When trying to understand the influences upon gasket selection we must break it down into specific factors which are the most influential: temperature, pressure, fluid (chemical compatibility), gasket thickness, stress to seal, storage and handling. Additional factors are influential in some unique applications and specialty equipment. The following sections look at these factors independently, however, they must all eventually be balanced and evaluated as a whole to make a recommendation. It very important to involve your gasket manufacturer for detailed analysis.
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In general, the temperature of the application fluid is assumed to be the temperature of the gasket. While that is mostly assumed negligible, there are other variables that may be significant in specific applications such as insulation, extreme weather conditions and external heat sources, etc.
Gaskets are affected by temperature in three ways: gross physical characteristics, mechanical and chemical resistance properties. General temperature limits of both gasketing (What is Gasketing Page) products and components are provided in the Table 1 below
Table 1 outlines the approximate minimum and maximum temperature limits for common gasketing materials and individual components, based on generally accepted application parameters. While every attempt has been made to provide reasonably accurate values, the data contained herein is a guide and should not be used as a sole source of information for any given product group or type. Variations within the category descriptions between manufacturers, manufacturing methods and material combinations, can significantly affect the listed limits and care must be exercised when such circumstances arise. Please consult the specific manufacturer of the product in question for specific information as it relates to the application.
Consideration should be given regarding the continuous operating temperature of the system. The general temperature limits listed above are generally considered upper and lower limits to the given material or material type under continuous operating conditions. Many manufacturers publish specific pressure-temperature curves for their various non- metallic products as these products are most susceptible to pressure and temperature fluctuations. It should be noted that a given continuous operating temperature limit provided by a manufacturer does not necessarily coincide with a constant continuous operating pressure; rather the relationship is dynamic and should be considered as such. Consult with the manufacturer’s specific information as it pertains to individual operating conditions within the system .
Common Brasses
-269C (-452F)
260C (500F)
Copper
-269C (-452F)
316C (600F)
Aluminum
-269C (-452F)
427C (800F)
Stainless Steel, Type 304
-254C (-425F)
760C (F)
Stainless Steel, Type 316
-254C (-425F)
760C (F)
Stainless Steel, Type 317
-198C (-325F)
760C (F)
Stainless Steel, Type 321
-254C (-325F)
760C (F)
Stainless Steel, Type 347
-254C (-325F)
871C (F)
Soft Iron, Carbon Steel
-29C (-20F)
538C (F)
Alloy 20 (UNS N)
-198C (-325F)
871C (F)
Titanium
-59C (-75F)
C (F)
Nickel
-198C (-325F)
760C (F)
Monel® 400 (UNS N)
-198C (-325F)
816C (F)
Inconel® 625 (UNS N)
-254C (-325F)
C (F)
Hastelloy® (UNS N)
-254C (-325F)
C (F)
1 Categories of flexible graphite styles and limits in this chart are referenced from PVRC-SRC.
2 Limits vary depending on the configuration of the binder and fiber-reinforcing system. The minimum range provided is based upon the glass transition temperature (ASTM STP ) of the elastomer binder in these types of products. Manufacturers and manufacturing methods vary; when combined with other materials, the minimum temperature limit published by the manufacturer will take into consideration their unique formulation and experiences in static seal applications which may allow for lower limits than those published here. The maximum range provided generally considers the various typical configurations offered by major manufacturers.
3 Maximum temperature limit provided is generally based upon manufacturers agreed maximum continuous operating limits.
4 The maximum temperature limit of rubbers/elastomers can be highly dependent on the operating pressure of the application. Generally a PxT limit is set at <20,000 (psig x oF) for this product group. Consult the manufacturer for specifics based on the application.
5 Minimum temperature limits used in this table for metals are referenced from ASME B31.3 Process Piping as it is considered that the metallic gasket and/or the metallic gasketing components will be limited to the minimum temperatures of the process piping design. Please note that the allowable stress values for metals are greatly reduced at elevated temperatures. Please consult with the manufacturer for specific details based on the application. Additionally, the limits provided are based on the specific material identified: do not use these values for all sub-styles of a particular metal i.e. 304H is different than 304 and 316L is different than 316.
