I. Introduction
In oil and gas, coal chemical, overseas general contracting, and complete equipment supporting projects, a large number of pipeline fittings and flanges adopt the ASME American standard pound rating Class system.Unlike the national standard PN nominal pressure system, Class is not a fixed pressure value but a pressure-temperature rating curve system dependent on material and temperature. Class150 does not mean 150psi, and Class300 is not equal to 300psi. This is the most core underlying logic of pound-rated procurement.
In procurement practice, there are generally four typical errors: First, directly using the static conversion of "Class150≈PN20, Class300≈PN50" and replacing directly without distinguishing temperature and material;Second, only checking the design pressure, ignoring the maximum operating temperature, and not performing temperature-pressure derating verification, while the actual allowable pressure drops significantly under high-temperature conditions;Third, the pound ratings of flanges, fittings, gaskets, and bolts are inconsistent, and different pound ratings are mixed in the same sealing joint;Fourth, confusing pound rating with SCH wall thickness, assuming that a high pound rating necessarily corresponds to thick-walled piping, and ignoring the independent verification requirement for pipe wall thickness.These errors are often exposed only during arrival installation, hydrostatic testing, and supervision inspection stages, causing returns, rework, schedule delays, and even safety risks of medium leakage.
The procurement determination of pressure class ratings is essentially not a matter of table lookup and conversion, but a systematic determination method centered on design operating conditions, using the ASME code temperature-pressure tables as the benchmark, and taking the unified matching of the entire sealing node as the bottom line.This article sorts out the standardized pressure class determination steps, clarifies the key inspection points, boundary conditions, and misconception prevention for each stage, and forms a determination method suitable for direct use by procurement positions, for the entire process of inquiry, material requisition, technical agreement preparation, and incoming goods acceptance.
II. Underlying Logical Differences Between the Class Rating System and the PN System
The PN nominal pressure system has a relatively low reference temperature; commonly used PN in China specifies allowable pressure at the reference temperature, and the allowable pressure decreases slowly as the temperature rises;The Class rating system has different reference temperatures for different ratings: Class 150 has a reference temperature of 260°C, and Class 300 and above have a reference temperature of 454°C. The allowable pressure corresponding to the class rating decays more significantly as the temperature rises, and the material group directly determines the load-bearing capacity under the same class rating.
For the same Class, the temperature-pressure ratings of A105 carbon steel, 304 stainless steel, and chrome-molybdenum alloy steel are all different, and a single general conversion table cannot cover all materials.At the same time, the class rating carries two layers of meaning: first, pressure-bearing capacity; second, flange structural dimensions.The bolt circle, number of bolt holes, flange thickness, and sealing face dimensions of ASME B16.5 flanges are jointly determined by the class rating and NPS. Even if the room-temperature pressure-bearing values of two ratings are close, different class ratings cannot be directly connected; forced assembly will cause sealing failure.
Procurement determination must be separated into two main lines: equivalent pressure-bearing capacity determination and structural dimension matching determination.Pressure-bearing equivalence only addresses whether the strength is sufficient;structural matching determines whether the flanges can connect; during cross-system conversion, the two main lines must not be confused.For domestic projects implementing American standards and for Chinese-foreign standard alignment and retrofit projects, errors most easily occur in flange dimension matching.
III. Standardized Determination Process for Pound-Rating Pressure Procurement
(I) Step One: Extract the Complete Operating Condition Boundary and Lock Down the Determination Input Conditions
The starting point for pound-rating determination is not pressure, but the complete set of operating condition parameters. Before requisitioning materials and requesting quotations, procurement must extract from the piping line list, P&ID, single-line diagram, and specification: design pressure, maximum operating temperature, minimum temperature, medium composition, GC rating, material group, whether there is severe cyclic loading, hydrogen sulfide stress corrosion, vacuum conditions, etc. With only the operating pressure and no temperature, the class rating cannot be determined.
Special distinction: use the design pressure and maximum design temperature for table lookup, not the normal operating conditions;Transient pressures such as water hammer, startup/shutdown surges, and pressure-relief fluctuations shall be included in the safety margin.Vacuum/negative-pressure pipelines cannot simply use a low class rating based on atmospheric pressure; the risk of external pressure buckling must be evaluated, and the rating is usually raised by one class.The operating condition parameters shall be attached as an annex to the class rating determination and written into the technical agreement, serving as the basis for the supplier's quotation and for later dispute resolution.
(II) Step 2: Retrieve the ASME temperature-pressure rating table according to material group
ASME B16.5 and B16.9 divide forging and piping fitting materials into multiple material groups; A105 belongs to Group 1.1, 304/316L stainless steel belongs to the corresponding austenitic group, and chrome-molybdenum steel is grouped separately.When looking up the table, first match the material group, then use the maximum design temperature as the horizontal axis to read the maximum allowable working pressure of that Class at that temperature; this pressure must be greater than or equal to the pipeline design pressure.
