Gasket Types and Selection Guide – How to Choose the Right Flange Gasket

 

Gasket Types and Selection Guide

1. Introduction

In any piping system, a flange is only as good as the gasket between them.
A poorly selected gasket can lead to leakage, blowout, or chemical attack — even if the flanges are perfectly aligned.
This guide explains the major types of gaskets, their materials, and how to select the right one for your operating conditions.

---

2. What Is a Gasket?

A gasket is a sealing element placed between two flanges to prevent leakage of fluid or gas under compression.
It compensates for flange surface irregularities and maintains tightness under pressure and temperature fluctuations.

Primary Functions:

• Maintain a seal under varying pressure and temperature.
• Compensate for minor flange misalignments.
• Resist chemical or thermal degradation over time.

---

3. Main Gasket Categories

Type	Description	Typical Pressure Range	Common Material	Application
Non-Metallic	Soft materials compressed easily	Low–Medium	Rubber, PTFE, Graphite	Water, air, chemical lines
Semi-Metallic	Combination of soft and metal layers	Medium–High	Spiral wound, Metal jacketed	Steam, oil, refinery lines
Metallic	Solid metal construction	High–Extreme	SS, Inconel, Copper	High pressure, high temperature

---

4. Non-Metallic Gaskets

Non-Metallic Gaskets

4.1. Compressed Fiber Gasket

Made from aramid or graphite fibers bound with elastomers.
Ideal for water, oil, or low-pressure steam.

Pros: Cheap, flexible, easy to cut.
Cons: Limited to about 400°F (200°C).
Example: Garlock Blue-Gard, Klinger C-4400.

---

4.2. PTFE Gasket

PTFE flange gasket used in chemical process lines.

Chemically inert and ideal for corrosive environments.
Often used in chemical, food, and pharmaceutical industries.

Pros: Excellent chemical resistance, wide temperature range.
Cons: Cold flow under high pressure; not suitable for heavy bolts.
Max Temp: 260°C.

---

4.3. Graphite Gasket

Excellent for thermal cycling and high-temperature service.
Often reinforced with stainless steel foil for added strength.

Pros: Resists oxidation and pressure cycling.
Cons: Brittle and can crumble if mishandled.
Max Temp: Up to 450°C.

---

5. Semi-Metallic Gaskets

5.1. Spiral Wound Gasket (SWG)

Spiral wound gasket with outer centering ring for Class 150–600 flanges.

The most widely used type in industrial piping.
Formed by winding metal (usually SS 304/316) and soft filler (graphite/PTFE) alternately.

Pros: Handles wide range of pressure and temperature.
Cons: Requires precise flange surface and bolt load.
Standard: ASME B16.20.

---

5.2. Metal Jacketed Gasket

Soft filler material enclosed by a thin metal jacket.
Common in heat exchangers and vessel manways.

Pros: Good chemical resistance, high mechanical strength.
Cons: Not reusable; requires smooth flange faces.

---

6. Metallic Gaskets

RTJ gaskets – octagonal and oval cross-section types for API flanges.

6.1. Ring Type Joint (RTJ)

A precision-machined metal ring used in high-pressure flanged joints (API, ASME B16.20).
The gasket sits in a groove on each flange and provides metal-to-metal sealing.

Shape Types:

• R type (Oval, Octagonal) – standard for ASME 1500–2500#
• BX, RX type – for API flanges (up to 20,000 psi)

Pros: Excellent sealing at extreme pressures.
Cons: Requires precise alignment and surface finish.

---

7. Gasket Selection Criteria

Parameter	Consideration
Pressure & Temperature	Choose metallic or semi-metallic for high-pressure systems.
Media Compatibility	PTFE for chemicals, Graphite for steam, Rubber for water.
Flange Type	RF or FF flanges use soft gaskets; RTJ flanges require metallic rings.
Bolt Load	Ensure sufficient compression without damaging soft gaskets.
Reusability	PTFE and Graphite can be reused in low-stress applications; Spiral Wound and RTJ cannot.

---

8. Field Tips

• Always match gasket size and standard to the flange (ASME 150# vs 300#).
• Avoid over-tightening soft materials like PTFE — they deform easily.
• For corrosive or mixed-service lines, consider PTFE envelope gaskets (PTFE outer + graphite inner).
• Never reuse a metallic gasket after disassembly unless manufacturer allows it.

---

9. Common Failures and Causes

Failure Type	Likely Cause
Blowout	Under-tightened bolts or soft gasket under high pressure
Leakage after startup	Uneven bolt torque or flange misalignment
Chemical degradation	Wrong material selection (e.g., NBR in acid line)
Oxidation damage	Graphite gasket exposed to high oxygen flow

---

10. Conclusion

The gasket is often the smallest and cheapest part of a piping system — yet the first one to fail when chosen wrong.
Always verify the standard, pressure rating, and material compatibility before installation.
When in doubt, consult the ASME B16.20 / B16.21 standards for selection limits and recommended torque.

 

Flange Types and Their Applications – Weld Neck, Slip-On, Blind, Socket Weld, Threaded, Lap Joint

 

Flange Standards & Dimension Chart – ASME / JIS / DIN Comparison

 

Pressure Vessel Testing & Inspection – What Really Happens on Site