Understanding the Two Gasket Families
When you specify a flange gasket, the first decision is whether the application calls for a metallic or a nonmetallic design. The ASME standards that govern these products are clear about their scope: ASME B16.20 covers metal ring-joint, spiral-wound, and metal-jacketed gaskets, while ASME B16.21 covers nonmetallic flat gaskets. Both standards define dimensions, tolerances, and marking requirements, but they do not tell you which gasket is right for your process. That decision rests on a careful evaluation of service conditions, flange design, and assembly practices.
For EPC engineers and procurement managers, the distinction matters because the two families behave very differently under pressure, temperature, and chemical exposure. Metallic gaskets are typically used in high-pressure or high-temperature services where nonmetallic materials would fail. Nonmetallic gaskets, on the other hand, are often chosen for lower-pressure applications where chemical compatibility and cost are primary concerns. However, there is no universal rule—each joint must be evaluated on its own merits.
What ASME B16.20 and B16.21 Actually Cover
ASME B16.20 specifies materials, dimensions, tolerances, and markings for three types of metallic gaskets: ring-joint, spiral-wound, and metal-jacketed. These gaskets are dimensionally suitable for flanges described in ASME B16.5, B16.47, and API 6A. The standard also covers spiral-wound and metal-jacketed gaskets for raised-face and flat-face flanges. Importantly, the standard does not guarantee that a gasket will perform in every service; it only ensures dimensional compatibility and consistent manufacturing.
ASME B16.21 covers nonmetallic flat gaskets, including types, sizes, materials, dimensions, tolerances, and markings. These gaskets are dimensionally suitable for flanges described in the referenced flange standards. Again, the standard does not address chemical compatibility or pressure-temperature limits—those are left to the engineer.
Both standards are maintained on a stabilized basis, meaning they are no longer actively revised but remain in effect. This stability is beneficial for procurement because it provides a fixed reference for inspection and quality control.
The Role of ASME PCC-1 in Joint Assembly
Selecting the correct gasket is only half the battle. ASME PCC-1 provides guidelines for pressure-boundary bolted flange joint assembly (BFJA). It applies to flanged joints with ring-type gaskets that are entirely within the bolt circle. The standard emphasizes that gasket selection and assembly are part of a service-specific joint procedure. It includes guidance on training and qualification of assemblers, alternative bolt-up patterns, and troubleshooting leak-tight performance.
For buyers, this means that even the best gasket will fail if the joint is not assembled correctly. When you purchase gaskets, you should also consider the assembly procedure that will be used. If your project does not have a qualified bolted joint assembly procedure, you may need to develop one in accordance with PCC-1.
Key Selection Factors: What to Evaluate
Before you choose between metallic and nonmetallic gaskets, you must evaluate the following:
- Fluid compatibility: The gasket material must be resistant to the process fluid. For aggressive chemicals, nonmetallic materials like PTFE may be suitable, but for high-pressure steam or hydrocarbons, metallic gaskets are often required.
- Temperature and pressure: Metallic gaskets generally handle higher temperatures and pressures than nonmetallic ones. However, spiral-wound gaskets can be used in moderate services with appropriate filler material.
- Flange facing and surface finish: The gasket must match the flange facing (e.g., raised face, flat face, ring joint) and the surface finish must be appropriate for the gasket type. Nonmetallic gaskets typically require a smoother finish than metallic ones.
- Fire and emissions requirements: Some services require fire-safe gaskets or low-emission sealing. Metallic gaskets often provide better fire resistance, but nonmetallic gaskets with special fillers can also meet these requirements.
- Assembly load: The gasket must be able to withstand the bolt load without crushing or relaxing. PCC-1 provides guidance on calculating the required assembly load.
Procurement Checks and RFQ Inputs
When you issue an RFQ for gaskets, include the following information to ensure you receive the correct product:
- Flange standard and size: Specify the flange standard (e.g., ASME B16.5) and the nominal pipe size and pressure class.
- Gasket type: Indicate whether you need a ring-joint, spiral-wound, metal-jacketed, or nonmetallic flat gasket.
- Material specification: For metallic gaskets, specify the metal and filler material. For nonmetallic gaskets, specify the material grade (e.g., PTFE, rubber, compressed fiber).
- Dimensions: Provide the gasket dimensions or reference the standard that defines them.
- Marking requirements: Ensure the gasket is marked in accordance with the applicable standard.
- Quantity and packaging: Specify the quantity and any special packaging requirements.
During inspection, verify that the gaskets meet the dimensional tolerances and marking requirements of the relevant standard. Check for any damage during shipping, as gaskets are easily deformed.
Common Risks and How to Avoid Them
One common risk is selecting a gasket based solely on price or availability without verifying its suitability for the service. Another is assuming that a gasket certified to ASME B16.20 or B16.21 is automatically safe for your process. These standards only cover dimensional and material requirements, not performance under specific conditions.
To mitigate these risks, always consult the gasket manufacturer's data sheets and compare them with your service conditions. If you are unsure, consider conducting a joint integrity test or consulting a specialist.
Conclusion
Choosing between metallic and nonmetallic flange gaskets is a decision that requires careful analysis of service conditions, flange design, and assembly practices. ASME B16.20 and B16.21 provide the dimensional and material standards, while ASME PCC-1 guides the assembly process. By evaluating all factors and including the right information in your RFQ, you can avoid costly failures and ensure leak-tight performance.
For more information on flanges and gaskets, explore our products page or contact us for assistance.
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