CL150 Spiral Wound Gasket: The Complete Technical Guide to ASME B16.20 Standard, Dimensions, Materials, and Installation

Introduction

In the complex landscape of industrial piping systems, where leak-free integrity is non-negotiable, the spiral wound gasket (SWG) stands as one of the most trusted and widely specified sealing solutions. Among the various pressure ratings available, the CL150 (Class 150) spiral wound gasket holds a particularly important position—it is the standard choice for low-to-moderate pressure raised face (RF) flanged connections in oil refineries, petrochemical plants, power generation facilities, and general process piping systems-

What distinguishes the CL150 spiral wound gasket is its ability to maintain a reliable seal across wide temperature ranges and in the presence of corrosive fluids, all while accommodating the slight flange irregularities and thermal cycling that inevitably occur in real-world operating conditions. As the most widely used semi-metallic gasket type for raised face flanges, the spiral wound gasket is governed primarily by ASME B16.20, the American standard that defines metallic gaskets for pipe flanges

This article provides a comprehensive technical overview of the CL150 spiral wound gasket, covering its construction and component anatomy, governing standards, material selection criteria, critical dimensional data, and essential installation best practices.

1. What Is a Spiral Wound Gasket and How Does It Work?

A spiral wound gasket is a semi-metallic sealing device consisting of alternating layers of a profiled metal strip (typically V-shaped or W-shaped) and a softer non-metallic filler material spirally wound together to form a continuous, resilient sealing element. The metal winding provides structural strength and mechanical resilience, while the filler material conforms to flange surface irregularities to create a tight, durable seal under compression

For CL150 applications, the gasket is designed to operate at lower seating stress requirements compared to higher pressure classes. Products such as the Flexitallic Style LS® (Low Stress) series are specifically engineered so that compression and sealing requirements are achieved under very low seating stress conditions, making them ideal for Class 150 and Class 300 flanges as well as other non-standard low-pressure flange applications

The key to the spiral wound gasket‘s exceptional performance lies in what sealing engineers term “controlled deformation and recovery.” When bolt load is applied, the V-shaped metal windings act like a precision spring system—they compress in a predictable manner, allowing the soft filler material to flow into and seal microscopic flange surface imperfections. When the system undergoes thermal expansion, pressure surges, or bolt relaxation, the metal windings exhibit elastic recovery, maintaining the required sealing contact stress even as the flange gap changes. This dynamic sealing behavior is what makes spiral wound gaskets uniquely capable of handling the cyclic thermal and pressure conditions encountered in process piping.

2. Construction and Component Anatomy

The spiral wound gasket employs a multi-component design optimized for performance, precise alignment, and long-term reliability. A complete CL150 spiral wound gasket assembly typically consists of three distinct components:

2.1 The Sealing Element (Winding Strip + Filler)

The core sealing element is formed by precision-winding a profiled metal strip—most commonly stainless steel 304 (SS304) or 316 (SS316) —together with a resilient filler material such as flexible graphite or PTFE (polytetrafluoroethylene) The winding process creates alternating metal and filler layers that, when compressed between flange faces, allow the filler to flow and fill any surface imperfections while the metal provides the necessary structural support and recovery characteristics.

2.2 Outer (Centering) Ring

The outer ring—also called the centering ring—serves several critical functions. First, it facilitates precise alignment of the gasket within the bolt circle of the raised face flange. Second, it acts as a compression-limiting stop, preventing over-compression and potential crushing of the delicate sealing element. Third, it provides additional radial strength to the overall gasket assembly. The outer ring is typically manufactured from carbon steel, which is painted or otherwise coated for corrosion protection and is color-coded according to the gasket material for easy field identification.

