Know About Weld Neck Flanges

Weld neck flange is a type of flange with a neck and a round tube transition that is welded to the pipe. It is characterized by its resistance to deformation and good sealing performance. It is often used in pipelines that transport expensive, flammable, and explosive media. Weld neck flanges are used to connect pipes or equipment. It provides a strong, reliable seal under harsh conditions such as high temperature, high pressure, and large temperature and pressure fluctuations. It reduces stress concentration and facilitates pipe removal, maintenance, and replacement. Its unique tapered neck design effectively transfers stress and ensures the strength of the joint between the flange and the pipe.

 

Structural characteristics:

 

• Neck-Tapered Transition: This flange connects to the pipe wall through a tapered neck extending from the flange plate.

 

• Double-Sided Welding: This method uses double-sided welding to connect pipes, providing higher joint strength.

 

Functions of Weld Neck Flanges

 

Connection and Sealing: Weld neck flanges are used as fittings to connect pipes or valves. Their primary function is to provide a durable, leak-proof connection between the two surfaces. They are butt-welded to the pipe using a tapered hub, creating a connection with the same strength as the pipe itself.

 

High-Strength Structural Connection: The core advantage of a weld neck flange is that it is more than just a connector; it is a pipe component capable of withstanding extremely high loads and ensuring the structural integrity of the piping system. It smoothly transitions from a thick flange plate to a wall thickness that matches the pipe, creating a continuous, rigid structure. The long tapered neck increases the bending and torsional stiffness of the entire flange connection, effectively resisting deformation and maintaining flange face parallelism and bolt hole concentricity, thereby protecting the sealing surfaces from damage.

 

Suitable for Harsh Environments: Neck weld flanges can withstand pressure ratings from Class 150 to Class 2500 and even higher. They are also resistant to alternating stresses caused by temperature fluctuations, pressure surges, and mechanical vibration. Due to its exceptional sealing reliability and structural integrity, it is a mandatory requirement or preferred choice for pipeline systems transporting high-risk media such as toxic, highly flammable, explosive, highly corrosive, or radioactive materials, minimizing the risk of leakage.

 

Easy Inspection And Maintenance: The tolerance between the flange outer diameter and the bolt hole center distance is controlled to ±0.8 mm, facilitating disassembly and reassembly of the seal. Butt welds are one of the most mature and reliable forms of industrial welding. Positioned on the pipe body, away from the flange sealing surface and bolt holes, they provide ample welding clearance and facilitate full penetration.

 

Weld Neck Flange Types

 

Raised Face (RF): The raised face weld neck flange features a flat surface elevated slightly above the bolt circle. Sealing is achieved by compressing a soft or semi-metallic gasket between the two mating raised faces under bolt load. This is the most common type, effective where extreme leak tightness isn't paramount.

 

Ring Type Joint (RTJ): Characterized by a groove in its face designed to accommodate a solid metal ring gasket, the Ring Type Joint Weld Neck flange creates a robust metal-to-metal seal. Under bolt load, the ring gasket plastically deforms and seats tightly within the grooves of the mating flanges. This design has superior resistance to gasket blowout and the ability to maintain seal integrity under severe thermal cycling and vibration.

 

Flat Face (FF): The Flat Face Weld Neck flange has a sealing surface entirely flush with the bolt circle. It requires a full-face gasket covering both the sealing surface and bolt holes to distribute the bolt load across the entire flange face, preventing damage to brittle materials like cast iron. Primarily used for low-pressure applications, its key advantage is preventing flange fracture in weaker materials but it offers lower pressure capability and sealing performance compared to RF or RTJ types; crucially, it must never be mated with a raised face flange.

 

Tongue and Groove (T&G): Tongue and Groove Weld Neck flanges come in pairs: one flange has a raised, narrow ring ("tongue") and the other has a matching recess ("groove") machined onto their sealing faces. A flat gasket is placed within the groove, and compression occurs as the tongue enters the groove during bolting.

 

Male and Female (M&F): Male and female weld neck flanges also come in pairs. The male face has a raised, flat surface extending beyond the bolt circle, while the female face has a matching recess of the same depth. A flat gasket is placed within the female recess and compressed by the male face during bolting.

 

Weld Neck Flange Dimensions

 

The dimensions of weld neck flanges are not fixed but vary according to the nominal diameter (DN), pressure rating, and manufacturing standard. These dimensions typically include the flange's outer diameter, neck diameter, bolt hole diameter and center diameter, and the outer diameter of the connecting pipe. Specific parameters should be referenced to the applicable standard and specific model.

 

Common Dimensional Parameters

 

Nominal Size (DN): Indicates the nominal diameter of the flange and also represents the pipe size, such as DN100, DN150, etc.

 

Pressure Class: Indicates the pressure the flange can withstand, such as Class 150, Class 300, PN16, PN40, etc.

 

Flange Outside Diameter: The diameter of the flange itself.

 

Neck Size: The transition between the connecting pipe and the flange; its dimensions vary depending on the pipe diameter and standard.

