When to use lap joint flange?

What is a lap joint flange?

A Lap Joint Flange is a two piece device that is much like a weld-neck flange but also like a loose slip-on flange. One piece is a sleeve called a ‘Stub-end” and is shaped like a short piece of pipe with a weld bevel on one end and a narrow shoulder on the other end called the hub. The hub is the same outside diameter as the raised face (gasket contact surface) of a weld neck flange. The thickness of the hub is normally about ¼” to 3/8″. The back face of the hub has a rounded transition (or inside fillet) that joins the hub to the sleeve.
The other piece of a Lap Joint Flange is the backing flange. This flange has all the same common dimensions (O.D., bolt circle, bolt hole size, etc.) as any other flange however it does not have a raised face. One side, the backside, has a slight shoulder that is square cut at the center or pipe hole. The front side has flat face and at the center hole an outside fillet to match the fillet of the “Stub-end” piece. The flange part of the Lap-joint flange assembly is slipped on to the stub-end prior to the sleeve being welded to the adjoining pipe or fitting. The flange itself is not welded or fixed in any way. It is free to spin for proper alignment with what ever it is joining to.
The “Stub-end” can normally be purchased in two lengths. There is a short version, about 3″ long and a long version of about 6″ long. It is prudent for the piping designer to know which version is in the piping specification.
Because of it’s two piece configuration, the Lap Joint Flange offers a way to cut cost or simplify work. The cost saving comes when the piping system requires a high cost alloy for all “wetted” parts to reduce corrosion. The sleeve or Stub-end can be the required higher cost alloy but the flange can be the lower cost forged carbon steel.
The lap joint flange can be rotated which can be useful when fixing issues with bolt hole alignment. When designing a piping system, lap joint flanges should not be considered solely to alleviate poor alignment during construction. Good design practice should not need to include poor construction quality. However, if the piping needs to be frequently dismantled for inspection or cleaning, consideration should be made for lap joint flanges. They give the ability to swivel flanges and to align bolt holes which simplifies the assembly of large diameter or unusually stiff piping.
Lap joint flanges are usually used in low pressure applications and are not suitable when there are high loads on the flange pair. Some types of piping require the use of lap joint flanges. For example, metallic pipe that has been plastic lining may have lap joint flanges.
Using lap joint flanges might be an option for saving costs when the piping is made of exotic materials. By using a lap joint flange, the wetted materials would consist of the exotic materials and the flange would be carbon steel. Since the flange doesn’t ever come in contact with the process fluid, it would not be affected by the fluids.

Buying tips
Certain factors to consider are as follows:
  • Size
  • Design Standard
  • Material
  • Normal Pressure
  • Face Type
  • Durability
  • Corrosion Resistant

Key Features:

  • Recommended in applications that require frequent dismantling of the flanges and pipe.
  • Used with a matching stub-end insert.
  • Can rotate to allow for an easy alignment of bolt holes.
  • Not recommended in extreme or high-pressure temperature applications.

Advantages of Lap Joint Flanges:
Lap Joint flanges have certain special advantages over other flanges. These are as follows:

  • Lap Joint flanges have the freedom to swivel around the pipe. This facilitates the lining up of opposing flange bolt holes.
  • Lack of contact with the fluid inside the pipe leads to the greater durability of these flanges.
  • In piping systems which corrode or erode quickly, the lap joint flanges may be salvaged for re-use.
  • The pressure-holding capacity of lap joint flanges is little. But it is better than that of slip-on flanges.
  • The Lap Joint works as a backing ring on the stub.
  • The main advantage of lap joint flange is that the bolt holes can be aligned with the matching flange after the welds have been completed.
  • They allow rotational alignment capability.

The lap joint flange is practically identical to a slip-on flange except it has a radius at the intersection of the bore and flange face. This radius is necessary to have the flange accommodate a lap joint stub end. Normally, a lap joint flange and a lap joint stub end are mated together is an assembly system.

Standards, Dimensions & Weight
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  • ASME/ANSI B16.5 Class 150 LJ
  • ASME/ANSI B16.5 Class 300 LJ
  • ASME/ANSI B16.5 Class 600 LJ
  • ASME/ANSI B16.5 Class 900 LJ
  • ASME/ANSI B16.5 Class 1500 LJ
  • ASME/ANSI B16.5 Class 2500 LJ

Standards:  ANSI, MSS, API, AWWA, DIN, JIS, BS and GB.

Process:  free forging, plate cutting.

Sizes range:  1/2″ to 64″ (and much larger size according to customers’ demands).

Printing:  Carbon steel and alloy steel with yellow print, black print, oil or zinc.

Packing:  in wooden cases, wooden pallets or as per customers’ special requirement, Standard Export Package.

* We can also manufacture special application flanges and forgings that customers require.

