How to select the right gasket for a piping application? To choose the proper flange gasket, piping engineers should take into consideration the following 4 key factors: fluid type, process temperature/pressure, fugitive emissions Laws, other general considerations. While gaskets are relatively cheap components of the overall piping system, they are critical for its reliability and integrity: saving a few bucks with gaskets may create way larger disasters in the mid-long term, be careful!
FLANGE GASKETS SELECTION
Flange gaskets shall be selected based on multiple process factors, such as operating temperature/pressure, type of fluid conveyed by the pipeline, flange type, size, pressure rating, material grade, and specifications. Let’s now dive into each of these factors:
The first criteria to select the right type of gasket is, of course, the type of fluid conveyed by the pipeline – and the fluid temperature and pressure.
Different fluids require different gaskets materials to make sure that the flanged joint performs over a long period of time.
The chemical resistance chart shows how different flange gasket materials resist specific temperatures, pressures, and fluids.
Compressed Gaskets Chemical Resistance Chart
|Gasket Material||Max temperature (F)||Max Pressure (psi)||Gasket Thickness||Gasket Service Recommended|
|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.|
Recommended Gasket Service Materials
|Gasket Material||Max temperature (F)||Max Pressure (psi)||Creep Relaxation (%)||Recommended Gasket Service|
|Non-Asbestos/EPDM||800||1200||15||Steam/oxygenated solvents/mild organic acids/alkalis. Excellent aging properties.|
|Non-Asbestos/Hypalon||400||900||40||Strong organic & inorganic acids/oils/aromatic hydrocarbons, powerful oxidizing agents.|
|Non-Asbestos/Neoprene||800||1200||15||Fuel/oils. Good general purpose material.|
|Non-Asbestos/Nitrile||800||1500||15||Steam/oil/fuel/solvent. Excellent general-purpose material with wide chemical resistance.|
|Non-Asbestos/Nitrile (with metal insertion)||850||2500||15||Hot gases. High load/stress environments. Available with galvanized low carbon steel foil or mesh insert.|
|Non-Asbestos/SBR||800||1500||15||Steam/industrial gases, Low pressure/temperature.|
|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 for 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.|
|Stainless Steel 304||1000||A general-purpose, soft, corrosion-resistant, non-magnetic stainless that will not harden under heat.|
|Stainless Steel 316||1000||Not as strong as 304, but more corrosion-resistant in chemical solutions (except for a limited range of oxidizing acids)|
|Stainless Steel 321||1600||Stronger than 304. Used when similar performance to 304 is needed at higher temperatures.|
|Stainless Steel 347||1600||More corrosion-resistant and harder than 321.|
|Stainless Steel 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 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|
PROCESS TEMPERATURE AND PRESSURE
A second factor to consider to select the right gasket material is the working temperature and pressure of the piping system.
In particular, the gasket material shall be able to withstand the highest temperature and pressures expected for the process (highest expected temperature-pressure 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.
Gaskets and temperature
The gasket should not creep at the highest temperature and pressure expected for the process, otherwise, the flanged joint would become ineffective generating leaks.
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.
FUGITIVE EMISSIONS LAWS
The fugitive emission laws in the country of installation shall be also taken into consideration when selecting the right type of gasket for a process.
Indeed, more stringent fugitive emissions laws may drive specific decisions about the flanges and the gaskets to be used for the flanged joints of the process.
OTHER GENERAL ASPECTS
The other key factors to consider to select the type of gasket to use are:
a) Pipeline vibration and oscillation: the gasket shall withstand the oscillations and the vibrations that may affect the pipeline
b) Fluid contamination risk: For some applications, it is important to use gaskets that do not contaminate the fluid conveyed by the pipeline (for example; pharmaceutical and food applications, or gas pipelines)
c) Flanges corrosion: Some flange materials, such as austenitic stainless steel, are subject to stress corrosion cracking. This fact shall be considered when selecting the gasket type and material
d) Integrity: Toxic fluids require totally leak proof seals to prevent soil contamination or threats to human life. As a consequence, tighter and stronger gaskets may be the natural choice for these types of applications (example spiral wound gasket with outer ring vs. non-asbestos gasket)
e) Financial risk incurred in case of gasket failure: Although gaskets have a relatively low-cost impact on total piping materials costs, the selection of wrong gaskets may generate huge financial impact and savings in this respect may be dangerous both for the contractor, which may be subject to penalties, and the end-user
As a general reference, the following table shows the recommended types of gaskets by service, pipeline temperature and pressure rating, and flange facing types:
WHAT FACTORS AFFECT THE PERFORMANCE OF A GASKET?
