Information about gaskets, gasket materials and gasket types is given in an article titled Gaskets – Materials and Types. In this article information is give on test procedures, standards, gasket selection, information on flange and bolting, installation procedure, storage of gaskets, and useful information on joints.
Click on a link below to jump to a topic or read all.
Test Procedures
Gasket Standards
Gasket Selection
Flange and Bolting Information
Installation Procedure
Storage of Gaskets
Useful Information on Joints
Test Procedures
Gaskets are tested to find out their physical properties. Various important ASME and DIN test procedures for testing of gaskets are as under.
Compressibility and Recovery of Gasket Material – ASTM Designation: F36
This method covers determination of the short-time compressibility and recovery at room temperature of sheet gasket materials. Good recovery upon release of load is indicative of torque retention of a gasketed joint. Compressibility and recovery as defined by ASTM are two worthwhile physical property criteria for supplier and purchaser to agree upon as routine tests.
Creep Relaxation of Gasket Material – ASTM Designation: F38 Method B
ASTM F38 provides a means for measuring the amount of creep relaxation of a gasket material at a stated time after a compressive stress has been applied. This method is designed to compare related products under controlled conditions in regard to their ability to maintain a given compressive stress as a function of time. A portion of the torque loss on the bolted flange is a result of creep relaxation. The result of creep relaxation is loss of thickness of a gasket, which causes bolt torque loss, resulting in leakage.
Fluid Resistance of Gasket Materials – ASTM Designation: F146
These methods provide a standardized procedure for measuring the effect of immersion on physical properties of non-metallic gasketing materials in specified fluids under defined conditions of time and temperature. They are not applicable to the testing of vulcanized rubber (they are tested as per D471). This test can be used as a routine test when agreed upon between the supplier and purchaser.
Sealability of Gasket Materials – ASTM Designation: F37
Test methods A and B provide a means of evaluating fluid sealing properties at room temperature. Method A is restricted to liquid measurements and Method B (most common) can be used for both gas and liquid measurements. These test methods are suitable for evaluating the sealing characteristics of a gasket product under differing compression flange loads. Since this physical property is important for proper function of a gasket, it should be used as an acceptance test when test methods are agreed upon between supplier and purchaser. These precise measurements of leakage rates are designed to compare gasketing products under controlled conditions. The leakage measured comes either through the gasket, or between the gasket and the flange faces, or both. In most cases, the leakage measured is a result of leakage through the gasket.
DIN designation 3535 provides a means of measuring leakage of a gas through a gasket. The apparatus used in this method is considerably more versatile than that used in ASTM F37.
Tension of Non-metallic Gasket Materials – ASTM Designation: F152
The Universal Tester is used to determine the tensile strength of non-metallic gasketing products. The types of products covered are those containing various organic fibers, inorganic fibers, flexible graphite, or fluorocarbons as described in F104. F152 is not applicable to the testing of vulcanized rubber (they are tested as per D142). The measurement of tensile strength characterizes various classes and grades of products of a given type. It also aids the purchaser in determining whether the gasketing product approved for a given application is being manufactured to acceptable quality. Tensile strength is not necessarily the most important function of a gasket material. Expanded graphite for example is relatively weak, though it performs very well as a gasket material.
Torque Retention – DIN 52913
This test is designed to determine the torque retention capabilities of gasketing products, when subjected to the compression load and operating temperature as defined by the test procedure. The test consists of applying a predetermined load on the test gasket via a tension screw, then heating the gasket/flange assembly to the desired temperature (there is no internal pressure). The standard test period is either sixteen (16) hours or one hundred (100) hours. At the end of the required time period, the compression load which is left acting on the test gasket is measured. This allows one to calculate the torque retention capabilities of various gasketing products.
Blowout of Gasket Products – (No ASTM Designation)
Gasket manufacturers also carry out blowout resistance test on gaskets at varying pressures and temperatures. Test results are used to provide P (psig or bar) x T (deg. F or deg. C) values for various products.
Go to Top
Gasket Standards
Various standards are used to specify gasket dimensions and properties as under.
Standards for Gasket Dimensions
Cut Gasket
ASME B 16.21 – for ASME flanges as per ASME B 16.5.
