A gasket is specified from a chemical compatibility chart, the material is correct for the fluid, and the joint leaks anyway. On the next turnaround the specification moves to a more expensive material that also leaks. The chart was never wrong. It answered a question about chemistry while the joint was failing on mechanics — too little seating stress, a flange face cut for a different gasket, or a channel cover deflecting away from a pass-partition rib. Gasket selection is a joint design problem that contains a material decision.
IN BRIEF
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A gasket seals because it is compressed, not because it is compatible. Chemical resistance screens candidates; the gasket stress the joint can deliver decides whether it holds.
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Construction and material are separate decisions. Construction determines how a gasket generates and holds a seal; material determines what it can be exposed to.
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The flange is part of the seal. Facing type, confinement, surface finish and flange stiffness decide how much bolt load reaches the gasket and whether it stays there.
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Published pressure and temperature maxima are not an operating envelope. The upset, steam-out and cycling cases often select the gasket, not the design point.
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In a multi-pass exchanger the pass-partition rib is the harder seal. A partition leak is internal, so it degrades thermal duty without showing itself.
On this page: what the joint must do · process envelope · constructions · flange geometry · bolt load · exchanger joints · pass-partition leakage · mistakes · selection sequence · FAQs
What a gasketed joint has to do
A gasket does two mechanically different jobs, governed by different numbers.
The first is seating. On assembly, bolt load must compress the gasket enough to conform it to the flange faces and close the leak paths across the surface texture. Minimum seating stress is a property of the construction — a few thousand psi for restructured PTFE, an order of magnitude more for a solid metal ring.
The second is holding operating stress. Once the vessel is pressurized, internal pressure pushes the flanges apart. What remains after that end load, after the bolts relax and the gasket creeps and the flange rotates, is what holds the seal. A gasket that seated perfectly at ambient can leak at temperature with no chemical attack involved.
That sets the working sequence: process conditions → joint and flange design → gasket construction → gasket material → available gasket stress → a seal that survives operation. Running it backward, by choosing a gasket and fitting it to whatever flange exists, selects a gasket without knowing what stress it will see.
Process conditions set the envelope, and the envelope is not two independent numbers
A data sheet’s separate pressure and temperature maxima are not an operating envelope. Garlock states it directly in its GYLON catalog: operating temperature and pressure values may not be attained simultaneously. Some manufacturers publish a combined P×T limit instead, and that figure falls as sheet thickness rises — thicker sheet is more forgiving of a rough or warped face and less capable at pressure, because there is more material to extrude and relax.
The conditions that decide the gasket are often outside the design case anyway: startup and shutdown excursions, steam-out temperature, vacuum during drain-down, cycling frequency, and whether the environment is oxidizing.
Construction and material are two decisions, not one
Construction determines how a gasket generates and holds a seal; material determines what it can be exposed to.
| Construction | Typical strengths | Important limitations | Joint considerations |
|---|---|---|---|
| Compressed non-asbestos sheet | Low seating stress; conforms to imperfect faces | Binder-limited temperature; creep relaxation | Low to moderate pressure raised- and flat-face joints |
| PTFE — filled, restructured, expanded | Resistance across nearly the full pH range; low seating stress | Continuous service near 500°F (260°C); cold flow; filler governs resistance | Must not be over-compressed; forms a tight crevice |
| Flexible graphite sheet | Very high temperature in steam and inert service; low relaxation | Oxidizes in air, at a rate that matters below the published limit | Normally reinforced for handling and blowout resistance |
| Spiral wound | Wide pressure range; recovers under cycling; dimensions in ASME B16.20 | Higher seating stress than sheet; buckles inward without an inner ring | Outer ring centers it; inner rings required for several filler and class combinations |
| Grooved metal with facing layers | Broad usable stress range; tolerates a rougher face; can be refaced | Facing material sets the temperature limit, not the core | Used where double-jacketed gaskets were traditional, ribbed joints included |
| Double-jacketed | Long-established exchanger construction; integral partition ribs | Needs a smoother flange face; limited recovery under cycling | Seals at the metal inner lap, so face finish and flatness matter more |
Where published material limits mislead
Two traps recur. PTFE’s chemical breadth belongs to the PTFE itself — Gore publishes resistance across all media pH 0–14, excepting molten alkali metals and elemental fluorine, but in a filled grade the filler governs. Its creep relaxation varies widely between styles of one nominal material, so the style number carries more information than the material name. Flexible graphite — GRAFOIL is a registered trademark of NeoGraf Solutions, not a generic term — has an oxidation limit that behaves as a rate rather than a wall, falling as intended service life lengthens; the Fluid Sealing Association puts the practical limit near 700–850°F for facing material but 950°F in a spiral wound gasket.
