GASKETS FOR FLANGES
Seal gaskets create a static seal between the faces of two flanges in a piping arrangement. Gaskets, in whatever material from compressed and soft sheets to metal, fill the microscopic spaces and the irregularities existing on the faces of the flanges as they get bolted and tightened, forming a strong seal that prevents fluids and gases from leaking out of the pipeline.
The correct installation of gaskets in between flanges is one of the most important contributing factors to a good sealing or a pipeline and its leak-proof performance. Of course both the flange and the gaskets shall not be damaged during the construction of the pipeline and the joining of the pipes.
There are several types of gaskets and specifications, depending on the operational parameters of the process, the flanges and the material grades involved in the system. The installation of gaskets between flanges is a mandatory requirement, i.e., it would be not advisable nor safe to tighten flanges without gaskets.
There are three main families of gaskets, depending on the construction material:
NON METALLIC GASKETS
Non-metallic Gaskets, otherwise defined flat cut gaskets or soft gaskets or compressed sheet gaskets or asbestos free gaskets, are usually composite sheet materials used with flat-face flanges (FF) and raised face flanges (RF) in low pressure and low-class pipelines. Non-metallic gaskets are manufactured with aramid fiber, glass fiber, elastomer, PTFE, graphite etc. Full-face gasket types suit flat face flanges, whereas flat-ring gasket suit raised face flanges. The ASME B16.21 Norm covers sizes, types, materials, dimensions, dimensional tolerances, and markings of non-metallic flat gaskets.
SEMI METALLIC GASKETS
Semi-Metallic gaskets are built with a combination of metal and non-metallic materials. The metal guarantees strength while the non-metallic component provides conformability and salability to the gasket. The most common semi-metallic gaskets are the spiral wound gaskets (introduced by Flexitallic at the beginning of the past century) and Kammprofile gaskets, and a variety of metal-reinforced graphite gaskets.
Spiral wound gaskets are available in different configurations depending on the number of rings and material grades combination of the inner rings and the outer rings of the gasket. Semi-metallic gaskets can withstand most operating conditions (pressure and temperature conditions), and are used on raised face flanges (RF), male-and-female flanges (M&F), and tongue-and-groove flanges (T&G). Semi metallic gaskets can be grouped in the following categories:
SPIRAL WOUND GASKETS
Flexitallic USA launched on the market spiral wound gaskets back in 1912 to cope with the demand for leak proof flanges seals in condition of increasing temperatures and pressures. Spiral wound gaskets are designed to withstand the mechanical stress generated by severe and fluctuating service conditions on the flanges, the gaskets and the bolting sets of the flanged pipeline joints. A spiral wound gasket has, as key sealing element, a formed metal strip. The sealing strips or fillers are usually made of graphite, even if different materials such as PTFE may be used, and the windings are generally made of stainless steel. The gasket is fitted into a steel ring of a specific thickness. When the gasket is fitted with a flange and a bolt load is applied, the steel outer ring of the gasket creates the sealing. To further improve the pressure rating of the spiral wound gasket, a steel ring may be added in the internal part of the gasket (the so-called inner ring). This gives an additional compression and provides a heat and corrosion barrier protecting the gasket windings and the flange from erosion. The inner ring material may be, or not, of the same of the winding metal. The usage of a solid metal inner ring is required by ASME B16.20 for pressure class 900, NPS 24 and larger, pressure class 1500 NPS 12 and larger, pressure class 2500, NPS 4 and larger.
METAL JACKETED GASKETS
As their name goes, metal jacketed gaskets are manufactured by joining a metallic outer shell in to a metallic or a non-metallic filler. The filler material attributes resilience to the gasket while the metal jacket protects the filler and is in charge of withstandaing the pressures, temperatures and corrosion of the process. Metal jacketed gaskets are generally used for heat exchangers, pumps, and valves. They require a smooth flange surface finish, high bolt loads, and flat flanges to seal effectively.
Kammprofile gaskets, alternatively named "Grooved gaskets" or “Camprofile gaskets”, are used in many industrial applications as well as in power generation and nuclear (sealing of flanges or sealing of heat exchangers, superheaters and other pressure equipment). In general, Kammprofile gaskets are preferred in all cases when pressure and temperatures are constantly high and, therefore, strong bolt loads are involved. A Kammprofile gasket consists of a metal core (generally stainless steel) with concentric grooves on either side with sealing materials. The sealing layers can be graphite, PTFE, CAF or other non ferrous metals like Silver; Bronze and Aluminum, depending on the service conditions and process parameters.
Metal gaskets are fabricated out of metal, and the most common type are the ring joint gasket (RTJ gasket) that are used in connection with RTJ flanges. Ring Type Joint gaskets create a seal when a bolting force is applied to the flange and the softer material of the gaskets gets “squeezed” into the micro fine structure of the flange (which is made of a harder material) creating a very tight and lasting seal (metal to metal seal or metal to metal connection). Ring type gaskets were developed to ensure lasting and strong seals for flanges, valves, piping spools and vessels at high pressure and temperature conditions (typical of the petrochemical industry).