BS EN 12560 Part 1 (formerly BS 7076 Part 1) – for BS flanges as per BS 1560.
BS 3063 – for BS flanges as per BS 10.
BS EN 1514 Part 1 – For DIN flanges as per DIN / EN 1092 (formerly BS 4504).
Spiral Wound Gaskets
ASME B 16.20 – for flanges as per ASME standard.
BS 3381 – for flanges as per BS standard.
BS EN 12560 Part 2 – for DIN / BS type of flanges.
API Ring Joints
ASME B 16.20 – for flanges as per ASME standard.
BS EN 12560 Part 5 – for flanges as per DIN / BS standard.
Corrugated Metallic Type
BS EN 12560 Part 4 – for flanges as per DIN / BS standard
Standards for Gasket Properties
Gaskets are tested for properties and types are specified in accordance with various standards as under.
ASME Standards – F 36, F 37, F 38, F 152, F 146, F 104
BS 7531, BS 2815, BS 1832, BS 125
DIN 3535, DIN 52913, DIN 3754
French standard NFT 48001
IS 2712 (Now BIS, Bureau of Indian Standard)
Depending upon the application, Indian standard IS2712-1998 have specified grades of compressed asbestos fibres (CAF) as follows:
| Grade | Application | Temp. And Pressure |
|---|---|---|
| IS2712/1998 W/1 | Water, steam and for some chemical high service conditions | Upto 350o c and 130 Bar |
| IS2712/1998 W/2 | Water, steam and for some chemical medium service condition | Upto 350o c and 40 Bar |
| IS2712/1998 W/3 | Water, steam and for some chemical low service condition | Upto 250o c and 30 Bar |
| IS2712/1998 O/1 | Oils – high service conditions | Upto 350oc and 130 Bar |
| IS2712/1998 O/2 | Oils – medium and nominal service conditions | Upto 300oc and 80 Bar |
| IS2712/1998 A/1 | Acids – highly corrosive | Upto 250oc and 100 Bar |
Go to Top
Gasket Selection
Primarily gasket selection is based upon temperature of media, pressure of media, compatibility of gasket material for the media and application. It is suggested that gasket shall be selected for above parameters in following sequence.
Temperature
In selection processes, the temperature of the fluid at the gasketed joint should be considered first. This will reduce the number of product candidates quickly, especially as temperatures go from 200°F (95°C) to 1000°F (540°C). Gasket shall withstand system temperature without serious impairment of its performance properties. When system operating temperatures approach a particular gasket material’s maximum continuous operating temperature limit, an upgrade to a superior material is suggested. In some situations cryogenic temperatures must also be considered.
Application
The most important information under application is the type of flange (flat face, raised face, tongue and groove, ring joint flange, etc.), flange metallurgy (steel, nonmetallic, glass lined, etc.) and bolts used. The number, size and grade of bolts used in the application determines the load available. The surface area being compressed is calculated from the gasket contact dimensions. The load from the bolts and the contact area of the gasket result in the compressive load available to seal the gasket. Selection of gasket shall also take care of minor misalignment, flange bowing (flange bending), and flange surface imperfections like – waviness, grooves, scoring and finish. Gasket shall have sufficient strength to resist crushing under the applied load, and maintain its integrity when being handled and installed.
Media
Gasket material shall chemically resist the system fluid to prevent serious impairment of its physical properties.
Pressure
Gasket shall be strong enough to resist internal fluid pressure.
Notes:
Maximum temperature and pressure capabilities do not necessarily operate to gather for all gasket thicknesses and it is recommended that pressure and temperature are considered simultaneously using P x T Limit of a gasket for different gasket thicknesses.
For gaskets cut from sheets, always use the thinnest material which the flange arrangement will allow, but thick enough to compensate for unevenness of the flange surfaces, their parallelism, surface finish and rigidity etc. The thinner the gasket, the higher the bolt load which the gasket can withstand, and the less the loss of bolt stress due to relaxation. Also the gasket area will be lower which will be exposed to attack from the internal pressure and aggressive media. In view of this, ensure that the gasket is as thin as possible. As a rule of thumb, gasket performance decreases as material thickness increases.