2.3 Inner Ring

The inner ring is an optional but highly recommended component, particularly for applications involving elevated temperatures, pressure fluctuations, or corrosive service conditions. When present, the inner ring serves to protect the sealing element from direct fluid impingement, prevents inward buckling of the windings, and provides additional structural integrity. The inner ring also acts as a thermal and erosion barrier, shielding the filler material from the erosive effects of high-velocity fluid flow. The inner ring is typically manufactured from stainless steel (SS304, SS316, or SS316L) to provide excellent corrosion resistance

Gasket configurations are designated by standardized codes. CG denotes a spiral wound gasket with an outer centering ring only (suitable for standard RF flange applications), while CGI designates a gasket with both an outer centering ring and an inner ring (recommended for more demanding service conditions). Some high-performance applications may also specify a CGI-Plus configuration with additional sealing enhancements.

3. Governing Standards: ASME B16.20 and Related Specifications

The design, dimensions, materials, and marking requirements for CL150 spiral wound gaskets are predominantly governed by ASME B16.20: Metallic Gaskets for Pipe Flanges. This standard covers spiral wound gaskets, ring type joint (RTJ) gaskets, and Kammprofile gaskets used with ASME B16.5 and ASME B16.47 flanges

The standard provides detailed specifications for:

• Inner and outer diameter dimensions by NPS and pressure class

• Winding metal and filler material options

• Outer ring (centering ring) dimensions and color-coding requirements

• Inner ring dimensions and application conditions requiring its use

• Gasket type designations (CG, CGI)

• Permanent marking requirements

For European applications, the parallel standard EN 12560-2:2013 specifies the dimensions, design, types, designation, materials, and marking of spiral wound gaskets for use with type A flat face or type B raised face flange facings complying with EN 1759-1 for Class designations Class 150 through Class 1500-. This standard applies to nominal sizes DN 15 to DN 600, with Class designation 2500 covering sizes up to DN 300

The key companion standard is ASME B16.5, which defines the dimensions and pressure-temperature ratings of the flanges that B16.20 gaskets are designed to fit (NPS 1/2 through NPS 24). For larger diameter applications, ASME B16.47 covers flanges from NPS 26 through NPS 60. Understanding this standards ecosystem is essential for proper gasket selection and specification writing.

4. Material Selection: Metal Windings and Filler Types

Material selection for CL150 spiral wound gaskets must balance the mechanical requirements of the metal winding with the sealing and chemical compatibility demands of the filler material.

4.1 Winding Strip Materials

The metal winding strip is typically manufactured from austenitic stainless steels, with SS304 and SS316 being the most common choices for standard industrial applications. The selection between these grades depends on the specific service conditions:

• SS304 (UNS S30400): Provides good general corrosion resistance and is suitable for most standard process applications. It offers excellent formability for the winding process and is the most economical stainless steel option for spiral wound gaskets.

• SS316 (UNS S31600): Offers enhanced corrosion resistance, particularly in chloride-containing environments, due to its molybdenum content. SS316 is the preferred choice for offshore applications, coastal installations, and services involving brackish water or marine atmospheres.

• SS316L (UNS S31603): The low-carbon variant provides improved resistance to intergranular corrosion after welding or elevated temperature exposure, making it ideal for high-temperature process applications

• Specialty Alloys: For highly corrosive or extreme temperature applications, materials such as Monel (nickel-copper alloy), Inconel (nickel-chromium alloy), titanium, or duplex stainless steels may be specified for the winding strip.

4.2 Filler Materials

The filler material is critical to the sealing performance of the gasket. Two primary filler types dominate CL150 applications:

Flexible Graphite Filler:
Flexible graphite (also known as expanded graphite or graphite foil) is the most widely used filler material for spiral wound gaskets in Class 150 service. Manufactured from high-purity natural graphite flakes that have been chemically treated and expanded, flexible graphite offers:

• Exceptional temperature capability from cryogenic conditions (-240°C / -400°F) up to 450°C (842°F) in oxidizing atmospheres and even higher in non-oxidizing environments

• Excellent chemical resistance to a broad spectrum of process media, including steam, hydrocarbons, heat transfer fluids, and most chemicals across a pH range from 0 to 14-

• Outstanding conformability and compression characteristics that enable effective sealing under low bolt loads

• Low creep relaxation, maintaining sealing stress over extended service periods

Flexible graphite fillers are manufactured to purity levels typically exceeding 98% elemental carbon, with high-purity grades (>99%) available for nuclear and other critical applications.