6 The minimum temperature limit for PTFE products has been listed at the temperature just below the points where liquid nitrogen and liquid oxygen will become a solid. This reference point is used in the publication due to the fact that these two substances are the most common cryogenic liquids used in industry. The maximum temperature limit for PTFE products is based on manufacturer’s experience of maximum continuous operating temperature.
7 The minimum temperature limit of rubbers/elastomers has been set to approximately the glass transition temperature (ASTM STP ) of the specific material as this change of state from flexible to “glass-like” is considered the limiting factor when using them on any given application.
8 Phyllosilicates come in various types from virtually pure to mixtures with various fillers and binders (for sheet processing and handleability). The maximum temperature range provided in this table is based on published maximums by major manufacturers of this product type.
Metallic and semi-metallic gasket thickness generally does not alter the gasket pressure and temperature (PT) rating. Therefore, gasket thickness is not a critical factor in gasket selection for metallic and semi-metallic gaskets. Gasket thickness is typically dependent only on gasket construction due to constraints in manufacturing or handling. For example, spiral wound gaskets are generally 3.2mm (1/8”) thick for smaller diameters and up to 7.2mm (0.285”) for larger diameters.
In contrast, the thickness of non-metallic (soft) gaskets affects the gasket pressure and temperature (PT) rating. In general, thicker gaskets will have both a lower pressure rating and a lower temperature rating. This reduction may be compensated with additional compressive load (up to the maximum for the gasket at the given thickness). The maximum compressive load may decrease with increasing gasket thickness.
For non-metallic (soft) gaskets, selecting a gasket thickness can be based on the flange material and/or the flange condition. For metallic flanges, the flange condition may determine the selection of gasket thickness. For special flange materials, such as fiber- reinforced plastic (FRP) and glass-lined steel, other factors, such as flange strength, flange cracking drive and gasket thickness selection.
For non-metallic (soft) gaskets, the goal is to select the thinnest material able to compensate for flange irregularities. (i.e. flange damage, flange warping, uneven flange surfaces, lack of flange parallelism, etc.). Unless the flanges are new, assessing the flange condition can only be done when a bolted flange connection is disassembled. Therefore, it is often difficult to plan for which gasket thickness is best for your particular flange connection. Some general guidance is provided below with respect to the advantages of thin and thick gaskets.
Advantages of a Thin Gasket
Generally thin gaskets are 1.5mm (1/16”) or thinner. A thinner in-service gasket provides the following benefits:
Advantages of a Thick Gasket
Generally, thick gaskets are 3.2mm (1/8”) or thicker. Typically a thicker gasket can “travel” more upon installation, allowing the gasket to fill in flange irregularities. The amount of “travel” is dependent on both the gasket compressibility and gasket thickness. To illustrate this concept, a 3.2mm (1/8”) gasket compressed 10%, will “travel” 0.32mm (0.”); while, a 6.4mm (1/4”) gasket compressed 10%, will “travel” 0.64mm (0.025”). Therefore, the 6.4mm (1/4”) can accommodate twice the amount of flange irregularities. Compressibility is a term often used to describe the gasket’s ability to compress and takes into account compressive load and gasket type.
As with most industrial products, there are many unique applications and pieces of specialty equipment. They often have their own product, material, and specification requirements in combination and in addition to those referenced in 1-6 above. Though it is impossible to consider all the unique applications and specialty equipment, several common ones are briefly discussed below in order to highlight their specific gasketing requirements.
