A gasket is a compressible material, or a combination of materials, which when clamped between two stationary members prevents the passage of the media across those members. The gasket material selected must be capable of sealing mating surfaces, resistant to the medium being sealed, and able to withstand the application temperatures and pressures.

The Problem of Defining a Gasket

Gaskets are commonly made of a flexible material such as rubber, paper, or cork. In fact, there are many different materials that gaskets can be made out of and so gaskets cannot simply be defined by means of the material that they are made out of. Gasket materials are, however, typically flexible, as they are usually compressed between two other surfaces. Having said this, it is also possible to find gaskets made of metal: such as spiral wound or camprofile gaskets.

A similar problem arises when we try and define a gasket by its function. Gaskets can be used for many different purposes, such as: sound and noise reduction, anti-vibration, packaging, hygiene, sealing, and as supports and mountings. For these reasons gaskets are found in many different products and in many different industries. For instance, gaskets can be found inside of: cars, planes, boats, trains, pumps, electrical equipment (from electronic tills and hi-fi equipment to lighting, to industrial electrical transformers). Gaskets find application in industries as diverse as food processing, petrochemical, pharmaceutical, aerospace, automobiles, water and oil and gas.

The properties of a gasket are typically those of the material out of which it is manufactured. For this reason gasket materials are selected for their characteristics: such as their resistance to chemicals, acids, alkaline, extreme temperatures, pressures, and their ability to withstand different environments (such as deep-sea, mining, and even in space). Similarly, the specification of a gasket is defined by its operating environment.

Gaskets for Sealing Purposes

Perhaps the most common use of a gasket is as a component of a sealing system, in which one of the major functions of the gasket is to create a seal between two other surfaces. For instance, pipe gaskets are used to create a seal between two pieces of pipe when laying a pipeline. The exact function of a gasket in this respect is to prevent the escape or ingress of fluids (liquids or gases) even at extreme pressure and temperature. Yet the function of a gasket is not only to provide a seal to prevent liquids or gases escaping, but it is also an important safety device. In this respect it is important for the gasket to be the weakest component of the sealing system, so that it fails in response to a problem. The consequences of a gasket not failing in response to a problem are far greater than simple gasket failure. If the gasket does not fail in response to a build in pressure then the consequence is potential bursting of the pipe and/ or explosion.

It is also important that the gasket allows for essential maintenance, by allowing the sealing surfaces to be separated and reassembled. For this reason it is important for the gasket to remains in a good a condition for as long as possible. The life of a gasket can be significantly improved by selecting the right material for the application, and by taking into account all of the features of the environment in which it will be used.

Conclusion

As we have seen, it's not easy to define what a gasket is: as they can be made out of many different materials, perform many different functions, and be used in many different industries. Having said this, the most common use of a gasket is to interface between two surfaces to improve the function of the components.

If you have any technical questions then please contact us.

Gasket function can be divided into two stages

• Gasket compression during Assembly : The gasket needs to be properly seated during assembly. In order to achieve this, the bolt stress should be sufficiently higher than the internal pressure, hydrostatic end force, and minimum required bolt stress for gasket seating.
  
• Gasket resiliency and recovery during a running plant : In a running plant, the gasket needs to maintain resiliency and recovery properties in order to maintain the same initial bolt stress and fight against hydrostatic end force and flange movements due to pressure-temperature fluctuations.

Factors influencing Gasket Function

Many factors can influence gasket function. External factors such as bolts and flanges should also be considered in conjunction with gaskets as they influence sealing performance.