The most common type of RTJ gaskets is the “style R”, which is machined to tight manufacturing tolerances to ensure correct installation with standard ASME B16.5 and API 6B ring joint faced flanges. The material of RTJ gaskets shall match the flanges material but is shall have a lower hardness than the material of the flanges: this to ensure that the gasket, and not the flange, gets plastically deformed within the groove of the flange as a bolting pressure is applied to the flanges. “R style” oval and octagonal RTJ gaskets can seal pressures up to 6.250 psi by ASME B16.20 and up to 5.000 psi according to the API 6A pressure ratings. RTJ type is frequently used as valve cover gasket. Besides the “R style” ring joint gaskets, the other two popular designs are the “Style BX” and “Style RX” RTJ gaskets.
RING JOINT GASKETS STYLE R, STYLE RX, STYLE BX
- FLAT GASKET: NPS 1/2'' - 60’’, Class 150 to 2500 LB
- SPIRAL WOUND GASKET: NPS 1/4'' - 60'', Class 150 to 2500 LB
- RING JOINT GASKET: R11 to R105 (type R, oval or octagonal), RX 20 to RX215 (API 6A type 6B), BX150 to BX303 (API 6A type 6)
SELECTION TABLE RING JOINT GASKETS STYLE R (SIZE DESIGNATION)
|NOMINAL PIPE SIZE||PRESSURE CLASSES|
|ASME B16.5||API 6B||ASME B16.47 Series A|
|150#||300# – 600#||900#||1500#||2500#||720# - 960# (1)||2000#||3000#||5000#||10000#||150#||300# - 600#||900#|
SELECTION TABLE RING JOINT GASKETS STYLE RX (SIZE DESIGNATION)
SELECTION TABLE RING JOINT GASKETS STYLE BX (SIZE DESIGNATION)
|16 3/4"||-||-||BX-161 BX-162||BX-162||BX-162||-|
|FLAT||ASME B16.5 (gaskets for ASME ANSI B16.5 flanges), ASME B16.21 (gaskets for ASME ANSI B16.47 Series A and MSS SP 44 flanges), BS 3293, ASME B16.21 (gaskets for ASME ANSI B16.47 Series B flanges)|
|SPIRAL WOUND||ASME B16.20 (gaskets for ASME ANSI B16.5 flanges), ASME B16.20 (gaskets for ASME ANSI B16.47 Series A and MSS SP 44 flanges), ASME B16.20 (gaskets for ASME ANSI B16.47 Series B flanges), NF E 29.900-3 (gaskets for ASME ANSI B16.47 Series A flanges)|
|RING JOINT (RTJ)||ASME B16.20 (gaskets for ASME ANSI B16.5 flanges), ASME B16.20 (gaskets for ASME ANSI B16.47 Series A and MSS SP 44 flanges), ASME B16.20 (gaskets for ASME ANSI B16.47 Series B flanges), ASME 16.20 (gaskets for API 6A type 6 and 6B flanges.|
Main ASME norms related to gaskets:
- ASME B16.20 Metallic Gaskets for Pipe flanges, i.e. Ring-Joint, Spiral-wound, and Jacketed: This Standard covers the dimensional tolerances, materials, dimensions and markings of metal ring-joint gaskets, spiral-wound gaskets, and metal-jacketed gaskets. These gaskets are dimensionally suitable for use with flanges described in reference flange standards ASME B16.5, ASME B16.47, API Specification 6A, and ISO 10423. This Standard also covers spiral-wound metal gaskets and metal-jacketed gaskets for use with raised-face (RF) and flat-face (FF) flanges.
- ASME B16.21 Non Metallic Flat gaskets for Pipe Flanges: This Standard covers types, sizes, materials, dimensions, dimensional tolerances, and markings for nonmetallic flat gaskets. These gaskets are dimensionally suitable for use with flanges described in the referenced flange standards.
GASKETS MATERIAL GRADES
Gaskets are manufactured with the following materials:
|GASKET TYPE||MATERIAL GRADE|
|NON METALLIC GASKET||Aramid NBR binder gasket, carbon graphite reinforced gasket, glass reinforced (steam service gasket), PTFE bi axially oriented (silica filler), SBR gasket (styrene-butadiene), CR-chloroprene (Neoprene gasket), EDPM gasket (ethylene propylene), fluorocarbon (Viton gasket), graphite gasket, BUNA-n-rubber (nitrile, NBR gasket), natural rubber gasket, vegetable fiber sheet, silicon, chlorosulfonated polyethelene (Hypalon gasket), aramid neoprene binder (oil service gasket), aramid + SBR binder (premium type gasket), graphite + ss316/316l insert, PTFE bi axially oriented (with hollow glass microsphere) blue, PTFE bi axially oriented (pigment free)-gray, expanded PTFE gasket, PTFE joint sealant, PTFE envelope slit type, PTFE envelope milled type, PTFE envelope formed type, MICA sheet, ceramic fiber|
|METALLIC GASKET||Soft iron-D, low carbon steel-S, ASTM A182 F304, ASTM A182 F304L, ASTM A182 F304H, ASTM A182 F316, ASTM A182 F316L, ASTM A182 F316H, ASTM A182 F321, ASTM A182 F347, ASTM A182 F410, ASTM A182 Gr. F5, ASTM A182 Gr. F51 (duplex), ASTM B564 UNS N06625 (inconel 625), ASTM B564 UNS N08825 (Incoloy 825), N7M (Hastelloy B), ASTM A182 UNS S31803 (Superduplex)|