Lap Joint Flanges

INTRO

Lap Joint Flanges comprise a stub end butt-welded to the pipe and a loose backing flange that slides over it, these flanges address the need for frequent dismantling and maintenance.

They solve issues related to welding complexities and material compatibility by allowing the backing flange to be made from a more affordable material like carbon steel, while the stub end matches the pipe’s grade.

This design not only facilitates easier assembly and disassembly but also reduces costs without compromising functionality. Ideal for systems where space is constrained and frequent access is necessary,

Standard：
GB/T9119-2010 HG/T20592-2009 HG/T20615-2009 DIN ASME BS JIS

Material：
304：F304 S30408 S30408II S30408III S30400
304L：S30403 S30403II 30403 30403II
316L：31603 31603II 31603III
2507（F53） 2205（F51） TP310S（06Cr5Ni20） TP321（06Cr18Ni11） S22053III=2205III T321-32168=321

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DESCRIPTION DOWNLOAD

Lap joint flanges feature a flat face and are always used in conjunction with a stub end.
A lap joint flange slips over the pipe and seats on the back of the stub end and the two are kept together by the pressure of the bolts.

Manufacturer Intro

Longan Flange is a stainless steel lap joint flange manufacturer specializing in flange-and-stub-end assemblies for systems requiring frequent disassembly. Our lap joint flanges with stub ends cover DN15 to DN600 in Class 150, 300, and 600 per ASME B16.5 and DIN. The stub end is available in SS304, 304L, 316L, duplex 2205, and super duplex 2507, while the backing flange can be manufactured in a more economical stainless grade to reduce project cost. Lap joint flange dimensions for each pressure class are listed below. All components forged and machined in-house. Quote within 24 hours.

Stub End + Backing Flange: Match the Right Material to Each Part

The core advantage of a lap joint flange is that the two components don’t need to be made from the same material. Only the stub end contacts the process media — the backing flange simply provides bolting force. This lets you specify a corrosion-resistant grade only where it’s actually needed, reducing overall material cost without compromising performance.

Stub End (Wetted Part)

Butt-welded to the pipe and in direct contact with the process media. Available in SS304, 304L, 316L, duplex 2205, and super duplex 2507 to match your pipe’s corrosion resistance requirements.

Backing Flange (Structural Part)

Slides freely over the stub end and provides the bolting force to seal the joint. Can be manufactured in a more economical grade, since it never contacts the media — reducing total assembly cost.

100% Traceability: Our Multi-Stage Quality Control Process

PMI Analysis

100% Spectral detection

Forging Control

Grain flow optimization

CNC Precision

Dimensional & fit accuracy

Final Inspection

Stub end / flange fit check

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MOQ from 1 Piece

Standard sizes available from a single piece — ideal for trial orders and replacement purchases.

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Quote Within 24 Hours

Technical quotations and drawing reviews delivered within 24 hours of your inquiry.

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ISO 9001 / ISO 14001 / ISO 45001 / CE / PED

Manufactured in a certified facility, fully compliant with ASME B16.5 and EN 1092-1. MTC per EN 10204 3.1 supplied with every order.

Where Lap Joint Flanges Are Specified

Where hygiene, traceability, and easy disassembly matter, Longan produces lap joint flanges (alongside slip-on and socket weld flanges) in SS304/316L with smooth machined finishes and full MTC documentation — well suited to piping systems that require frequent inspection, cleaning, or reconfiguration.

FAQ

What is the advantage of a lap joint flange with stub end?

A lap joint flange with stub end allows the backing flange to rotate freely for easy bolt-hole alignment, simplifying installation in tight spaces. The stub end can be made from a corrosion-resistant grade (such as 316L or duplex 2205) while the backing flange uses a more economical material, reducing total project cost.

What sizes and classes are available?

We manufacture stainless steel lap joint flanges from DN15 to DN600 in Class 150, 300, and 600 per ASME B16.5. Matching stub ends are supplied in the same material grade. Lap joint flange dimensions for each class are listed in the specification tables above.

Do you supply the stub end together with the flange?

Yes. We manufacture both the lap joint flange and the stub end in-house, ensuring precise dimensional compatibility. Both components are forged from certified stainless steel at our Wenzhou facility.