Application parameters need to be first considered before selecting a gasket. These include

• Temperature in the system
• Pressure in the system
• Corrosive nature of media
• Viscosity of media

The following parameters should be considered for gasket material selection

• Compression characteristics
• Recovery characteristics
• Chemical resistance
• Stress Retention
• Shouldn't corrode/ damage flanges
• Blow out resistant
• Creep resistant
• Ease of handling

The above need to be complimented by proper plant maintenance practices. These include the following

Flange Condition

• Flange Surface Finish: Different gaskets require different flange finishes. The flange finish should be concentric or phonographic and correspond to the gasket being installed.
• Flange Surface Damage: Flange surface damage can lead to a leakage path, and should be avoided.
• Flange Flatness: An out of flatness flange or a non-parallel flange would lead to uneven compression on the gasket, and thus affect performance.

Bolting

Bolt stress during assembly: In order to maintain a seal, it is extremely important that proper bolt stress is maintained during assembly. Proper bolting equipments should be considered to maintain these stresses.

Bolting Procedure: In order to maintain uniform bolt stress, it is recommended to follow the below sequence. Bolting should also be done in four stages with 33% torque during the first stage, 66% torque during the second stage, 100% torque during the third stage, and a 100% retorque during the fourth stage.

Bolt Lubrication: Good lubrication is necessary to maintain uniform torque. In the absence of proper lubrication, torque will be lost to friction.

Guide to basic Gasket Design

Basic information concern gasket design can be found in this section. The information here is offered as a general guide only, please check the details of your application and material specifications carefully.
The gasket is only one component of a sealing system. The other components are the mating surfaces, the clamping method, the internal and external environments. First we need to consider the general function of a gasket:

Prime Function

• To allow two surfaces to be adequately mated and sealed.
• Provide easy separation of the mated parts at service intervals.
• Prevent escape or ingress of fluids (gas or liquid) even at extreme pressures and temperatures.

Major Requirement

• Impermeable : Must not allow leakage through the material, even under pressure.
• Resilience and Strength: Must conform and moulditself into all irregularities on flange surfaces whilst having sufficient tensile strength to resist blow-out under operating conditions.
• Recovery : Must seal when tightened down (and often crushed) but recover to maintain a seal when the flanges move under mechanical, temperature or pressure forces.
• Creepage : Must not creep, spread or extrude under conditions of high bolt pressure or high contained fluid pressure – even at high temperatures.
• Chemicals : Must not be attacked or weakened by a wide range of fluids even when exposed for extended periods at high temperatures.
• Temperature : Must remain resilient for long periods of time at low or high temperatures.
• Contamination : Must not contaminate the sealed fluids – especially important in the pharmaceutical and food industries.

Gasket Design Guide

Step 1 : Material Selection

• Evaluate the application, the likely temperatures, pressures, and any chemicals.
• For dust sealing with low bolt loading consider a foam rubber.
• Lids and sumps can be sealed with cork.
• At low temperatures and pressures select a rubber suitable for the environment.
• Uses above 100 centigrade but at low pressures could be accommodated by a specialist rubber.
• For aggressive chemicals below about 240 centigrade PTFE can often help.
• Various non-asbestos jointings are available for conditions up to 450 centigrade and 160 bar.
• Consider graphite for steam with a variable process cycle and mica for extreme temperatures.

Step 2: The Flange and the Gasket

Once the material is chosen the flange and bolting need to be designed to ensure the following: The gasket is compressed evenly over the whole surface area. Thin flanges or excessive distance between bolt holes can result in some portions of the gasket being crushed whilst other areas are not sufficiently loaded to prevent leakage or blow-out.

The flange and gasket will appear thus
Vary the width of narrow flanges. If the bolts are widely spaced and the flange width is constant then excessive loading will occur on the limited material around the bolts.

For Example : This Design Will Even The Loading

Note: Sheet metal flanges can be stiffened by forming a lip. Soft materials can be protected by using compression stop (could be a washer).

Additional Requirements for Dynamic Sealing

• Fitting : Often must flex or compress during assembly of parts and then maintain a seal with no means of assistance to provide post assembly pressure.
• Friction : Must not burn or wear when in contact with rotating or sliding components.
• Sealing : Always a compromise. Most dynamic seals must leak to function. It is the leaking fluid which provides the buffer between the moving parts and acts as a lubricant.

