Full Face vs Ring Gasket: How Flange Facing Decides Contact Area and Bolt Load

A maintenance crew had four hours to close a leaking DN100 Class 150 steam line. The store room sent out a full-face gasket, the bolt holes lined up perfectly, and the joint passed a cold water test. Two hours after warm-up it was weeping again at roughly 8 bar. The material was correct and the torque wrench had been calibrated that morning. The gasket had simply been laid across a 1.6 mm raised face it was never designed to bridge, so the bolts loaded the outer rim of the flange instead of the sealing face.

The rule that prevents this is short: choose the gasket by flange facing, not by pipe size or pressure class. Full-face gaskets are made for flat-face flanges. Ring gaskets are made for raised-face and tongue-and-groove flanges. Bolt torque, gasket thickness, material and price all follow from that first decision.

The comparison below covers the points that actually decide a purchase order: contact area, bolt load, facing compatibility, corrosion exposure and the failure modes that show up most often in sealing enquiries from refineries, chemical plants and power stations.

Full face vs ring gasket: the comparison that matters

If you only have thirty seconds before a quotation deadline, this table is the short answer. Everything after it explains the consequences behind each row.

First, clear up the naming: flat ring, ring joint, and "ring gasket"

Half of the confusion in a purchase email is linguistic. "Ring gasket" can mean two very different products:

• Flat ring gasket — cut from sheet, spiral wound or kammprofile, with no bolt holes and an outside diameter smaller than the bolt circle. This is the everyday partner of a raised-face flange.
• Ring joint gasket (RTJ) — a solid metal oval or octagonal ring that seats in a machined groove in the flange face. It seals by line contact and metal-to-metal wedging, not by a broad compressed area.

The distinction matters because the two are not interchangeable and rarely share a price bracket. Above Class 900, or wherever sheet material would creep under thermal cycling, the conversation shifts from flat rings to ring joint gaskets, groove numbers and hardness limits.

Ring Joint Gaskets for High-Pressure Flanged ServiceMetallic RTJ gaskets for high-pressure and high-temperature flanges, made to API 6A, API 17D and ASME B16.20 in R, RX, BX and other styles.View Product →

Where the gasket actually touches the flange

A full-face gasket is shaped like the flange itself: same outside diameter, bore in the middle, bolt holes punched through. Because the bolts pass through the gasket, the clamping force is delivered across the entire face, inside and outside the bolt circle.

A ring gasket stops at the bolt circle. The bolts never touch it, and the only load it receives is the load that reaches the raised face or groove.

Why a full-face gasket needs roughly three times the bolt load

Seating stress is bolt load divided by contact area. Double the area at the same torque and you halve the stress on the sealing material. That single relationship explains most of the practical difference between the two styles.

Work it through on a standard NPS 4 (DN100) Class 150 flange: 229 mm outside diameter, 157 mm raised-face diameter, 102 mm bore on Schedule 40 pipe, eight bolts. A full-face gasket for that flange has about 31,100 mm² of net contact area once the eight bolt holes are removed. A flat ring gasket on the same flange has about 11,200 mm². The full-face version is roughly 2.8 times larger.

Keep the bolt load identical and the full-face gasket sees only about a third of the seating stress the ring gasket sees. To reach the same stress you would need close to 2.8 times the bolt load — often more than the flange class, the bolt size or the available wrench can deliver.

This is also why full-face gaskets are usually specified on lower-pressure, flat-face flanges where the design bolt load is generous relative to what the gasket needs, and why thicker soft sheet (3 mm rather than 1.5 mm) is common in those joints: a softer, thicker sheet reaches usable seating stress at modest torque.

When a full-face gasket is the right choice

• Flat-face flanges. Class 125 and 150 cast iron valves, pumps, water boxes and manways are flat-faced by design, and a full-face gasket is the intended partner.
• Cast iron and bronze bodies. These crack when a narrow ring concentrates load near the bore. Spreading the clamp over the whole face is a mechanical necessity, not a preference.
• FRP, GRP, PVC and PTFE-lined piping. The wide contact area lowers the stress on a soft or brittle flange face and protects the liner edge.
• Aggressive media. A full-face gasket covers the flange face completely, leaving no exposed annulus between gasket OD and bolts where crevice corrosion can start.
• Insulating gasket kits. Cathodic protection joints use full-face geometry with bolt sleeves and washers to break electrical continuity across the flange pair.
• Utility duty. Cooling water, instrument air, low-pressure steam and similar services where bolt load is not the limiting factor.

Material selection usually decides the rest. If the medium is the open question, this overview of non-metallic gasket applications in industrial service is a practical follow-up.

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When a ring gasket is the right choice

• Raised-face steel flanges. ASME B16.5 flanges carry a 1.6 mm raised face up to Class 300 and about 6.4 mm from Class 400 upward. That raised band is the sealing surface, and only a ring gasket sits on it.
• Tongue-and-groove and male-and-female facings. The groove confines the ring; a full-face sheet would be trapped between the faces and crushed unevenly.
• Higher pressure and temperature. A narrow, controlled contact width keeps seating stress high with the bolt load actually available.
• Spiral wound and kammprofile construction. With inner and outer rings, these are the ring family used in critical process service where a sheet gasket would relax.

Put a full-face gasket on a raised-face flange and it bridges the raised band. Tightening bends the flange, damages the facing and still leaves the sealing element under-loaded — the exact failure described at the start of this article.

Spiral Wound Gaskets and VariantsIncludes W, RW, RWI, WI, RH and RS styles to ASME B16.20 and EN 1514-2, using metal windings and soft fillers for varied sealing service.View Product →

Four mistakes that appear in almost every second review

1. Mixing style and facing. A full-face gasket on a raised face, or a hard ring gasket on a flat-face cast iron flange, causes bending, cracking or a leak that no amount of retorquing will fix.
2. Copying torque values across styles. A torque table written for a ring gasket assumes a small contact area. Applied to a full-face gasket it under-compresses the joint, and the leakage appears only at operating temperature.
3. Checking the wrong diameter. What matters is the gasket bore against the pipe bore, not against the flange outside diameter. A gasket protruding into the flow creates turbulence, erosion and a false reputation for the material.
4. Ordering "ring gasket" without detail. Flat ring or ring joint? Soft iron, low-carbon steel, 316L or alloy 825? Which groove number? A two-word line item is the most common cause of a wrong delivery.

Installation discipline closes the loop: clean and inspect the facing, lubricate the bolting, tighten in a star or cross pattern in three passes, and finish at the specified torque rather than at "tight enough".

Frequently asked questions