Gasket Dimensions

Spiral wound gaskets are the most common gasket type for raised face flanges in oil and gas piping systems. ASME B16.20 defines the dimensions, materials, and markings for metallic gaskets used with ASME B16.5 and B16.47 flanges. The gasket dimensions depend on the NPS and the flange pressure class.

When Spiral Wound Gaskets Are Used

Spiral wound gaskets are specified for standard raised face (RF) flanged joints in pressure classes 150 through 2500. They consist of alternating layers of metal strip (typically 304SS or 316SS) and a soft filler (graphite or PTFE), with an outer centering ring (carbon steel) and an optional inner ring (stainless steel).

Spiral Wound Gasket Dimensions: Class 150

NPS	DN	Gasket ID (mm)	Gasket OD (mm)	Centering Ring OD (mm)	Gasket Thickness (mm)
1/2	15	22.4	34.8	47.8	4.5
3/4	20	27.7	42.9	57.2	4.5
1	25	34.0	50.8	66.5	4.5
1-1/2	40	49.3	73.2	82.6	4.5
2	50	62.0	92.1	104.6	4.5
3	80	90.7	127.0	133.4	4.5
4	100	115.9	157.2	171.4	4.5
6	150	168.1	215.9	228.6	4.5
8	200	219.1	269.7	282.4	4.5
10	250	273.1	323.9	342.9	4.5
12	300	323.9	381.0	400.1	4.5
14	350	355.6	412.8	431.8	4.5
16	400	406.4	469.9	489.0	4.5
18	450	457.2	533.4	546.1	4.5
20	500	508.0	584.2	603.3	4.5
24	600	609.6	692.2	711.2	4.5

Spiral Wound Gasket Dimensions: Class 300 and 600

NPS	DN	CL 300 Gasket ID (mm)	CL 300 Gasket OD (mm)	CL 300 Ring OD (mm)	CL 600 Gasket ID (mm)	CL 600 Gasket OD (mm)	CL 600 Ring OD (mm)
1	25	34.0	50.8	66.5	34.0	50.8	66.5
2	50	62.0	92.1	104.6	62.0	92.1	104.6
3	80	90.7	127.0	133.4	90.7	127.0	133.4
4	100	115.9	157.2	171.4	115.9	157.2	184.2
6	150	168.1	215.9	228.6	168.1	215.9	247.7
8	200	219.1	269.7	282.4	219.1	269.7	304.8
10	250	273.1	323.9	342.9	273.1	323.9	368.3
12	300	323.9	381.0	400.1	323.9	381.0	419.1
16	400	406.4	469.9	489.0	406.4	469.9	539.8
20	500	508.0	584.2	603.3	508.0	584.2	647.7
24	600	609.6	692.2	711.2	609.6	692.2	762.0

The gasket ID and OD (wound element) are often the same across classes. The centering ring OD differs because it must fit inside the bolt circle of the specific flange class. Class 600 bolt circles are larger than Class 300 for NPS 4 and above, allowing larger centering rings.

Material Designations

Spiral wound gaskets are identified by color-coded centering rings per ASME B16.20:

Filler Material	Winding Metal	Color Code	Max Temp
Flexible graphite	304SS	Yellow	650 C (1200 F)
Flexible graphite	316SS	Green	650 C (1200 F)
PTFE	304SS	White	260 C (500 F)
PTFE	316SS	Coral	260 C (500 F)
Mica	Alloy 625	Turquoise	980 C (1800 F)

Inner Ring Requirements

An inner ring prevents buckling of the spiral winding into the pipe bore under vacuum or high-pressure cycling. ASME B16.20 recommends an inner ring for:

• NPS 4 and above in Class 150
• NPS 3 and above in Class 300
• All sizes in Class 600 and above
• All vacuum services
• Services subject to thermal cycling

Pro Tip

Always specify “CG” (with inner ring) or “CGI” style gaskets in the purchase order for critical services. The inner ring material should match or be compatible with the process fluid. For flange face finish requirements, ASME B16.5 specifies 3.2-6.3 micrometers Ra for raised face flanges used with spiral wound gaskets.

Read the full guide to pipe flanges