- Introduction
Author: Beijing Guojie Pipeline Co., Ltd.
As the export business of domestic pressure piping equipment continues to grow, oil and gas, chemical, power station, and offshore engineering EPC foreign-related projects extensively use bilingual Chinese-English drawings, material take-off (MTO) lists, inquiry documents, and material test certificates (MTC).Pipeline fittings include pressure-bearing components such as elbows, tees, reducers, flanges, caps, and blind plates, which are components of special equipment, and their parameter descriptions require strong engineering rigor.Chinese and English parameters are not simple word-for-word translations; behind them correspond two independent standard systems: the national standard GB adopts metric DN nominal diameter and PN nominal pressure;the ASME American standard adopts NPS nominal pipe size, Class pound-grade pressure, and SCH wall thickness schedules.In procurement practice, many practitioners simply equate DN directly with NPS and directly convert PN to Class, ignoring differences in outside diameter bases, resulting in wrong specification selection, mismatched wall thickness, and incorrect sealing face descriptions, ultimately leading to fittings that cannot be assembled upon arrival, failed pressure tests, and resulting schedule delays and economic losses.
The core goal of Chinese-English parameter comparison for pipeline fittings is to achieve terminology consistency, parameter identifiability, and standard system matching, rather than mere literal translation.Many drawing abbreviations, material grades, non-destructive testing codes, and flange sealing face types have fixed industry-conventional expressions in Chinese and English engineering documents and cannot be translated literally by ordinary machine translation.For example, "Class 150" cannot simply be translated as 150 kg pressure, and SCH40 cannot be directly equated with a fixed wall thickness in the national standard; such conceptual misunderstandings are a frequent problem in foreign-related procurement.
This article builds a Chinese-English parameter comparison foundation from eight dimensions: nominal size, pressure rating, wall thickness series, pipe fitting categories, material grades, connection forms, inspection and testing terminology, and common drawing abbreviations; it analyzes common comparison misunderstandings and establishes a Chinese-English parameter verification workflow for inquiry, drawing interpretation, technical agreement preparation, and factory document review, providing standardized terminology reference for foreign trade procurement of pipes and fittings and material management in foreign-related engineering projects.
II. Chinese-English Comparison of Pipe Nominal Sizes and Basic Logic
Nominal size is the most basic parameter of pipes and fittings and is divided into the metric DN system and the national standard, and the imperial NPS system corresponding to the ASME American standard; both are dimensionless designations and are not directly equal to the actual inner diameter of the pipe.
DN, Chinese: nominal diameter, English: Nominal Diameter, defined by national standard GB/T 1047, with a unit system of metric millimeters.
NPS, Chinese: nominal pipe size, English: Nominal Pipe Size, defined by ASME B36.10M, imperial system.
The most critical basic knowledge point here: the same numerical DN and NPS have the same outer diameter; for example, NPS 4 corresponds to DN100, and the beveled outer diameters of the two fittings are the same, allowing butt connection;However, NPS is not the inch inside diameter; NPS4 does not equal a 4-inch inside diameter—it is merely a designation code.
In bilingual drawings and procurement lists, a common description format is: DN100 (NPS4), indicating that the fitting is marked with both nominal designation systems, for use in international projects.Procurement personnel must distinguish: if only DN is written, it is a metric project;if only NPS is written, it is a project under the US standard system; one cannot simply judge interchangeability based on equal numbers.
Supporting outside diameter abbreviations: OD (Outside Diameter) outside diameter, ID (Inside Diameter) inside diameter, WT (Wall Thickness) wall thickness; these three abbreviations appear widely in single-line diagrams, P&ID drawings, and product data sheets. Many procurement lists mistakenly use inside diameter as the basis for selection; for pressure piping fittings, selection should be determined primarily by the combination of outside diameter + wall thickness, not inside diameter.
Common misconception: DN50 being equivalent to NPS2 only means the matching outside diameters are the same; it does not mean pressure rating and wall thickness automatically match, and fittings cannot be directly replaced across standards.
III. Chinese-English Comparison System for Pressure Ratings
Nominal pressure is divided into two systems: PN (metric pressure under the national standard) and Class (pound-grade pressure under the American standard), and the two cannot be converted simply and linearly.
PN: Nominal Pressure, based on the unit MPa, widely used in domestic industrial piping, heating, and water supply and drainage piping, such as PN16, PN25, and PN40.