Both materials also act on the flange they seat against. Every gasket forms a crevice, and the material changes the chemistry inside it rather than removing it. Work by Francis and Byrne on stainless steels in chlorinated seawater found PTFE creates a more severe crevice than aramid fibre because it deforms to fit the faces, and describes graphite stimulating local acidification. Graphite purity and leachable chloride are therefore specifiable in chloride-bearing service, and crevice chemistry belongs with alloy selection.
Spiral wound rings, and two names that are not specifications
On spiral wound gaskets the rings are frequently misread. The outer ring centers the gasket; treating it as a compression stop is a common error, and compressing the gasket onto it leaves a nearly flat metal gasket with little recovery for cycling. The inner ring stops the windings buckling into the bore, and ASME B16.20 requires one for all PTFE-filled gaskets, for graphite-filled gaskets unless the purchaser specifies otherwise, and for the larger sizes in Classes 900, 1500 and 2500.
Two names deserve settling internally. Garlock is a manufacturer, not a generic term for compressed sheet. And “double ring” is not a defined term in the sealing industry — absent from the ESA/FSA Glossary of Sealing Terms, it is used for a spiral wound gasket with inner and outer rings, for a double-jacketed gasket, and for an exchanger joint with a peripheral gasket plus a partition rib. Neither names a construction.
The flange is part of the sealing system
Flange geometry decides how much bolt load reaches the gasket, where it acts, and whether it stays there.
| Joint arrangement | Typical gasket geometry | Important selection consideration |
|---|---|---|
| Raised face | Ring gasket inside the bolt circle | Unconfined; bolts act outboard, so bolt load rotates the flange and can unload the gasket ID |
| Flat face | Full-face gasket past the bolt holes | Material outboard of the bolt circle limits rotation; outside the scope of Appendix 2 |
| Male and female | Semi-confined gasket | Female face locates the gasket and partially restrains it against extrusion |
| Tongue and groove | Totally confined gasket | Groove depth controls compression; needs a finer face finish than a raised face |
| Ring-joint groove | Solid metal ring, self-energizing | Tighter sidewall finish limit; reuse discouraged, the ring work-hardens on first seating |
| Dovetail groove | Elastomeric O-ring | A retention feature for a seal in a vertical or overhead face, not general confinement |
Ring gaskets and full-face gaskets are not substitutes
The difference is where the gasket sits relative to the bolt circle. In a raised-face joint the gasket is inside it, the bolts pull on a cantilever, and the flange rotates about the gasket. In a flat-face joint the full-face gasket extends outboard of the bolt holes and acts as a fulcrum limiting that rotation. ASME Section VIII, Division 1 Mandatory Appendix 2 is explicit about which case it analyzes: its rules apply to gaskets entirely within the circle enclosed by the bolt holes.
Swapping one for the other changes the joint that was analyzed. A ring gasket in a joint designed around a full-face gasket removes the outboard support — the mechanism behind cracked castings when a raised-face steel flange meets brittle gray cast iron, and the reason TEMA requires full-face gaskets there. A full-face gasket in a joint sized for a ring does the opposite, spreading the load over far more area and landing below the intended seating stress.
Confined joints, and what confinement actually buys
TEMA, answering an official inquiry, stated that “any gasket with a solid metal outer retaining ring that prevents a direct radial leak path to the environment in the event of gasket extrusion or ‘blowout’ meets the intent of the TEMA ‘confined’ joint.” Confinement can come from the flange, as in tongue-and-groove or male-and-female facings, or from the gasket, as with a solid outer ring. What it buys is radial restraint against extrusion and a barrier to atmosphere if the sealing element fails. It does not eliminate leakage, and it does not reduce the seating stress the gasket needs. Dovetail grooves are narrower still — Parker’s O-ring design handbook describes the dovetail as an elastomeric O-ring retention feature, expensive to machine and not generally recommended.
Surface finish is specified by the facing, not by habit
Face finish is a requirement of the flange standard, not of the gasket, and it varies by facing type — 125 to 250 µin average roughness on standard raised and flat faces, tighter on confined facings. Conformability then decides which construction suits: soft and semi-metallic gaskets key into the serrated band, while metal-jacketed and solid metal gaskets need a smoother face. An existing flange’s finish can rule out a construction before anything else.
Bolt load and gasket stress: why tightening harder is not a strategy
Every property above is realized through bolt load, the least precisely controlled variable in the joint.
