Author: Beijing Guojie Management Co., Ltd.
I. Standard Origins and Core Definitions of the Two Pressure Systems
In the field of industrial pressure-bearing equipment, PN nominal pressure (kilogram class) and Class pressure rating (pound class) are the two major mainstream pressure classification systems operating in parallel worldwide, corresponding respectively to the European standard EN/national standard system and the American standard ASME system, and are widely used in the full range of pressure-bearing products such as pipes, elbows, tees, flanges, pipe caps, valves, and pressure vessels.The root cause of the vast majority of foreign trade order errors, on-site mismatches, acceptance failures, and replacement failures is confusion over the conversion logic of the two systems, simply substituting numerical equivalence for standard equivalence.
PN (Nominal Pressure) is the common nominal pressure system used in European standards, national standards, and Russian standards, with the unit bar (kilogram-force), a reference temperature of 120°C, and represents the maximum allowable long-term working pressure of the medium under 120°C working conditions.The core logic of the PN system is calibration at normal temperature, margin at low temperature, and derating at high temperature, with regular classification and clear linearity, suitable for standardized selection in Eurasian engineering.
Class (Lb pound class) is the core pressure system of the American standards ASME B16.5 and B16.9, with no direct pressure unit, and classifies pressure-bearing capacity by rating.Its core feature is high-temperature reference design: Class150 uses 260°C as the calibration reference, and higher grades such as Class300 and above use 454°C as the design temperature.The essence of the American standard system is high-temperature pressure retention with a large surplus at room temperature, and its safety redundancy is far higher than that of the PN rating with the same numerical value. This is also the core underlying reason why the two systems cannot be directly replaced with equivalent values.
II. Core Underlying Differences Between Class and PN (Key Prerequisites for Conversion)
To accurately master the conversion logic, one must abandon the "numerical comparison" mindset and first understand the three essential differences, which are the foundation of all engineering conversions and foreign trade replacements.
First, the design reference temperature is completely different.The PN system sets pressure ratings at 120°C and is suitable for conventional plumbing and heating, room-temperature oil and gas, and ordinary industrial media;The Class system takes high-temperature working conditions as its design baseline, and the actual pressure-bearing capacity of products at room temperature will be significantly higher than the nominal rating.For example, Class300 bears 50 bar at 454°C, and at room temperature of 20°C it can bear more than 75 bar, far exceeding products of the same PN standard level.
Second, the pressure-temperature decay curves are inconsistent.PN products' pressure drops rapidly and linearly as temperature rises, leaving insufficient pressure margin under high-temperature working conditions;Class products degrade gently at high temperatures and offer excellent high-temperature resistance, making them suitable for high-temperature steam, high-heat oil and gas, and refining and petrochemical units.At the same numerical rating, American-standard products have greater wall thickness, material redundancy, and structural strength.
Third, the applicable scenarios and certification systems of the standards differ.PN is compatible with the EN, GOST, and GB systems and is mainly used for conventional engineering in Europe and Asia;Class is compatible with the ASME and API systems and is mainly used for petroleum, chemical, energy, and high-end EPC projects. The flange sealing faces, bolt hole spacing, and structural dimensions of the two systems are not fully interchangeable; they are only approximately equivalent in pressure and must not be swapped or assembled at will.
III. Industry-Standard Conversion Comparison Table (Room-Temperature Engineering Equivalence)
The commonly used approximate equivalence standard at room temperature of 20°C for industrial foreign trade, drawing conversion, and procurement selection is an industry-recognized compliant comparison relationship that can be used for quotations, stocking, and replacement, and is also the most commonly used practical conversion basis for enterprises:
Class150 ≈ PN20 (commonly used in engineering as a substitute for PN16)
Class300 ≈ PN50 (commonly used in engineering as a substitute for PN40)
Class600 ≈ PN100
Class900 ≈ PN150
Class1500 ≈ PN250
Class2500 ≈ PN420
Key point to note: The simplified formulas circulating online, "1Class ≈ 6.6PN, 1Class ≈ 7.5PN," are only rough estimates and must not be used for formal production, drawing submission for approval, or replacement of high-pressure equipment; they may only serve as a preliminary reference for inquiries.
IV. Precise Conversion Logic and Operating Condition Correction Rules
1. Normal temperature and normal pressure working conditions (≤120°C)
For conventional water supply and drainage, normal-temperature gases, normal-temperature crude oil, and ordinary transmission pipelines, the engineering equivalent comparison table can be used directly for replacement.In this case, Class products have ample pressure-bearing margin, and replacing PN products is fully safe and compliant; this is also the most commonly used replacement scenario for foreign trade exports.When the project has no special high-temperature requirements, the owner, supervisor, and customs all recognize this equivalent conversion relationship.
