Gaskets and Flange Face Interview Questions and Answers (2026): Spiral-Wound, RTJ and Face Finish

Q1What is a gasket, and how does it actually seal a flanged joint?

Short answer: A gasket is a compressible element clamped between two flange faces. Bolt load squeezes it until it flows into the surface irregularities of the faces (seating), and enough compressive stress must remain on it in operation to resist internal pressure and stop leakage through the gasket body and across its faces.

A good interview answer names the two leak paths: interfacial leakage between gasket and flange face, and permeation through the gasket material itself. Seating closes the first; a dense, well-chosen material limits the second. In service the gasket must keep sealing while pressure tries to separate the flanges (hydrostatic end force), while temperature relaxes the bolts and gasket, and while piping loads add bending. A gasket is therefore a component that has to be stressed correctly — not just “fitted”.

Q2Which properties decide whether a gasket material is suitable?

Short answer: Compressibility and recovery, creep relaxation resistance, sealability, chemical compatibility, temperature limit, pressure (blow-out) resistance and the bolt load needed to seat it.

• Compressibility and recovery (ASTM F36): how easily it fills face imperfections and how much it springs back when the joint breathes under thermal and pressure cycles.
• Creep relaxation (ASTM F38): loss of gasket stress over time at temperature — the main reason soft gaskets leak after start-up.
• Sealability (ASTM F37 or EN 13555 leakage tests): leak rate at a given stress and pressure.
• Chemical and temperature resistance: of every component — filler, binder, metal winding and rings.
• Blow-out resistance: strength against being pushed out radially, improved by inner rings, inserts or confinement.

Q3How are gaskets classified?

Short answer: By material and construction into three families — non-metallic (soft), semi-metallic (composite) and metallic. By shape, flat gaskets are either full-face (covering the whole flange face, with bolt holes) or ring gaskets that sit inside the bolt circle.

Q4What is the difference between a full-face gasket and an inside-bolt-circle (ring) gasket?

Short answer: A full-face gasket covers the entire flange face and has bolt holes; it is used on flat-face flanges. A ring (inside-bolt-circle, IBC) gasket fits inside the bolts and is centred by them; it is used on raised-face flanges.

Full-face gaskets protect brittle or flexible flanges — cast iron, ductile iron, FRP/GRE and plastic — by spreading the bolt load over the whole face so the flange does not bend and crack. On raised-face steel flanges a full-face gasket would sit on the low outer area and waste bolt load, so an IBC ring gasket is used instead.

Q5Which standards govern gaskets for pipe flanges?

Short answer: ASME B16.20 (metallic and semi-metallic: ring joint, spiral-wound, grooved/kammprofile, jacketed), ASME B16.21 (non-metallic flat gaskets), API 6A (RX and BX rings for wellhead equipment) and ASME PCC-1 (assembly). European projects use the EN 1514 series and EN 13555 test parameters.

Q6How is a gasket written on a pipe class or purchase order?

Short answer: With type, size, class, standard, materials of each component and thickness — enough that two suppliers would deliver the same item.

For a sheet gasket the description changes to material grade, thickness (normally 1.5 mm), and full-face or ring form to ASME B16.21. A vague entry such as “gasket, 6 in, 300#” is a classic cause of wrong material arriving at site.

Q7Why are asbestos gaskets no longer used?

Short answer: Asbestos fibre is a proven carcinogen and is banned or phased out in most countries. Compressed asbestos fibre (CAF) sheet and asbestos-filled spiral-wound gaskets have been replaced by CNAF, flexible graphite, PTFE and mica-based materials.

If an old pipe class still lists “CAF” or “asbestos filler”, raise it as a specification error. During maintenance, treat old gaskets of unknown composition as possibly asbestos-containing and follow the site’s hazardous-material procedure when removing them.

Q8Which factors decide gasket selection for a new service?

Short answer: Fluid (chemistry, toxicity, fugitive-emission limits), design and upset temperature, design pressure and flange class, flange type and facing, available bolt load, thermal or pressure cycling, and fire-safety requirements.

A practical memory aid is STAMPED: Size, Temperature, Application (flange type, facing, bolting), Media, Pressure, Extras (fire-safe, emissions, cycling) and Delivery (availability, cost). Temperature and media usually eliminate most options before pressure is even considered.