Caution:
Choosing wrong sealing product can result in propevtydamace C.d -dor7erkkus0ersknandinh1rydmpheP$Sel!ctKon CknsK$erAtioN7 fmr Ga7ket3 ar%: oh4 d
Important:
Always use a good quality gasket from a reputable supplier, because the cost of a gasket is insignificant when compared to the cost of downtime or safety considerations.
Go to Top
Flange and Bolting Information
A joint may fail even though the gasket material itself may be correct due to flange, bolting or improper installation.
Flanges
There are limits on the degree of flange surface imperfection that can be sealed successfully with a gasket. Large nicks, dents, or gouges must be avoided, since a gasket cannot properly seal against them. The surface finish of a flange is described as roughness, lay and waviness.
Roughness is read in millionths of an inch as the average of the peaks and valleys measured from a midline of the flange surface. This is expressed either as rms (root mean square) or AA (arithmetic average). The difference between these two methods of reading is so small that they may be used interchangeably.
Lay is the direction of the predominant surface roughness pattern. Example: multidirectional, phonographic spiral serrations, etc.
Waviness is measured in thousandths or fractions of an inch. Basically, it is the departure from overall flatness.
Generally most manufacturers provide recommendations about appropriate flange surface finishes for particular gasket materials. Recommended values of roughness are as under.
Spiral Wound Gaskets: 125-250 rms
Jacketed or Metal Clad Gaskets: 63-80 rms
Solid Metal Gaskets: 63-80 rms
It is recommend that a flange be machined using a 1/16" radius, round nosed tool to have 30-55 serrations per inch in a concentric or spiral pattern.
The lay of the finish should follow the midline of the gasket if possible. Take, for example, concentric circles on a round flange. Every effort should be made to avoid lines across the face, such as linear surface grinding allowing a direct leak path.
Waviness is seldom a problem under normal conditions. There are two areas that must be watched since excessive waviness is very difficult to handle. The first area is glass-lined equipment where the natural flow of the fused glass creates extreme waviness. Often the answer here is to use thick and highly compressible gasket. The second area of concern is warped flanges. If warpage is caused by heat or internal stresses, remachining is generally sufficient. However, warpage due to excessive bolt loads or insufficient flange thickness results in what is generally called bowing. The solution is to redesign for greater flange rigidity. Sometimes backer plates can be added to strengthen the design without having to replace the parts. Another solution would be to add more bolts.
Bolting
For the majority of flange and gasket joints, the fasteners which provide the compressive pressure on the flanges (and through this onto the gasket) are normally bolts or studs in tension.
Fasteners exhibit stress relaxation behaviour dependent upon their material of construction. This will have a marked effect on the load they are able to generate on the flange / gasket assembly under operating conditions. Consequently, when selecting the fasteners to use for a particular application, always consider the temperature variations which the fasteners will experience in service. Recommended values are as under.
| Recommended fastener working temperatures | ||
|---|---|---|
| Material | Temperature °C (°F) | |
| Minimum | Maximum | |
| Carbon steel | -20 (-4) | 300 (572) |
| B7, L7 | -100 (-148) | 400 (752) |
| B6 | 0 (32) | 500 (932) |
| B8 | -250 (-418) | 575 (1067) |
| B16 | 0 (32) | 520 (968) |
| B17B | -250 (-418) | 650 (1202) |
| B80A | -250 (-418) | 750 (1382) |
Tension loads above the elastic limit will produce some permanent deformation. The fastener will not return to its original length and its effectiveness as a spring clamp will be impaired. In view of this, select fasteners with sufficient yield strength to ensure they are within their elastic limit at the required load.
The tension in the fastener is generated by tightening nuts along the threads of the fastener. The threads therefore play a major role in the clamping operation, and care must be exercised to maintain their integrity. Threads will strip when the axial forces on the fastener exceed the shear strength of the threads.
Threads strip more readily when fastener and nut material are of the equal strength. For optimum safety, use a nut which has a specified proof load 20% greater than the ultimate strength of the fastener. In this way, the fastener will break before the nut threads will strip. A break is easier to detect than a stripped thread.
Flat, hardened washers should always be used with fasteners to reduce significantly the friction between a turning nut and the joint components. This improves the consistency of the torquing operation improving accuracy and reducing the torque required. Use the same material for the washers and that of nuts.
