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Material code: The material flowing within the pipe is represented by specified uppercase English abbreviations. For the letter codes of materials, refer to \"Material Codes and Abbreviations on Piping and Instrumentation Diagrams\" (HG 20559.5-93). For material codes that are required in a project but not listed in the regulations, they shall be established by the person in charge of the process system based on the project requirements, and approved by the design manager before being used in the project. The process (main item) number and pipeline sequence number for the project: It consists of two or three units, and is generally composed of numbers or numbers with letters attached (the letters occupy one digit and are the same size as the numbers); note ①. a. Process numbering unit of the project: The process (main item) number of a project is assigned by the project itself; it is an identification code allocated to each process (main item) within the system, expressed as two digits, such as 01, 10, etc. b. Pipeline sequence number unit: The sequence number is a specific identifier that assigns a sequential order to a particular material medium within a process (main item). For each material medium in every process step, numbering starts at 01 (or 001). The pipeline sequence number is represented by two (or three) digits; when the pipeline number exceeds 999, it is expressed using four digits. c. Serial number unit: When there is an exactly identical system within a process (main item) – meaning that the equipment, instruments, pipes, valves, and fittings of all systems are identical – then for these identical repeated pipe numbers, aside from the serial number unit, all other parts and components of the pipe number are exactly the same. The serial number is represented by a single uppercase English letter, with the letters O and I generally not being used. The piping for equipment and fittings that serve as backups to each other (such as pumps, filters, instruments, bypasses, etc.) and for equipment that is arranged in parallel and is of the same size and overlaps with one another (such as parallel heat exchangers, etc.) are not covered by the use of serial numbers for identification. Note: ① In a combined plant design, if it is necessary to identify a unit, the second part of the pipe number can consist of three (or four) digits; the first digit represents the unit identification number, expressed as two (or three) digits, such as 01. The second unit is the process number of the project, represented by two digits, such as 01. The third unit is the serial number, represented by uppercase English printed letters, such as “A”. Unit 4 is the pipeline sequence number, represented by two (or three) digits, such as 010. Example: Pipe size: expressed in terms of the pipe’s nominal diameter. a. For pipes with metric dimensions, such as DN100 and DN150, they are simply denoted as 100 and 150; the unit “mm” for metric dimensions is omitted. b. For pipes with imperial dimensions, such as welded steel pipes, the nominal diameter is also used; for example, 2\" is expressed as 50, with the unit \"mm\" being omitted. c. Other methods of representing pipe sizes shall be used only with the approval of the design manager, in accordance with the project’s characteristics and requirements, and shall be indicated on the first page of the P&ID. Pipeline grade: Composed of three units. The pipeline grade is established by the pipeline materials specialists based on the characteristics and requirements of the project, and is provided to the process systems specialists for use in labeling. The pipeline grade consists of three elements, as described below: a. First element: the nominal pressure grade code of the pipeline, represented by uppercase English letters. b. Unit 2: Serial numbers are represented by Arabic numerals, starting from 1. c. Unit 3: Pipe material categories, represented by uppercase printed English letters. Note: ② The pipe grade consists of several units, and the content, meaning, and representation methods of each unit are specified in the regulations on pipe grades prepared by the pipeline materials industry. The codes for heat insulation, thermal insulation, fire protection, and sound insulation are represented by specified one (or two) uppercase English letters; the corresponding code letters can be found in \"Codes for Heat Insulation, Thermal Insulation, Fire Protection, and Sound Insulation in P&ID Diagrams\" XHG 20559.6-1993). If the pipeline has no requirements for insulation, heat preservation, fire protection, or sound isolation, this section is omitted from the pipeline designation.