Spiral Wound Gasket

What a Spiral Wound Gasket Is and What Is Being Decided

The construction explains the behavior. A thin metal strip, typically 0.15 to 0.25 mm thick, is rolled into a V or W shaped cross section and wound in a continuous spiral with a softer filler strip laid into the V of each turn. The result is a resilient ring: under bolt load the angled plies flatten slightly and store spring energy, the filler flows into the serrations of the flange facing and seals, and the metal spiral holds its shape against internal pressure. The sealing element is then finished as a product in one of three commercial forms: bare winding for confined grooves, winding plus a solid outer centering ring, or winding with both outer ring and an inner ring on the bore side. Each form has a flange geometry it belongs to, and choosing the wrong form is the most common specification error on the drawing.

What is actually being decided in this purchase is a system purchase in disguise. A gasket line sets the joint's seating stress demand, its blowout resistance, its chemical and thermal envelope, and its reusability, which is nil; the flange sets the facing finish and rigidity; the bolting sets the actual stress delivered. The buyer commits to style, winding alloy, filler, and thickness in one line, and a weak line shows up as a leak on the first thermal cycle, discovered at 2 a.m. a year after startup. Those decisions are checkable at RFQ stage from the maker tables, which is what the rest of this page does.

Against its neighbors the spiral wound gasket occupies deliberate ground, and the full comparison belongs in the selection chapter. Sheet gaskets serve the economy end at low class and moderate temperature. Kammprofile gaskets, serrated solid metal with a facing layer, offer tighter control of seating stress at the high end. Metal ring gaskets belong exclusively to ring type joint flanges at extreme pressure. Jacketed styles serve heat exchanger duty. The general gasket page remains the authority on those non SWG styles; this page is the deep reference for the wound construction and its purchase.

Four families of standards organize the datasheet. ASME B16.20 governs dimensions, tolerances, and marking of the gasket for ASME B16.5 flanges, and the two standards must be quoted together because the gasket is meaningless without its flange class. DIN 2699 fixes the corresponding German form, and DIN 3535 with EN 13591 define leakage and test practice referenced in European tightness arguments. Fire qualification for the assembled valve or joint follows API 607 or ISO 10497, and it is a qualification of assemblies, not of the gasket alone, a distinction honest datasheets respect. A quotation that names none of these standards is a price per piece, not a sealing specification.

The RFQ should also name the station, because the gasket rides on hardware someone else bought. Every flanged joint is a system of three parts, gasket, flange, and bolting, all specified against the piping class of the line, and every pipe fitting and valve in that class carries the same facing and drilling so the spool assembles without adaptation. Facing finish matters more than buyers expect: the winding needs the standard serrated raised face, 125 to 250 AARH, to bite into. Neighboring pressure boundary parts appear on the same drawings and are often specified by the same engineer: a rupture disk holder seats its disk on machined surfaces, an expansion joint carries its own bolting pattern, and a diaphragm seal on a pressure instrument or any instrument tapping brings its own small flanges. Naming the whole joint in the RFQ, style, facing, bolt material and lubrication, and tooling, transfers the compatibility check to the supplier, which is where it belongs.

How the Winding Seals and Carries Load

The winding is a spring, and its behavior under load is the whole engineering story. As bolt load rises, the angled metal plies flatten elastically and then partly plastically, the filler compresses into the flange serrations, and the gasket thickness shrinks. Manufacturers publish a target seating stress on the winding, roughly 97 to 207 MPa, which is 14 to 30 ksi in the older unit system, with crush damage beginning around 35 ksi, about 240 MPa, where plies flatten permanently and the spring is gone. Seating below the target leaves the filler unconformed and the joint leaks on test; seating above the crush point destroys the resilience the design exists to provide. The window between those numbers is wide but real, and the bolt procedure exists to land inside it.

The compression numbers make the spring visible. Standard windings come in 3.2 mm and 4.5 mm thicknesses, and vendor assembly tables call up an assembled compression of roughly 1.2 to 1.5 mm on the standard 4.5 mm element, expressed in practice as a torque sequence rather than a micrometer reading. In operation the joint breathes: internal pressure adds an end load that unloads the gasket, temperature expands the bolts and flange relative to each other, and cycling walks the assembly stress downward. The winding's stored springback absorbs those excursions, which is why a properly seated SWG survives thermal cycling that destroys a flat sheet, and why a crushed winding, with no spring left, leaks on the first cycle after retorque is forgotten.