###CLASS 150###

Nominal size		Steel Pipe outer diameter	Flange outer diameter		Connection size				flange thickness	Flange inner diameter	Flange neck big end	Flange height	Rounded comers	Butt welding ring		Theotical weight
DN	NPS	A	D		Bolt hole center circle diameter K	Bolt hole diameter L		Bolt hole quantity n(pieces)	C	B	N	H	R1	high H	outer diameter d	Kg
15	1/2	21.3	90	88.9	60.3	16	15.7	4	11.2	22.9	30	16	3	51	34.9	0.38
20	3/4	26.9	100	98.6	69.9	16	15.7	4	12.7	28.2	38	16	3	51	42.9	0.55
25	1	33.7	110	108	79.4	16	15.7	4	14.3	34.9	49	17	3	51	50.8	0.76
32	1 1/4	42.4	115	117.3	88.9	16	15.7	4	15.9	43.7	59	21	5	51	63.5	1.01
40	1 1/2	48.3	125	127	98.4	16	15.7	4	17.5	50.0	65	22	6	51	73.0	1.3
50	2	60.3	150	152.4	120.7	18	19.1	4	19.1	62.5	78	25	8	64	92.1	2.03
65	2 1/2	76.1	180	177.8	139.7	18	19.1	4	22.3	78.5/75.4	90	29	8	64	104.8	3.25
80	3	88.9	190	190.5	152.4	18	19.1	4	23.9	91.4	108	30	10	64	127.0	3.81
100	4	114.3	230	228.6	190.5	18	19.1	8	23.9	116.8	135	33	11	76	157.2	5.25
125	5	139.7	255	254	215.9	22	22.4	8	23.9	144.4	164	36	11	76	185.7	6.02
150	6	168.3	280	278.4	241.3	22	22.4	8	25.4	171.4	192	40	13	89	215.9	7.4
200	8	219.1	345	342.9	298.58	22	22.4	8	28.6	222.2	246	44	13	102	269.9	12.1
250	10	273	405	406.4	362.0	26	25.4	12	30.2	277.4	305	49	13	127	323.8	16.4
300	12	323.9	485	482.6	431.8	26	25.4	12	31.8	328.2	365	56	13	152	381.0	26.1
350	14	355.6	535	533.4	476.2	30	28.4	12	35.0	360.2	400	79	13	152	412.8	34.5
400	16	406.4	595	596.9	539.8	30	28.4	16	36.6	411.2	457	87	13	152	469.9	44.6
450	18	457	635	635	577.9	33	31.75	16	39.7	462.3	505	97	13	152	533.4	48.7
500	20	508	700	698.5	635.0	33	31.75	20	42.9	514.4	559	103	13	152	584.2	61.6
600	24	610	815	812.8	749.3	36	35.1	20	47.7	616.0	663	111	13	152	692.2	86.6

###CLASS 300###

Nominal size		Steel Pipe outer diameter	Flange outer diameter		Connection size				flange thickness	Flange inner diameter	Flange neck big end	Flange height	Rounded corners	Butt welding ring		Theotical weight
DN	NPS	A	D		Bolt hole center circle diameter K	Bolt hole diameter L		Bolt hole quantity n(pieces)	C	B	N	H	R1	high H	outer diameter d	Kg
15	1/2	21.3	95	88.9	66.7	16	15.7	4	14.3	22.9	38	22	3	51	34.9	0.62
20	3/4	26.9	115	98.6	82.6	18	15.7	4	15.9	28.2	48	25	3	51	42.9	1.1
25	1	33.7	125	108	88.9	18	15.7	4	17.5	34.9	54	27	3	51	50.8	1.33
32	1 1/4	42.4	135	117.3	98.4	18	15.7	4	19.1	43.7	64	27	5	51	63.5	1.65
40	1 1/2	48.3	155	127	114.3	22	15.7	4	20.7	50.0	70	30	6	51	73.0	2.44
50	2	60.3	165	152.4	127.0	18	19.1	8	22.3	62.5	84	33	8	64	92.1	2.83
65	2 1/2	76.1	190	177.8	149.2	22	19.1	8	25.4	78.5/75.4	100	38	8	64	104.8	4.25
80	3	88.9	210	190.5	168.3	22	19.1	8	28.6	91.4	117	43	10	64	127.0	5.78
100	4	114.3	255	228.6	200.0	22	19.1	8	31.8	116.8	146	48	11	76	157.2	9.55
125	5	139.7	280	254	235.0	22	22.4	8	35.0	144.4	178	51	11	76	185.7	12.2
150	6	168.3	320	279.4	269.9	22	22.4	12	36.6	171.4	206	52	13	89	215.9	15.5
200	8	219.1	380	342.9	330.2	26	22.4	12	41.3	222.2	260	62	13	102	269.9	24.1
250	10	273	445	406.4	387.4	30	25.4	16	47.7	277.1	321	95	13	254	323.8	34.4
300	12	323.9	520	482.6	450.8	33	25.4	16	50.8	328.2	375	102	13	254	381.0	50.4
350	14	355.6	585	533.4	514.4	33	28.4	20	54.0	360.2	425	111	13	305	412.8	70.9
400	16	406.4	650	596.9	571.5	36	28.4	20	57.2	411.2	483	121	13	305	469.9	89.5
450	18	457	710	635	628.6	36	31.75	24	60.4	462.3	533	130	13	305	533.4	111
500	20	508	775	698.5	685.8	36	31.75	24	63.5	514.4	587	140	13	305	584.2	137
600	24	610	915	812.8	812.8	42	35.1	24	69.9	616.0	702	152	13	305	692.2	204

