Slip-On Flange Dimensions

Slides over the outside of the pipe and is fillet-welded both inside and outside. The bore is sized to the pipe OD rather than to a schedule, so a single slip-on fits any wall thickness in that NPS.

Standard: ASME B16.5
Abbreviation: SO
Pressure classes: 6
Size range: NPS 1/2 – NPS 24
Typical service: Low-pressure, non-cyclic utility and water service.

Slip-On Flange in section, and a Class 150 flange from the front. The section shows what each column of the dimension tables measures, with welds in orange; it is not to scale. The front view is an NPS 6 Class 150 flange drawn to scale, with its bolt holes straddling the centrelines.

Dimensions by pressure class

Class 150NPS 1/2 – 2420 sizes Class 300NPS 1/2 – 2420 sizes Class 400NPS 1/2 – 2420 sizes Class 600NPS 1/2 – 2420 sizes Class 900NPS 1/2 – 2419 sizes Class 1500NPS 1/2 – 2 1/27 sizes

Strengths

Cheaper than weld neck and tolerant of small alignment errors
One bore fits every schedule in the size
Shorter than a weld neck, useful in tight spaces

Limitations

Two fillet welds instead of one butt weld; harder to inspect
Roughly a third of the fatigue life of a weld neck

Class 150 dimensions

Dimensions are given in inches, with the millimetre equivalent beneath in grey.

ASME B16.5 Class 150 slip-on flange dimensions — the most commonly specified class.
Size	Flange OD	Thickness	Bolt circle	Bolts	Bolt dia.	Raised face OD
NPS 1/2	3.5088.90 mm	0.389.65 mm	2.3860.45 mm	4	1/2 in	1.3835.05 mm
NPS 3/4	3.8898.55 mm	0.4411.18 mm	2.7569.85 mm	4	1/2 in	1.6942.93 mm
NPS 1	4.25107.95 mm	0.5012.70 mm	3.1279.25 mm	4	1/2 in	2.0050.80 mm
NPS 1 1/4	4.62117.35 mm	0.5614.22 mm	3.5088.90 mm	4	1/2 in	2.5063.50 mm
NPS 1 1/2	5.00127.00 mm	0.6215.75 mm	3.8898.55 mm	4	1/2 in	2.8873.15 mm
NPS 2	6.00152.40 mm	0.6917.53 mm	4.75120.65 mm	4	5/8 in	3.6291.95 mm
NPS 2 1/2	7.00177.80 mm	0.8120.57 mm	5.50139.70 mm	4	5/8 in	4.12104.65 mm
NPS 3	7.50190.50 mm	0.8822.35 mm	6.00152.40 mm	4	5/8 in	5.00127.00 mm
NPS 3 1/2	8.50215.90 mm	0.8822.35 mm	7.00177.80 mm	8	5/8 in	5.50139.70 mm
NPS 4	9.00228.60 mm	0.8822.35 mm	7.50190.50 mm	8	5/8 in	6.19157.23 mm
NPS 5	10.00254.00 mm	0.8822.35 mm	8.50215.90 mm	8	3/4 in	7.31185.67 mm
NPS 6	11.00279.40 mm	0.9423.88 mm	9.50241.30 mm	8	3/4 in	8.50215.90 mm
NPS 8	13.50342.90 mm	1.0626.92 mm	11.75298.45 mm	8	3/4 in	10.62269.75 mm
NPS 10	16.00406.40 mm	1.1228.45 mm	14.25361.95 mm	12	7/8 in	12.75323.85 mm
NPS 12	19.00482.60 mm	1.1930.23 mm	17.00431.80 mm	12	7/8 in	15.00381.00 mm
NPS 14	21.00533.40 mm	1.3133.27 mm	18.75476.25 mm	12	1 in	16.25412.75 mm
NPS 16	23.50596.90 mm	1.3835.05 mm	21.25539.75 mm	16	1 in	18.50469.90 mm
NPS 18	25.00635.00 mm	1.5038.10 mm	22.75577.85 mm	16	1 1/8 in	21.00533.40 mm
NPS 20	27.50698.50 mm	1.6241.15 mm	25.00635.00 mm	20	1 1/8 in	23.00584.20 mm
NPS 24	32.00812.80 mm	1.8145.97 mm	29.50749.30 mm	20	1 1/4 in	27.25692.15 mm

Higher classes are on the Class 300 page and above.

Guides and comparisons for this page

Weld neck vs slip-on flangeOne butt weld against two fillet welds.Slip-on vs threaded flangeWelded or screwed attachment for low-pressure service.Lap joint vs slip-on flangeA loose backing flange against a welded one.Choosing a flange face typeRaised face, flat face or ring-type joint, by service.Flange bolt torqueTarget torque from bolt stress, for every B16.5 bolt size.Pressure-temperature deratingReading a P-T table and interpolating between temperatures.

Common questions

When should I use a slip-on flange?

Low-pressure, non-cyclic utility and water service.

What pressure classes does a slip-on flange come in?

ASME B16.5 publishes the slip-on flange in Class 150, Class 300, Class 400, Class 600, Class 900 and Class 1500, in sizes NPS 1/2 through NPS 2 1/2 in Class 1500, and through NPS 24 in the other classes. It does not publish a slip-on flange in Class 2500. Above NPS 24 large-diameter flanges are covered by ASME B16.47 instead.

Does the flange face type change these dimensions?

The flange outside diameter, bolt circle and bolting are the same whatever the face. What changes is the facing itself and the flange thickness measurement: B16.5 thickness excludes the raised face, so a Class 300 raised-face flange is 0.06 in thicker overall than the tabulated figure. Ring-type joint flanges are grooved instead and are dimensioned separately.