Go to Top
Installation Procedure
The most common cause of leaky gasketed joints is improper installation procedures. Care shall be taken at all stages given below.
Cutting of Soft Gaskets
First cut / punch bolt holes. Cut the bolt holes slightly larger than the bolts, to ensure proper seating. Never cut out a gasket by hammering material against the flange. Use a good cutter to cut the required shape of the gasket. Ensure that the inside diameter of the gasket is not less than the inside diameter of the process line to minimize obstruction of the process line.
Handling of Gaskets
When working in the field, carry cut gaskets carefully. If you bend the gasket it will be damaged. Always transport large diameter metallic and semi-metallic gaskets to the installation site on its mounting / packing.
Tools Required and Cleaning
Tools will be required to both clean the flange and tension the fasteners.
Clean fasteners with a wire (ideally brass) brush to remove dirt on the threads. After removing old gasket, clean flange surface of all debris using a wire brush (use stainless steel bristles on alloy components) or a brass scraper (a scraper can be made from a sheet of brass, ~5 mm (0.2 in) thick x 50 mm (2 in) wide, which is filed and shaped to a 45° chisel across the width). Using a hammer, lightly tap the scraper into the flange grooves to remove debris.
Flange spreaders may be used to make gap between flanges for cleaning them. Two spreaders are required for a joint. Use mechanical / hydraulic type spreader based on force required to open flanges.
The tensioners will require regular calibration and may include torque wrench, hydraulic or other tensioners. Instruments to measure tension may include a micrometer, or
Ultrasonic equipment.
Visual Inspection
Inspect various components as under.
Fasteners / nuts / washers – after cleaning examine them to assure freedom from defects such as burrs or cracks.
Flange assembly – inspect the flange surfaces for defects, such as radial scores and warping. Ensure that the flange surfaces are sufficiently flat and parallel.
Gasket – check that the correct gasket is available (suitable for the service, size,
thickness). Examine the gasket prior to installation to ensure that it is free from defects.
If any defect is observed, replace defective components with a good alternative.
Lubrication
It is estimated that in the absence of a suitable lubricant, up to 50% of the torque effort may be used to merely overcome friction. Effectively, this would mean that the same torque applied to non-lubricated fasteners on a joint might provide markedly different loads on each one. Therefore, lubrication is essential when torque is used as the control for setting tension in the joint. After cleaning, lubricate fastener threads and all bearing surfaces (underside of bolt heads, nuts, washers) with a quality lubricant such as an oil and graphite mixture. Ensure that lubricant does not contaminate either flange or gasket faces.
Gasket Installation
Carefully insert the new gasket between the flanges to prevent damage to the gasket surfaces and center it. Do not use tape to secure the gasket to the flange. If it is necessary to secure the gasket to the flange, use a light dusting of spray adhesive (e.g. 3M type 77). Do not use jointing compounds or release agents on gasket / flange faces.
Bolt / Stud Tightening Pattern
One of the most difficult jobs is to produce the correct assembly pressure on the gasket, low enough to avoid damaging the gasket, but high enough to prevent a leak in the seal. It is vitally important to control accurately the amount of force applied to any particular flange arrangement. Always use a torque wrench or other controlled-tensioning device for tightening. The sequence in which bolts or studs are tightened has a substantial bearing upon the distribution of the assembly pressure on the gasket. Consequently always torque nuts in a cross bolt tightening pattern. Always run the nuts or bolts down by hand. This gives an indication that the threads are satisfactory (if the nuts will not run down by hand, then there is probably some thread defect – check again and, if necessary, replace defective parts). Now torque the joint using a minimum of 5 torquing passes, using a cross-bolting sequence for each pass, as shown below.
Pass 1 – Tighten nuts loosely by hand in the first instance, according to the cross bolt tightening pattern, then hand-tighten evenly.
Pass 2 – Using a torque wrench, torque to a maximum of 30% of the full torque first time around, according to the cross bolt tightening pattern.
Pass 3 – Torque to a maximum of 60% of the full torque, according to the cross bolt tightening pattern.
Pass 4 – Torque to the full torque, according to the cross bolt tightening pattern.
Pass 5 – Final pass at full torque, in a clockwise direction on adjacent fasteners.