Gaskets can be classified into three categories: soft cut, semi-metallic and metallic types. The physical properties and performance of a gasket will vary extensively, depending on the type of gasket selected and the materials from which it is manufactured. Physical properties are important factors when considering gasket design and the primary selection of a gasket type is based on the following :

• Temperature of the media to be contained
• Pressure of the media to be contained
• Corrosive Nature of the Application
• Criticality of The Application

1) Soft Cut Gasket

Sheet materials is used in low to medium pressure services. With careful selection these gaskets are not only suitable for general service but also for extreme chemical services and temperatures.

2) Semi - Metallic Gasket

These semi - metallic gaskets consist of both metallic and non-metallic materials. The metal provides the strength and the resilience of the gasket and the non-metallic component provides the conformable sealing material. These gaskets are suitable for low and high pressure and temperature applications. A wide range of materials are available.

2.1) Spiral Wound Gasket

FILLERS MATERIALS	MAX. WORKING TEMPERATURE
Graphite	600°C
Graphite 99.8% purity	1200°C
Non asbestos	550°C
PTFE	250°C

Commonly Used Winding Material

Winding Material	Maximum Temperature	ASME B16.20 / Colour Coding
Carbon Steel	500°C	Silver
304 Stainless Steel	650°C	Yellow
316L Stainless Steel	800°C	Green
Duplex UN S31803	800°C	N/A
347 Stainless Steel	800°C	Blue
321 Stainless Steel	800°C	Turquoise
Monel 400	450°C	Orange
Nickel 200	315°C	Red
Titanium Gr 2	350°C	Purple
Hastelloy B-2/B-3	450°C	Brown
Hastelloy C-276	450°C	Beige
Inconel 600	1000°C	Gold
Inconel 625	450°C	Gold
Inconel X-750	1000°C	N/A
Incoloy 825	450°C	N/A
Zirconium	500°C	N/A
Super Duplex	600°C	N/A
254 SMO	600°C	N/A
Titanium Gr7	350°C	N/A
Hastelloy C-22	450°C	N/A
Hastelloy G-31	450°C	N/A
Alloy 20	600°C	N/A

Thickness of Spiral Wound Gasket

Initial thickness	After Compression
2.5mm	2.0 mm
3.2mm	2.5 mm
4.5mm	3.0 -3.2 mm
6.4mm	4.6 - 4.8 mm
7.2mm	4.8 - 5.00 mm

Note : Maximum temperature ratings are based upon hot air constant temperatures. The presence of contaminating fluids and cyclic conditions may drastically affect the maximum temperature range.

2.2) Camprofile Gaskets

Sealing Layer Materials And Sealing Stresses

The following table gives information regarding different types of materials offered as sealing layer materials by Parikh Gaskets. Also given is recommended seating stress range for reliable and effective performance:

Material	Temp (Deg.C)		Max. Operating Pressure (Bar)	Gas Tightness	Application	Seating Stress
	Min	Max				Min (N/mm2)	Optimum (N/mm2)	Max (N/mm2)
Graphite	-200	600	250	Good	Aggressive Media	20	90	400
PTFE	-200	250	100	Good	Aggressive Media	20	90	400

Core Thickness

When a Camprofile is replacing an existing gasket (eg. spiral wound gasket); Parikh Gaskets recommends a 4mm thick core to prevent unnecessary stresses on existing pipe lines. For new system, we recommend 5 mm thick cores. The value should be taken into account at the design stage.

Pipe system	Core thickness	Seated Thickness (Core + 2 sealing layers)
Existing	4mm	5.0mm to 5.2mm
New	5mm	6.0mm to 6.2mm

General

Parikh Gaskets recommends the following best practices to ensure correct gasket fitting.

• Care in material selection.
• Care of flange faces.
• Do not use jointing compounds.
• Re-torque after 24 hours or one process cycle.