PTFE (Polytetrafluoroethylene) Filler:
PTFE-filled spiral wound gaskets are specified for applications requiring superior chemical resistance, particularly in strongly oxidizing environments, concentrated acids, or ultra-pure process streams. PTFE fillers offer:

• Near-universal chemical inertness, resistant to virtually all industrial chemicals except molten alkali metals and elemental fluorine at elevated temperatures

• Temperature capability from cryogenic conditions (-240°C / -400°F) up to 260°C (500°F)

• Compliance with FDA requirements for food-grade and pharmaceutical applications

• Excellent non-stick and low-friction properties

The selection between graphite and PTFE fillers should be based on a thorough evaluation of the process fluid composition, operating temperature range, and pressure cycling conditions.

5. Dimensions and Pressure Ratings

The standardized dimensions of CL150 spiral wound gaskets ensure proper fitment and sealing performance across the full range of standard pipe sizes. The table below presents the critical dimensional data for Class 150 raised face flanges per ASME B16.20:

Data source: ASME B16.20 standard dimensions

Standard Thickness: The standard thickness for spiral wound gaskets is 0.175 inches (4.5 mm) , which includes the compressed sealing element-. Alternative thicknesses of 3.2 mm and 6.4 mm are also available for specialized applications, with thickness tolerance of ±0.13 mm measured across the metallic portion of the gasket (excluding any protruding filler material)-

Clearance Considerations: Proper gasket sizing must account for nominal outer diameter clearance. For gaskets up to 60" O.D., a nominal clearance of 1/16" is standard; from 60" to 80" O.D., 5/64" clearance is allowed; above 80" O.D., 3/32" nominal O.D. clearance is permitted-. These clearances ensure the gasket can be installed without binding against flange surfaces while maintaining proper alignment.

6. Installation Best Practices for CL150 Spiral Wound Gaskets

Proper installation is essential to achieving the full sealing potential of a spiral wound gasket. Even the highest-quality gasket will fail if installation procedures are not followed with care and precision.

Pre-Installation Inspection:
Before installation, thoroughly inspect the flange faces, bolts, nuts, and the gasket itself. Check for any defects such as surface imperfections, warping, misalignment, or damage to the bolt holes. The flange surface finish should be within the range recommended by the gasket manufacturer, typically Ra 3.2–6.3 μm for standard raised face flanges-. Excessively rough flanges may damage the gasket during compression, while overly smooth surfaces may not provide sufficient friction to prevent gasket movement.

Flange Surface Preparation:
Cleanliness is critical. Flange faces must be completely free of dirt, rust, oil, grease, and old gasket residue. Ensure that bolt and nut threads are thoroughly clean and properly lubricated. Apply bolt lubrication to the threads and the nut face, but never apply grease or bolt lubricant directly to the gasket or the flange sealing surface, as this can cause the gasket to extrude from the flange under compression or create a lubricated path for leakage

Gasket Handling and Placement:
Handle spiral wound gaskets with care. The outer centering ring is robust, but the inner sealing element is relatively delicate. Carefully position the gasket onto the flange, using the centering ring to ensure proper alignment with the bolt holes. Take care not to nick, tear, or otherwise damage the gasket during positioning, as even minor damage can compromise sealing effectiveness

Bolt Tightening Sequence:
This is arguably the most critical step in gasket installation. Always follow a cross-pattern (star pattern) bolt tightening sequence to apply pressure evenly across the gasket face. Tighten bolts in two to three progressive passes, gradually bringing the flange faces together. The first pass should apply approximately 30% of the final target torque, the second pass approximately 60%, and the final pass 100% of the specified torque value. This incremental approach prevents uneven loading that could permanently distort the gasket or cause localized crushing of the sealing element

Final Verification and Documentation:
After the final torque pass, perform a full-circle verification using a calibrated torque wrench to confirm that all bolts have achieved the specified torque value. Document the installation torque values, bolt tightening sequence, gasket manufacturer and lot number, and the date of installation for future reference and quality assurance purposes.

