Excepciones temporales de las normas avanzadas

The metal windings stiffen the gasket (hence the higher required seating stresses) and make it less susceptible to crushing than gaskets where the entire bolt load is carried by the compressible gasket material.

There are few, if any, welds in the gasket assembly.

EDS-2004/FL-68

Spiral Wound Gaskets

„ Commonly used in refineries due to excellent sealing performance through a wide range of temperatures, pressures, liquid or gaseous atmospheres (including hydrogen), and flange finishes

„ Layers provide many sealing surfaces and a labyrinth path in the direction of leakage

„ Forgiving to seal stress variations (e.g., overstress) and less prone to relax over time

These gaskets are now specified in accordance with ASME B16.20. Up until the early 1990’s, API 601 governed gasket design and characteristics. Most API 601 requirements have been incorporated into ASME B16.20, so little was lost by the change. One notable difference is that API 601 specified default materials. If nothing else was called for, they were required. B16.20 does not have default materials so the designer must be careful to call out the filler, winding, and ring material for every gasket.

Type 304 stainless steel was the default winding material specified by API 601. It is still the most common material and is recommended for general use. Type 316L, or stabilized (Type 321 or 347) stainless steel may be required where sensitization, and intergranular stress corrosion cracking, is a concern.

Graphite windings may be unsuitable for oxidizing atmosphere services over 800°F because the graphite “dissolves”. For spiral wound gaskets, consider using another material, e.g., ceramic or mica, for the first few windings, where oxygen exposure is possible. The remaining windings may then be graphite.

Specialty filler materials are often tailored for a specific type of application. They may not work well, and may even be outright dangerous, in another service. They are often a blend of a number of components, and sensitive to variations in the proportions or quality of those materials.

EDS-2004/FL-69

Spiral Wound Gaskets

(continued)

„ Asbestos used to be the “universal” filler material

„ Filler materials are now commonly a non asbestos material

– Graphite and sometimes Teflon (for low

temperature applications) are the most common

– Specialty materials are available for particular conditions

„ Filler material must be specified for each gasket (there is no default)

„ Windings are commonly 304 Stainless Steel

– Must be compatible with the internal atmosphere

Stability refers to handling for installation.

The outer ring aligns the gasket by fitting against the inside edge of the flange bolts.

As noted in the jacketed gasket discussion, handling of large spiral wound gaskets can be difficult - they can “spring” apart and wobble around. Jacketed gaskets are sometimes specified for large diameter, low pressure services.

EDS-2004/FL-70

Spiral Wound Gaskets

(continued)

„ Gaskets have an outer ring for stability

„ By using the flange bolts as a guide, the outer ring is used to center the gasket

„ The outer ring also helps resist blowout and acts as a limit stop, preventing crushing of the outer windings from overbolting

„ The outer ring is normally carbon steel, protected against corrosion

As describer on slide 63, large diameter spiral wound gaskets are difficult to handle, especially in a vertical orientation. An inner ring helps keep them from unraveling.

Use of a jacketed gasket is also often considered.

The inner ring also assists in maintaining the integrity of the ID of the gasket as the flanges rotate about the outer edge of the gasket and tend to push the gasket inward.

It also provides additional gasket strength to resist blowout. For some filler materials (e.g., Teflon), an inner ring may be required by the governing standard (ASME B16.5).

Inner rings are also advisable when the mating flanges are different metallurgy's with differing coefficients of thermal expansion. As the system heats and cools, the differing movements may put the gasket into shear.

ASME B16.5 requires inner rings for some gaskets in high pressure services (≥ 24 inch Class 900, ≥ 12 inch Class 1500, and ≥ 4 inch Class 2500). UOP’s criteria is to provide an inner ring for all Class 900 and higher flanges.

EDS-2004/FL-71

Spiral Wound Gaskets

(continued)

„ An inner ring is provided for additional (handling) stability for large gaskets

„ An inner ring is also used to protect the flange surface from corrosion due to the internal atmosphere

„ Use the same material as for the windings

„ An inner ring is frequently used for Class 900 and higher flanges to resist inner deflection and

possible gasket buckling due to the high bolt loads present

– The bolt load tends to squeeze the outer portion of the gasket and push the gasket inward

EDS-2004/FL-72

Spiral Wound Gaskets

(continued)

„ Graphite (and Teflon) filler materials act as incompressible fluids as the flanges press upon the gasket

„ As the gasket is squeezed, the filler material presses outward into the windings and ring

„ This can cause bucking of the windings and a failure of any gasket

„ Asbestos is a system of compressible fibers and does not create radial forces as it is compressed

„ All winding and ring materials must be specified (is no default)

When the bolts for high pressure flanges are tightened, the large forces present may slightly deform the flanges and create a inward component of the force acting on the gasket. As noted on the slide, an incompressible filler material, such as graphite or Teflon, can lead to a similar problem. Asbestos, on the other hand, is a fibrous, compressible, material. Compression of the gasket does not create radial forces on the windings or ring.

The gasket inner ring needs to be checked to insure that this force will not damage the gasket (i.e., buckle the inner ring). Outer rings, and even the windings, may also be subject to buckling. This is the reason that, in high pressure service, the outer ring often has a wavy (non-planar) appearance.

The inner ring and winding material must be compatible with the internal atmosphere of the vessel or piping. As noted previously for the filler material, ASME B16.20 does not specify default materials for the ring(s) and windings. The designer must be careful to call them out. Often the outer ring is made of a low alloy material (carbon steel if the temperature is below 1000°F) because it is not exposed to the internal atmosphere.

Gaskets have standardized dimensions and tolerances specified in ASME B16.20 in accordance with Classes compatible with ASME B16.5 (and B16.47).

Seating of the gasket yields the gasket materials. Parts may even be crushed. When opened, the gasket does not return to its original dimensions or regain its original elasticity. Therefore, it cannot be reliably reseated and must be replaced.

EDS-2004/FL-73

Spiral Wound Gaskets

(continued)

„ Durable

„ Works with a wide range of fairly rough flange finishes

„ Easily replaced because it rests upon a flat surface and is less affected by flange distortions

„ Gaskets must be replaced each time the flange is opened

The finish is the smoothness of the metallic surface of the flange face that mates against the gasket. This is the sealing surface. The facing is both the sealing surface finish and the geometry (e.g., ring joint grooves, tongue and groove, raised face).

If the metallic surface is too rough, the peaks and valleys of that surface will be too large for the gasket to deform or flow and fill for seating.

If the metallic surface is too smooth, the gasket deformations will be too large and they will not match the surface deformations of the flange face. The gasket cannot

“bite” into the flange surface and may slide or be deformed inward by the bolt load.

A surface that is too smooth can be as bad, or worse, i.e., difficult to seal, than one that is too rough. Spiral wound gaskets have been shown to be susceptible to this problem.

B16.5 requires 55 grooves per inch so that a leak would have to pass at least 45-55 grooves for a 1-inch wide gasket. The finish is produced by cutting small concentric or spiral grooves, producing a series of peaks and valleys opposing a radial leak. A lapping or back-and-forth method of finish production is not acceptable because it produces some radial grooves, enhancing leakage.

EDS-2004/FL-74