- Valve gaskets and sealing elements are the most failure-prone components in any flanged valve or piping system — and gasket selection based on pressure class alone, without considering temperature, chemistry, surface finish, and bolt load, is the leading cause of premature joint failure and fugitive emissions.
- The range of gasket types — compressed non-asbestos fibre, spiral wound, ring type joint, and expanded PTFE — each have specific performance envelopes defined by pressure, temperature, fluid chemistry, and flange surface finish requirements that must all be satisfied simultaneously for reliable sealing.
- Correct installation practice is as important as correct gasket selection — the right gasket installed incorrectly, with wrong bolt torque or sequence, will leak just as reliably as the wrong gasket installed correctly.
I’ve investigated flanged joint leaks across many types of industrial plant, and the pattern is consistent: the majority of flange leaks are not caused by the wrong gasket material — they’re caused by inadequate bolt load, incorrect installation sequence, damaged flange faces, or gaskets reused after a previous disassembly. The gasket gets the blame, but the root cause is usually something else entirely. Understanding what actually makes a flanged joint seal — and what makes it fail — is the foundation of good valve and piping system specification.
The Sealing Mechanism in Flanged Joints
A flanged joint seals by compressing the gasket between two flange faces to a stress level that fills all surface irregularities and provides a continuous seal. The bolt load provides the compressive force — the gasket material yields under this load, flowing into the microscopic surface irregularities of the flange face and creating a fluid-tight contact. For the joint to remain sealed throughout its life, the gasket must maintain adequate seating stress under all operating conditions including thermal cycling, vibration, and pressure fluctuations.
Two key parameters define a gasket‘s sealing performance: the minimum seating stress required to achieve initial sealing (the “y” value), and the minimum stress required to maintain sealing under operating pressure (the “m” factor, defined as the ratio of minimum seating stress to operating pressure). A gasket with a high y value needs more bolt load for initial sealing; a gasket with a high m factor needs more residual bolt load to resist the hydrostatic end force trying to separate the flanges under pressure.
ASME BPVC Section VIII Appendix 2 and ASME PCC-1 provide the calculation framework for flanged joint design. For most standard process applications, following the manufacturer’s torque tables is sufficient. For critical high-pressure, high-temperature, or fugitive-emission-sensitive joints, a full ASME PCC-1 bolt load calculation is the correct approach.
Gasket Types: Performance Envelopes
Compressed Non-Asbestos Fibre (CNAF) gaskets — made from aramid, glass, or carbon fibre with a nitrile or EPDM binder — are the workhorses of low-to-medium pressure process sealing. Appropriate for Class 150 flanges in water, steam, compressed air, and mild chemical service at temperatures up to 250°C (depending on the binder material). They are NOT suitable for high-pressure service (Class 600 and above), high-temperature steam, or hazardous/toxic chemical service where any leakage is unacceptable. CNAF gaskets are the cheapest option and entirely appropriate for their design envelope — but they’re frequently misapplied to higher-pressure or more aggressive service than they can reliably seal.
Spiral Wound Gaskets (SWG) are the standard for ASME B16.5 Class 300 and above, and for Class 150 in elevated temperature, steam, or hazardous service. The V-shaped metallic winding (typically 304SS or 316SS) alternating with a filler (flexible graphite for most process applications, PTFE for clean/food service) provides resilience — the gasket springs back as the joint sees thermal cycles and pressure fluctuations, maintaining seating stress through operational variations that would cause CNAF to relax and leak.
Spiral wound gaskets come in three standard configurations: with inner ring only (IR), with outer centering ring only (OR), and with both inner ring and outer ring (IR+OR). The IR+OR configuration is the standard for most critical applications — the outer ring locates the gasket centrally on the bolt circle, and the inner ring prevents the winding from buckling inward under high bolt load. Always specify IR+OR for Class 600 and above.
Ring Type Joint (RTJ) gaskets are used in Class 600 and above for elevated temperature and pressure service, and are the standard for all Class 900, 1500, and 2500 flanges. The metallic ring (oval or octagonal cross-section) seats in a precision-machined groove in the flange face, and the ring material is specified softer than the flange face material to achieve sealing by plastic deformation of the ring’s contact areas. Correct material pairing is critical — soft iron rings in 316SS grooves, 316SS rings in duplex grooves. Getting the hardness relationship wrong results in a joint that cannot be sealed regardless of bolt load.
Expanded PTFE (ePTFE) gaskets have revolutionised sealing in chemical, food, and pharmaceutical service. Unlike sheet PTFE (which is prone to cold flow under high bolt loads), ePTFE maintains its seating stress through a unique microstructure of stretched fibrils that provide resilience. ePTFE gaskets conform to damaged or irregular flange faces better than any other gasket type, have very low y and m values (sealing at low bolt loads, ideal for glass-lined flange connections), and are chemically inert to virtually all process fluids. For sensitive chemical and pharmaceutical applications, ePTFE is the standard modern specification.
Installation Best Practices
The correct gasket installation procedure is: clean and inspect flange faces (no radial scratches, pits, or deposits); centre the gasket on the flange using the bolt holes or centering ring; hand-tighten all bolts to maintain face parallelism; torque in a cross-pattern sequence to 25% of target torque; repeat at 50%, then 75%, then 100% of target torque; for steam and high-temperature service, re-torque after the first heat-up cycle when the gasket has consolidated under heat and bolt load.
Never reuse gaskets. A compressed gasket has yielded to the specific surface profile of the flanges it was compressed against. When removed and reinstalled, it will not conform correctly to those surfaces again, and will either leak immediately or fail earlier than a new gasket. The cost of a new gasket is negligible; the cost of a repeat leak and flange re-work is not. Replace every gasket every time a joint is opened, without exception.