Flange gaskets can be classified in multiple ways. The most common taxonomy is based on the gasket material type:

• Non-metallic gaskets or soft gaskets (flat, soft, sheet and die-cut gaskets)
• Semi-metallic gaskets (Kammprofile, Spiral Wound, and Metal Jacketed gaskets)
• Metallic gaskets (Ring Joint gaskets, type R, RX, and BX)

ASME B16.21 covers types, sizes, materials, dimensions, tolerances, and markings for nonmetallic flat gaskets for pipe flanges. ASME B16.20 covers metallic and semi-metallic gaskets for pipeline flanges.

THE 5 GASKETS SELECTION FACTORS

Flange gasket shall be selected based on multiple process parameters, such as operating temperature/pressure, type of fluid conveyed by the pipeline, flange type, size, pressure rating, material grade, and specifications. The key factors to consider to select proper gaskets are:

1 – TYPE OF FLUID CONVEYED BY THE PIPELINE

The flange gasket should not be affected by the fluid conveyed by the pipeline over the whole range of operating conditions (pipeline operating temperature and pressure). The chemical resistance chart shows how different flange gasket materials resist specific temperatures, pressures, and fluids.

Gaskets Chemical Resistance Chart

Gasket Material	Max temperature (F)	Gasket Service Recommended
Copper	600	Excellent for steam. Excellent chemical resistance except for powerful oxidizing agents.
Brass	500	Excellent for steam. Excellent chemical resistance except for powerful oxidizing agents.
GHL	212	Excellent chemical resistance except for powerful oxidizing agents. Available laminated or homogeneous.
Grafoil ®	800	Excellent chemical resistance except for powerful oxidizing agents. Available with 316 SS Foil, Mesh or Tang Core. Available laminated or homogeneous.
Titanium	1000	Similar strength to 300 series stainless, but tougher and much less dense. Excellent resistance to chloride solutions (sea water) and bleaching solutions.
Soft Iron, Low Carbon Steel	1000	Soft. Will corrode in water. Mostly used where immersed in liquid hydrocarbons.
Stainless Steel, Type 304	1000	A general-purpose, soft, corrosion-resistant, non-magnetic stainless that will not harden under heat.
Stainless Steel, Type 316	1000	Not as strong as 304, but more corrosion-resistant in chemical solutions (except for a limited range of oxidizing acids)
Stainless Steel, Type 321	1600	Stronger than 304. Used when similar performance to 304 is needed at higher temperatures.
Stainless Steel, Type 347	1600	More corrosion-resistant and harder than 321.
Stainless Steel, Type 410	1200	Commonly referred to as “Chrome”. This stainless will harden when heat-treated. It is highly magnetic, hard and strong, but not very corrosion-resistant.
Stainless Steel, Type 430	1400	More corrosion-resistant than 410, but will not harden when heat-treated. This stainless is soft and no stronger than 300 series stainless.
Nickel	1400	Exhibits good corrosion and erosion resistance at moderate temperatures.
Monel®	1500	A family of nickel/copper alloys that offer greater corrosion and erosion resistance than nickel alone. Particularly useful in seawater applications.
Inconel®	2000	A family of nickel/chromium alloys that are non-magnetic and take corrosion resistance to elevated temperatures.
Hastelloy®	2000	A family of Nickel/chromium/molybdenum alloys for use in highly aggressive chemical environments at elevated temperatures

2 – PROCESS TEMPERATURE

The selected flange gasket should have a reasonable service life expectancy considering the most demanding conditions that the pipeline may experience (highest expected temperature-pressure conditions for high-temperature applications, and lowest temperatures for low-temperature applications).

The temperature-pressure ratings of the common gasket materials are shown in the image below as a general reference.

Gasket materials are designed to compress under load to seal the flanges. However, to retain such seal, the gasket should be able to keep a proper sealing surface and not creep significantly (an adverse phenomenon that may be activated by temperature).

3 – PROCESS PRESSURE

The flange gasket should be able to withstand the maximum pressure expected in the pipeline; this is often the test pressure, which can be at least 2 times the flange rating at ambient temperature.

4 – FUGITIVE EMISSIONS LEGISLATIVE CONSTRAINTS

More and more stringent procedures and legislation are coming about in the field of allowed fugitive emissions, and such constraints shall be duly considered during the design of a pipeline and its flanged joints.

5 – OTHER ASPECTS

The other key factors to consider for a proper gasket selection are listed below:

a) Pipeline vibration and oscillation: the gasket shall withstand the oscillations and the vibrations that may affect the pipeline

b) Fluid erosion: a properly selected flange gasket shall withstand the erosion and the corrosion coming from the fluid conveyed by the pipeline. This aspect impacts either the gasket mat, rial, type and size

c) Fluid contamination risk: For some applications, it is important to select flange gaskets materials that do not contaminate the fluid conveyed by the pipeline (for example; pharmaceutical and food applications, or gas pipelines)

d) Flanges corrosion: Some flange materials, such as austenitic stainless steel, are subject to stress corrosion cracking. Therefore, the flange gasket selection should consider the possible impurities that may induce corrosion of the flange’s metal

e) Integrity: When the integrity of a gasket is of prime importance (e.g. when sealing a highly toxic chemical), the choice of the gasket may be influenced by the requirement for a larger safety margin. As an example, a spirally wound gasket with an outer retaining ring may be selected in place of a compressed asbestos fiber gasket.

f) Potential financial impact of gaskets failures: Although a gasket is a relatively inexpensive item in the overall pipeline cost, due care should be given to the proper selection of gaskets to prevent leakages and failures that may generate way larger financial impact

As a general reference, the following table shows the recommended types of gaskets by service, pipeline temperature and pressure rating, and flange facing types: