GASKETS FOR FLANGES
Gaskets for flanges are used for pipelines’s flanged connections. Gaskets can be grouped into three main families: soft (nonasbestos fibers), semi-metallic (Camprofile, jacketed and spiral wound) and metallic (ring joint style R, RX, and BX). In this article, we define gaskets, give an overview of the most common types, suggest how to select the right gasket and, recall ASME B16.20 and ASME B16.21 – the two standards that define the dimensions of gaskets for petrochemical use.
WHAT ARE GASKETS?
Flange gaskets create a static seal between the faces of the matings flanges of a flanged connection. Flange gaskets, regardless their manufacturing material and type, fill the spaces and the irregularities existing on the faces of the bolted flanges to ensure a leak-free connection.
Gaskets generate a very strong seal that prevents fluids and gases from leaking out of the flanged connection. The installation of gaskets between flanges is a mandatory requirement, i.e. it is not possible and safe to tighten flanges without using gaskets (flanges shall never be bolted without gaskets!).
MAIN TYPES OF GASKETS
There are several types of gaskets for flanged connections. The selection of the correct type of gasket depends on multiple process parameters, such as the temperature, the pressure and the media flowing through the pipeline, the type of flanges and their material grade, the dimensions and the manufacturing norms for the flanges used to join the pipes. The three main families of flange gaskets (based on their construction material) are:
- metallic gaskets (ring joint and spiral wound type, i.e. RTJ and SW)
- semi-metallic gaskets (“Camprofile” and jacketed gaskets)
- non-metallic gaskets (flat, soft, sheet and die-cut gaskets)
HOW TO CHOOSE GASKETS
The factors to consider to select the correct type of gasket for a flanged connection are:
The gasket should obviously not be affected by fluid being sealed over the whole range of operating conditions. The chemical resistance chart shows the resistance of many common jointing materials to a variety of chemicals. This is a guide only and should any doubt exist, then the gasket manufacturer should be consulted.
The Chemical Resistance Chart for Gaskets
|Material||Temp. (F)||P Max (psi)||Thick. (in.)||Applications/Features|
|Butyl||-40 to 225||150||1/16 to 1/4||Gases, inorganic acids & alkalis. Excellent weather/abrasion resistance.|
|EPDM||-40 to 212||150||1/16 to 1/4||Water, steam, animal/vegetable oils, oxygenated solvents. Excellent weather resistance.|
|Natural (Pure Gum)||-20 to 140||100||1/32 to 1||Acids, organic salts & alkalis. Non-toxic. Abrasion resistant. Soft.|
|Neoprene||-20 to 170||150||1/32 to 2||Oil/gasoline. Excellent weather resistance.|
|Neoprene – Cloth Inserted||-20 to 170||150||1/32 to 1/4||Oil/gasoline. Excellent weather resistance. Handles movement. High tensile strength.|
|Nitrile (NBR, Buna-N)||-25 to 170||150||1/32 to 2||Oil/Aromatic fuels, mineral, animal and vegetable oils, solvents and hydraulic fluid. Available in commercial, premium and FDA grades.|
|SBR (Red Rubber)||-20 to 170||150||1/32 to 1/4||Air, hot/cold water.|
|SBR – Cloth Inserted||-20 to 170||150||1/16 to 1/4||Air, hot/cold water, saturated/ low-pressure steam. Excellent for high compression loads. Handles movement.|
|Silicone||to 400||150||1/32 to 1/4||High-temperature air or water (not oil or steam). Soft. Available in FDA grade.|
|Vinyl||20 to 160||150||1/16 to 1/4||Water, oxidizing agents. Excellent weather/abrasion resistance|
|Viton||to 400||150||1/32 to 1/4||Oil/Aromatic fuels, mineral, animal and vegetable oils, solvents and hydraulic fluid.|
mild organic acids/alkalis. Excellent aging properties.