After the five basic torquing passes are completed, it may be beneficial to repeat pass 5 until no further rotation of the nut is observed. The final tightening must be uniform, with each bolt pulling the same load.
Hydraulic tensioners are often used to preload fasteners. In this method when the tensioner load is applied, the nut is run down against the joint (finger tight). The hydraulic pressure is then released and the tensioner removed.
Another way to tighten large bolts is to insert a heating rod in a hole drilled down through the centre of the bolt. As it heats up, the bolt expands length wise, and the nut can be run down against the joint (finger tight). The heater is now removed, and as the bolt cools, it shrinks, so developing tension.
Re-tightening
For the majority of materials in the flange system (including gaskets, fasteners, nuts, washers), relaxation sets in after a fairly short time. For soft gasket materials, one of the major factors is usually the creep relaxation of the gasket. These effects are accentuated at elevated temperatures. Due to this the compressive load on the gasket is reduced, increasing the possibility of a leak. Consequently, some engineers recommend that fasteners should be re-tightened (to the rated torque) 24 hours after the initial assembly. Re-tightening shall always be carried out at ambient temperature. However, this is an area of conflicting views!
Elastomer-based CAF gasket materials continue to cure in service, especially on start up as the operating temperature is reached. Once fully cured, gasket materials may become embrittled and liable to cracking under excessive load, and this is especially the case with elastomer-based asbestos-free materials. As it is impossible to predict the time for embrittlement, always consult the manufacturer for advice about re-tightening. As a general rule do not re-torque an elastomer-based asbestos-free gasket after it has been exposed to elevated temperatures.
Go to Top
Storage of Gaskets
Although many gasket materials can be used safely after storage for many years, ageing will have a distinct effect on the performance of certain types of gasket materials. Primarily, this is a concern with materials which are bonded with elastomers. They in general should not be used after about 5 years from the date of manufacture. If required, they shall be used only after careful inspection. Materials with elastomeric binders will inevitably deteriorate over time, and even more quickly at higher ambient temperatures. Degradation is also catalysed by intense sunlight. Since graphite and PTFE materials contain no binders, sheets and gaskets of these materials have a virtually indefinite shelf life. In general,
- During storage gaskets should not be subjected to extreme heat or humidity – store in a cool, dry place, away from direct sunlight, water, oil and chemicals.
- Store sheet materials flat.
- Avoid hanging gaskets – they may distort. Store soft gaskets flat. Large diameter spiral wound gaskets should be retained on their mounting board.
- Gaskets should be kept clean and free from mechanical damage (for maximum protection, store in sealed poly bags).
Go to Top
Useful Information on Joints
In this section other useful information related with a joint is given.
Flange Insulation Sets
Flange insulation sets are installed in pipeline systems to isolate the flow of electrical current. For example – in order to ensure efficient operation of cathodic protection systems for stainless steel pipelines, it is necessary to divide the pipelines into manageable lengths. The installation of flange insulation set at flanged joint ensures effective sealing and isolation from electric current. Different types of sets are available for different types of flanges.
Flange insulation sets comprise the following components which ensure that full electrical isolation is achieved.
- Central insulating gasket which is fitted between the flanges.
- Insulating sleeve per flange bolt.
- Insulating washers per flange bolt.
- Metal backup washers per flange bolt.
The components are manufactured from insulating materials with high compressive strength and good stability
Direct Tension Indicators
Direct tension indicators provide the means to measure bolt tension (bolt load). They are manufactured as per ASTMF959 and F959M. They are generally used with structural fasteners. The Direct Tension Indicator (DTI) is a specially hardened washer with protrusions on one face. The DTI is placed under the bolt head or nut, and the protrusions create a gap. As the bolt is tensioned, the clamping force flattens the protrusions, reducing the gap.
DTI's stay on the job, providing permanent visual and measurable proof that the bolt is correctly tensioned to specification. Gap corresponds to bolt load verified by a test certificate traceable to NIST.