Lap joint flange’s drawing:
20171223102437 59331 - When to use lap joint flange?
1. Lap Joint flange 2. Stub End 3. Butt weld 4. Pipe or Fitting

These flanges are nearly identical to a Slip On flange with the exception of a radius at the intersection of the flange face and the bore to accommodate the flanged portion of the Stub End.

Lap Joint Flange Material:
  • Cabon steel: A105, A350 LF2
  • Stainless steel: A182 F304/F304L/F316/F316L/F321
  • Alloy Steel: ASTM A 182, GR F1, F11, F22, F5, F9, F91

  • Nickel Alloys: Monel 400 & 500, Inconel 600 & 625, Incolloy 800, 825, Hastelloy C22,C276

Dimensional Tolerances of Lap Joint Flanges ASME B16.5

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Dimensions are in millimeters unless otherwise indicated.

OUTSIDE DIAMETER

≤ 24 = 1.6 mm
> 24 = ± 3.2 mm

INSIDE DIAMETER

 10 = ± 0.8 mm
 12 = + 1.6 mm / – 0 mm

DIAMETER OF CONTACT FACE

1.6 mm Raised Face = ± 0.8 mm
6.35 mm Raised Face
Tongue & Groove / Male-Female = ± 0.4 mm

DRILLING

Bolt Circle = 1.6 mm
Bolt Hole Spacing = ± 0.8 mm
Eccentricity of Bolt Circle with Respect to Facing
 2½ = 0.8 mm max.
 3 = 1.6 mm max.

DIAMETER OF COUNTERBORE

Same as for Inside Diameter

OUTSIDE DIAMETER OF HUB

 12 = + 2.4 mm / – 1.6 mm
 14 = ± 3.2 mm

THICKNESS

 18 = + 3.2 mm / – 0
 20 = + 4.8 mm / – 0

LENGTH THRU HUB

 18 = + 3.2 mm / – 0.8 mm
 20 = + 4.8 mm / – 1.6 mm

When to use lap joint flange?

lap joint flange 2 - When to use lap joint flange?

1. Because of the structure of a Lap joing flange,  it can swivel around the stub end and pipe lining.    When the piping system is assembe and disassemble frequently,  it is better to use a Lap joint flange. It means the flange can work even the two flanges bolt holes are misalignment.
2. In a corrosive situation, the flange joints need to be exchange very soon. To a lap joint flange, only the stub end is touch with the pipe and fluid, the backing flange no need to touch it .  It means you could only replace the stub end , no need to replace the backing flange, so the lap joint flange can decrease the cost of the piping systems.
3. The backing flange and the stub end is seperated, so we can use two different materials for the two pieces. It can work for more complicated application. 

Lap Joint Flange Uses
Economy
Because a lap joint flange has a two piece configuration, it offers a way to cut cost when piping systems requires a high cost alloy for all “wetted” parts to reduce corrosion. In this situation, it is only required for the stub-end to be can be made of the higher cost corrosion-resistant material, where the flange itself can be the produced from lower cost steel.

Ease of Work
By using lap joint flanges, work can be simplified in situations that require frequent and rapid disassemble and assembly during the operation of a plant. The ability to spin that backing flange compensates for misalignment of the bolt holes during assembly.

Physical Features
Flange
1.  The backside, has a slight shoulder that is square cut at the center or pipe hole
2.  The front side has a flat face with a filleted (rounded) center hole to match the filleted back face of the stub end. Here the stub end will wrap tightly around the center hole of the flange.

20171223103021 14795 - When to use lap joint flange?

Stub End
1.  Shaped like a short piece of pipe with a weld bevel on one. This portion of the stub end is also called the sleeve.
2.  Narrow shoulder on the flange facing end called is the hub. The back face of the hub has a rounded transition (or inside fillet) that joins the hub to the sleeve.

20171223103056 30510 - When to use lap joint flange?

Source: China Lap Joint Flange Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@pipelinedubai.com

Please notice that you might be interested in the other technical articles we’ve published:

  • What are Lap Joint Flanges

  • What are Blind Flanges

  • What are Specialty Flanges

  • How to get high quality flanges

  • The Differences between socket welding flanges and butt welding flanges

  • An Introduction to Large Diameter Carbon Steel Flanges

  • Surface Treatments of Stainless Steel Flanges

  • What Is the Lap Joint Flange with Welding Neck Collar

  • How to get high quality alloy steel pipes

  • How to get high quality pipe fittings

  • How to get high quality bellow expansion joints

  • DIFFERENCE BETWEEN A PIPE ELBOW AND A PIPE BEND

  • HOW TO GET HIGH QUALITY STEEL PIPES

  • When to use slip on flanges


References:

  • https://www.extreme-bolt.com/blog/when-is-it-best-to-use-a-lap-joint-flange/
  • https://www.yaang.com

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