What is the difference between Type C and Type D metal spiral wound gaskets

Type C and Type D are the two standard metal spiral wound gasket configurations used on bolted flanged joints. Both seal with the same V-shaped element — alternating layers of preformed metal strip and soft filler such as flexible graphite or PTFE — and the real difference is the set of metal rings around that element. The rings decide the pressure class, the flange face and the assembly behaviour, so the two types are not interchangeable on every joint. The comparison below covers construction, ring function, selection and installation of these spiral wound gaskets.

How Type C and Type D Are Built

Both types start from the same sealing element: a V-shaped winding of preformed metal strip (commonly 304 or 316L stainless steel) with a filler of flexible graphite, PTFE or a graphite-PTFE combination. Dimensions, tolerances and marking follow ASME B16.20. The ring set is what separates the types.

• Type C (code CG): sealing element plus one outer centering ring; the inner diameter of the winding stays open.
• Type D (code CGI): an inner ring, the sealing element and an outer centering ring; the inner ring closes the bore-side gap.
• Basic element (code A or R): winding alone, used where the flange groove already locates the gasket, as in tongue-and-groove or male-and-female joints.
• Winding thickness is picked per joint size and pressure class — common values are 3.2, 4.5, 4.8, 5.5 and 7.5 mm — and the inner ring of a Type D set is thinner (for example 3.2/2 or 4.8/3.0 mm).

What the Outer and Inner Rings Do

Each ring protects the winding from a different failure mode, so the choice follows service conditions rather than price.

• Outer centering ring: centers the gasket on the bolt circle during assembly, keeps the winding from being blown outward by internal pressure and acts as a compression stop when the bolts are torqued.
• Inner ring (Type D): supports the inner turns of the winding so they cannot buckle toward the bore, blocks filler extrusion and reduces the effect of pressure and thermal cycling on the seal.
• Type D therefore holds its sealing stress more steadily through start-up, shutdown and temperature swings, while Type C is quicker to place and check during assembly.

Choosing Between Type C and Type D

Start from the flange face and the pressure class, then confirm the medium and the operating temperature. For pipe flanges rated Class 150 or Class 300, Type C is normally sufficient and faster to place during assembly.

Move to Type D whenever the winding needs support from the inside: high pressure classes, high temperature, steam, thermal cycling, or any joint where filler extrusion into the bore would be a problem. The table summarizes the typical split.

Table: typical application ranges. Final selection must be confirmed against flange rating, temperature, medium and the allowable bolt load of the joint.

• Check gasket dimensions against the flange facing and ASME B16.20 before ordering.
• Place the gasket so the outer ring centers it on the bolt circle; never let a bolt bear on the winding.
• Tighten the bolts in a cross pattern in at least three passes to the target torque, then re-check torque after the joint reaches operating temperature.
• Fit a new gasket every time the joint is opened — the winding takes a permanent set and will not seal a second time.

FAQ

Which flange faces can take a Type C or Type D spiral wound gasket?

Spiral wound gaskets fit raised face (RF), flat face (FF), tongue-and-groove and male-and-female faces, with Type C and Type D used mainly on RF and FF flanges. Ring joint grooves take a solid metal ring gasket instead, so a spiral wound gasket is not the right part there.

Why does Type D need an inner ring?

The inner ring supports the inner turns of the V-shaped winding. Without it, high pressure and thermal cycling push those turns toward the bore and squeeze the filler out of the windings, sealing stress drops and the joint leaks.

Can a spiral wound gasket be reused after the joint is opened?

No. The winding and the filler take a permanent set at first bolt-up, so a reused gasket will not reach the required sealing stress. Fit a new gasket at every reassembly.

Graphite or PTFE filler — how do I choose?

Flexible graphite handles higher temperatures and steam and is the usual pick for refinery and power service. PTFE suits chemical media and joints that need low seating stress, but it creeps under heat, so it is not the first choice for hot service.