I. Overview of the American Standard Flange Pressure Rating System

American standard fittings and flanges utilize the Class rating system established by the American Society of Mechanical Engineers (ASME), which is distinct from Europe's PN pressure rating system. This system classifies pressure ratings using pound-class (LB) units, consisting of seven standard levels: Class 150LB, 300LB, 400LB, 600LB, 900LB, 1500LB, and 2500LB. Among these, Class 150LB serves as the lowest pressure rating within the system, establishing the baseline threshold for industrial piping systems.

The pressure rating is determined based on the Maximum Allowable Working Pressure (MAWP) at a reference temperature of 38°C (100°F) as specified by the ASME B16.5 standard. It is worth noting that the Class numerical value does not directly correspond to the working pressure value; rather, it represents the structural strength class of the flange. For instance, a Class 150LB flange has an actual pressure-bearing capacity of approximately 1.96 MPa at room temperature, which is equivalent to 20 kilograms-force per square centimeter.

II. Detailed Technical Parameters of Class 150LB

As the lowest pressure standard in the American standard flange system, Class 150LB possesses the following typical characteristics:

Basic Parameters of American Standard Flange Class 150LB

Flanges of this rating are typically manufactured using ASTM A105 carbon steel or ASTM A182 F304 stainless steel, with the sealing face primarily in the Raised Face (RF) form. In the petrochemical sector, Class 150LB flanges are widely applied in transport systems for non-hazardous media such as low-pressure steam, circulating water, and compressed air.

III. Pressure Unit Conversion Relationships

The conversion between American standard and metric pressure units is a critical piece of knowledge in engineering applications. For a Class 150LB flange, the conversion of its pressure value follows this relationship:

1 psi=0.00689 MPa

From this, we get:

150 psi=150×0.00689=1.034 MPa

However, the actual marked pressure is rounded to 2.0 MPa to account for safety factors and engineering conventions. The following general industry algorithms are used for conversions between different ratings:

When the pound class is ≤ 300LB:

MPa=75LB​

When the pound class is > 300LB:

MPa=60LB

IV. Key Points in Engineering Selection Practice

Although Class 150LB is the lowest pressure standard for American standard flanges, the following points must still be considered in practical engineering applications:

Significant Impact of Working Temperature on Pressure-Bearing Capacity: When the medium temperature exceeds 38°C, corrections must be made according to the temperature-pressure derating curves specified in ASME B16.34. For example, under a working condition of 200°C, the allowable working pressure of a Class 150LB flange drops to approximately 1.3 MPa.

Medium Characteristics Dictating Material Selection: For corrosive media such as sour oil and gas, even if the pressure falls within the Class 150LB range, high-strength, corrosion-resistant materials like ASTM A694 F52 should be considered. In refinery practices, sour environments often implement the "upgraded selection" principle, utilizing Class 300LB as the actual minimum standard.

Flange Structural Form Affecting Sealing Performance: Welding Neck (WN) flanges are more suitable for situations with pressure fluctuations than Slip-On (SO) flanges. This structural difference is frequently overlooked in low-pressure systems, yet it is a critical factor in preventing leakage accidents.

V. Correspondence with National Standard (GB) Flanges

The PN20 flange in the National Standard GB/T 9119 series is the closest to Class 150LB, but important differences exist between the two:

Dimensional Tolerances: American standard flanges have stricter bolt hole circle diameter tolerances (±0.8mm vs ±1.5mm).

Sealing Face Roughness: The American standard RF face requires Ra≤3.2μm, which is higher than the national standard requirement of Ra≤6.3μm.

Pressure-Temperature Ratings: The American standard provides complete temperature-pressure curves, whereas the national standard only marks the room-temperature pressure.

Due to these differences, detailed verification must be conducted during replacement or substitution; simple pressure rating matching can introduce latent safety hazards. In multinational projects, it is recommended to prioritize the original design standard system.

VI. Standard Evolution and Special Applications

The latest version, ASME B16.47-2020, has eliminated the Class 400LB rating, establishing Class 150LB to 2500LB as the standard pressure sequence. In specialized fields such as LNG receiving stations, although the working pressure is typically below 1.6 MPa, the actual minimum standard for selected flanges is often upgraded to Class 300LB paired with austenitic stainless steel materials due to low-temperature operating conditions (-162°C).

The nuclear power sector has developed its own distinct "Nuclear Grade Flange" standards. Although their lowest pressure rating refers to Class 150LB, additional special requirements such as impact tests and intergranular corrosion tests have been added, reflecting the extended development of basic standards driven by industry-specific applications.