Example: For an A105 material Class150 flange, the allowable pressure at room temperature is approximately 19.6 bar;when the temperature rises to 260°C, the allowable pressure drops to around 10 bar.If the pipeline design pressure is 12 bar and the operating temperature is 260°C, Class150, which appears adequate at room temperature, no longer meets the requirements under high-temperature conditions and must be upgraded to Class300.This is also the biggest limitation of static conversion tables: the converted values apply only to room temperature, and all fail under high-temperature conditions.
(III) Step 3: Safety margin verification and preliminary class selection
For chemical, oil and gas, high-temperature, and highly toxic media GC1 pipelines, a safety margin must be added on top of theoretical compliance.For conventional stable operating conditions, a margin of 1.1 to 1.2 times is recommended;for cyclic loading, media corrosion, frequent pressure fluctuations, and hydrogen service pipelines, a margin of 1.25 to 1.5 times should be used;Important interlock pipelines and tank inlet/outlet pipelines should be directly upgraded by one pressure class.
After margin verification, the preliminary pressure class is formed.Prefer standard series Class150, 300, 600, 900, 1500, 2500, and avoid non-standard intermediate grades. Non-standard pressure classes have long supply cycles, few sources of matching gaskets and bolts, and high costs.Procurement personnel must not blindly pursue high pressure classes. The higher the pressure class, the flange thickness, weight, and cost increase simultaneously, and support loads and installation costs increase, causing unnecessary waste.
(IV) Step 4: Consistency determination of the complete sealing joint (key control point)
For the same flange joint, the pressure classes of the four components—flange, butt-welded pipe fitting, gasket, and fastener—must be unified. This is a high-frequency management failure point in procurement.In many projects, flanges are Class300 while matching pipe fittings are supplied only as Class150, or the gasket pressure class is too low, creating a weakest-link weak point.Determination rule: for one sealing joint, all pressure-bearing matching components shall be no lower than the selected pressure class;The temperature-pressure rating capability of gaskets, ring gaskets, and bolts must not be lower than that of the flange body itself.
At the same time, verify that the sealing face types match; RF raised face, RTJ ring joint face, and MFM male-female face must not be mixed arbitrarily;RTJ ring groove flanges must be paired with dedicated ring gaskets of the same pressure class; low-pressure-class ring gaskets cannot meet high-pressure sealing requirements.This judgment rule is independent of the pressure-bearing calculation and is a mandatory structural matching item; if it is not satisfied, the item is directly judged as not supplyable.
(5) Step Five: Separate verification of SCH wall thickness and pressure class
There is a widespread misconception in procurement: once the pressure class is selected, the pipe SCH wall thickness automatically matches.In reality, Class is the pressure rating of flanges and forged pipe fittings, while SCH is the steel pipe wall thickness schedule; the two are two independent systems.Even if the flange is selected as Class 300, the wall thickness of straight pipes, butt-welded tees, and elbows must still be calculated separately according to GB/T20801 or ASME B31.3, verifying corrosion allowance, forming thinning, and negative deviation; the pipe wall thickness cannot be directly inferred from the flange pressure class.
When preparing the procurement list, the four parameters NPS, SCH, material, and Class shall be marked independently, and the technical agreement shall separately stipulate the acceptance standards, so as to avoid the supplier inferring wall thickness from the pound rating, which could lead to the risk of insufficient pipe wall thickness.
(6) Step Six: Determination of Equivalent Conversion between International Standard Pound Ratings and PN
When a project requires interfacing American standard pound ratings with national standard PN, direct numerical equivalence conversion is prohibited; the three principles of equivalence determination shall be implemented: First, at the same design temperature, the allowable pressure of pound-rated components shall be ≥ the rated pressure of PN components;Second, priority shall be given to using reducing conversion flanges and transition nipples, rather than forcibly butting them directly;Third, the equivalent scheme must be approved in writing by the design unit, forming a material substitution sheet, which shall be retained and archived by the procurement department.
When used only as a rough reference during the inquiry stage, a room-temperature comparison table may be used, but in technical agreements, drawings, and supervision and inspection documents, approximate conversion values must not be used as a formal basis.Retrofit projects and domestic projects supporting imported equipment are most prone to interfacing errors here.
(7) Step Seven: Determination of Verification for Pound-Rating Documents, Marking, and Pressure Testing
Once the class rating is selected, procurement adds data verification clauses to the technical agreement and incoming inspection: the steel stamps on flanges and forged products must clearly indicate Class, material group, and heat number;the quality certification documents shall include a compliance statement with the ASME pressure-temperature table for the corresponding material group;the hydrostatic test pressure shall be executed in accordance with ASME B16.5, not the PN test pressure rules.Passing the pressure test does not mean the class rating selection is correct; the pressure test is a short-term strength verification, while the pressure-temperature rating capability is the basis for long-term operation.