Recommendations

• Choose as thin a material as possible.
• Clean the flange faces and remove any dirt on the studs / bolts /nuts.
• Lubricate the threads and the nut face. If possible use a lubricant based on:
• Before assembly ensure that the nuts run freely on the threads.
• Carefully fit the gasket taking care not to damage the gasket surface.
• Tighten the bolts in the recommended sequence (eg diametrically opposite) to about half load.
• Check visually that the flanges are uniformly loading the gasket.
• Tighten to the recommended torque for the flange system.
• After 24 hours of operation, or one process cycle, re-tighten to the correct torque (the gasket will have relaxed).
• If the flange has not been in service and is unlikely to be so in the short term then still re-tighten.

Use of Graphic & other Anti-Stick coatings

• Care should be taken to apply a very thin coating.
• Not recommended where flanges are worn or where high pressures (in excess of 17 bar) are expected.
• Do not use graphite grease.

A list of Questions That we Frequently Encounter From Customers :

What is the right material for my application?

The right material for your application will be determined by what you are going to use it for. The easiest way to decide which material is right for your application is to use our Gasket Material Selector and Prosperities of Gasket Materials charts. If in doubt, please contact us for advice

What thickness material should I specify?

As a general rule you should choose the thinnest material possible for your application, as the pressure exerted on the gasket will increase with its thickness. The thickness of the material will be dictated by the state of the flanges in question, and the compression required to seal.

How do I work out the Outer Diameter (O.D.) and Inner Diameter (I.D.) of my gasket?

The inner dimension of your gasket is the size of the hole or bore. To calculate the outer dimension of your gasket, measure the outer dimension of the gasket (the full diameter of the gasket).

How do I determine which Class or Table of gasket I need?

The easiest way to work out which class or table of gasket you need is to consult our Gasket Dimensions Guide.

How do I work out the Pitch Circle Diameter (P.C.D.) of my gasket?

The pitch circle diameter is the diameter that specifies the centre of bolt holes of your gasket. The P.C.D. is measured from the centre of the bore.

Below can be found a glossary of basic gasket terms. For further technical advice, please do not hesitate to contact us.

