Spiral Wound Gasket
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A spiral wound gaskets is categorized as semi-metallic gasket that is typically used for high pressure applications. The sealing element is formed by winding two materials (one for sealing, one for resilience) into thin v-shaped spirals. One material used in the sealing element is usually a metal and the other, referred to as a filler, which is a soft nonmetallic material, typically Teflon, aramid fiber or graphite. Both are chosen for compatibility with the sealing media and their ability to withstand the operating conditions. Different configurations exist to handle different flange designs and operating conditions. These gaskets have good recovery and good tolerance for flange-surface finish irregularities. The sealing action is the result of a combination of the flow of the metal and soft filler plies when the gasket is compressed. They are particularily suited for assemblies subject to extremes in joint relaxation, temperature or pressure cycling, shock, or vibration.[1]
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How they Work
A spiral wound gasket can be considered a heavy duty spring in series with the bolts, nuts, washers, and flange. Its spring like structure enables the gasket to recover under variable loads. Seating stress forces the filler into the flange imperfections, creating a seal. Flexible metallic windings maintain seal integrity when flange movement occurs.
History
1912 - Asbestos spiral wound gaskets were invented to provide a better sealing method than other compressed asbestos fibre gaskets.
1980s - Non-Asbestos alternative filler materials were introduced.
Styles
There are 4 main styles that a spiral wound gasket can have. Other special styles exist for specific applications.
CR
A solid metal ring is used as a centering device and a compression stop for the windings. This is a general purpose gasket for use with flat face and raised face flanges. An inner ring is recommened for applications above class 600.
CRIR
A solid metal outer ring is used as a centering device and a compression stop for the windings. A solid metal inner ring is used as an additional compression stop, and provides a heat and corrosion barrier, protecting gasket windings and preventing flange erosion. It is suitable for use with flate face and raised face flanges. This style is specified for class 900lb and abover and wherever corrosive or toxic media is present. Inner rings are used for a host of reasons. They prevent build up of solids between the internal diameter of the gasket and flange bore, reduce turbulent flow of process fluids, provide greater stability, additional compression limiting stop for the windings, protect against heat and corrosion, and facilitates winding for larger diameter gaskets. They also serve to prevent inward buckling of the windings and filler when the gasket is overtorqued.
R
Inner and outer diameters are reinforced with several plies of metal without filler to give strength and stability. This style is suitable for tongue and groove, male and female, flat face recess flanges or valve bonnets.
IR
A solid metal inner ring acts as a compression stop and provides a heat and corrosion barrier, protecting gasket windings and preventing flange erosion. This style is suitable for male and female pipe flanges, and custom made boilers and heat exchangers.
Low Stress Spiral
There is concern that low strength flanges, such as Class 150, are not suitable for some spiral wound gaskets. The low stress gasket (LS type) comprises an overfill of filler material which extends beyond the metallic spirals. The result is that the gasket will compress to its optimum sealing thickness at a significantly reduced seating stress. However, with the LS type of gasket there is a layer of unreinforced filler that is subject to the full operating conditions of the flange. In this region, the gasket is not blow out resistant.
A low-density spiral wound gasket is a better choice, where more filler material is wound within the sealing element, allowing the gasket to seal with less sealing stress.
Gasket Style Comparison
The following chart lists different manufacturers specific coding for spiral wound gasket styles.
| Type | SEVAL | 
| Leader | Lamons | Flexitallic | Garlock | Teadit |
| Outer Ring Only | SP12 | CR | SR | WR | CG | RW | 913 |
| Windings Only | SP11 | R | S | W | R | SW | 911 |
| Inner Ring Only | SP13 | RIR | SI | WI | RIR | | 911M |
| Inner and Outer Rings | SP14 | CRIR | SRI | WRI | CGI | RWI | 913M |
| Low Stress Controlled Density | | | STD GSKT | WR-LP | LS / LSI | CD/RW | |
| | | | | | | | |
| Low Stress Density Inner Ring with Pass Partitions | SP-S13 | HTX | SP | WP | HE | | |
| Low Stress Density Outer Ring with Pass Partitions | SP-S12 | HTX-CR | SRP | WRP | HE-CG | | |
| Hot Application | | | WRI-HTG | HOT | HEAT SHEILD | |
Materials
Typical metals used as the metallic component of the winding, inner and outer rings:
Carbon Steel
304 Stainless Steel
316 Stainless Steels
347 Stainless Steel Alloy 20
Monel
Nickel
Titanium
Hastalloy
Inconel
Typical Fillers used as the sealing component of the winding:
Flexible Graphite
PTFE (Poly-Tetra-Fluoro-Ethylene)
Mica
Ceramic
Compressed Asbestos
Compressed Non-Asbestos Fillers
Gasket Selection
Proper selection of a spiral wound gasket is critical to its success in the application. Many different styles of gasket and material types are available to provide a wide range of options to provide the best sealing option. To select a suitable gasket for the application, information about the application must be analysed. Typically, the most information that can be collected, the more success you will have in determining your best option.
