What is the Difference Between Class 300 and Class 600 Flanges? Pressure Ratings, Specifications & Applications

Flanges are crucial components in pipeline systems, enabling the secure connection of pipes, valves, and other equipment. Understanding the distinctions between class 300 and class 600 flanges is essential for engineers and procurement professionals. 

This article compares these two flange classes based on pressure ratings, specifications, and applications. Such information will help guide the right selection for your system or business.

Pressure Rating Differences

The pressure rating of a flange determines its ability to withstand internal pressures at specific temperatures. Both Class 300 and Class 600 are governed by ASME B16.5 flange ratings. These define their maximum allowable working pressure ratings.

Maximum Allowable Working Pressure

• Class 300: 740 psi at 100°F
• Class 600: 1,480 psi at 100°F

These ratings reflect the capacity of each flange to manage pressure at standard temperatures. Higher ratings for ANSI 600 make it suitable for demanding applications.

Detailed Dimensions for ASME B16.5 Flange>>>

Temperature Derating Factors

Flange pressure-temperature limits are affected by temperature increases. For instance:

• A class 300 flange may tolerate reduced pressure at elevated temperatures compared to room temperature.
• Material-specific curves in the ASME B16.5 flange ratings guide engineers in estimating derating impacts.

Material Grade Influences

Flange material significantly affects pressure ratings. Carbon steel flange specifications are common, but high-grade alloys might be necessary for extreme conditions.

Temperature Effects

Temperature significantly impacts a flange’s performance, necessitating careful consideration.

Pressure Ratings at Elevated Temperatures

While class 300 flanges are adequate for moderate temperature systems, class 600 flanges handle more severe conditions. This is due to their superior pressure-temperature tolerances.

Maximum Temperature Limits

The material composition determines a flange’s upper-temperature limit. For example, carbon steel withstands temperatures up to approximately 800°F before requiring alternative alloys.

Material-Specific Derating Curves

When selecting between Class 300 and Class 600 flanges, it’s crucial to understand how pressure ratings decrease as temperature rises. 

Engineers rely on derating curves to predict flange performance under varying temperatures, ensuring safe operations. Below is a comparison chart illustrating how the pressure ratings for carbon steel and stainless steel 316 flanges differ between these two classes at various temperature ranges.

Temperature (°F)	Class 300 (Carbon Steel)	Class 600 (Carbon Steel)	Class 300 (SS316)	Class 600 (SS316)
-20 to 100	740 PSIG	1480 PSIG	720 PSIG	1440 PSIG
200	680 PSIG	1360 PSIG	620 PSIG	1240 PSIG
400	580 PSIG	1160 PSIG	540 PSIG	1080 PSIG
600	425 PSIG	850 PSIG	410 PSIG	820 PSIG
800	Not Recommended	Not Recommended	260 PSIG	520 PSIG

Physical Characteristics

The physical design of a flange affects its application and compatibility.

Dimensional Differences

Key Factors in Comparing Class 300 vs. Class 600 Flange Dimensions:

• Flange thickness: Flange thickness increases with class rating to handle higher pressure. Based on ANSI/ASME standards, here are approximate minimum thicknesses for carbon steel flanges: Class 300: ~1.38 inches and Class 600: ~2.12 inches
• Bolt circle diameter and number of bolt holes: Higher-rated flanges require more bolts and a larger bolt circle to distribute stress effectively. Below are example specifications for NPS 6 flanges:

Class	Bolt Circle Diameter (inches)	Number of Bolt Holes
Class 300	26.00	12
Class 600	36	20

• Face-to-face dimensions: Face-to-face dimensions also vary slightly between classes to align with performance requirements. For example:

Class 300 (NPS 6): 1/16th inch 

Class 600 (NPS 6): 1/4th inch

Material Requirements

• Minimum Material Grades: Class 300 and Class 600 flanges are typically made from the following materials, chosen for their ability to withstand specific operating conditions:

1. Carbon Steel (e.g., ASTM A105)

• Pressure Tolerance: Up to ~1480 PSIG for Class 600 at ambient temperature.
• Temperature Tolerance: Suitable for temperatures ranging from -20°F to ~800°F.
• Reason for Use: Cost-effective and widely available; suitable for low to moderate temperature and pressure applications.

2. Stainless Steel 316 (e.g., ASTM A182 F316)

• Pressure Tolerance: Up to ~1440 PSIG for Class 300 and ~2750 PSIG for Class 600 at ambient temperature.
• Temperature Tolerance: Performs well in temperatures ranging from -150°F to ~1500°F.
• Reason for Use: Superior resistance to corrosion and scaling at high temperatures, making it ideal for chemical and high-temperature environments.

3. Alloy Steel (e.g., ASTM A182 F22)

• Pressure Tolerance: Comparable to stainless steel under similar ratings.
• Temperature Tolerance: Handles extreme temperatures up to ~1200°F.
• Reason for Use: High strength and durability for demanding environments, including those with high thermal cycling.

