What Is a Flange? Types & Selection

Flange Essentials

A flange connects pipes, valves, pumps, and equipment into a piping system. Flanges are classified by shape (weld neck, blind, socket weld, threaded), facing type (RF, FF, RTJ), material (cast/forged), and specification (ASME B16.5/B16.47 for US, EN 1092-1 for Europe).

Classification of pipe flange types

What Is a Flange?

Definition of Pipe Flange

A flange is a plate or ring that provides a mounting surface for connecting pipe sections or attaching pipes to valves, pumps, and other equipment. When bolted together with a gasket between them, flanges create a leak-proof seal.

Flanges are produced by casting or forging steel-in oil and gas applications, forged steel flanges are preferred for their superior strength.

Flange Parameters

Parameter	Description	Selection Notes
Type & Specification	Shape/design: weld neck, slip-on, blind, threaded, lap joint. 13 types available	Specify ASME B16.5, B16.47, EN 1092-1, JIS, or DIN
NPS	Nominal Pipe Size-standardized size designation	For WN/SW flanges, also specify pipe schedule
Pressure Class	Max allowable pressure at temperature: 150, 300, 600, 900, 1500, 2500	Class determines flange thickness and bolt pattern
Facing Type	FF (flat face), RF (raised face), RTJ (ring joint)	RF is ASME B16.5 default; RTJ for high P/T
Material Grade	A105 (CS), A182 F304/316 (SS), A182 F11/22 (alloy)	Must match pipe material and service conditions
Cathodic Protection	Flange isolation kits prevent galvanic corrosion	Required where dissimilar metals meet

Common Mistake

Mixing ASME B16.5 and EN 1092-1 flanges on the same line - they look similar but bolt holes don’t align. If you’re buying from multiple sources, standardize on one specification from the start.

Common Ordering Mistake

For welding neck and socket-weld flanges, always specify the pipe schedule-this information is frequently missing from RFQs and POs but is necessary for proper bore matching.

What Is a Flanged Joint?

A flanged joint connects two pipe sections or equipment by bolting flanges together with a gasket between them. Hundreds-even thousands-of flanged joints are required in a typical piping system.

Component	Function
Flanges	Circular disks with bolt holes; welded or threaded to pipe ends
Gasket	Sealing element (rubber, graphite, PTFE, metal) between flanges
Bolts/Nuts	Compress the gasket to create a leak-proof seal

Advantage	Description
Leak-proof	Proper assembly prevents fluid/gas escape
Easy maintenance	Can be disassembled without cutting or welding
Flexibility	Accommodates slight misalignments
P/T tolerance	Designed for specific pressure and temperature conditions

Connection types:

• Welded (WN, SW, SO, LJ): For high P/T applications and sizes >2”
• Threaded: For small diameters without vibration or thermal cycling

Assembly Requirements

Flanged joints must be assembled by trained personnel per EN 1591 Part 1-4. Improper assembly causes dangerous leaks.

Field Note

When tightening flange bolts, always follow a cross-pattern (star sequence) and do at least three passes - 30%, 60%, then 100% of target torque. Rushing this step is the #1 cause of flange leaks during commissioning.

Flanges for Piping Systems

Standard flange types include welding neck, blind, socket weld, lap joint, and threaded. Special types include swivel, expander/reducer, Nipoflange/Weldoflange, and orifice flanges.

Application	Recommended Flange Type
High P/T, permanent joints	Weld neck, slip-on, socket weld
Low pressure, no vibration	Threaded
Bolt alignment, cost savings on exotic alloys	Lap joint with stub end

Flanges Specifications (ASME, API, EN)

Standard	Coverage	Sizes
ASME B16.5	Pipe flanges and flanged fittings	NPS 1/2 - 24, Class 150-2500
ASME B16.47	Large diameter flanges (Series A & B)	NPS 26 - 60, Class 75-900
ASME B16.36	Orifice flanges	Class 300-2500
ASME B16.48	Spectacle blinds, spades, spacers	Class 150-2500
ASME B16.1	Cast iron flanges	-
API 6A	Wellhead/Christmas tree equipment	Material classes AA-HH
API 6B/6BX	RTJ and extreme high-pressure flanges	-
EN 1092-1	European pipe flanges	Type 01, 05, 11, 13

Design notes:

• MSS SP-44: Use for pipeline flanges with materials not listed in ASME B16.5
• Unlisted materials: Design per ASME Section VIII Div 1, Appendix 2