Class (abbreviated CL): Pressure Class, ASME B16.5 flange standard, Class150, 300, 600, 900, 1500, 2500.
The most common error in Chinese-English document translation: translating Class150 as 150bar or 150 kilograms of pressure.In fact, the allowable pressure corresponding to Class pound grades varies with temperature; the allowable pressure decreases at high temperatures, and it can only be checked against tables; there is no fixed mathematical conversion formula.In procurement technical documents, the correct bilingual expression is: Class150 (PN20), for reference comparison only; the technical agreement must clearly specify the governing standard, and the reference comparison value must not be used as a design basis.
Related pressure terms: Design Pressure, Operating Pressure, Test Pressure, Hydrostatic Test, Pneumatic Test, Vacuum, Low Pressure, Medium Pressure, High Pressure.
Pressure piping classification GC (GB/T20801): GC1, GC2, GC3, in English Grade GC pressure piping; this is a domestic special equipment classification, and overseas projects generally do not use GC classification; overseas projects mostly use Piping Class.
IV. Wall Thickness Schedule SCH Chinese-English Parameter Comparison
SCH (Schedule Number) wall thickness series is a core parameter of American standard pipes and fittings; in Chinese: wall thickness schedule number.SCH is not a direct wall thickness in millimeters; it is a dimensionless number defined in the ASME B36.10M steel pipe standard. Common grades include SCH20, SCH40, SCH80, and SCH160.Matching designations include STD (Standard) standard wall thickness, XS Extra Strong thickened, and XXS Double Extra Strong double thickened.
Chinese descriptions: standard wall thickness STD, thickened XS, extra-thick XXS;English abbreviations are written directly on drawings and procurement lists.
SCH wall thickness and national standard wall thickness tables can be cross-referenced against each other, but there is no simple conversion formula; for the same NPS, the larger the SCH number, the greater the wall thickness.Bilingual procurement lists must indicate both NPS and SCH; SCH alone must not be written.
Common procurement mistake: writing "SCH40 wall thickness" in Chinese, which loses the standard version after machine translation;or directly mapping SCH40 to a fixed millimeter wall thickness from the national standard, ignoring the wall thickness differences caused by the material.In a bilingual MTO material table, an example of the standard way to write it: NPS4, SCH40, A234 WPB.
V. Chinese-English comparison of fitting category names
Elbow, abbreviated as EL.90° Elbow;45° Elbow;Long Radius Elbow (ELL, R=1.5D);Short Radius Elbow (ELS, R=1D);180° Return Bend;Reducing Elbow.
Tee, abbreviated as T;Straight Tee;Reducing Tee;Lateral Tee.
Reducer;Concentric Reducer (CR);Eccentric Reducer (ER).
Cap (CP);Blank (BLK);Spectacle Blind.
Stub End;Reinforcing Pad / Branch Outlet.
Flange (FL): Welding Neck Flange (WNF);Slip-on Flange (SOF);Socket Welding Flange (SWF);Threaded Flange;Blind Flange.
Flange sealing face: Raised Face (RF); Flat Face (FF); Ring Type Joint (RTJ); Male & Female Face (MFM).
Pipe fitting category names are high-frequency content in inquiries and MTO lists, and the Chinese and English names must be matched with drawing codes.In foreign trade orders, many wrong-shipment problems stem from non-standard translation of the Chinese term "reducer" (reducer), where colloquial literal translation is used instead of the industry-standard "Reducer," making it unrecognizable to overseas customers.
VI. Chinese-English Comparison of Pipe Material Grades
Carbon Steel (CS), commonly used grades: Q235, 20#, English 20G, A105 (forged flanges), A234 WPB (butt-welded carbon steel pipe fittings).
Stainless Steel (SS): 304 / A403 WP304;316L / A403 WP316L.
Alloy Steel (AS): A234 WP11, WP22 and other high-temperature alloy pipe fittings.
Key points for material terminology comparison: domestic grades and ASTM grades cannot be arbitrarily equated; 20# steel is not equal to A105; A234 WPB is a dedicated grade for forged carbon steel pipe fittings, while A105 is a flange forging grade, and the two cannot be mixed up.In bilingual technical agreements, material grades must be fully marked with ASTM or GB standards; writing only Chinese grades is invalid in overseas documentation, while writing only ASTM grades requires conversion to national standard explanations for domestic special equipment filing.
Supporting material-related vocabulary: Corrosion Allowance;Yield Strength;Tensile Strength;HIC Hydrogen Induced Cracking.