Oxygen Service
All organic and inorganic materials react with both gaseous and liquid oxygen at certain pressures and temperatures. This can cause serious fires and/or explosions. Because of this inherent danger, it is important to select gasket materials which have been tested and certified for use under these severe conditions. Perhaps the most recognized testing and certifying body in the gasket industry is the Federal Institute for Materials Testing & Research (BAM) located in Berlin, Germany. There are many sub-handling requirements for all products in oxygen service, but most applications require the base gasket material to be BAM certified. ASTM publishes two guides (G63, G94) which establish a system for evaluating non-metallic and metallic materials for oxygen service. Table X1.1 in G63, lists several materials commonly used as gaskets in oxygen service, such as PTFE, graphite, and sponge chloroprene (CR) elastomer. Guide G94 tends to not list accepted metals as the determination of such is a very complex process, however, the listed best practice by ASTM for metal selection is to use the least reactive material available with the highest oxygen indices.
Chlorine Service
Chlorine is an aggressive oxidizer that reacts with many metals and organic materials. Service conditions, including contact with dry or wet chlorine, must be taken into account when assessing proper gasket selection. The Chlorine Institute (CI) Pamphlet 95 – Gaskets for Chlorine Service and Euro Chlor in Europe list several gasketing products which have been found as acceptable for use. The Chlorine Institute and Euro Chlor do not endorse any of the listed products; rather the publication reflects information obtained from CI companies in their use and/or evaluation of the gasket or gasket material.
Ethylene Oxide
Ethylene Oxide (EO) is considered very reactive and must be given special attention when selecting proper sealing products. The reactive process is referred to as polymerization and occurs naturally with many products. EO rapidly attacks and breaks down many of the organic polymers and elastomers used to make gaskets and one of the most important points to consider is the rate of deterioration of any selected material. The most visible organization making suggestions on safe products for use with EO is the American Chemistry Council and their publication Ethylene Oxide. Through the American Chemistry Council’s Ethylene Oxide/Ethylene Glycols Panel membership experience, it has been found that the preferred gasket type is a spiral wound gasket with 304SS outer/inner rings, 304SS windings and pure (98% min.) flexible graphite filler. Where spiral wound gaskets are not practical or possible for use, the next choice is often flexible graphite sheets laminated onto a tanged metal core insert. Virgin PTFE has been found to be an acceptable material where the gasket can be captured to minimize creep, however, filled PTFE products are often unsuitable due to effects caused by polymerization of the EO with the filler material (such as glass and barium sulfate).
Fire Safe Requirements
In many refinery and power plants there is the requirement for gaskets to be rated as “fire safe”. One of the more common gasket industry fire test procedures is known as the API
607 Modified Fire Test, which is an adaptation of a fire test for valves. It is common that most metallic and semi-metallic gaskets are accepted as being “fire rated”. There are non-metallic gasketing products such as flexible graphite gaskets which have passed fire tests. In addition, there are a few compressed elastomer-based fiber and PTFE gaskets that have also passed fire tests. Consultation with your gasket manufacturer is important when requiring a fire safe gasket, as they may have many styles for you to choose from to suit your budget and performance criteria.
Nuclear Applications
There are many traditional applications found in the nuclear industry which can use common gasketing types, however, gasket requirements located on the reactor side of the process have very different requirements which must always be carefully scrutinized for safety reasons. Perhaps the most common gasket type used in the nuclear industry is some combination of stainless steel and flexible graphite. Typically there are high purity requirements for the flexible graphite component of the gasket and often special oxidation inhibitors are also required. The nuclear industry has its own regulating bodies and gasket standards can vary depending on operator, reactor type, and region/country. It is important to verify local plant standards and coordinate suitable materials from your gasket manufacturer for these applications.
Food and Pharmaceutical Application
Though these industries are very distinguishable, gasketing specifications and requirements for these two industries are often based upon toxicological tests and component “leaching” tests. The common standards used in these industries include, but are not limited to:
There are many gasket products which meet the codes and standards mentioned above and the user should consult their operations standards, local regulatory bodies and gasket manufacturer for assistance.
Clean/Ultra-Pure Water
Due to the requirement of this industry for ultra-clean equipment (piping, valves, fittings and gaskets), most gaskets can be used in these pure water applications providing they have been cleaned to some specific level as required by the end-user. In many circumstances cleaning procedures similar to those for oxygen service can be employed and will meet the local requirements, however, always consult with the manufacturer and verify that the gasket supplier’s cleaning procedures meets end-user requirements. Typically end-users will perform additional cleaning procedures before placing equipment in service.