Material Specifications

ASTM Standard	Material Type
A105	Carbon steel (high temp)
A182	Alloy and stainless steel
A350	Low-temperature carbon steel

Dimensional Tolerances

Standard	Key Tolerances
ASME B16.5/B16.47	OD (±1.6mm for NPS 24), thickness, bore diameter, bolt hole spacing, surface finish
API 6A	PSL requirements, material classes, temperature ratings
API 6B/6BX	RTJ groove depth/width, pitch diameter, gasket surface finish

Flange Materials

General Overview

Key material grades must match the pipe material:

Application	Flange Material	Matching Pipe
High-temperature CS	ASTM A105	A53, A106, API 5L
Low-temperature CS	ASTM A350 LF1/2/3	A333
High-yield CS (pipelines)	ASTM A694	API 5L X42-X80
Alloy steel	A182 F1 to F91	A335 P-grades
Stainless steel	A182 F304/316/321	A312
Duplex/Super Duplex	A182 F51/F53/F55	A790/A928

General requirements:

• Flanges shall be castings, forgings, or plates
• Bolting per ASME B16.5, Table 1B
• Pipeline flanges: carbon equivalency per ASME B31.4/B31.8

Material Selection by Service

Material	Temperature Range	Notes
A105 (CS)	-29°C to 425°C	Standard high-temp CS
A350 LF2 (CS)	Below -29°C	Impact tested per B31.3
A694 (High-yield CS)	-	Matches API 5L X42-X65
A182 F11 (Alloy)	To 593°C	1¼Cr-½Mo
A182 F5 (Alloy)	To 650°C	5Cr-½Mo
Cast iron (A126 Cl.B)	-	Fire systems only; FF face required

Material Restrictions

• Cast iron flanges: Not permitted in process piping within plant battery limits
• Carbon steel: Max service temperature 425°C
• NACE service: Specify “NACE MR0175” for H₂S environments

Pro Tip

For stainless steel flanges in offshore or coastal environments, always request dual certification (e.g., A182 F316/F316L). The “L” grade gives you weldability without re-testing, and the cost difference is negligible.

SS interchangeability: 304L/316L may substitute for 304/316 if physical requirements are met. Types 321 and 347 are compatible.

Flanges Marking Standard

Flange markings ensure traceability and proper selection. Required markings per ASME/API:

Marking	Information
Manufacturer	Name or trademark
Material	ASTM grade (e.g., A105, A182 F304)
Pressure Class	150, 300, 600, 900, 1500, 2500
Size	NPS or DN
Bore	If different from nominal
Heat Code	Traceability to material batch and test records
Standard	ASME B16.5, B16.47, API 6A, etc.
Facing	RF, FF, RTJ (if applicable)
Special	”NACE” for sour service (MR0175/ISO 15156)

Flange Bolting Chart

Correct stud bolts and nuts are required for leak-free flange connections. The bolting chart below specifies the number, diameter, and length of bolts for each flange size and pressure class.

Bolting Essentials

• Use correct bolt material compatible with flange and service environment
• Follow proper tightening sequence (cross-pattern)
• Apply specified torque values-over-tightening damages gaskets; under-tightening causes leaks

Bolting Chart (Class 150 to 600)