Torque is an indirect measurement. It sets preload through friction, captured in a nut factor that shifts with lubrication, thread condition and washer use, so the same torque on two bolts can produce materially different preload. Tightening one bolt also relaxes its neighbors, which is why joints are tightened in multiple passes and why PCC-1’s alternative patterns exclude spiral wound, double-jacketed and ring-joint gaskets from the fast circular pattern.
The flange has a stiffness limit, not only a stress limit. ASME added a mandatory rigidity criterion to Appendix 2 because flange designs were rotating enough to leak without being overstressed by calculation. Past that point, added bolt load buys rotation rather than gasket stress, and on an exchanger the rotation preferentially unloads whatever sits farthest from the bolts. The m and y gasket factors do not close the gap — they size the flange and the bolting — while ASME PCC-1 handles assembly separately, bounding bolt stress by bolt yield, by the stress that would damage the flange, and by a target gasket stress the manufacturer supplies.
The joint then relaxes. Soft sheet gaskets lose a significant fraction of bolt load to creep at ambient temperature where semi-metallic constructions lose very little, so retorque guidance is gasket-specific. Temperature adds a second mechanism: an austenitic stainless flange expands faster than a low-alloy bolt, and that thermal stress added to preload can exceed the bolt’s yield, so the bolt yields hot and the joint leaks on cooldown.
Heat exchanger joints are harder than piping flanges
Shell-and-tube exchangers concentrate every difficulty above into joints that are larger, less stiff, more thermally cycled, and sometimes unreachable.
TEMA addresses gaskets in RCB-6. Gaskets must have a continuous periphery with no radial leak paths, and minimum peripheral widths run 3/8 in. (9.5 mm) for shells through 23 in. and 1/2 in. (12.7 mm) above. Class R requires metal jacketed or solid metal gaskets for internal floating head joints, for all joints at 300 psi and above, and in hydrocarbon service; Classes B and C permit composition gaskets on external joints below that.
Accessibility is a design decision with a maintenance consequence. Fixed-tubesheet rear heads carry no gasketed joint on the shell side, while the S and T floating-head arrangements carry an internal head cover gasket that cannot be reached without pulling the bundle — so its selection has to survive a full run, not a convenient inspection interval. The TEMA class comparison covers how that interacts with the rest of the shell-and-tube design.
Pass-partition gaskets and cross-pass leakage
In a multi-pass channel the perimeter gasket keeps process fluid inside the exchanger and the partition rib keeps the passes separated. Only one of those is straightforward.
The rib’s job is harder for three reasons at once. It has less seating width — TEMA RCB-6.4 sets the partition web minimum at 1/4 in. (6.4 mm) through 23 in. shells and 3/8 in. (9.5 mm) above, one step narrower than the peripheral minimum at the same shell size. It shares one bolt circle with the perimeter gasket, so a single bolt load has to seat both. And it sits where the cover deflects most. CalGavin notes that internal pressure deflects these components, relaxing the gasket and allowing leakage, and that the problem is most frequent on large exchangers at low pressure — where the cover is big and flexible and the available bolt load is modest.
The dimension that matters most is not specified. Asked what the flatness tolerance is for the pass-partition gasket surface area, TEMA replied that “this tolerance is not specifically addressed in the Standards and we advise user to exercise sound engineering judgement.” The relationship between the rib seating surface and the peripheral seating surface — the thing that decides whether one bolt load seats both — lives in fabrication practice and the purchaser’s specification.
Mixing constructions compounds it. A study reported by 3S-Superior Sealing Services found that different gasket styles in the same pass-partition application have different compression rates and different minimum sealing stresses, producing varying gasket stress and varying leakage on one joint. One bolt load cannot put two dissimilar gaskets at their respective optima, which is why one-piece constructions exist: double-jacketed and grooved metal gaskets can be made with integral partition ribs.
A partition leak does not announce itself. It is an internal leak between passes, not a leak to atmosphere. The seal is enclosed inside the exchanger body, so it reduces efficiency and output without being apparent, and CalGavin notes that bypassing is hard to diagnose because it occurs only during operation, suggesting infrared imaging or tracer injection for online detection. The diagnostic signature is worth remembering: bypass produces poor thermal duty and lower-than-expected pressure drop together, where fouling raises pressure drop as duty falls.
Common gasket-selection mistakes
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Selecting from a compatibility chart alone. It screens out wrong materials and says nothing about seating stress.
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Reading pressure and temperature maxima as independent. Look for a P×T limit or a pressure-temperature diagram.
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Treating all graphite, or all PTFE, as one material. Reinforcement, purity, filler and processing change creep, handling and blowout resistance more than the material name suggests.
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Substituting a ring gasket into a full-face joint, or the reverse. Both change the analyzed load path, and one of them cracks gray cast iron.