Heat Exchangers
Shell and tube heat exchangers pose a unique challenge, due to the relatively low allowable gasket stress designed into the equipment, combined with the temperature cycles and temperature changes (creating a condition called radial shear) from chamber to chamber. Research indicates that there can be a net loss of 20% or more in initial bolt load during the increase in operational temperature of the joint. Care needs to be exercised when making gasket changes in these types of systems.
Many shell and tube heat exchangers come from the factory equipped (OEM) with metal jacketed gaskets, mostly for costs, which usually do not give optimum performance over an extended period of time. Kammprofile, expanded PTFE (ePTFE) and flexible graphite gaskets, can be suitable alternatives to these OEM gaskets.
Kammprofile gaskets (stainless steel serrated cores with flexible graphite covering layers) offer significantly lower required seating stress, resiliency to temperature fluctuations and radial shear inherent in the application. With the varying designs of shell and tube heat exchangers and required partition bars, it is important to communicate measurements and drawings to your gasket manufacturer in order to ensure precision alignment with your equipment.
ePTFE gaskets can be a gasketing option for heat exchanger design that does not require a metal component to the gasket. ePTFE gaskets offer the benefit of conformability and chemical compatibility. Heat exchanger gaskets can be cut from ePTFE sheets. In addition, the use of ePTFE tapes can be a practical solution as the gasket can be formed- in-place around the outside diameter and across the cross-bar(s).
Often in the field, heat exchanger surfaces get damaged, causing difficulty in sealing the metal gasket specified in the original heat exchanger design. When heat exchanger surface re-surfacing is not possible during an outage, graphite joint sealant and ePTFE can be an effective repair method.
While not necessarily a frequent occurrence, recognizing the flanges’ stress limits of a given bolted flange connection is an important consideration that must be made to ensure integrity of the seal.
Metallic flanges
Metallic flanges are employed most extensively because of their robustness and wide range of suitable mechanical, thermal, and chemical resistance properties. Proper function of a bolted flange connection must also take into consideration the mechanical limits of metallic flanges. If these are exceeded through overstressing it can lead to various forms of damage from excessive flange rotation to failure by Gross Plastic Deformation (GPD) causing the loss of the integrity of the seal. This can occur if flanges are assembled at bolt loads beyond the limits of the flange. In that case, the flanges become the limiting factor for sealing performance. A discussion of the various stresses and the point at which they affect flange damage is beyond the scope of this publication but has been documented in several publications as noted below.
Until relatively recently an established method for practical determination of the limiting stresses for flanges had not been addressed. Earlier research was focused only on bolt and gasket stress limits. The publication of ASME PCC-1 now includes a comprehensive methodology that enables the determination of bolt stress limits taking into consideration pipe flange limits as well. This is included in Appendix O of the PCC-1 document which takes into account not only the type of flange but also the composition of the metal. An example is provided to show what conditions of bolt stress exceed maximum flange stress limits for NPS steel Weld neck Flanges.
Non-metallic Flanges
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Fiberglass reinforced plastic (FRP), plastic and other non-metallic piping and flanges are often used in the Chemical Process Industry (CPI). Application of these types of flanges is usually driven by chemical resistance and lower cost compared with the more exotic metals. Design practices for FRP flanges are described in ASME Standard RTP-1- and are beyond the scope of this publication. Appendix NM-9 Installation of RTP Vessels provides guidelines for gaskets, fasteners, torque, lubrication, and other factors related to the bolted flange connection. Mechanical properties of these materials differ substantially from those of metals, and none approach the strength, stiffness, resistance to impact damage, temperature capability and robustness of metals. As a result, flange strength limits bolt torque and consequent gasket stress. Failure modes differ from those of metals because creep and fatigue can result in failure at much lower stresses.
Elastomeric gaskets historically have been applied to these flanges because of their low sealing stress. PTFE is being applied where superior chemical resistance is required but requires a higher sealing stress contributing to some sealing problems with current flange designs. These are being addressed through revisions in flange design methodology.