Q9What is a CNAF gasket, and where is it used?

Short answer: Compressed non-asbestos fibre — aramid, glass or carbon fibres with mineral fillers bound by an elastomer such as NBR or SBR, calendered into sheet. It is the general-purpose replacement for asbestos sheet in water, air, oils and low-pressure utility service, mostly Class 150 and 300.

Its weakness is the rubber binder: at higher temperature it hardens, so the gasket loses recovery and stress relaxes. Manufacturers therefore publish a P×T chart rather than separate pressure and temperature maxima. For continuous steam or hot oil above roughly 200–250 °C, an interviewer will expect you to move to flexible graphite or a spiral-wound gasket.

Q10What is the P×T value of a sheet gasket?

Short answer: The product of operating pressure and temperature that a sheet material can tolerate. The manufacturer’s chart is split into zones: a safe zone, a zone that needs technical review, and a zone where the material must not be used.

The figure is a selection guide, not a design value. Two services with the same P×T can behave very differently, so always plot both the pressure and the temperature on the chart and check the thickness the chart applies to — thinner gaskets usually earn a larger safe zone.

Q11Why is a 1.5 mm sheet gasket preferred over a 3 mm one?

Short answer: A thinner gasket has less area exposed to internal pressure, creeps and relaxes less, resists blow-out better and permeates less. Use the thinnest gasket that can still take up the flatness and finish variations of the flange faces.

3 mm sheet is kept for rough, uneven or slightly warped faces — older cast-iron or large fabricated flanges — where 1.5 mm cannot fill the gaps. On new, machined raised-face flanges 1.5 mm (1/16 in) is the normal choice.

Q12What types of PTFE gasket are available, and why is virgin PTFE avoided?

Short answer: Virgin (skived) PTFE, filled and structurally modified PTFE, and expanded PTFE (ePTFE). Virgin PTFE cold-flows (creeps) under bolt load, so joints lose stress and leak; filled and expanded grades greatly reduce this.

• Silica- or glass-filled PTFE: strong acids and solvents — not hydrofluoric acid or hot strong caustic, which attack the filler.
• Barium-sulphate-filled PTFE: strong caustics and many acids, and often used in HF service.
• Expanded PTFE (ePTFE): very soft and conformable, ideal for glass-lined, FRP, plastic and damaged flanges that cannot take high bolt load.

PTFE is not fire-safe and should not be used where the pipe class demands a fire-tested joint.

Q13What are the strengths and limits of flexible graphite gaskets?

Short answer: Flexible graphite seals well over a very wide temperature range, resists most chemicals, does not creep, and is fire-safe. Its limit is oxidation in air at about 450 °C and attack by strong oxidisers such as nitric acid, oleum and chromic acid.

Specify industrial grade with at least 98 % carbon and low leachable chloride (and nuclear grade where stainless steel is sensitive to chlorides). Pure graphite sheet is fragile, so it is supplied laminated with a 316 stainless tanged insert or flat foil that gives handling strength and blow-out resistance. In steam or inert gas, graphite can work at much higher temperatures than in air.

Q14How do you choose an elastomer (rubber) gasket?

Short answer: By the fluid and temperature: NBR for oils and fuels, EPDM for water, steam condensate and many chemicals but never hydrocarbons, FKM for hydrocarbons, acids and higher temperature, and neoprene (CR) for refrigerants and general duty.

Rubber gaskets are limited to low pressure, low temperature and full-face use on cast iron, plastic and lined pipe. A frequent field mistake is installing EPDM on a line that later carries oil — it swells and fails within days.

Q15Can a sheet gasket be used on a Class 600 flange?

Short answer: Usually not. Most pipe classes limit soft sheet gaskets to Class 150 and 300. From Class 600 upwards the default is a spiral-wound, kammprofile or ring-type joint, because soft sheet can be crushed by the higher bolt loads and can blow out under the higher pressure.

Some high-performance graphite laminates with metal inserts are qualified for higher classes, but only when the manufacturer’s data and the project specification both allow it.

Q16Which gasket is recommended for high-temperature, high-pressure service?

Short answer: The spiral-wound gasket (SWG) to ASME B16.20 is the industry default for high-temperature and high-pressure raised-face flanges up to Class 2500. For the most severe service — very high pressure, wellheads, large cycling — ring-type joints are used.