VII. Comparison of Chinese-English Terminology in Connection Types, Inspection and Testing
Connection methods: Butt Weld (BW);Socket Weld (SW);Threaded End (NPT);Flanged Connection.
NDT Non-Destructive Testing: RT Radiographic Testing;UT Ultrasonic Testing;MT Magnetic Particle Testing;PT Liquid Penetrant Testing.
MTC Material Test Certificate, material test certificate, core documentation for foreign trade orders.EN10204 3.1B, 3.2 certificates, commonly translated in China as 3.1 certificate and 3.2 certificate; this numbering belongs to the European material certificate standard, and the numbers are retained directly in both Chinese and English documents.
Heat Treatment (HT);Normalizing;Annealing;Quenching;Tempering.
PMI Positive Material Identification, on-site material verification, a high-frequency inspection item for supervision in overseas projects.
VIII. Comparison of Common English Abbreviations in Piping Drawings and Material Lists
Piping Isometric Drawing (ISO drawing); P&ID Piping and Instrument Diagram; MTO Material Take-off; BOM Bill of Material; DWG Drawing; Rev Revision number; Lot batch; Heat No., the heat number is the core identifier for pressure piping traceability, and the Chinese and English certificates must correspond one-to-one.
ITP Inspection and Test Plan, which is the core document for supervision.WPS Welding Procedure Specification welding procedure specification.
IX. Analysis of Common Misunderstandings in Chinese-English Parameter Comparison
9.1 Misunderstandings About Nominal Size
DN and NPS having equal numbers only means the outer diameter is the same; it does not mean the entire set of parameters is equivalent.DN100=NPS4 only indicates outer diameter matching; wall thickness, pressure rating, and governing standard still need to be checked separately.Many procurement personnel directly replace DN with NPS without checking SCH and Class, resulting in mismatched flanges.
9.2 Misunderstandings About Pound Class to PN Conversion
There is no unique conversion formula between Class and PN; temperature affects pressure-bearing capacity.Do not directly write Class300=PN50 as a technical judgment basis; it can only be used as a reference. The technical agreement must lock in the execution of ASME B16.5 and must not rely on approximate conversion.
9.3 Misconceptions in translating wall thickness SCH
SCH is only a series number. SCH40 has different actual millimeter wall thicknesses under different NPS specifications and cannot be translated as a fixed thickness; do not simply write SCH40=6mm.
9.4 Misconceptions in literal translation of material grades
Grades such as No. 20 steel and A234 WPB are fixed standard designations and must not be translated literally.Machine translation often produces incorrect material names. Procurement documents must manually verify ASTM/GB grades to prevent errors in material descriptions.
9.5 Misconceptions in simplifying certificate terminology
Abbreviations such as MTC, EN10204 3.1, PMI, and NDT are commonly used in international industry; do not force all of them to be translated into long Chinese sentences;When translating domestic documents, retain abbreviations and mark the full Chinese name at first occurrence.
X. Workflow for Chinese-English Parameter Verification of Pipes and Pipe Fittings
Establish a standardized cross-reference verification process, apply it to inquiries, technical clarifications, order reviews, and factory document reviews, to avoid risks of translation and parameter mismatches.
Step 1: Confirm the project standard system, distinguish whether it is a national standard metric project or an ASME American standard international project, lock in whether the DN/PN or NPS/Class system is adopted, and determine the drawing language baseline.
Step 2: Extract the core parameter list—nominal size, pressure rating, wall thickness, material, fitting type, sealing face, connection method—and compare each item in Chinese and English.
Step 3: Terminology verification, use industry-standard terminology, reject direct machine translation, verify the meaning of abbreviations, and check the consistency of parameters in the MTO material list and single-line diagrams.
Step 4: Cross-standard equivalence determination. If there is a mutual comparison between national standards and American standards, the piping professional engineer shall perform the equivalence assessment and form a written technical clarification; procurement personnel are prohibited from making equivalent substitutions on their own.
Step 5: Document consistency review. The Chinese and English parameters and heat numbers in the MTC material certificates, drawings, and purchase orders shall remain consistent, and the Chinese and English descriptions in the certificates shall not conflict.