###CLASS 600###

Nominal size		Steel Pipe outer diameter	Flange outer diameter		Connection size				flange thickness	Flange inner diameter	Flange neck big end	Flange height	Rounded corners	Butt welding ring		Theotical weight
DN	NPS	A	D		Bolt hole center circle diameter K	Bolt hole diameter L		Bolt hole quantity n(pieces)	C	B	N	H	R1	high H	outer diameter d	Kg
15	1/2	21.3	95	88.9	66.7	16	15.7	4	14.3	22.9	38	22	3	76	34.9	0.72
20	3/4	26.9	115	98.6	82.6	18	15.7	4	15.9	28.2	48	25	3	76	42.9	1.25
25	1	33.7	125	108	88.9	18	15.7	4	17.5	34.9	54	27	3	102	50.8	1.5
32	11/4	42.4	135	117.3	98.4	18	15.7	4	20.7	43.7	64	29	5	102	63.5	2
40	11/2	48.3	155	127	114.3	22	15.7	4	22.3	50.0	70	32	6	102	73.0	2.92
50	2	60.3	165	152.4	127.0	18	19.1	8	25.4	62.5	84	37	8	152	92.1	3.55
65	21/2	76.1	190	177.8	149.2	22	19.1	8	28.6	78.5/75.4	100	41	8	152	104.8	5.23
80	3	88.9	210	190.5	168.3	22	19.1	8	31.8	91.4	117	46	10	152	127.0	6.95
100	4	114.3	275	228.6	215.9	26	19.1	8	38.1	116.8	152	54	11	152	157.2	14.4
125	5	139.7	330	254	266.7	30	22.4	8	44.5	144.4	189	60	11	203	185.7	24.3
150	6	168.3	355	279.4	292.1	30	22.4	12	47.7	171.4	222	67	13	203	215.9	28.5
200	8	219.1	420	342.9	349.2	33	22.4	12	55.6	222.2	273	76	13	203	269.9	43.1
250	10	273	510	406.4	431.8	36	25.4	16	63.5	277.4	343	111	13	254	323.8	70.5
300	12	323.9	560	482.6	489.0	36	25.4	20	66.7	328.2	400	117	13	254	381.0	86.1
350	14	355.6	605	533.4	527.0	39	28.4	20	69.9	360.2	432	127	13	305	412.8	100
400	16	406.4	685	596.9	603.2	42	28.4	20	76.2	411.2	495	140	13	305	469.9	145
450	18	457	745	635	654.0	45	31.75	20	82.6	462.3	546	152	13	305	533.4	177
500	20	508	815	698.5	723.9	45	31.75	24	88.9	514.4	610	165	13	305	584.2	225
600	24	610	940	812.8	838.2	51	35.1	24	101.6	616.0	718	184	13	305	692.2	318

Frequently Asked Questions (FAQs)

What are lap joint flanges used for?

Applications where ease of alignment and assembly is crucial, such as in piping systems with frequent disassembly or inspection.

What are the benefits of using lap joint flanges?

Ease of installation and alignment, ability to accommodate pipe misalignment, and the flexibility of using a backing flange to match different pipe sizes or configurations.

How do lap joint flanges differ from other flange types?

Consist of two parts: a loose backing flange and a lap joint flange, allowing for easier alignment and adjustments without welding.