2. Medium-temperature working conditions (120°C–260°C)
This temperature range is the critical deviation range between the two systems, and it is prohibited to directly apply the normal-temperature comparison table.The pressure of PN products begins to decay significantly, while Class150 still maintains its rated pressure-bearing capacity.Under these working conditions, Class150 performance is significantly better than PN20, and it is strictly prohibited to use PN20 as a reverse replacement for Class150; otherwise, insufficient pressure-bearing capacity, sealing leakage, and safety hazards will occur.
3. High-temperature conditions (>260°C)
For high-temperature steam, high-temperature media in refining and chemical processing, and high-heat process pipelines, the Class rating must be taken as the governing criterion.The PN system has largely lost its pressure-retaining validity under these conditions; only Class300 and higher ratings can meet the high-temperature pressure-retention requirements.For high-temperature conditions, there is no simple PN equivalent substitution; matching can only be done American standard to American standard.
V. Core Conversion Misconceptions in Foreign Trade Procurement and Engineering Selection
Misconception 1: Direct numerical equivalent substitution
Many companies mistakenly believe that PN16 = Class150 and PN40 = Class300.Strictly according to the standard definitions, PN16 is nominally 16 bar, while Class150 at room temperature actually withstands close to 20 bar; they are not equivalent.Low-requirement civil engineering may be approximately substituted, but oil and gas, chemical, and pressure-bearing special equipment strictly prohibit numerical equivalent substitution.
Misconception 2: Bidirectional indiscriminate interchangeability
The industry's only compliant interchange logic: Class may be equivalently substituted downward for PN, but PN may not be substituted upward for Class.American-standard products have greater safety redundancy and stronger high-temperature performance, and may substitute European-standard products of the same grade;Conversely, European-standard products have insufficient high-temperature pressure-bearing capacity and small wall-thickness margins, and absolutely must not be used to replace American-standard equipment in reverse.
Misconception 3: Ignoring temperature correction and directly applying the comparison table
All fixed comparison tables apply only to normal-temperature working conditions. Once medium- or high-temperature working conditions are involved, all approximate conversions become invalid, and the pressure-bearing grade must be re-verified according to the standard temperature-pressure curve. This is also the main reason for acceptance failure in high-pressure projects.
Misconception 4: Pressure equivalence equals universal dimensions
Equivalent pressure ratings do not mean that flange dimensions, sealing faces, and installation dimensions are interchangeable.There are fixed differences between Class and PN flanges in bolt hole spacing, sealing face height, and flange thickness. Meeting the pressure requirement alone does not enable on-site assembly. During selection, the two dimensions of pressure equivalence and structural interchangeability must be distinguished.
VI. Practical specifications for foreign trade supply, drawing conversion, and project replacement
For pipeline and pipe fitting foreign trade B2B, exports to the EU, Russian-standard renovation, and localized replacement in US-standard projects, Beijing Guojie Management Co., Ltd. summarizes a standardized implementation process.
First, the order benchmarking stage: when a customer provides US-standard Class drawings, priority is given to matching domestic production standards according to the equivalent PN rating. For normal-temperature working conditions, the standard comparison table is applied directly; for high-temperature working conditions, the selection is upgraded by one grade to eliminate the risk of insufficient pressure bearing.
Second, contract marking specifications: foreign trade contracts, technical agreements, and quality inspection reports must be marked with "normal-temperature working condition Class ×× equivalent to PN ××" to clarify the applicable scope of working conditions and avoid acceptance disputes.
Finally, the principle for rectification and replacement: for localized replacement of old US-standard equipment, priority should be given to using one grade higher PN rating as a fallback;for domestic equipment exported to US-standard markets, production must strictly follow the Class rating, without downgraded equivalence.
VII. Conclusion
The conversion between Class ratings and PN kilogram ratings is by no means a simple mathematical numerical conversion, but rather a systematic engineering benchmarking involving temperature baselines, design redundancy, adaptation to operating conditions, and differences in standards systems.The two pressure systems each have their own applicable scenarios: the PN system is regular and universal, suitable for conventional Eurasian engineering; the Class system is high-temperature stable, has high safety redundancy, and is suitable for high-end energy and chemical projects.
Foreign trade pipeline enterprises, engineering procurement personnel, and technical personnel must abandon the simple conversion mindset and distinguish the core boundaries between normal-temperature equivalence and high-temperature dedicated use, forward substitution and reverse prohibition, and pressure equivalence and dimensional universality.A precise grasp of this conversion logic can effectively avoid risks such as selection errors, production rework, on-site assembly failures, and foreign trade rejection claims, and comprehensively improve the standardization, compliance, and professionalism of cross-border supply of industrial pipeline fittings.