Yield Strength
For the bolt to operate like a spring, the yield strength must be in balance with the joint. Bolts that apply loads higher than necessary are harmful for the system, as are those that apply insufficient load. The applied load must be sufficient to guarantee the correct gasket seating stress and lower than the flange strength and gasket limit.
When fasteners and joint components are put under tension by tightening the nut (thus inducing a load on the gasket), the fastener and joint components will deform. The fastener will increase in length as the tension in it increases.
The initial stretch of the bolt/stud is the elastic region, within which permanent deformation of the fastener will not occur, even with repeated loading and unloading. The highest tensile force which can be withstood without permanent deformation is known as the elastic limit (also called proof load or yield strength). Fasteners perform most effectively within their elastic region.
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. At the maximum limit, the ultimate strength of the fastener is also known as the tensile strength.
The mechanical properties of bolts are sometimes expressed in terms of property class. For carbon and alloy steel, it consists of two figures (Cf EN ISO 898-1)
For stainless steel, it consists of the combination of a letter and a number (Cf En ISO -1)
Temperature Capability
A correlation exists between the metallurgical and chemical properties of the bolt material and its temperature capability (i.e. in-use temperature range). The bolt material selection must be based on the yield strength needed as well as the design temperature and flange material. It is crucial to have both bolts and flanges made of materials with similar thermal expansion for proper performance of the fasteners.
Chemical Compatibility
The compatibility of the bolt material with the process fluid and the environment must be considered. Bolts can be subject to chemical corrosion which over time can limit the performance of the fastener and cause problems with disassembly. Bolts can also be subject to electrochemical corrosion and as such, materials should be selected accordingly.
Hardness
Though hardness is not often verified, it is an important consideration for critical applications in nuclear, oil rigs and aerospace. Hardness is an important indicator of whether the required metallurgical processes were followed during manufacture of the fastener. This is an important property check because determination of the fastener material’s chemical composition alone is insufficient to ensure that material properties requirements, such as yield strength, are met. It also ensures the fastener will not be prone to failure, particularly due to Stress Corrosion Cracking (SCC) or brittle fracture at its operating conditions.
For each bolt/stud specification, there is an associated nut specification. Since both parts interact, it is important to refer to the nut standard as well. The ASTM standard for nuts related to the above for bolts and studs is:
The EN/ISO standards for nuts related to the above for bolts and studs are:
The tension in the fastener (and hence the compressive pressure on the gasket) is generated by tightening nuts along the threads of the bolt. 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. The main factors which determine stripping strength are:
The threads on a larger bolt/stud are “longer” per turn and have thicker roots than the threads of a smaller bolt/stud. This means that the per-thread area which must be sheared to strip the threads is greater on a larger bolt/stud, which means greater stripping strength. Increasing the length of engagement between threads increases the cross-sectional area of the material which must be sheared to strip the threads.
Threads strip more readily when bolt/stud and nut materials 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 bolts/studs. When done this way, the bolts/studs will break before the nut threads strips. Remember, a break is easier to detect than a stripped thread!
Also note the effect of “galling”, which is the cold welding (partial or full) of one heavily loaded surface against another. It is encountered when the surfaces are brought together so intimately that molecular bonds form between mating parts, for example, between a nut and a bolt. This occurs when surfaces are highly loaded, when threads are a tight fit, when lubricants have migrated or dried out and when threads are damaged. This is compounded at high operating temperatures or when corrosion has occurred. It is difficult to eliminate galling, but the following may help:
· use coarse threads, rather than fine
· use the correct lubricant
· select materials for bolts/studs and nuts which in combination are resistant to galling, such as cold drawn 316 stainless steel on cold drawn 316 stainless steel, 400 steel nuts and 316 fasteners, etc.
Finished hex nuts are the most common type. Heavy hex nuts are used in high temperature and high-pressure applications. This is the most common type of nut for flanged joints. Heavy hex nuts are slightly larger and thicker than finished hex nuts.