• Boiler Door Joint : A boiler door joint is a gasket, or seal, for a boiler door (such as on a traction or steam engine). Boiler door joints can be supplied as Topog-E type joints, or manufactured from woven-proof glass fibre.
• Bore : The bore specifies the inner dimension (I.D.) of the gasket.
• Carbon Steel (C.S.) : Carbon steel is steel in which the main alloy is carbon. Carbon steel is used to manufacture spiral wound gaskets (S.W.G).
• Class : The class of a gasket specifies its standard sizes/ rating (E.g. P.N.16, P.N.40, CL.150, etc.).
• Cork : A gasket material manufactured from the cork tree. Cork material is typically combined with rubbers to give it greater resistance to chemicals and solvents. Cork is a low compression jointing.
• EPDM : EPDM is a synthetic rubber used to manufacture gaskets. EPDM gaskets are suitable for use with water; a grade for use with drinking water is also available. EPDM is suitable for use up to 120 degrees centigrade (°C).
• Foam : Foam, or sponge, is rubber that has been formed into an air-filled matrix structure. It is typically used to manufacture environmental, and dust, seals for equipment.
• Full Faced (F.F.) : A full faced gasket is a gasket that covers the full face of the flange. A full faced gasket will have bolt holes, as the bolts will run through the flange and the gasket.
• Gasket : A generic term for a part the fits between two pieces of metal, typically for sealing purposes. Gaskets are commonly, although not always, made of flexible and compressible material.
• Grade X : Short hand for British Standard BS7531X. These materials are suitable for use up to a medium pressure grade of 100 bar and to 400 degrees centigrade (°C).
• Grade Y : Short hand for British Standard BS7531Y. These materials are suitable for use up to a medium pressure grade of 70 bar and to 350 degrees centigrade (°C).
• Graphite : Graphite is a gasket material commonly used with steam.
• Inner Bolt Circle (I.B.C): I.B.C. gaskets, or joints, fix within the diameter of the bolt circle. Otherwise known as raised face joints (R.F.), they are contrasted with full faced gaskets (F.F.) which cover the face of the flange and have bolt holes.
• Inner Diameter (I.D.) : The inner diameter specifies the inner dimension, or bore size, of the gasket.
• Neoprene : Neoprene is a synthetic rubber used to manufacture gaskets. Neoprene gaskets are commonly used for exposed environment applications, such as those involving sea water.
• Nitrile : Nitrile is a synthetic rubber used to manufacture gaskets. Nitrile gaskets are commonly used for applications involving oils and fuels.
• Non-Asbestos : Non-asbestos materials replace asbestos gasket materials. These materials typically have a kevlar, glass, or carbon base, and are otherwise known as compressed fibre gasket materials.
• O- Ring : An o-ring is a loop of elastomer with an o-shaped cross-section.
• Outer Diameter (O.D.) : The outer diameter specifies the outer diameter of the gasket.
• Paper : Gasket paper, otherwise known as cellulose paper or oil paper jointing, is a paper impregnated with chemicals to make it resistant to oils, fuels, and solvents.
• Pipe Gaskets : Pipe gaskets are those which fit the standard range of pipes, and are specified by their: bore size, thickness, table/ class, and whether they are full faced or raised face joints (I.B.C.).
• Pitch Circle Diameter (P.C.D.) : The circle passing through the centre of the bolt holes (typically four holes equally spaced on a 150mm P.C.D.).
• Pressure Rating : The pressure the material can seal against.
• PTFE : PTFE is a plastic used for making gaskets. PTFE is extremely chemically inert, and as such is resistant to chemicals and corrosives. PTFE also has a low co-efficient friction.
• PTFE Envelope : A PTFE Envelope is an 'envelope' of PTFE designed to line the bore of a pipe gasket. PTFE is an extremely inert material but it is also comparatively expensive. Having a PTFE bore-liner is therefore a way of gaining the chemical resistance of PTFE without the associated costs. Such envelopes can be fitted to rubber, or non-asbestos, gaskets.
• Raised Face Joint : A raised face joint, otherwise known as an I.B.C. (inner bolt circle) gasket, is a ring that sits within the diameter of the bolts holding the flanges together. It is contrasted with a full faced (F.F.) gasket which covers the face of the flange and has bolt holes.
• Ring Joint/ Ring Type Joint (R.T.J) : Is a metal ring of oval or octagonal section, usually made of soft iron or stainless steel.
• Rubber : An elastic material that comes in natural and synthetic forms.
• Shore : Shore A, (otherwise known as durometer), specifies the hardness of material (e.g. 60 SH).
• Silicone : Silicone is a synthetic rubber used to manufacture gaskets. Silicone can withstand 200°C, and is chemically inert, which gives it application in the food processing industry.
• Spiral Wound Gaskets (S.W.G) : Spiral wound gaskets are made of a metal coil, or winding, with a material filler. Spiral wounds allow for a higher bolt loading of the pipe flanges. Spiral wounds can withstand very high pressures.
• Stainless Steel (S.S.) : Stainless steel is steel alloy resistant to corrosion.
• Table/ Flange Dimensions : These define gaskets to fit particular flanges (e.g. table D. P.N. 16, A.S.A. 150 etc.).
• Thickness : The thickness of the gasket. It is best of go for the thinnest gasket possible given your application, as the pressure on the gasket will increase in proportion to its thickness.
• Viton : Viton is a synthetic rubber used to manufacture gaskets. Viton can withstand temperatures up to 250°C, and is a very inert but expensive rubber. Viton gaskets are typically used in extreme environments, in the presence of acids and corrosives.