 

Bolt Holes: Includes the number and diameter of the bolt holes, as well as the center diameter of the bolt holes.

 

Steel Pipe Outside Diameter (A): Refers to the outer diameter of the connecting pipe to the flange weld end, also known as the flange weld end outside diameter.

 

Class 150 Weld Neck Flange Dimensions - ASME B16.5

 

DN	NPS	Flange outer diameter	Flange inner diameter	Flange thickness (min.)	Flange height	Bevel outer diameter	Neck Diameter	Seal face diameter	Seal face height	Number of bolt holes	Screw hole diameter	Bolt hole center diameter
A	B	C	D	E	F	G	f	n	I	J
15	1/2	90	15.8	9.6	46	21.3	30	34.9	2	4	15.7	60.3
20	3/4	100	20.9	11.2	51	26.7	38	42.9	2	4	15.7	69.9
25	1	110	26.6	12.7	54	33.4	49	50.8	2	4	15.7	79.4
32	1 ¼	115	35.1	14.3	56	42.2	59	63.5	2	4	15.7	88.9
40	1 ½	125	40.9	15.9	60	48.3	65	73.0	2	4	15.7	98.4
50	2	150	52.5	17.5	62	60.3	78	92.1	2	4	19.1	120.7
65	2 ½	180	62.7	20.7	68	73.0	90	104.8	2	4	19.1	139.7
80	3	190	77.9	22.3	68	88.9	108	127.0	2	4	19.1	152.4
90	3 ½	215	90.1	22.3	70	101.6	122	139.7	2	8	19.1	177.8
100	4	230	102.3	22.3	75	114.3	135	157.2	2	8	19.1	190.5
125	5	255	128.2	22.3	87	141.3	164	185.7	2	8	22.4	215.9
150	6	280	154.1	23.9	87	168.3	192	215.9	2	8	22.4	241.3
200	8	345	202.7	27.0	100	219.1	246	269.9	2	8	22.4	298.5
250	10	405	254.6	28.6	100	273.0	305	323.8	2	12	25.4	362.0
300	12	485	304.8	30.2	113	323.8	365	381.0	2	12	25.4	431.8
350	14	535	-	33.4	125	355.6	400	412.8	2	12	28.4	476.3
400	16	595	-	35.0	125	406.4	457	469.9	2	16	28.4	539.8
450	18	635	-	38.1	138	457.0	505	533.4	2	16	31.8	577.9
500	20	700	-	41.3	143	508.0	559	584.2	2	20	31.8	635.0
600	24	815	-	46.1	151	610.0	663	692.2	2	20	35.1	749.3

 

Slip On Flange vs Weld Neck

 

There are two types of neck flanges: butt-weld neck flanges and slip on neck flanges. There are some subtle differences between them. Let's learn more about them.

 

Introduction

 

Butt weld neck flange is a flange with a tapered neck that is butt-welded to a cylinder or pipe. It is a one-piece flange.

 

The full name of a slip on neck flange is a slip-on neck steel pipe flange. It allows pipes and fittings to be extended into the flange and connected to equipment or pipelines via fillet welds.

 

Differences

 

Different Weld Types:

 

The welds of butt weld neck flanges can be radiographically inspected, while those of slip-on neck flanges cannot. The weld between a butt-weld neck flange and the pipe is a girth weld, while the weld between a slip-on neck flange and the pipe is a fillet weld.

 

A butt weld is a single butt girth weld, while a slip on weld is two fillet girth welds. The difference between the two is that the weld between a butt-weld neck flange and the pipe is a Class B seam, while the weld between a slip-on neck flange and the pipe is a Class C seam.

 

Material Difference:

 

Neck weld neck flanges are typically machined from forged steel. Slip on neck flanges, on the other hand, are machined from ordinary steel plates of the required thickness.

 

Nominal Pressure Difference:

 

Neck weld neck flanges have a higher nominal pressure rating of 1-25 MPa. Slip on neck flanges have a lower nominal pressure rating of 0.6-4 MPa compared to neck weld flanges.

 

Weld Neck Flange Uses

 

Weld neck flanges are primarily used in piping systems subject to high temperatures, high pressures, low temperatures, and large pressure or temperature fluctuations. These flanges are particularly useful in harsh operating conditions requiring high stress, repeated loads, and extremely tight sealing requirements, such as in the petrochemical industry, where pipelines transport flammable, explosive, or toxic media.

 

High-pressure, high-temperature, and low-temperature pipe connections: The tapered neck of the weld neck flange provides an effective transition, achieving excellent strength and rigidity, making it suitable for piping systems subject to high pressure and high temperature media.

 

Extreme temperature and pressure fluctuations: Its structural design also reduces stress concentration, allowing it to better withstand stress changes and maintain sealing performance during large temperature and pressure fluctuations.

 

Pipelines carrying flammable, explosive, and toxic media: In the petroleum and chemical industries, these flanges are often used in pipeline connections transporting flammable, explosive, and toxic media to ensure safety and reliability.