Industry standards provide nomenclature for gaskets for standard flanges.
Nominal Pipe Size (NPS)
Pressure Class (Class)
Ring or Full Face Gaskets
Example: NPS 2, Class 150 Full Face Gasket
Nominal Diameter (DN)
Nominal Pressure (PN)
Inner Bolt Circle (IBC) or Full Face Gaskets
Example: DN10 PN40 IBC gasket
ASME provides two standards to consider when specifying standard gaskets for standard flanges made to ASME B16.5 and ASME B16.47 (Series A and B)
CEN provides a standard to consider when specifying standard gaskets for standard flanges made to EN Flanges and Their Joints – Circular Flanges for Pipes, Valves, Fittings and Accessories, PN Designated and made to EN Flanges and Their Joints – Circular Flanges for Pipes, Valves, Fittings, and Accessories, Class Designated
Concerning pressure class designation, it is a frequent misconception that the pressure class is the gasket rating. Class 300 simply refers to a Class 300 flange design and does not necessarily mean the gasket is good up to 300 psi in the actual application. Similarly, PN40 simply refers to a PN40 flange design and does not necessarily indicate the gasket is good for use up to 40 bar. The allowable application pressure of the flange is dependent on temperature and material, which is typically provided in the associated flange standard (such as ASME B16.5) A rule of thumb, for ASME flanges, the maximum pressure at ambient temperature specified for Class 150 is 290 psi and for all the other pressure classes it is 2.5 times the class designation (i.e. Class 300 is 750psi).
All joints leak at some low but measurable rate. Therefore, the acceptable leak rate for a joint design should be defined by the type of fluid contained, the hazardous nature of the contained fluid, the laws controlling emissions where the joint is installed, the value of the leaking product and other factors.
Tightness Classes
Tightness classes were developed to define the leak rates. These tightness classes are generally defined in the Americas as class T1 through T5 as proposed in the Room Temperature Tightness (ROTT) test procedure. A T2 leak rate is called a standard tightness class and has a helium leak rate of 0.002 mg/sec mm of the gasket outside diameter. The higher the tightness class, the lower the mass leak rate. With each full level of tightness (going from T2 to T3 for example) reducing the leak rate by 2 orders of magnitude, so a T3 tightness class joint would have a leak rate of 0. mg/sec mm.
Gasket tightness is a measure of a gasket’s ability to control leak rate in a joint at a given load. Given all other variables are equal a tighter gasket requires higher internal pressure to drive the same rate of fluid through the joint. Tightness also may be considered as the internal pressure needed to cause a small leak rate in a joint.
Tightness class is defined differently in Europe. Per EN--1, the leak rate is defined by L with a subscript indicating the mass leak rate. For example L1.0 would be a helium mass leak rate of 1 mg/ (sec m) of the mean gasket circumference. The chart below shows the relative correlation between the American “T” tightness class and the European “L” tightness class.
Design Codes/Standards are used to design bolted flange connections. In general, these codes/standards provide guidance on design and selection of flanges, bolts, and gaskets to ensure adequate performance of the joint. Some codes/standards have implemented the concept of joint tightness. Below is a discussion of two main design codes/standards and their respective design gasket factors. Other concepts used in industry are also briefly discussed.
ASME Boiler and Pressure Vessel Code
ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 and 2 contain rules for the design and construction of unfired pressure vessels. The design of bolted flanges requires that gasket constants referred to as maintenance (m) factor and seating (y) stress be used in the calculation. The recommended values given for the gaskets listed in the code are non-mandatory. However, these constants, m and y, must be used in the code formulas unless the designer can justify the use of other values for these constants. Values for constants of specific gaskets are included in Division 1, Table 2-5.1. Additionally, gasket manufacturers publish m and y values for their own specific gasket materials and styles.
NOTE: There is currently no industry standard test to determine the m and y gasket constants, so many gasket manufacturers have developed individual test procedures based on the withdrawn ASTM F586 test method. There is no approved ASME alternative to the code that requires use of these constants.