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Specifying face finish from a drawing template. The requirement depends on the facing type, and metal-jacketed gaskets need a smoother face than the serrated band.
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Tightening harder to stop a leak. Past the flange’s rigidity limit, added bolt load buys rotation rather than gasket stress.
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Ignoring bolt-to-flange expansion mismatch. A joint that seals cold can yield its bolts hot and leak on cooldown.
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Watching only for external leakage on a multi-pass exchanger. Cross-pass leakage degrades thermal duty silently, and seepage at a gasket line is a late-stage indicator.
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Reusing gaskets. ASME PCC-1 says reuse is not recommended, with a carve-out for grooved metal substrates that have been reconditioned and refaced; manufacturers and the ESA/FSA are stricter. PCC-1 is recognized good practice rather than a construction code, so how firmly it binds depends on what the purchase specification invokes.
A selection sequence, and what the fabricator needs
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Characterize the service, not just the design point — composition including trace species, temperature and pressure ranges, vacuum, upset and steam-out conditions, cycling frequency.
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Identify the joint — facing type, confinement, seating dimensions, flange and bolt material, and whether it is external or internal to the equipment.
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Establish the available gasket stress — bolt area and permissible bolt stress, flange rigidity, and on an exchanger channel, how load divides between perimeter and partition rib.
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Choose the construction that fits that stress and the expected movement, then the material within it — chemistry, temperature in the actual atmosphere, and what it will do to the flange face.
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Confirm face finish and flatness against the chosen construction and the condition of existing flanges.
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Set the assembly requirement — target bolt stress, tightening pattern, lubrication and retorque, per ASME PCC-1.
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Check what the specification invokes. Code, TEMA class, owner or EPC specifications and a mandated manufacturer are four separate constraints.
The information that shortens a gasket conversation with a fabricator is short: fluid composition including trace oxidizing species and halides; design and operating temperature and pressure plus upsets; cycling frequency; joint type, facing and gasket dimensions, with existing flange drawings where they survive; any specified material, construction or mandated manufacturer; bolt material and size; code and TEMA class; and on repair work, the leakage history of the joint.
Harris Thermal designs and fabricates ASME Section VIII pressure vessels and shell-and-tube heat exchangers where the gasketed joint is part of the mechanical design rather than a detail resolved afterward. That includes machining serrated gasket surfaces on body flanges up to 16 ft 6 in. in diameter and milling pass-partition grooves in tubesheets, in-house thermal and mechanical design, and repair, retubing and re-rating where flange faces and joint geometry are given rather than chosen.
FAQs about industrial gasket selection
How should an engineer choose a gasket for an industrial pressure application?
Work from the joint outward. Define the process conditions including upsets and cycling, identify the flange facing and confinement, establish the gasket stress the bolting can deliver, then choose a construction that seals at that stress and a material that survives the chemistry. A compatibility chart screens materials; it does not select a gasket.
What is the difference between a ring gasket and a full-face gasket?
A ring gasket sits inside the bolt circle on a raised-face flange; a full-face gasket extends past the bolt holes on a flat-face flange. Material outboard of the bolt circle resists flange rotation, while a ring gasket leaves the flange free to rotate about it. TEMA requires full-face gaskets on cast iron because the unsupported gap under a ring gasket can crack the casting.
What is a confined gasket joint?
A joint in which a solid metal element prevents a direct radial leak path to atmosphere if the gasket extrudes or blows out. TEMA accepts either confinement machined into the flange, such as tongue-and-groove, or a solid metal outer retaining ring on the gasket. Confinement resists extrusion and blowout; it does not reduce the seating stress required.
Why do heat exchanger pass-partition gaskets matter?
Because a partition leak is invisible. The rib seals pass-to-pass inside the channel, so a failure lets fluid bypass the tube bundle and degrade thermal duty with no external leakage. It has less seating width than the perimeter gasket, shares the same bolt circle, and sits where the cover deflects most. TEMA specifies no flatness tolerance for that surface.
Does flange surface finish affect gasket selection?
Yes. ASME flange standards require 125 to 250 µin average roughness on standard raised and flat faces, not more than 125 µin on tongue-and-groove, and not more than 63 µin in a ring-joint groove. Soft and semi-metallic gaskets key into the serrated band; metal-jacketed and solid metal gaskets need a smoother face, so an existing flange can rule out a construction.
Can a gasket be reused?
Generally no. ASME PCC-1 states that reuse is not recommended, and that only a new gasket reliably provides the plastic deformation and elastic recovery an effective seal requires. The one exception is a grooved metal substrate reconditioned and refaced. Manufacturers and the ESA/FSA advise against reuse without exception.