7. Typical Applications of CL150 Spiral Wound Gaskets

The CL150 spiral wound gasket finds extensive application across a diverse range of industrial sectors where reliable, leak-free flange sealing is essential. Common applications include:

• Oil and Gas Refineries: Process piping for crude oil distillation, catalytic cracking, reforming units, and hydrotreating operations; tank farm piping and transfer lines; utility piping systems carrying steam, cooling water, and instrument air

• Petrochemical Plants: Chemical reactor piping, distillation column connections, heat exchanger flanges, and general process lines handling a wide variety of hydrocarbons and chemical intermediates

• Power Generation: Steam turbine auxiliary piping, boiler feedwater systems, condensate return lines, and cooling water circuits in both conventional thermal and combined-cycle power plants

• Chemical Processing: Reactors, storage tanks, and transfer lines handling acids, alkalis, solvents, and other corrosive chemicals across a pH range from 0 to 14

• Offshore Platforms: Topside process piping, utility systems, and wellhead flowlines where resistance to marine atmospheres and corrosive production fluids is essential

• Shipbuilding and Marine: Fuel transfer systems, ballast piping, seawater cooling circuits, and engine room auxiliary piping

• Pharmaceutical and Food Processing: Hygienic process lines where PTFE-filled gaskets provide FDA-compliant, non-contaminating sealing for ultra-pure product streams

• Water and Wastewater Treatment: Process piping, pump connections, and valve flanges in treatment plants handling potable water, process water, and various treatment chemicals

8. Pressure-Temperature Ratings and Operating Limits

Understanding the pressure-temperature envelope for CL150 spiral wound gaskets is essential for safe and reliable operation. The table below provides typical operating limits for standard material combinations:

Source: Flexitallic Style LS® specifications

Important Notes on Operating Limits:

• The maximum temperature limits listed are for the gasket material itself. The actual service temperature may be further limited by the flange material, bolting material, or process safety considerations.

• Graphite filler is suitable for steam service and most hydrocarbon applications but should not be used in strongly oxidizing environments at elevated temperatures, where PTFE filler is the preferred alternative.

• CL150 gaskets with graphite filler are capable of sealing at temperatures up to 450°C in non-oxidizing atmospheres. In oxidizing conditions, the maximum continuous operating temperature should be limited to approximately 450°C to prevent oxidation of the graphite filler.

• For applications involving temperature cycling, gaskets with inner rings (CGI configuration) are strongly recommended to maintain sealing integrity during thermal expansion and contraction cycles.

9. Gasket Marking and Field Identification

ASME B16.20 requires permanent marking on the gasket or its packaging to enable proper field identification and verification. The marking must include the manufacturer‘s name or trademark, the nominal pipe size and pressure class (e.g., NPS 4 CL150), the gasket type designation (e.g., SWG-CGI), the winding and ring material (e.g., 316), and the filler material (e.g., Graphite)

Additionally, the outer centering ring is typically color-coded according to the winding material for rapid visual identification in the field. Common color codes include:

• Yellow: Stainless Steel 304 windings

• Green: Stainless Steel 316 windings

• Blue: Stainless Steel 321 windings

• Gray: Carbon Steel windings

This standardized marking and color-coding system allows field verification during piping inspection and ensures the correct gasket is installed for each flanged joint, reducing the risk of material mismatch and subsequent gasket failure

Conclusion

The CL150 spiral wound gasket represents a mature, field-proven sealing technology that continues to be the gasket of choice for raised face flanged connections in low-to-moderate pressure industrial applications. Its unique combination of a resilient metal winding and conformable filler material, standardized dimensions per ASME B16.20, and broad material compatibility make it an exceptionally versatile sealing solution. By carefully considering the specific service conditions—temperature, pressure, chemical exposure, and thermal cycling—and adhering to proper material selection and installation practices, engineers and maintenance professionals can achieve reliable, long-term flange sealing performance with CL150 spiral wound gaskets across the full spectrum of industrial piping applications.