• Metallurgical Considerations

1. Chemical Composition Control: Ensures a consistent alloy blend to provide uniform mechanical properties, such as tensile strength, yield strength, and toughness. For example, carbon steel flanges maintain a low carbon content (<0.35%) to balance weldability and strength, while stainless steel incorporates chromium (~16-18%) for corrosion resistance.
2. Structural Integrity: Focus on grain size and phase distribution, achieved through precise heat treatment, to prevent failure under cyclic loads. Flanges often undergo normalization or quenching and tempering to refine grain structures, increasing their resilience to stress and thermal expansion.

• Heat Treatment Specifications

Heat treatment for flanges includes normalization, annealing, or quenching and tempering. The process depends on the material. ASTM A105 flanges are normalized at ~1650°F to improve toughness. 

Alloy steel flanges are tempered to relieve stress. Strict temperature control prevents warping or other defects. Stainless steel may need solution annealing at ~1900°F to restore corrosion resistance.

• Material Testing Protocols

Mechanical testing ensures flanges can handle expected loads. Tests include tensile strength, yield strength, and impact testing. 

For example, a Class 600 ASTM A182 F316 flange must have a tensile strength of at least 515 MPa and a yield strength of 205 MPa. Nondestructive testing (NDT), such as radiographic or ultrasonic methods, detects internal flaws. 

Hardness testing confirms successful heat treatment. Manufacturers must provide material test certificates (MTCs) and comply with ASME B16.5 or API 6A standards.

Cost Implications

Flanges with higher pressure ratings, such as class 600, typically have increased costs due to material demands and manufacturing complexity.

Class 600 flanges cost more due to higher-grade materials, increased material volume, and manufacturing complexity. Premium materials, like ASTM A182 F316, can cost 20–30% more as they offer the strength and corrosion resistance needed for high-pressure applications. Class 600 flanges require thicker walls, larger outer diameters, and more bolt holes, resulting in up to 50% greater material use than Class 300. 

Additionally, stricter material availability, longer lead times, and precise machining tolerances increase production costs. Extensive heat treatments and stringent quality control further add to their premium price.

Material Costs

• Raw material availability: Scarcer high-grade materials increase costs.
• Volume requirements: Larger production runs might reduce unit costs but are less common for specialized classes.

Installation Considerations

1. Gasket Requirements

Gaskets differ significantly between Class 300 and Class 600 flanges due to pressure and temperature demands:

• Class 300: Typically uses soft gaskets, such as rubber or non-metallic materials, suitable for lower pressures and temperatures.
• Class 600: Requires high-performance gaskets like compressed graphite with stainless steel insertion or expanded PTFE, designed to handle higher pressures and temperatures.

Installation Guidelines: Always use a new gasket, ensure it is properly centered between flanges, and confirm it is clean and free from damage to prevent leaks and maintain a proper seal.

2. Bolting Specifications

Bolting requirements differ significantly between Class 300 and Class 600 flanges to accommodate higher pressure ratings:

• Number of Bolts: Class 600 flanges often require more bolts to distribute higher stresses evenly.
• Bolt Diameter: Larger bolt diameters are typically specified for Class 600 flanges to handle increased loads.
• Bolt Grade: Class 600 bolts use stronger, heat-treated grades, such as ASTM A193 Grade B7, compared to lower-grade bolts used for Class 300.

3. Torque Requirements

Torque requirements increase with higher flange classes due to thicker flanges and stronger gaskets. Proper torque application ensures a leak-free seal:

• Torque Application Process: Tighten bolts in a star or cross pattern, gradually increasing torque in increments to ensure uniform pressure.
• Specific Torque Values: Class 600 flanges often require 25–50% more torque than Class 300 flanges, depending on bolt size and gasket type.
• Post-Installation: Recheck and retighten bolts after the system reaches the operating temperature to address any relaxation or gasket compression.

Application Guidelines

Choosing between class 300 and class 600 depends on operating conditions.

Class 300 Applications

• Moderate pressure systems
• Standard process piping
• General industrial use
• Common service conditions

Class 600 Applications

• High-pressure systems
• Critical process services
• Severe operating conditions
• Special service requirements

Why Choose LONGAN for Your Flange Needs?

LONGAN is a trusted name in flange manufacturing, offering unmatched craftsmanship and advanced technology to meet diverse pipeline requirements.

Our Production Advantages

• Flexible manufacturing adapts processes for efficiency and quick delivery.
• 6S Management System upholds industry-leading standards for quality and work environments.
• Optimized pre-production meetings ensure flawless execution, enhancing quality, productivity, and customer satisfaction.

For all your class 150, class 300, or ANSI 600 flange needs, LONGAN guarantees reliable, high-quality solutions. Whether it’s standard components or custom requirements, we provide stable connections tailored to your pipeline system.

Let LONGAN be your partner in building safe and efficient pipeline infrastructures!

Reference

1. Experimental leak testing of 16-inch Class 300 RFWN flange with and without external bending moment; George Bibel, T. Fath, and W. Palmer; 2001 (https://www.researchgate.net/publication/291440904)
2. Finite Element Analysis on the Contact Stress of Class 600 Flange Joints; Min-Young Lee, Doo-Hyung Jeong, and Byung-Tak Kim; 2017 (https://www.researchgate.net/publication/312078397)