Valve Gasket Selection: Types, Materials, and Pressure-Temperature Limits

Introduction: Why Gaskets Matter

A gasket is a compressible material placed between two flanges (or between valve body and bonnet) to create a pressure-tight seal. A wrong gasket choice causes leaks, fugitive emissions, and flange joint failure. Gasket selection must consider fluid type, temperature, pressure, and flange facing.

This article explains gasket types (sheet gaskets, spiral wound gaskets, ring joint gaskets), materials (PTFE, graphite, metallic), and how to select the correct gasket for your valve flange joint.

Sheet Gaskets: Non-Metallic

Sheet gaskets are cut from compressed sheet material (PTFE, graphite, rubber, or fiber-reinforced). They are used for low-pressure, low-temperature service (PN 10 to PN 40, up to 200 degrees C for PTFE, 450 degrees C for graphite).

PTFE sheet gaskets: Excellent chemical resistance, suitable for pH 0 to 14. Limited temperature (minus 200 to plus 200 degrees C). Cold flow (creep) under compression. Graphite sheet gaskets: High temperature (up to 450 C in oxidizing atmosphere, 3000 C in non-oxidizing). Not suitable for strong oxidizers (nitric acid, concentrated sulfuric acid).

Spiral Wound Gaskets (SWG): The Industry Workhorse

Spiral wound gaskets are the most widely used gasket for refinery, petrochemical, and power plant service. They consist of a V-shaped metal winding strip (typically stainless steel 304 or 316) and a soft filler (graphite or PTFE), with inner and outer retaining rings.

SWG can handle high pressure (up to Class 2500) and high temperature (graphite filler: up to 450 C, PTFE filler: up to 200 C). The metal winding provides strength, and the soft filler provides sealability. SWG compresses under bolt load and recovers when the flange separates (because of thermal cycling or pressure fluctuation).

Ring Joint (RTJ) Gaskets: High Pressure and High Temperature

Ring joint gaskets are solid metal rings (oval or octagonal cross-section) that fit into a grooved flange (ring joint facing). They are used for high-pressure, high-temperature service (Class 600 to Class 2500, up to 800 degrees C).

RTJ gaskets are made from soft iron, low-carbon steel, stainless steel (304, 316, 347), or Inconel (625, X750) depending on the service. The gasket deforms plastically under bolt load and creates a metal-to-metal seal. RTJ gaskets are reusable only if the contact surfaces are undamaged.

Gasket Material Selection: Chemical Compatibility

PTFE: Compatible with almost all chemicals except molten alkali metals and fluorine gas. Suitable for strong acids (sulfuric, hydrochloric, nitric) and caustics. Temperature limited to 200 degrees C.

Graphite: Chemically inert to most media except strong oxidizers. Suitable for steam, hot oil, hydrocarbons, and most chemicals. Temperature limit 450 C (oxidizing), 3000 C (non-oxidizing). Not suitable for highly oxidizing media (nitric acid above 20 percent concentration).

Pressure-Temperature Rating: ASME B16.20 and ASME B16.21

ASME B16.20 covers metallic gaskets (spiral wound, ring joint, camprofile). ASME B16.21 covers non-metallic flat gaskets. Both standards specify the pressure-temperature ratings for gaskets based on the flange rating and the gasket material.

Always select a gasket with a pressure-temperature rating that exceeds the maximum operating condition. For thermal cycling service, select a gasket with good recovery (such as spiral wound with graphite filler). For cyclic service, sheet gaskets may fail because of poor recovery.

Gasket Installation Best Practices

Practice 1: Clean flange faces. Remove old gasket material, corrosion, and scratches. Practice 2: Center the gasket on the flange. The gasket must be centered on the raised face (for RF flanges) or in the groove (for RTJ flanges).

Practice 3: Tighten bolts in a cross pattern (star pattern) to the specified torque. Do not over-tighten, which crushes the gasket. Practice 4: Retorque after the first thermal cycle (for spiral wound gaskets). The gasket relaxes after the first heating, and retorquing restores the proper compression.

Common Gasket Selection Mistakes

Mistake 1: Using PTFE gaskets for high-temperature steam. PTFE creeps and loses sealability above 200 degrees C. Use graphite spiral wound gaskets for steam. Mistake 2: Using graphite gaskets for strong oxidizers. Graphite burns in nitric acid. Use PTFE or specialty oxidized-resistant gaskets.

Mistake 3: Using flat sheet gaskets for high-pressure Class 600 service. Sheet gaskets cannot withstand high bolt loads. Use spiral wound or ring joint gaskets. Mistake 4: Reusing ring joint gaskets. RTJ gaskets are designed for one-time plastic deformation. Reusing them causes leaks.

Conclusion: Select Gasket Based on Operating Condition

Gasket selection is a balance of chemical compatibility, temperature, pressure, and flange type. By following the guidelines in this article and referring to ASME B16.20 / B16.21, you can select gaskets that provide reliable, leak-free service.

At Wenzhou Wofer Valve Co., Ltd., we supply valves with appropriate gaskets for your service conditions. Contact us today for a valve quotation with gasket material specified for your application.

Contact Us

Ted Wang

Wechat / WhatsApp: +86 18267833722

Email: sales@wofervalve.com

Website: www.wofervalve.com

Wenzhou Wofer Valve Co., Ltd.