The outer centering ring carry two duties that buyers under appreciate. First, centering: the ring ID locates on the flange bolt circle so the sealing element sits concentric with the bore, because an off center winding overhangs the bore, intrudes into the flow, and unwinds under turbulence. Second, blowout resistance: a confined or ring restrained gasket cannot eject, and the outer ring provides that confinement on a raised face flange where nothing else does. The ring is normally carbon steel for moderate service or the 304 and 316L stainless steel grades where the atmosphere or insulation sweating attacks carbon steel, and ring material is a line on the order, not a detail left to the maker.

Creep and relaxation differ by filler, and the difference drives retorque practice. Flexible graphite is dimensionally stable and relaxes only a few percent at temperature, so graphite filled joints usually need at most one retorque after the first thermal cycle where site practice calls for it. PTFE cold flows: under sustained bolt load it keeps creeping, above about 150 deg C visibly, so PTFE filled joints are specified with an inner ring to support the winding bore and are scheduled for a retorque after first heat up as standard practice. Mica and Thermiculite class fillers sit between, chosen for high temperature cycling where graphite would oxidize. ASME PCC-1 assembly practice puts numbers and sequences on all of this, and joints that matter deserve the PCC-1 discipline: target stress per the gasket maker table, passes in a cross pattern, calibrated tools.

Seating stress is delivered by bolts, and the arithmetic belongs in words at RFQ stage. The required bolt load is the gasket stress target times the gasket sealing area, plus the hydrostatic end load at design pressure, distributed over the bolts; torque is only a proxy for that load, frustrated by friction scatter of 25 percent or more unless lubrication and calibration are controlled. This is why the specification states bolt material, coating, lubricant, and tooling along with the gasket: a torque wrench number without those inputs is not reproducible. On large or critical joints, hydraulic tensioning removes most of the friction scatter, and the tensioner load is stated directly as stress, which is the quantity the gasket maker actually published.

Types, Construction, and Duty Boundaries

Winding alloy is the structural choice, and the commercial set is short. Type 304 stainless steel is the economy default for general process duty; 316L stainless steel adds molybdenum and chloride resistance and is the most specified winding in chemical service. Inconel 625 carries the high temperature and sour and acid duties where stainless loses strength or corrodes, at a multiple of the price. Monel 400 serves seawater cooling and hydrofluoric acid service, with a galvanic caution on carbon steel flanges. Titanium and other exotics exist for specific chemistry, quoted to order. The winding alloy also sets the upper temperature of the gasket as a structure, independent of the filler: a graphite filler outlasts a 304 winding that has gone weak, so the envelope is the lower of the two limits, and honest datasheets state both.

Filler is the chemical and thermal choice. Flexible graphite, inhibited for oxidation resistance, runs to about 450 deg C in oxidizing service and to the 540 deg C class in steam where inhibitors and low oxygen give it protection, and it seals across nearly the whole pressure range. PTFE is the clean and chemically inert option, flat to about 260 deg C, but it creeps under bolt load and always requires an inner ring by ASME B16.20 practice. Mica and Thermiculite class fillers carry the cyclic high temperature duties above graphite limits, to the several hundred degree range where oxidation has killed graphite and chemistry has killed PTFE. Ceramic fillers survive temperature but have been largely phased out of general specification on health handling grounds, a phase out worth stating plainly because legacy drawings still call them out.

Style codes describe what rings are fitted, and they map onto flange geometry. A basic winding, bare sealing element, belongs in confined spaces, tongue and groove or ring grooves, where the surrounding metal controls crush and centering. Style CG adds the solid outer centering ring and is the standard call for ASME B16.5 raised face flanges at CL150 through CL2500: the ring centers, takes the bolt hammering, and stops blowout. Style CGI adds an inner ring on the bore side, specified at CL900 and above, on cyclic and hot service, wherever graphite oxidation or media turbulence would attack the winding ID, and mandatorily with every PTFE filler because PTFE extrudes without support. An inner ring without an outer ring, sometimes marked RIR, appears on male and female flange pairs where the female step centers the assembly. The table below compresses the style logic.

Specialty forms extend the construction beyond round line flanges. Heat exchanger joints add a rib or partition bar across the winding to seal the pass partition lane of a tubesheet, built as a welded assembly in the same winding and filler materials. Boiler manholes and handholes take oval and rectangular shapes wound to pattern. Valve bodies close on the same parts: a gate valve bonnet, a globe valve cover, a ball valve body joint, and at high class the trunnion ball valve with its bolted body closure all take SWG or Kammprofile elements by maker choice, and a check valve cover or a bellows seal assembly carries smaller ones. A butterfly valve seals through its own resilient liner instead of a gasket at the disc, yet its flange connections still take wound gaskets off the line schedule, which is how the same part number appears on both sides of a valve station. On the instrument side, an instrument manifold or a needle valve joint at NPS 1/2 uses miniature windings where the line specification demands them, honestly a minor volume of the market but the same physics.

Duty boundaries deserve plain language. A SWG belongs on a raised face or confined flange with a serrated finish; it is not for a flat face flange without spacer ring review, because flat faces neither locate the ring nor guarantee the stiffness the winding needs. It is never for a ring type joint flange, whose machined groove takes a metal ring gasket of octagonal or oval section, an entirely different product. Glass lined steel and plastic flanges, FRP and PVDF, seal with Kammprofile or soft sheet because their low allowable bolt loads crush a winding before it seats. The SWG is a static seal only: it absorbs no relative motion, and any joint that must flex belongs to an expansion joint or a bellows design, not to the gasket.

Key Specification Parameters

The table below decodes the datasheet lines that decide whether a quoted gasket matches the joint. Each row is a purchase defining parameter; the paragraphs after the table explain what to demand and what a weak answer looks like.

The material matrix is where chemistry meets cost, and the pairing of winding and filler deserves the hardest questions, because both limits apply and the joint lives at the lower of the two. The matrix below covers the commercial set a specifier actually chooses among; exotic pairings are quoted to order and validated the same way.

Marking practice closes the purchase with a receiving check. Per ASME B16.20 practice the outer centering ring carries a color code identifying the winding alloy, yellow for 304, green for 316L, purple for the Inconel class, with assigned colors for Monel and other strips, and the ring is stamped with maker, size, and class. The color is a screening tool, not a certificate: mismarked and counterfeit strip is a documented industry problem, and on any service where alloy substitution matters, hydrogen, HF, sour, high temperature, the order demands EN 10204 3.1 material certificates on both winding and ring strip. A datasheet that omits the filler temperature class or the ring material is incomplete at any price.

Selection Decision Factors and RFQ Language

The decision sequence is short enough to run as a checklist in a specification meeting, and each step eliminates a class of quotations before price comparisons begin.

1. Read the flange before the gasket: facing type, raised face or ring joint, class, and the achievable serrated finish of 125 to 250 AARH, because the flange decides which styles are even admissible.
2. Set the temperature envelope including excursions and cyclic events, then choose filler: inhibited graphite, PTFE with its inner ring mandate, or a mica and Thermiculite class for hot cycling.
3. Set the chemistry, inside and outside the line, including insulation sweating and washdown, then choose winding and ring alloys from the chapter 4 matrix.
4. Apply the style logic: CG as the default on raised face, CGI at CL900 and above, on cyclic or oxidizing service, and always with PTFE.
5. Compute the bolt load at the target stress of 97 to 207 MPa plus the pressure end load, and confirm the flange and bolting can deliver it; if they cannot, the gasket style is wrong for the joint, not the bolt torque.
6. Write the assembly procedure into the purchase: cross pattern, three passes to 30, 70, 100 percent, calibrated tooling, and the retorque decision after first thermal cycle.
7. Fix the evidence: B16.20 dimensional compliance, marking, EN 10204 3.1 certificates where alloy matters, and fire qualification at assembly level where the spec requires it.
8. Plan gaskets as single use consumables with spares stocked per flange schedule, tagged and stored flat, before the first shutdown makes the point for you.

Against neighboring sealing products, the comparison a specifier actually needs runs as follows, and it belongs here in the selection chapter rather than in the page title. Against sheet gasket materials, the SWG wins above CL300 and above about 300 deg C, and it wins on blowout resistance outright because the winding and rings are metal; sheet wins the economy end at low class and clean utility duty. Against Kammprofile, the solid serrated core with graphite facing layers, the Kammprofile controls seating stress more precisely and holds tightness classes on stiff exchanger and high class flanges, while the SWG tolerates lower bolt loads and less perfect flanges; both are legitimate at CL600, and the decision usually follows flange rigidity and site standard; on a trunnion ball valve body closure at high class the same two styles split the duty the same way. Against metal ring gaskets for ring type joint flanges there is no contest and no choice, because the geometries do not interchange. Against jacketed gaskets on exchanger tubesheets, the ribbed SWG and the Kammprofile split the market by temperature and tightness class. The general gasket page remains the authority on sheet, RTJ, jacketed, and the other non SWG styles; this page closes the wound construction decision, which is the majority of metallic line flange joints in a modern plant.

The application map below turns that comparison into a table the project file can carry.

A gasket line in a flange schedule should be written once and reused. A complete line reads: quantity, size NPS, and class; style CG or CGI; winding alloy; filler; thickness, 3.2 mm or 4.5 mm; dimensional standard, ASME B16.20 or DIN 2699 form; ring materials; and the evidence set, marking per B16.20 and EN 10204 3.1 certificates on strip where the service is audited. State the flange facing finish available on the spool and the bolting, size, material, coating, lubricant, and tooling, because a gasket cannot outperform its bolt load, and name any adjacent hardware that changes the joint, a butterfly valve whose liner replaces one sealing plane, a bellows seal bonnet that swaps packing leakage for a gasketed closure, an expansion joint that alters nozzle loads, or a double block and bleed valve whose body joints share the spec. A quotation that quotes pieces without the filler temperature class and the seating stress table behind them is a price per piece, not a sealing proposal.

Installation, Standards, Failure Modes, and Maintenance

Installation decides whether the winding ever reaches the stress it was bought for. Verify the flange facing before assembly, finish between 125 and 250 AARH, flat, and free of the previous gasket's ghost, on the flange and on every pipe fitting end in the joint. Inspect the gasket: the outer ring color against the schedule, the winding edges undamaged, no dents that flatten plies before load is applied. Insert and center on the bolt circle, ring ID against the bolts, never against the bore. Run all nuts finger tight, then tighten in a cross pattern in three passes to 30, 70, and 100 percent of the target torque with a calibrated wrench, following ASME PCC-1 style practice on critical joints, or tension to the calculated stress directly. Record the tool, the passes, and the torque or load per bolt; on the joints that matter, that record is the baseline every future leak investigation wants.

Thermal practice follows the first heat up. Graphite filled joints relax a little and are retorqued after the first cycle where site practice or the maker table calls for it, hot retorque only where the spec permits and with the joint depressurized; PTFE filled joints are retorqued as standard because the filler creeps. And the rule that ends all arguments: a spiral wound gasket is single use. Once seated, the plies and filler take a set matched to that flange pair's exact flatness; reassembled elsewhere or even on the same pair after disturbance, they never recover the original stress distribution, and leaks arrive on the first excursion. Tag removed gaskets out of stores, stock spares per flange schedule, and treat the storeroom discipline, stored flat, dry, undamaged, no hooks through windings, as part of the sealing system.

The standards frame holds the whole file together. ASME B16.20 governs dimensions, tolerances, and marking; ASME B16.5 governs the flange the gasket lives on; DIN 2699 carries the European form and DIN 3535 with EN 13591 the leakage and test practice behind tightness claims; API 607 and ISO 10497 qualify fire performance at the assembly level; ASME PCC-1 disciplines the assembly itself. A specification that cites the standards by name transfers the burden of proof to the supplier, and a joint history that records stress, tooling, and retorque against those citations is the difference between a maintenance system and a pile of spare parts.

Failure modes in service follow a short and predictable list. Inward buckling of the winding from media turbulence, unwinding into the pipe bore, is the classic missing inner ring failure on pumping and erosive duty. Graphite oxidation above the envelope quietly removes the filler, leaving a leak path and a loose winding; the cure is inhibited graphite, an inner ring, or a Thermiculite class filler. Over compression crushes plies flat and deletes the spring; under bolting leaves the joint loose and it weeps on the first thermal cycle. PTFE cold flow extrudes the filler until the inner ring is the only thing left in the gap. Galvanic attack pairs the wrong winding with the flange, Monel on carbon steel in salt service being the textbook case, and inner ring corrosion does the same from the bore side. Counterfeit and mismarked strip, wrong alloy delivered as right, is a documented industry failure, and EN 10204 3.1 certification with receiving color and stamp checks is the defense. Most field leaks trace back to assembly stress, not to a defective gasket, which is why the procedure and its record carry as much weight as the part.

Maintenance of a gasket population is record keeping more than labor. Walk the flanges on a schedule and log weeping, any ring corrosion, insulation sweating damage, and the condition of bolts and nuts, since rusted bolting cannot deliver the stress the drawing demands; include the holder joints of a rupture disk and the body joints of any double block and bleed valve in the same walk, since they share the facing and bolt discipline. Audit stores annually against the flange schedule: correct styles and fillers in stock, exotics identified and segregated, nothing stored where windings get dented. At every joint break, inspect the flange face before the new gasket goes in, because the old gasket's ghost on the serrations is the next leak's starting point, and refacing or replacement is decided then, not after the second failure. Every event writes a line into the joint history, and that history, not the original brochure, is what specifies the next plant's flange schedule, which is how a purchasing file gets smarter instead of just older.