|400||900||40||Strong organic & inorganic acids/oils/aromatic hydrocarbons, powerful oxidizing agents.|
|800||1200||15||Fuel/oils. Good general purpose material.|
|800||1500||15||Steam/oil/fuel/solvent. Excellent general purpose material with wide chemical resistance.|
– Metal Inserted
|850||2500||15||Hot gases. High load/stress environments. Available with galvanized low carbon steel foil or mesh insert.|
|Pure PTFE||500||800||35 – 55||Excellent chemical resistance.|
|Filled PTFE||500||1200||11 – 40||Excellent chemical resistance.|
|Filled PTFE – Metal Inserted||500||2500||20||Excellent chemical resistance. 316 SS perforated core.|
|Expanded PTFE||600||3000||30||Excellent chemical resistance. Highly compressible.|
|Carbon or Graphite/Nitrile||840||1900||20||Excellent for steam. Excellent chemical resistance except powerful oxidizing agents.|
|Carbon or Graphite/SBR||900||2000||14||Excellent for steam. Excellent chemical resistance except for powerful oxidizing agents.|
|Pure Flexible Graphite||950||2100||5||Excellent chemical resistance except for powerful oxidizing agents. Available laminated or homogeneous.|
|Pure Flexible Graphite – Metal Inserted||950||2800||7||Excellent chemical resistance except for powerful oxidizing agents. Available with 316 SS Foil, Mesh or Tang Core. Available laminated or homogeneous.|
|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.|
|1000||A general-purpose, soft, corrosion-resistant, non magnetic stainless that will not harden under heat.|
|1000||Not as strong as 304, but more corrosion-resistant in chemical solutions (except for a limited range of oxiding acids)|
|1600||Stronger than 304. Used when similar performance to 304 is needed at higher temperatures.|
|1600||More corrosion-resistant and harder than 321.|
|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.|
|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 temper|
The gasket selected should have a reasonable life expectancy at the maximum temperature encountered (or the minimum temperature if for a low-temperature application). A broad indication of the temperature-pressure ratings of the common gasket materials is shown in the figure below. Gasket materials are designed to compress under load to achieve the initial seal. However, to retain that seal, the gasket should be able to resist flow (or creep) to prevent loss of surface stress by bolt reduction. This property is very important and is the one that most readily separates high quality from low-quality gaskets. Under ambient temperature conditions, most gasket materials do not creep significantly, but as the temperature rises beyond 100°C, creep becomes a serious consideration. For all applications but particularly for low-temperature applications, the following points should be observed:
- The gasket should be completely dry when installed (gaskets for such applications should be stored in a dry atmosphere).
- The required flange loading should be applied at ambient temperature.
The gasket has to be suitable for the maximum internal pressure experienced; this is often the test pressure, which can be > 2 times the flange rating at ambient temperature. Vacuum conditions need special considerations but as a guide:
- For coarse vacuum (760 torr to 1 torr): flat rubber or compressed asbestos fiber gaskets.
- For high vacuum (1 torr to 1×10-7 torr): rubber ‘O’ rings or molded rectangular seals.
- For very high vacuum (below 1×10-7 torr): specialized seals required.
Heightened awareness with regard to Health and the Environment is leading to new and more stringent standard procedures and legislation.The goal is to reduce emissions to target levels currently based on the best available technology. These targets will inevitably become tighter in the future. Large companies have been aware of the issues for a number of years and will have a major part to play in the future standards and legislation. As early as 1994, Fugitive Emissions had a major impact on meeting the challenge set by major petrochemicals companies. Some companies set their own high standards ahead of incumbent legislators.
There are many factors apart from those already considered that affect the selection of the correct gasket material and type:
- Cycling conditions.
- If the service conditions include frequent thermal or pressure cycles, then the gasket has to be resilient enough to allow for the flange movements and strong enough to resist the mechanical loading.
a) Vibration: If the pipeline is subjected to undue vibration, then the gasket has to withstand the mechanical effects involved.
b) Erosive Media: Certain media (e.g. solids suspended in liquids) can slowly erode gaskets leading to a much shorter life than expected. In such cases, choice of gasket material and selection of gasket dimensions are critical.
c) Risk of Contaminating the Fluid: Sometimes the effect of contaminating the fluid by leaching chemicals from the gasket should be considered. Typical examples are in the sealing of potable water, blood plasma, pharmaceutical chemicals, food, beer, etc.
d) Corrosion of Flanges: Some flange metals are prone to stress corrosion cracking (e.g. austenitic stainless steel).When using these, care should be taken to ensure that the gasket material does not contain an unacceptable level of leachable impurities which may induce corrosion. Such impurities include chloride ions.
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) Budget: Although a gasket is a relatively low priced item, the consequential expense of leakage or failure should be considered when deciding on quality, type, and material of the gaskets.
ASME B 16.20 / ASME B16.21
ASME B16.20 (metallic flange gaskets: ring joint, spiral wound, jacketed, “Camprofile”)
B16.20 offers comprehensive solutions applying to materials, dimensions, tolerances and marking. It addresses gaskets that are dimensionally suitable for use with flanges described in reference flange standards ASME B16.5, ASME B16.47, API Specification 6A, and ISO 10423 as well as with other ASME standards, such as the Boiler and Pressure Vessel Code and the B31 Piping Codes.
Notable revisions to the 2012 edition include a new chapter on Grooved Metal Gaskets with Covering Layers and updates to material tables.
ASME B16.21 (non-metallic gaskets, flat cut/soft)
This Standard covers types, sizes, materials, dimensions, tolerances, and markings for nonmetallic flat gaskets. These gaskets are dimensionally suitable for use with flanges described in the referenced flange standards.