The SWG wins because the V-shaped metal winding acts as a spring. It keeps recovering as the joint breathes with temperature and pressure cycles, while the soft filler does the actual sealing. Its built-in compression stop (the outer ring) also protects it from being crushed.

Q17How is a spiral-wound gasket constructed?

Short answer: A pre-formed V-shaped metal strip is wound together with a soft filler tape into a spiral sealing element. The first and last turns are spot-welded. A solid outer (centering) ring is fitted outside the element and, where required, a solid inner ring inside it.

Standard ASME B16.20 gaskets have a 4.5 mm (0.175 in) sealing element and 3.2 mm (0.125 in) rings. When tightened correctly, the element compresses to about 3.3 mm (0.130 in ± 0.005 in), close to the ring thickness, so the outer ring tells the fitter when compression is complete.

Q18What criteria decide the winding material of a spiral-wound gasket?

Short answer: The corrosiveness and concentration of the process fluid, the maximum (and upset) operating temperature, compatibility with the flange material, and cost. The winding must be at least as corrosion-resistant as the pipe and flanges.

Q19How is the filler material selected?

Short answer: By temperature and chemistry. Flexible graphite is the general-purpose filler; PTFE is used for aggressive chemicals at lower temperature; mica- or ceramic-based fillers are used above the oxidation limit of graphite.

Q20What do the outer and inner rings of a spiral-wound gasket do?

Short answer: The outer ring centres the gasket on the bolts, acts as a compression stop and resists blow-out. The inner ring stops the winding from buckling inwards into the bore, adds a second compression stop, fills the gap between flange bore and gasket to reduce turbulence and erosion, and shields the filler from the process.

Inward buckling is a real failure mode: loose turns can unwind into the pipe and be carried into control valves, pumps or compressors. That is why the inner ring is no longer optional in many services.

Q21When is an inner ring mandatory on a spiral-wound gasket?

Short answer: Under current ASME B16.20, an inner ring is required for all flexible-graphite- and PTFE-filled gaskets, and for every filler at the larger sizes of the higher classes.

Q22On which flanges should spiral-wound gaskets be used with care?

Short answer: On flat-face, cast-iron, FRP, plastic and lined flanges, and on light Class 150 slip-on, socket-weld or threaded flanges. These cannot develop the high seating stress an SWG needs without bending or cracking.

A large Class 150 flange may have too few, too small bolts to compress the winding to its stop. In such cases a kammprofile or a flexible graphite sheet gasket with insert is often the better choice — and an interviewer will like hearing you say “check the available bolt load”.

Q23What is a kammprofile gasket, and where is it preferred?

Short answer: A solid metal core with concentric serrations (grooves) on both faces, covered with thin layers of graphite or PTFE. It seals at low seating stress, tolerates thermal cycling and is used widely on heat exchangers and in place of SWGs where bolt load is limited.

Kammprofile (also called camprofile or grooved metal gasket with covering layers) is covered by ASME B16.20 and EN 1514-6. The core may have an integral or loose locating ring. The solid core cannot buckle, and it can often be refaced and reused after inspection — unlike an SWG.

Q24What are metal-jacketed gaskets used for?

Short answer: A soft filler (graphite or ceramic) enclosed in a thin metal jacket. Double-jacketed gaskets are the classic choice for heat-exchanger girth, channel and bonnet flanges because they can be made in large diameters and with integral pass-partition bars.

They need a high seating stress and a smoother face finish than SWGs (typically no rougher than about 2.5 µm Ra). Modern practice often adds graphite facings or replaces them with kammprofile gaskets for better tightness.

Q25What is a corrugated metal gasket?

Short answer: A thin metal sheet pressed with concentric corrugations, usually faced with graphite. The corrugations create several concentric sealing lines at moderate bolt load, making it useful for large-diameter, low-to-medium-pressure flanges and heat-exchanger covers.

Q26How should spiral-wound gaskets be stored and handled?

Short answer: Flat, in their original packaging, in a dry indoor store, labelled by size, class and material. Never hang them on nails or wire, bend them, or carry them by the winding.

Hanging distorts the outer ring and loosens the winding. Before installation, check the markings (size, class, winding, filler, standard), the colour codes, that the winding is tight in the ring, and that the faces are free from dents or radial marks.