XI. Application of the Chinese-English Parameter Comparison System in the Entire Procurement Process
11.1 Inquiry and Technical Clarification Stage
Foreign-related inquiry documents are generally prepared bilingually, with Chinese descriptions accompanied by English terminology.When customers send English ISO single-line diagrams, procurement personnel need to convert NPS, Class, SCH, and English names of pipe fittings into Chinese parameters recognizable for domestic production, while replying to customers with English clarification documents using unified terminology.Errors in parameter comparison will cause deviations in suppliers' understanding of quotations, resulting in missing quotation items and incorrect specifications.
11.2 Technical Agreement Preparation Stage
Technical agreements for foreign trade orders are often bilingual in Chinese and English, and the meanings of all pipe fitting parameters, inspection items, and certificate requirements in the Chinese and English clauses must be completely consistent.Terminology translation deviations will cause quality disputes later, for example, incorrect translation of sealing face type may result in the sealing face of supplied flanges not meeting customer requirements.
11.3 Production Supervision and Factory Documentation Stage
MTC material certificates are generally provided in English, with Chinese translations for domestic filing.The heat numbers, material grades, and test items in the certificate must correspond one-to-one between Chinese and English.Supervision personnel verify the English markings on the physical steel stamps against the certificate parameters to prevent contradictions between physical markings and certificate translations.
11.4 On-Site Acceptance Stage
On-site acceptance personnel for overseas projects verify the physical steel stamps on pipe fittings against English drawings; domestic projects use Chinese drawings; bilingual projects require simultaneous verification of both sets of parameters.The Chinese-English parameter comparison table serves as an auxiliary acceptance tool to quickly identify deviations in specifications, materials, and pressure ratings.
XII. Recommendations for Implementation and Management of the Parameter Comparison System
First, establish a bilingual Chinese-English terminology glossary for enterprise pipeline fittings, solidify fitting names, parameter abbreviations, material grades, and inspection terms, unify the language used by procurement, technical, and quality inspection teams, and avoid inconsistent translation versions among different personnel.
Second, implement a bilingual document review mechanism. All external English inquiries, technical agreements, and materials must be reviewed by professionals familiar with ASME and national standard piping, and translation software output must not be used directly.
Third, carry out regular terminology training, focusing on explaining concepts that are easily confused, such as DN/NPS, Class/PN, and SCH wall thickness, distinguishing literal translation from engineering standard definitions, and eliminating cognitive misunderstandings among procurement personnel.
Fourth, drawing version control: when bilingual drawings are revised, update the Chinese and English parameters simultaneously, and update the Rev version number simultaneously, to avoid mixing terminology from old and new versions.
XIII. Conclusion
The Chinese-English parameter comparison for pipeline fittings is not simple text translation work, but rather parameter conversion and terminology standardization work between two engineering standard systems.The DN/PN national standard metric system and the NPS/Class/SCH American standard imperial system have different logic in nominal size, pressure rating, and wall thickness definitions, and there are many conceptual pitfalls.Professional vocabulary such as fitting categories including elbows, tees, flanges, and reducers, material grades, non-destructive testing, and quality certificates has fixed expressions in the international industry and cannot rely on direct translation by ordinary translation tools.
In international EPC and foreign trade pipeline fitting procurement, confusion of parameter terminology and translation errors can directly lead to incorrect supply of fittings, assembly failure, and inability to pass acceptance, causing project losses.Establishing a complete Chinese-English parameter comparison foundation, building a standardized parameter verification process, and unifying terminology across MTO lists, single-line diagrams, technical agreements, and MTC certificates can effectively reduce cross-language and cross-standard procurement risks.Procurement, technical, and supervision personnel need to master the underlying logic of parameter comparison, distinguish between code meanings and actual dimensions, identify common conversion misunderstandings, ensure that parameter meanings in Chinese and English engineering documents are consistent, guarantee the technical accuracy of foreign trade pipeline fitting orders, and help domestic fitting products be smoothly delivered to overseas engineering projects.
References
[1] GB/T 1047, Definition and selection of DN (nominal size) for pipeline components [S].
[2] ASME B36.10M, Welded and Seamless Wrought Steel Pipe [S].
[3] ASME B16.5, Pipe Flanges and Flanged Fittings [S].
[4] GB/T12459-2017, Steel butt-welding seamless pipe fittings [S].
[5] Common Abbreviations for Petrochemical Piping Engineering SH/T 3902-2014[S].
[6] Li Xuedao. Practical Guide to Foreign Trade Procurement and Standard Comparison of Pressure Piping Components[M]. Beijing: China Machine Press, 2023.