NPS	CL 150 # Bolts	CL 150 Dia.	CL 150 Len.	CL 300 # Bolts	CL 300 Dia.	CL 300 Len.	CL 400 # Bolts	CL 400 Dia.	CL 400 Len.	CL 600 # Bolts	CL 600 Dia.	CL 600 Len.
1/2	4	1/2	2 1/4	4	1/2	2 1/2	4	1/2	3	4	1/2	3
3/4	4	1/2	2 1/2	4	5/8	3	4	5/8	3 1/2	4	5/8	3 1/2
1	4	1/2	2 1/2	4	5/8	3	4	5/8	3 1/2	4	5/8	3 1/2
1 1/4	4	1/2	2 3/4	4	5/8	3 1/4	4	5/8	3 3/4	4	5/8	3 3/4
1 1/2	4	1/2	2 3/4	4	3/4	3 1/2	4	3/4	4 1/4	4	3/4	4 1/4
2	4	5/8	3 1/4	8	5/8	3 1/2	8	5/8	4 1/4	8	5/8	4 1/4
2 1/2	4	5/8	3 1/2	8	3/4	4	8	3/4	4 3/4	8	3/4	4 3/4
3	4	5/8	3 1/2	8	3/4	4 1/4	8	3/4	5	8	3/4	5
3 1/2	8	5/8	3 1/2	8	3/4	4 1/4	8	7/8	5 1/2	8	7/8	5 1/2
4	8	5/8	3 1/2	8	3/4	4 1/2	8	7/8	5 1/2	8	7/8	5 3/4
5	8	3/4	3 3/4	8	3/4	4 3/4	8	7/8	5 3/4	8	1	6 1/2
6	8	3/4	4	12	3/4	4 3/4	12	7/8	6	12	1	6 3/4
8	8	3/4	4 1/4	12	7/8	5 1/2	12	1	6 3/4	12	1 1/8	7 1/2
10	12	7/8	4 1/2	16	1	6 1/4	16	1 1/8	7 1/2	16	1 1/4	8 1/2
12	12	7/8	4 3/4	16	1 1/8	6 3/4	16	1 1/4	8	20	1 1/4	8 3/4
14	12	1	5 1/4	20	1 1/8	7	20	1 1/4	8 1/4	20	1 3/8	9 1/4
16	16	1	5 1/4	20	1 1/4	7 1/2	20	1 3/8	8 3/4	20	1 1/2	10
18	16	1 1/8	5 3/4	24	1 1/4	7 3/4	24	1 3/8	9	20	1 5/8	10 3/4
20	20	1 1/8	6 1/4	24	1 1/4	8	24	1 1/2	9 1/2	24	1 5/8	11 1/4
22	20	1 1/4	6 1/2	24	1 1/2	8 3/4	24	1 5/8	10	24	1 5/8	12
24	20	1 1/4	6 3/4	24	1 1/2	9	24	1 3/4	10 1/2	24	1 7/8	13

NPS	CL 900 # Bolts	CL 900 Dia.	CL 900 Len.	CL 1500 # Bolts	CL 1500 Dia.	CL 1500 Len.	CL 2500 # Bolts	CL 2500 Dia.	CL 2500 Len.
1/2	4	3/4	4 1/4	4	3/4	4 1/4	4	3/4	4 3/4
3/4	4	3/4	4 1/2	4	3/4	4 1/2	4	3/4	5
1	4	7/8	5	4	7/8	5	4	7/8	5 1/2
1 1/4	4	7/8	5	4	7/8	5	4	1	6
1 1/2	4	1	5 1/2	4	1	5 1/2	4	1 1/8	6 3/4
2	8	7/8	5 3/4	8	7/8	5 3/4	8	1	7
2 1/2	8	1	6 1/4	8	1	6 1/4	8	1 1/8	7 3/4
3	8	7/8	5 3/4	8	1 1/8	7	8	1 1/4	8 3/4
3 1/2	-	-	-	-	-	-	-	-	-
4	8	1 1/8	6 3/4	8	1 1/4	7 3/4	8	1 1/2	10
5	8	1 1/4	7 1/2	8	1 1/2	9 3/4	8	1 3/4	11 3/4
6	12	1 1/8	7 1/2	12	1 3/8	10 1/4	8	2	13 1/2
8	12	1 3/8	8 3/4	12	1 5/8	11 1/2	12	2	15
10	16	1 3/8	9 1/4	12	1 7/8	13 1/4	12	2 1/2	19 1/4
12	20	1 3/8	10	16	2	14 3/4	12	2 3/4	21 1/4
14	20	1 1/2	10 3/4	16	2 1/4	16	-	-	-
16	20	1 5/8	11 1/4	16	2 1/2	17 1/2	-	-	-
18	20	1 7/8	12 3/4	16	2 3/4	19 1/4	-	-	-
20	20	2	13 3/4	16	3	21 1/4	-	-	-
22	-	-	-	-	-	-	-	-	-
24	20	2 1/2	17 1/4	16	3 1/2	24 1/4	-	-	-

Flange Weights

Class 150

NPS(Inches)	DN(mm)	FLANGE OD(mm)	THICKNESS(mm)	SLIP-ON(KGS)	WELD NECK (KGS)	BLIND FLANGE (KGS)
1/ 2	15	88.9	11.2	0.4	0.5	0.4
3/ 4	20	98.6	12.7	0.6	0.7	0.6
1	25	108	14.2	0.8	1	0.9
1 1/ 4	32	117.3	15.7	1	1.3	1.2
1 1/ 2	40	127	17.5	1.3	1.7	1.5
2	50	152.4	19.1	2.1	2.6	2.4
2 1/ 2	65	177.8	22.4	3.3	4.1	3.9
3	80	190.5	23.9	3.9	4.9	4.9
3 1/ 2	90	215.9	23.9	4.8	6.1	6.2
4	100	228.6	23.9	5.3	6.8	7.0
5	125	254	23.9	6.1	8.6	8.6
6	150	279.4	25.4	7.5	10.6	11.3
8	200	342.9	28.4	12.1	17.6	19.6
10	250	406.4	30.2	16.5	24	28.6
12	300	482.6	31.8	26.2	36.5	43.2
14	350	533.4	35.1	34.6	48.4	58.1
16	400	596.9	36.6	44.8	60.6	76.1
18	450	635	39.6	48.9	68.3	93.7
20	500	698.5	42.9	61.9	84.5	122.0
24	600	812.8	47.8	86.9	115	185.0