Q27What markings should a spiral-wound gasket carry?

Short answer: Manufacturer’s name or trademark, flange size and class, winding and filler material, ring materials (if not carbon steel), the gasket standard (ASME B16.20) and, for B16.47 sizes, the flange series (A or B).

These are stamped or printed on the outer ring and backed up by the colour code. If any marking is missing or conflicts with the colours, quarantine the gasket.

Q28What is a ring-type joint (RTJ), and how does it seal?

Short answer: A solid metal ring that sits in matching trapezoidal grooves machined into both flange faces. Bolt load wedges the ring into the grooves, and the ring plastically deforms to create a metal-to-metal line seal on the groove walls.

RTJ flanges are used for high pressure and temperature — commonly Class 900 and above, and for some Class 600 hydrocarbon or hydrogen services — and for wellhead equipment. Because the seal relies on plastic deformation of the ring, both the ring and the groove surfaces must be in perfect condition.

Q29What are R, RX and BX ring gaskets?

Short answer: R rings are oval or octagonal and fit ASME B16.5, B16.47 Series A and API 6B grooves. RX rings are pressure-energised, taller versions that fit the same 6B grooves. BX rings are pressure-energised rings for API 6BX flanges, which are made up face-to-face.

Q30Why must an RTJ ring be softer than the flange groove?

Short answer: So that the ring, not the groove, deforms. If the ring were harder it would dent the groove walls, creating leak paths and permanently damaging an expensive flange. Rings are normally specified at least about 30 HB softer than the groove.

Q31What surface finish is required in an RTJ groove?

Short answer: ASME B16.5 limits the side walls of the ring groove to 1.6 µm Ra (63 µin) or smoother. The ring itself is also finished to about 1.6 µm Ra, and both must be free of burrs, scratches and corrosion pits.

A single radial scratch across a groove wall can stop an RTJ from sealing. Groove damage is repaired by machining and must stay within the groove dimensions, otherwise the flange is replaced or the groove is weld-overlaid and re-machined by an approved procedure.

Q32Can an RTJ ring be reused?

Short answer: No. The ring has plastically deformed to the exact shape of one pair of grooves; reinstalling it will not rebuild the same contact and work-hardening makes it harder. Always fit a new ring, and inspect the grooves every time the joint is broken.

Q33What are lens rings and solid flat metal gaskets?

Short answer: A lens ring (DIN 2696) has spherical faces that seat on conical flange recesses, giving a line seal for very high-pressure service such as high-pressure hydrogen and ammonia synthesis. Solid flat metal gaskets are plain machined metal rings that need very high seating stress and smooth faces.

Q34What are welded membrane (weld-lip) seals?

Short answer: Two thin lips, one welded to each flange, that are seal-welded together at their outer edge after bolting. The bolts carry the load and the weld provides a zero-leak seal. They are used for toxic, lethal or hydrogen service where any leakage is unacceptable, at the cost of cutting and re-welding the lips to open the joint.

Q35Which flange facings are used, and which gasket suits each?

Short answer: Raised face (RF) with ring, spiral-wound or kammprofile gaskets; flat face (FF) with full-face soft gaskets; ring-type joint (RTJ) with metal rings; tongue-and-groove (T&G) and male-and-female (M&F) with confined gaskets.

Confined facings (T&G, M&F) hold the gasket against blow-out and are common on heat exchangers and valve bonnets, but the two flanges must be ordered as a matched pair.

Q36What is the raised-face height, and is it included in the flange thickness?

Short answer: 2 mm (1/16 in) for Classes 150 and 300, and 7 mm (1/4 in) for Class 400 and above. In ASME B16.5, the 2 mm face is included in the published flange thickness for Classes 150 and 300, but the 7 mm face is added on top of the thickness for the higher classes.

This catches people out when calculating stud lengths and spool dimensions — check which convention the dimension table uses before you order bolts.

Q37What types of serrated finish are used on flange faces?

Short answer: Spiral (phonographic or “stock”) serration — one continuous groove cut from the bore outwards like a gramophone record — and concentric serration, a series of separate circular grooves. Both bite into soft and semi-metallic gaskets to grip them and stop extrusion.

Q38When is a concentric serration specified instead of a spiral serration?