IV. Identification and Prevention Strategies for Common Pitfalls in Class Rating Procurement
Pitfall 1: The Class number equals the psi pressure, i.e., Class150 = 150psi.Correction: Class is a grade designation and is dimensionless; the allowable pressure for A105 at ambient temperature Class150 is approximately 285psi, not 150psi.
Pitfall 2: One conversion table applies to all temperatures and materials.Correction: Each material group has its own separate temperature-pressure curve; as temperature rises, pressure continuously decreases, and high-temperature operating conditions must be checked against the table.
Misconception 3: Once the flange pressure class is determined, fittings and gaskets can be downgraded.Correction: All pressure-bearing components at a sealing joint must have a unified pressure class; the weakest link determines the pressure-bearing upper limit of the entire joint.
Misconception 4: The higher the pressure class, the wall thickness will definitely be sufficient.Correction: SCH wall thickness is calculated independently; a high-pressure-class flange does not mean the straight pipe wall thickness is qualified.
Misconception 5: Equivalent conversion only considers room-temperature pressure-bearing capacity and ignores flange bolt circle dimensions.Correction: Pressure-bearing equivalence and dimensional mating are two separate matters; if dimensions do not match, installation is impossible even if strength is sufficient.
Misconception 6: Negative-pressure pipelines are selected according to normal-pressure low pressure classes.Correction: Negative pressure pipelines are governed by external pressure buckling, and the rating selection requires a separate verification, generally upgrading to a higher rating.
V. Methods for implementing rating determination in procurement documents
The rating determination conclusions need to be formalized in the inquiry sheet, technical agreement, procurement list, and drawing clarification minutes.Example of list annotation format: NPS2, Class300, A105, RF, with the straight pipe SCH annotated separately.The technical agreement shall specify the governing standards ASME B16.5 and B16.9, clearly state the material group, attach the temperature-pressure verification table adopted for this project as an appendix, and stipulate that the supplier must manufacture according to the temperature-pressure rating values corresponding to that rating, and must not simplify manufacturing based on approximate PN grades.
At the supplier quotation stage, procurement must verify whether the supplier's understanding of the rating is consistent with the determination conclusion, to prevent the supplier from interpreting it based on approximate room-temperature values and downgrading the configuration for high-temperature operating conditions.During supervision and factory acceptance, check the rating markings against the steel stamps and material certificates, and ensure that the material group in the certificates is consistent with the group used in the determination.When operating conditions change or the temperature increases, the entire rating determination process must be restarted, rather than continuing to use the original rating.
VI. Conclusion
The core of the procurement determination method for pipeline and fitting pressure ratings is to abandon the simple numerical conversion mindset and establish a seven-step determination logic: "working conditions first, table lookup for rating determination, margin reinforcement, complete-set consistency, independent wall thickness, equivalent approval, and documentation closed loop."The essence of Class ratings is a rating system that links material, temperature, and pressure; the same rating class has vastly different load-bearing capacities under different materials and temperatures, and simple conversion tables cannot be relied upon as a procurement basis.
In petrochemical, new energy, and overseas EPC projects, the risks brought by incorrect rating class selection are far higher than those of ordinary national-standard PN fittings.By mastering this determination method, procurement personnel can identify hidden risks in rating class selection throughout the entire process of material requisition, inquiry, contract signing, and arrival inspection, unify the technical understanding of rating classes between supply and demand parties, avoid problems such as flange connection failures, insufficient high-temperature pressure bearing capacity, and inconsistencies between documentation and supervision inspection, and achieve a balance among safety, compliance, and economy in rating class selection while meeting the safety bottom line for pressure pipeline special equipment.
References
[1] ASME B16.5-2023, Pipe Flanges and Flanged Fittings [S]
[2] ASME B16.9-2021, Factory-Made Wrought Butt-Welding Fittings [S]
[3] TSG 31-2025, Safety Technical Regulations for Industrial Pipelines [S]. State Administration for Market Regulation, 2025.
[4] GB/T 20801-2020, Pressure Piping Code: Industrial Piping [S]. Beijing: Standards Press of China, 2020.
[5] GB/T 12459-2017, Steel Butt-Welding Seamless Pipe Fittings [S]. Beijing: Standards Press of China, 2017.
[6] HG/T 20615-2009, American System Steel Pipe Flanges, Gaskets, and Fasteners [S]. Beijing: Chemical Industry Press, 2009.
[7] Li Gang. Practical Guide to Design and Selection of Pressure Piping Components [M]. Beijing: Atomic Energy Press, 2022.