Class 300

NPS(Inches)	DN(mm)	FLANGE OD(mm)	THICKNESS(mm)	SLIP ON(KGS)	WELD NECK (KGS)	BLIND(KGS)
1/ 2	15	95.3	14.2	0.6	0.8	0.6
3/ 4	20	117.3	15.7	1.1	1.3	1.1
1	25	124	17.5	1.4	1.5	1.4
1 1/ 4	32	133.4	19.1	1.7	2.0	1.8
1 1/ 2	40	155.5	20.6	2.5	2.9	2.7
2	50	165.1	22.4	2.9	3.4	3.2
2 1/ 2	65	190.5	25.4	4.3	5.2	4.9
3	80	215.9	28.4	5.9	6.9	6.8
3 1/ 2	90	228.6	30.2	7.3	8.7	8.7
4	100	254	31.8	9.6	11.2	11.5
5	125	279.4	35.1	12.3	15.1	15.6
6	150	317.5	36.6	15.6	19.1	20.9
8	200	381	41.1	24.2	29.9	34.3
10	250	444.5	47.6	34.1	42.7	53.3
12	300	520.7	50.8	49.8	61.8	78.8
14	350	584.2	53.8	69.9	85.8	105.0
16	400	647.7	57.2	88.1	106.0	137.0
18	450	711.2	60.5	109.0	131.0	175.0
20	500	774.7	63.5	134.0	158.0	221.0
24	600	914.4	70.0	201.0	230.0	339.0

Class 600

NPS(Inches)	DN(mm)	FLANGE OD(mm)	THICKNESS(mm)	SLIP-ON(KGS)	WELD NECK (KGS)	BLIND(KGS)
1/ 2	15	95.3	14.2	0.7	0.9	0.8
3/ 4	20	117.3	15.7	1.3	1.5	1.3
1	25	124.0	17.5	1.5	1.8	1.6
1 1/ 4	32	133.4	20.6	2	2.5	2.2
1 1/ 2	40	155.4	22.4	3	3.5	3.3
2	50	165.1	25.4	3.6	4.4	4.2
2 1/ 2	65	190.5	28.4	5.3	6.4	6.1
3	80	209.6	31.8	7	8.5	8.4
3 1/ 2	90	228.6	35.1	8.8	10.7	11
4	100	273.1	38.1	14.5	17.4	17.3
5	125	330.2	44.5	24.4	29.2	29.4
6	150	355.6	47.8	28.7	34.9	36.1
8	200	419.1	55.6	43.4	53.9	58.9
10	250	508	63.5	70.3	86.5	97.5
12	300	558.8	66.5	84.2	103.0	124
14	350	603.3	69.9	98.7	122.0	151
16	400	685.8	76.2	142.0	170.0	214
18	450	743	82.6	173.0	204.0	272
20	500	812.8	88.9	220.0	254.0	349.0
24	600	939.8	101.6	312.0	358.0	533.0

Class 900

NPS(Inches)	DN(mm)	FLANGE OD(mm)	THICKNESS(mm)	SLIP-ON(KGS)	WELD NECK (KGS)	BLIND(KGS)
1/ 2	15	120.7	22.4	1.8	1.9	1.8
3/ 4	20	130	25.4	2.4	2.6	2.4
1	25	149.4	28.4	3.6	3.7	3.6
1 1/ 4	32	158.8	28.4	4	4.3	4.1
1 1/ 2	40	177.8	31.8	5.5	5.9	5.8
2	50	215.9	38.1	10.2	10.8	10
2 1/ 2	65	244.3	41.1	13.9	15	14
3	80	241.3	38.1	11.6	13.7	13.1
4	100	292.1	44.5	19.7	22.5	26.9
5	125	349.3	50.8	31.9	37.4	36.5
6	150	381	55.6	41.1	47.7	47.4
8	200	469.9	63.5	70.7	81.3	82.5
10	250	546.1	69.9	101.0	119	122.0
12	300	609.6	79.2	133.0	157.0	173.0
14	350	641.4	85.6	153.0	180.0	206.0
16	400	704.9	88.9	185.0	217.0	259.0
18	450	787.4	101.6	258.0	292.0	367.0
20	500	857.3	108.0	317.0	362.0	463.0
24	600	1041.4	139.7	606.0	665.0	876.0