Short answer: For hydrogen, helium and other light, low-viscosity gases, for vacuum service, and for toxic fluids with strict leakage limits. A spiral groove is a single continuous path from the bore to the outside; if the gasket does not fill it completely, a light gas can follow it. Concentric rings have no such path.

Spiral (stock) finish remains the standard for general service because it is quicker and cheaper to machine and it seals reliably with soft and spiral-wound gaskets.

Q39How is flange face finish specified, and what is AARH?

Short answer: By arithmetic average roughness, Ra, in micrometres (µm) or microinches (µin), measured to ASME B46.1. AARH (arithmetic average roughness height) is the older name for the same quantity in microinches, so “125–250 AARH” means 125–250 µin Ra, or 3.2–6.3 µm Ra.

Q40What face finish does each gasket type need?

Short answer: Soft and spiral-wound gaskets need a serrated 3.2–6.3 µm Ra face. Harder, thinner metal gaskets need smoother faces: about 2.5 µm Ra or better for jacketed gaskets and 1.6 µm Ra or better for RTJ grooves and solid metal.

Q41Why is a face that is too smooth or too rough a problem?

Short answer: Too rough, and the gasket cannot fill the valleys, leaving leak paths. Too smooth, and soft or spiral-wound gaskets lose grip, slide outwards (extrude) and relax under pressure and temperature.

That is why the specification is a range (3.2–6.3 µm Ra), not a maximum, for soft and semi-metallic gaskets. Polishing a serrated face “to make it better” is a common mistake on site.

Q42How is face finish checked on site?

Short answer: With a portable stylus profilometer that reads Ra directly, or by comparing the face against a surface roughness comparator plate (visual and fingernail test). Groove pitch is checked by counting grooves over 25 mm with a magnifier.

Record the reading, the instrument and the location at four points around the face. For critical or disputed joints, a profilometer reading is the accepted evidence.

Q43Which flange face damage is unacceptable?

Short answer: Radial scratches, dents and pits that cross the gasket seating width are the most dangerous because they form a direct leak path. Circumferential marks along the serration are far less harmful. ASME PCC-1 Appendix D gives allowable defect sizes, with tighter limits for hard gaskets (spiral-wound, metal, RTJ) than for soft gaskets.

Faces that exceed the limits are re-machined (staying within the minimum flange thickness and raised-face height) or the flange is replaced. Never use grinders on a gasket seating face.

Q44Why should a flat-face flange not be bolted to a raised-face flange?

Short answer: When the bolts are tightened, the raised face acts as a fulcrum and bends the flat-face flange. Brittle cast-iron, ductile-iron or FRP flanges can crack. If the mating cannot be avoided, machine the raised face off the steel flange and use a full-face gasket.

Q45What are the gasket factors m and y?

Short answer: They are the classic gasket constants used in ASME Section VIII Division 1, Mandatory Appendix 2. y is the minimum seating stress needed to seat the gasket at assembly; m (the maintenance factor) is the ratio of the gasket stress that must remain in operation to the internal pressure.

Interviewers often follow up by asking what is wrong with m and y. The honest answer: they say nothing about the leak rate, temperature or gasket relaxation, which is why EN 13555/EN 1591-1 parameters (Qmin(L), QSmin(L), QSmax, PQR) and the ASTM F2836 tightness constants (Gb, a, Gs) are now preferred for critical joints.

Q46How should a flanged joint be assembled to ASME PCC-1?

Short answer: Clean and inspect the faces and fasteners, check alignment, fit a new gasket centred and dry, lubricate the threads and nut faces, then tighten in measured stages using a cross pattern before finishing with circular passes at full target.

Use the Aienginear bolt torque and flange calculator to estimate the tightening torque for a given stud size, target stress and nut factor.

Q47How is the target bolt stress chosen?

Short answer: High enough to seat the gasket and keep it tight in operation, but low enough not to crush the gasket, yield the bolts or over-rotate the flange. ASME PCC-1 Appendix O gives a method; many sites use a default bolt stress of roughly 40–70 % of bolt yield.

For A193 B7 studs, a common site default is in the region of 345 MPa (50 ksi) on the root area, adjusted by joint-specific checks. Using “standard torque tables” without knowing the gasket and lubricant is the most common cause of under- or over-loaded joints.

Q48What is the difference between torque tightening and hydraulic tensioning?