Bolted Flange Design Methodology
This methodology is based on the Taylor-Forge approach to flange design, which did not include the concept of tightness.
A flange must be designed to create sufficient compressive load on the gasket contact area, to create an initial seal with essentially no pressure in the vessel. The gasket must conform to the flange surface and be sufficiently compressed to compensate for internal voids or spaces that could be detrimental to a seal. The gasket stress required to achieve this initial seal is considered the y constant.
The m value allows the flange designer to determine the compressive load on the gasket required to maintain tightness when the vessel is pressurized. This value is considered a multiplier or maintenance factor. This constant is intended to ensure that the flange has adequate strength and available bolt load to hold the joint together, while withstanding the effects of hydrostatic end force or internal pressure. The design intent is that the flange and bolting will hold the flanges together under pressure and exert an additional stress on the gasket of m multiplied by the internal pressure.
The designer calculates the load required to seat the gasket (Wm2) using y, and calculates load required during operation (Wm1) using m and the design internal pressure. The flange design is then based on the larger of the two calculated values.
Critical Considerations
There are some critical considerations when using the m and y in the two design equations (Wm1 and Wm2); including the fact that they:
ROTT TEST PROCEDURE
While there are no design codes or standards that are based on gasket factors derived from ROTT, ROTT is often referenced/used to describe gasket sealing performance relative to tightness class in the Americas.
Test Methodology
The ROTT test is performed on a 4-inch NPS gasket using a ROTT test rig with helium gas as the pressurized medium. The test includes two parts.
Part-A represents initial joint tightening and gasket seating. The test includes 5 main gasket stress levels in psi; (S1), (S2), (S3), (S4), and (S5) psi, respectively. At each stress level, leakage is measured at 400 psig and 800 psig.
Part-B simulates the operating conditions by performing leakage rate measurements during unload-reload cycles, which are described below
Key Outputs
There are four key outputs from the ROTT test procedure:
Upon initial seating, gasket tightness normally increases with increasing gasket stress. The maximum tightness parameter, Tpmax, is simply the highest level of tightness achieved during the ROTT test. Normally, the Tpmax value corresponds to the maximum gasket stress level, S5. A high Tpmax is favorable.
“Gb” and “a” are obtained from the seating load sequence (Part-A) of the ROTT test. “Gb” represents the loading of the gasket at Tp =1, where Tp is the Tightness Parameter. “a” describes the rate at which the gasket develops tightness with increasing stress. For a given gasket material the “Gb” and “a” values are interdependent to determine the gasket stress. Low values of both “Gb and “a” are a favorable and they indicate that the gasket requires a lower gasket stress to achieve a given tightness level.
“Gs” is obtained from the load-unload cycles (Part-B) and is related to the operating conditions of the gaskets. “Gs” shows how sensitive the gasket is to the normal operating conditions such as vibration, shock load, pressure changes and other acts that try to reduce the gasket load and allow more leakage. A lower value of “Gs” is favorable, which indicates that the gasket is less sensitive to unloading.
CEN EN
CEN has published EN Flanges and Their Joint – Design Rules for Gasketed Circular Flange Connections which provides the European standard requirements for designing flanges. This document consists of four parts:
Design Methodology
The flange calculation described in CEN EN is a complex calculation that considers flange tightness. As a result, the gasket design parameters also incorporate tightness, L. Gasket manufacturers are responsible for publishing the parameters for their gaskets at the relevant test conditions.
The gasket parameters and associated test method are described in EN Flanges and Their Joints – Gasket Parameters and Test Procedures Relevant to the Design Rules for Gasketed Circular Flange Connections.
Note: the CEN EN gasket parameters only apply to Ring (IBC) gaskets, as a DN40 PN40 is the test specimen size.