Class 1500

NPS(Inches)	DN(mm)	FLANGE OD(mm)	THICKNESS(mm)	SLIP-ON(KGS)	WELD NECK (KGS)	BLIND(KGS)
1/ 2	15	120.7	22.4	1.8	1.9	1.8
3/ 4	20	130	25.4	2.4	2.6	2.4
1	25	149.4	28.4	3.6	3.7	3.6
1 1/ 4	32	158.8	28.4	4	4.3	4.1
1 1/ 2	40	177.8	31.8	5.5	5.9	5.8
2	50	215.9	38.1	10.2	10.8	10.1
2 1/ 2	65	244.3	41.1	13.9	15	14
3	80	266.7	47.8		19.9	19.1
4	100	311.2	53.8		29.9	29.9
5	125	374.7	73.2		55.4	58.4
6	150	393.7	82.6		68.4	71.8
8	200	482.6	91.9		117	122
10	250	584.2	108		194.0	210.0
12	300	673.1	124		288.0	316.0
14	350	749.3	133.4		380.0	420
16	400	825.5	146.1		485.0	558.0
18	450	914.4	162.1		644.0	760.0
20	500	984.3	177.8		775.0	965.0
24	600	1168.4	203.2		1232.0	1558.0

Class 2500

NPS(Inches)	DN(mm)	FLANGE OD(mm)	THICKNESS(mm)	SLIP-ON(KGS)	WELD NECK (KGS)
1/ 2	15	133.4	30.2	3.1	3.0
3/ 4	20	139.7	31.8	3.7	3.5
1	25	158.8	35.1	5.2	5.0
1 1/ 4	32	184.2	38.1	7.7	7.4
1 1/ 2	40	203.2	44.5	10.9	10.4
2	50	235	50.8	16.2	15.6
2 1/ 2	65	266.7	57.2	23.7	22.6
3	80	304.8	66.5	36.2	34.8
4	100	355.6	76.2	55.3	53.9
5	125	419.1	91.9	92.5	90.8
6	150	482.6	108	143.0	141
8	200	552.5	127	215.0	214
10	250	673.1	165.1	406.0	411.0
12	300	762	184.2	572.0	592.0

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Frequently Asked Questions

What is a pipe flange?

A flange is a plate or ring that connects pipe sections or attaches pipes to valves, pumps, and equipment. When bolted together with a gasket between them, flanges create a leak-proof, demountable joint. They are manufactured per ASME B16.5 (NPS 1/2 to 24) or ASME B16.47 (NPS 26 to 60).

What are the main types of flanges?

The six main flange types are: Weld Neck (best for high pressure, with a tapered hub for stress distribution), Slip-On (slides over pipe, fillet welded), Blind (closes pipe ends or vessel nozzles), Socket Weld (small bore, high pressure), Threaded (no welding required, for non-critical service), and Lap Joint (used with stub ends for easy alignment and disassembly).

What does flange class 150, 300, 600 mean?

Flange class (150, 300, 600, 900, 1500, 2500) indicates the pressure-temperature rating per ASME B16.5. Higher class = higher pressure capacity. Class 150 handles ~285 psi at ambient, Class 300 ~740 psi, Class 600 ~1480 psi. The actual allowable pressure depends on the flange material and operating temperature ; it decreases as temperature increases, following the P-T rating tables.

What is the difference between RF and RTJ flange faces?

RF (Raised Face) is the most common face type, used with spiral wound or sheet gaskets, suitable for applications up to Class 600. RTJ (Ring Type Joint) has a machined groove for metal ring gaskets and is used for high-pressure (Class 600 and above) and high-temperature applications that require metal-to-metal sealing for maximum leak resistance.

What flange material should I use?

Common materials: A105 (carbon steel, standard temperature), A350 LF2 (carbon steel, low temperature down to -46°C), A182 F304/F316 (stainless steel for corrosion resistance), A182 F11/F22 (alloy steel for high temperature), A182 F51/F53 (duplex/super duplex for corrosive and high-strength service). Selection depends on the process fluid, temperature range, and corrosion requirements.