The Flange Load
All gasket materials must have sufficient flange pressure to compress the gasket enough to insure that a tight, unbroken seal occurs. The flange pressure, or minimum seating stress, necessary to accomplish this is known as the “y” factor. This flange pressure must be applied uniformly across the entire seating area to achieve perfect sealing. However, in actual service, the distribution around the gasket is not uniform. The greatest force is exerted on the area directly surrounding the bolts. The lowest force occurs mid-way between two bolts. This factor must be taken into account by the flange designer.
Pressure in the Piping System
In service, as soon as pressure is applied to the vessel, the initial gasket compression is reduced by the internal pressure acting against the gasket (blowout pressure) and the flanges (hydrostatic end force). To account for this, an additional preload must be placed on the gasket material. An “m” or maintenance factor has been established by ASME to account for this preload. The “m” factor defines how many times the residual load (original load minus the internal pressure) must exceed the internal pressure. In this calculation, the normal pressure and the test pressure should be taken into account.
Temperature in the Piping System
The effects of both ambient and process temperature on the gasket material, the flanges and the bolts must be taken into account. These effects include bolt elongation, creep relaxation of the gasket material or thermal degradation. This can result in a reduction of the flange load. The higher he operating temperature, the more care needs to be taken with the gasket material selection. As the system is pressurized and heated, the joint deforms. Different coefficients of expansion between the bolts, the flanges and the pipe can result in forces which can affect the gasket. The relative stiffness of the bolted joint determines whether there is a net gain or loss in the bolt load. Generally, flexible joints lose bolt load.
The media being sealed, usually a liquid or a gas with a gas being harder to seal than a liquid. The effect of temperature on many fluids causes them to become more aggressive. Therefore, a fluid that can be sealed at ambient temperature, may adversely affect the gasket at a higher temperature. The gasket material must be resistant to corrosive attack from the fluid. It should chemically resist the system fluid to prevent serious impairment of its physical properties.
Gasket Outer Diameter
For two gaskets made of the same material and having the same width, the one with a larger outer diameter will withstand a higher pressure. Therefore, it is advisable to use a gasket with an external diameter that is as large as possible.
For a given material, it is a general rule that a thinner gasket is able to handle a higher compressive stresses than thicker one. However, thinner materials require a higher surface finish quality. As a rule of thumb, the gasket should be at least four times thicker than the maximum surface roughness of the flange faces. The gasket must be thick enough to occupy the shape of the flange faces and still compress under the bolt load. In situations where vibration is unavoidable, a thicker gasket than the minimum required should be employed.
In order to reduce the bolt load required to produce a particular gasket pressure, it is advisable not to have the gasket wider than is necessary. For a given gasket stress, a raised face flange with a narrow gasket will require less pre-load, and thus less flange strength than a full-face gasket. In general, high-pressure gaskets tend to be narrow.
Gasket Surface Finish
The surface finish of a gasket — which consists of grooves or channels pressed or machined onto the outer surface — governs the thickness and compressibility required by the gasket material to form a physical barrier in the clearance gap between the flanges. A finish that is too fine or shallow is undesirable, especially on hard gasket materials, because the smooth surface may lack the required grip, which will allow extrusion to occur. On the other hand, a finish that is too deep will yield a gasket that requires a higher bolt load, which may make it difficult to form a tight seal, especially when large flange surfaces are involved. Fine machining marks applied to the flange face, tangent to the direction of applied fluid pressure can also be helpful. Flange faces with non-slip grooves that are approximately 0.125 mm deep are recommended for gaskets more than 0.5 mm thick; and for thinner gaskets, grooves 0.065 mm deep are recommended. Under no circumstances should the flange-sealing surface be machined with tool marks extending radially across the gasket-sealing surface; such marks could allow leakage.
This factor is a measure of the material’s resiliency over a period of time, and is normally expressed as a percentage loss per unit of time. All gasket material will lose some resiliency over time, due to the flow or thinning of the material caused by the applied pressure. After some initial relaxation, the residual stress should remain constant for the gasket.