Important information that is required for the selection of a spiral wound gasket is the temperature, pressure of the system, chemical being run through the system, flange face style, flange size, flange material, flange clamping force limitations, number of bolts, and type of bolts.
Standards
ASME B16.20
This standard dictates a required level of specifications on compression under a defined bolt stress, construction of spiral element, dimensions and identification. It does not specify the purity of graphite and does not classify them by sealability.
Requirements of the ASME B16.20 specification say that the gasket must compress to 0.130"(3.3mm) +/- 0.005"(0.13mm) with a uniform bolt stress of 25,000 psi for 1/2", 3/4" and 1" for 150, 300 and 600 classes and 30,000 psi for all other sizes. The sealing element must have a specified number of windings (3) without filler material at both inner and outer diameters, the windings must also have a minimum of 3 welds at both inner and outer diameters. The gasket elements must be to the dimensions stated in the standard.
The gasket must also have the manufacturers name, B16.20, winding metal and filler material, pressure class, and nominal pipe size stamped onto the outer ring of the gasket.
Manufacturing Process
Inner and Outer Ring
The inner and outer ring is manufactured in one of two methods:
The first method involves cutting out the metal ring from sheet material by using a punch press, water jet or laser cutting machine. The ring is then put onto a lathe where a convex profile is machined onto the outer diameter edge for inner rings or a concave profile is machined onth to inner diameter edge for outer rings. This method is used for producing smaller rings up to 10", depending on the size of the lathe.
The second method begins by taking a long strip of metal and running it through a milling machine to produce the convex profile on one of the edges for the inner ring. Outer rings are profiled after the bending because buckling issues with the concave shape during the bending procedure.stage. Next the metal strip is run through a metal bending machine to form the correct radius of the ring. The bent strip is then measured to the correct radius and the excess length of the strip that is overlapping is cut off using a bandsaw so that the new ends of the strip meet together and form a complete ring. The strip is now in the shape of a ring but is still separated at its ends. To physically bond the ends they are typically TIG welded together and sanded down to form a complete, smooth ring. Following the formation of the complete ring, outer rings are then run through a milling machine to produce the concave profile on the inner diameter edge. This method is far more time consuming and requires a high level of skill to produce accurate results, but is capable of producing very large size rings.
Sealing Element
The sealing element is made by either a vertical winding process for smaller diameters (<10") or a horizontal winding process for larger diameters(>10"). Both processes are identical other than the orientation of the winding process. A horizontal winder has the capability of producing very large sealing elements, but it requires more floor space.
The process begins by setting an inner ring or a mandrel for 'R' style gaskets, on the main driving component of the winding machine. This component will rotate the inner ring or mandrel which will provide the necessary pulling force required to 'wind' the sealing element.
Next, a long thin metal strip, which will make up the winding of the sealing element, is fed into a set of profile rollers and then pulled towards the inner ring or mandrel setup. As the machine component rotates, the metal winding will be pulled through the profile rollers which will shape the metal strip into the correct winding profile. The metal winding is initially wrapped around the inner ring or mandrel once and its overlapping profile is welded in a minimum or 3 locations to physically bind the windings together. Although the ASME B16.20 standard specifies a minimum of 3 welds, it is a good practice to make numerous welds to provide the maximum binding strength of the inner diameter of the sealing element.
The metal winding is then wound around another 3 times, as per the ASME B16.20 specification, and the filler material is then added between the metal winding being wrapped and the started sealing element. The sealing element is wound a number of times to give the right width. When the sealing element is close to its final width, the filler material is removed from the winding process and the metal winding is then wrapped around the sealing element another 3 times. The process is finished off by welding the outer metal winding to the winding it overlaps.
Assembly
To finish a CRIR or CR style spiral wound gasket, the final stage is to snap the outer ring on around the sealing element. This is done by simply guiding the outer profiled edge of sealing element into profiled concave groove of the outer ring. If required, a mallet can be used to lightly tap the sealing element into the groove.
Installation Method
ASME VIII calculation is used to define a minimum bolt stress.
For most applications using standard flanges a bolt stress of 45,000 psi is used. This value is sufficient to take account of irregularities in the bolting procedure.
The joint should be bolted using the standard cross-bolting torquing sequence with a minimum of 3 passes. This bolting method provides a more uniform way of applying the clamping force of the flange to the gasket. Ideally, each bolt should have the same tension so that the seating stress is equal around the entire surface of the gasket.
Failure
Failure of spiral wound gaskets rarely occur because of quality problems. The most likely reasons for failure is the selection of an incorrect gasket for the application conditions, gasket was damaged in storage, handling or on installation, gasket was crushed by excessive load during assembly (buckling), gasket was reused, or gasket failed due to corrosion.
References
1. Daniel E. Czernik, Gaskets Design, Selection and Testing, McGraw-Hill, 1996, pg. 46, ISBN 0-07-015113-X
