pipe is distributed to various processes, are numbered according to the number of the farthest process. Assigned by the main supervisor to other individual processes, pipes serving that process are numbered using that process code; whereas pipes serving two or more processes continue to be numbered according to the farthest process. 2) Branch pipes from the process discharge device area, numbered according to the discharge process sequence. The branch pipes under its responsibility are assigned the number of the farthest process; the outlet branch pipes from two or more processes are also assigned numbers based on the number of the farthest process. 3) Numbers and labels of process pipes coming from or going out of the plant boundary zone (special cases): In cases where it is the main pipe for process pipes entering (leaving) the boundary zone, such main pipe should be assigned a project-specific “process step” number. (1) The pipe ends are equipped with end caps (or flange covers); as shown in Figure 5-12, the “process” number for the main pipe running between the process units entering and exiting the plant area is “7”. In Figure 5-12, the numbers and markings corresponding to the main process pipes running between the process units entering and exiting the plant area are also indicated. (2) The main pipes are fitted with equipment, instruments, as well as special pipes (valves) that require numbering, as shown in Figure 5-13. 5-13 Numbers and markings of the main process pipes for inter-process transfer within the equipment boundary area (III) (3) The project requires that the pipes used for inter-process transfer be considered as a single \"process\" in the engineering design, as shown in Figure 5-13. In Figure 5-13, the process number for the main pipeline that carries fluid in and out of the equipment area is “7”; the process numbers for the equipment, instruments, and special fittings located on this main pipeline are also “7”. The boundary between the pipe numbered “7” and the pipes numbered “1”, “2”, “3” from other processes can be located at any (specific) position on the main pipe within (or outside) those processes, and the boundary between the pipe numbers must be indicated on the PI diagram using a boundary symbol.