Reuse of a Gasket
A gasket’s function is to conform to flange high and low spots when compressed, and its ability to reseal decreases after it is compressed. Gaskets which contain rubber and which have experienced elevated temperatures will be even less likely to reseal. In view of this, it is recommended not to reuse a gasket. Even if the gasket appears to be okay, it is not worthwhile. The cost of a new gasket is minuscule compared to the cost of down time caused by a leak or blowout and the considerations of safety and environmental protection.
Joining of Gasket Sections
For making a large gasket, it is recommended to make a dovetailed joint instead of beveled joint.
Spacers in Flanges
Some installations require a very thick gasket to fill a large gap between flanges. It is recommended not to stack numerous gaskets in the same flange. Tests have shown that a better way to fill a 1/2" gap, for example, is to install a 1/16" gasket on each side of a 3/8" thick incompressible spacer ring. Ideally, the spacer ring shall be consistent with piping metallurgy, serrated, and cut to the same dimensions as the gasket. Higher minimum torque is recommended when using this type of arrangement.
Hydrostatic Testing Precautions
If hydrostatic tests are to be performed at pressures higher than those for which the flange was rated, higher bolt pressures must be applied in order to get a satisfactory seal under the test conditions.
For this, use high-strength alloy bolts (ASTM B 193 Grade B7 is suggested) during the tests. They may be removed upon test completion. Higher stress values required to seat the gasket during hydrostatic tests at higher than flange rated pressures may cause the standard bolts to be stressed beyond their yield points.
Upon completion of hydrostatic testing, relieve all bolt stress by 50% of the allowable stress.
Begin replacing the high-strength alloy bolts (suggested for test conditions) one by one with the standard bolts while maintaining stress on the gasket.
After replacing all the bolts, follow the tightening procedure recommended in the bolting sequence diagrams.
Troubleshooting Leaking Joints
One of the best methods for determining the cause of joint leakage is the careful examination of the gasket where the leakage occurred.
| Observation | Possible Remedies |
|---|---|
| Gasket badly corroded. | Select replacement material with improved corrosion resistance. |
| Gasket extruded excessively. |
Select replacement material with better cold flow properties. Select replacement material with better load capacity – i.e., more dense. |
| Gasket grossly crushed. |
Select replacement material with better load carrying capacity. Provide means to prevent crushing the gasket by use of a stop ring or redesign of flanges. |
| No apparent gasket compression achieved. |
Select softer gasket material. Select thicker gasket material. Reduce gasket area to allow higher unit seating load. |
| Gasket substantially thinner on OD than ID due to excessive flange rotation or bending. |
Provide stiffness to flange by means of back-up rings. Select softer gasket material to lower required seating stresses. Reduce gasket area to lower seating stresses. |
| Gasket unevenly compressed around circumference. | Make certain proper sequential bolt-up procedures are followed. |
| Gasket thickness varies periodically around circumference. |
Provide reinforcing rings for flanges to better distribute bolt load. Select gasket material with lower seating stress. Provide additional bolts if possible to obtain better load distribution. If flanges are warped, remachine or use softer gasket material |
Common Wrong Practices
- Reuse of old gasket (due to non availability of new gasket).
- Procurement of material by generic name with out specification (for example, neoprene sheet).
- Improper storage – Storage of gasket sheets in vertical rolls.
- Cleaning of flanges by chisel / hack saw blades (leading to flange damage).
- Cutting of gasket by hammering against flange / use of chisel instead of sharp cutter.
- Use of many gaskets to fill large gap between flanges.
- Use of one thickness and one type of gasket for all plant application.
- Application of grease on gasket / flange faces.
- Application of tape / thread to hold gasket in position.
- Use of ordinary fasteners instead of high tensile fastener (due to loss / damage to old fasteners).
- Use of dirty / rusted fasteners with out lubrication.
- Improper sequence of bolt tightening.
Acknowledgements:
Article titled Gaskets – Materials and Types and this article are written based on information from websites of Fluid Sealing Association, European Sealing Association, James Walker Moorflex Limited and Garlock Sealing Technologies. For more information please refer to their websites. Their site addresses are as under.
http://www.fluidsealing.com
http://www.europeansealing.com
http://www.jameswalker.biz
http://www.garlock.com
Site addresses of Indian gasket manufacturers are as under.
http://www.hindcompo.com
http://www.champion-india.com
http://www.jaygaskets.com
http://www.goodrichgasket.com