Definition: this factor to allows for the effect of the imposed load on the relaxation of the gasket between the completion of bolt-up and long term experience at the service temperature
Definition: the minimum gasket surface pressure on assembly required at ambient temperature in order to seat the gasket into the flange facing roughness and close the internal leakage channels so that the tightness class is to the required level L for the internal test pressure
after off-loading
Definition: the minimum gasket surface pressure required under the service pressure conditions, (i.e.) after off-loading and at the service temperature, so that the required tightness class L is maintained for the internal test pressure
Definition: the maximum surface pressure that may be imposed on the gasket at the indicated temperature without collapse or “crush”, compressive failure, unacceptable intrusion into the bore or damage of the stressed area of the gasket such that failure was imminent
Definition: this is the additional change in thickness of the gasket or sealing element due to creep between the completion of the loading and the end of the test period
The difference between a raised face and a flat face flange is that the raised face has a raised area that surrounds the pipe bore and the flat face flange doesn’t. These are two of the most commonly used types of flange faces. They can both be sealed with one of two different types of gaskets, the ring type and the full-face gasket. This post will provide some details on the raised face flange, the flat face flange, and the two types of gaskets used with them.
The raised face is the most common flange face type used. It’s called a raised face because it has a raised surface above the bolting circle where the gasket is placed. Sealing this type of flange face is accomplished by compressing a soft, flat, or semi-metallic gasket between mating flanges in the raised area of the flanges.
Raised face (RF) flanges are common in process plant applications but can be used in almost all applications. They are suitable for use in high and low temperatures and pressures. The purpose of the RF flange is to focus more pressure on the smaller gasket area, which increases the pressure containment capabilities of the joint.
Flanges are sized according to pressure ratings. The higher the pressure rating of the RF flange, the bigger the flange diameter, the number of bolts it requires, and the thicker both the flange itself and the raised face are.
Our simplex, duplex, and Y-strainers can come with raised face flange connections.
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Flat face (FF) flanges are similar to the RF flanges, but they do not have the raised area like the RF flange. Instead, the whole surface is flat. That means that the gasket used with the FF flange has full contact with the whole surface where two flanges are mated. The gaskets for FF flanges are typically made from non-metallic materials like Viton (a brand of fluoroelastomer) or EPDM (ethylene-propylene diene monomer); it’s uncommon for them to be metallic.
The purpose of the FF flange is to avoid the bending moment that is put on the flange as the bolts are torqued. Some flange materials can easily break at this time, such as cast iron and fiberglass. FF flanges eliminate this problem.
Generally speaking, flat face flanges are used in less arduous applications, like low-pressure water piping. They are commonly used in low temperature and pressure environments such as pump suctions or water treatment flanges.
At Commercial Filtration Supply, we carry a variety of simplex and duplex models that are constructed with flat face flange connections (found here and here). Contact us to learn more about how we can help you find exactly what you need.
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Sealing raised face or flat face flanges allows for two different types of gasket shape, the full-face and the ring-type. It’s important to know which type is best for your application and to understand the measurements you will need to know when you buy them.
The rig type gasket can be used for both the RF and FF flanges. It sits around the pipe bore and inside of the flange bolts, and in the RF flange, it is positioned on the raised surface. The ring type gasket is a drop-in gasket, which means it can be installed without taking apart the whole joint. It also requires less material and cutting than a full-face gasket, though it can be harder to clamp into position.
The measurements you need to know for ring type gaskets are the ID (the inside diameter or pipe bore size), the OD (the outside diameter), and the thickness of the gasket.
The full-face gasket is typically used with flat face flanges but can also be used with RF flanges. It sits on the raised flange faces but has the same outside diameter of the flange. For that reason, it has to have holes for the bolts securing the flanges to pass through. This makes aligning the gasket easier, but it does mean that the entire joint has to be taken apart for installation. The full-face gasket is better at stopping dirt from getting into the joint because it extends all the way out to the OD.
The measurements you need to know for the full-face type gasket are the ID, the OD, the bolt circle diameter, number of bolt holes, and the gasket thickness.
Choosing the right type of flange face is important, especially when the application is high pressure or temperature. In those situations, it’s best to use an RF flange, as they are designed to withstand more demanding environments. The FF flange is better for operations that are lower pressure and temperature and in systems where cast iron, fiberglass, or other materials that may break as the bolts are torqued are being used.
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