Short answer: Torque tightening turns the nut against friction, so only about 10 % of the applied torque stretches the bolt — accuracy depends on lubrication (nut factor). Hydraulic tensioning stretches the stud directly and then the nut is run down, giving more uniform and accurate preload, especially on large studs.

Tensioning needs extra stud length (about one diameter) above the nut and must allow for load transfer loss when the tool is removed. It is preferred for large, high-class and critical joints such as heat-exchanger girth flanges and reactor nozzles.

Q49Can a gasket be reused?

Short answer: No. Soft, spiral-wound and RTJ gaskets all deform permanently when seated and lose recovery. A used gasket will not reseat at the same stress, so every broken joint gets a new gasket. Kammprofile cores are the exception — they can be refaced by the manufacturer and refitted with new facing layers.

Q50What are the most common causes of flange leaks, and how are they fixed?

Short answer: Uneven or insufficient bolt load, wrong gasket, damaged or wrong face finish, misalignment forced together by the bolts, reused gaskets, and gasket relaxation after thermal cycling.

Q51What alignment is required before bolting up?

Short answer: The flanges must be brought together without using the bolts to pull them into line. ASME PCC-1 Appendix E gives typical limits: centreline offset about 1.5 mm, difference in flange gap around the circumference about 0.8 mm, and bolt-hole rotational misalignment about 3 mm.

Excessive misalignment is corrected in the piping (supports, cold spring, re-fit), not by force. Pulling a joint together locks a bending moment into it that the gasket will feel as uneven stress for its whole life.

Q52What are flange insulation (isolation) gasket kits?

Short answer: Kits that electrically isolate two flanges — for cathodic protection or to prevent galvanic corrosion between dissimilar metals. Each kit has a dielectric gasket (often G10 or G11 glass-reinforced epoxy with an elastomer or PTFE seal), insulating sleeves for every bolt, insulating washers and steel backing washers.

Type E kits use a full-face gasket, Type F a ring inside the bolts, and Type D an oval insulating ring for RTJ flanges. After assembly, test the isolation with a suitable insulation tester before the line is filled.

Q53A Class 300 superheated steam line at 420 °C keeps leaking at a flange. What do you check?

Short answer: The gasket type and filler (it should be an SWG with 316L or 321 winding, flexible graphite filler and an inner ring), the face finish (3.2–6.3 µm Ra serrated, no radial damage), the bolting material and target bolt stress, the tightening record, alignment, and piping loads from thermal expansion.

Then confirm the gasket temperature is below the graphite oxidation limit, and check whether the joint is relaxing with each start-up — a sign that a check pass, hydraulic tensioning or a higher preload is needed.

Q54Which gasket would you choose for 98 % sulphuric acid and for hydrofluoric acid?

Short answer: For concentrated sulphuric acid, a filled PTFE (often silica-filled) or PTFE-envelope gasket, or an SWG with a suitable alloy winding and PTFE filler. For HF, barium-sulphate-filled PTFE sheet or an SWG with Alloy 400 (Monel) winding and PTFE or graphite filler — never glass- or silica-filled PTFE.

Always finish the answer with “confirmed against the gasket manufacturer’s chemical compatibility data and the owner’s specification” — interviewers expect that discipline in chemical service.

Q55During a shutdown you find a spiral-wound gasket fitted on a mirror-smooth raised face. Is it acceptable?

Short answer: No. A spiral-wound gasket needs a serrated face of 3.2–6.3 µm Ra to grip it. On a polished face it can slide and extrude under pressure. Either re-machine the face to a serrated finish or change to a gasket suited to smooth faces, such as a kammprofile with graphite facing, after engineering approval.

Q56How would you specify gaskets for a new hydrogen service pipe class?

Short answer: Spiral-wound gaskets with 316L winding, flexible graphite filler and inner and outer rings for RF flanges up to the class limit, RTJ (octagonal) or welded seals for high-pressure or critical joints, concentric serrated faces, and assembly to ASME PCC-1 with qualified assemblers and leak testing.

Hydrogen’s small molecule and wide flammability range make tightness the priority: minimise flanged joints, specify low-emission gaskets where fugitive-emission limits apply (for example VDI 2290 / TA-Luft), and include helium or nitrogen-helium leak testing in the commissioning plan.