When to use 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.
Certain factors to consider are as follows:
- Design Standard
- Normal Pressure
- Face Type
- Corrosion Resistant
- 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
- 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:
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
Dimensions are in millimeters unless otherwise indicated.
≤ 24 = 1.6 mm
≤ 10 = ± 0.8 mm
DIAMETER OF CONTACT FACE
1.6 mm Raised Face = ± 0.8 mm
Bolt Circle = 1.6 mm
DIAMETER OF COUNTERBORE
Same as for Inside Diameter
OUTSIDE DIAMETER OF HUB
≤ 12 = + 2.4 mm / – 1.6 mm
≤ 18 = + 3.2 mm / – 0
LENGTH THRU HUB
≤ 18 = + 3.2 mm / – 0.8 mm
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
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.
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.
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.
Source: Network Arrangement – China Lap Joint Flange Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.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.)
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Please notice that you might be interested in the other technical articles we’ve published:
What is the difference between lap joint flange and slip on flange?
lap joint flanges are very similar to a slip-on flange, with the main difference being that it has a curved radius at the bore and face to house a lap joint stub-end. Lap joint flanges and stub-end assemblies are typically used in situations where frequent dismantling is required for for inspection.
What is a lap joint flange used for?
Lap joint pipe flanges slide directly over the pipe and are most commonly used with stub end fittings. A pipe is usually welded to the Stub End and the Lap Joint pipe flange is free to rotate around the stub end.
Lap Joint pipe flanges are often used for applications that require frequent dismantling.
How to Install a Slip on Flange?
Pipe is assembled with other pipe, fittings, and flanges either by welding or threading. There are specific codes describing the permissibility of threading or welding. With welding you also need to be aware of several options. Flanges and fittings are either slip-on or weld-neck. Slip-on fittings slip onto the end of the pipe. The flange is then welded around the contact points on the inside and outside of the pipe and the flange. Slip-on flanges are not considered as strong a joint as weld-neck or butt-welded connections. With butt-welded or weld-neck flanges, the two pieces—flange and pipe—are prepped and then welded together, with full penetration (a welder carefully lays a bead and builds up layers around the entire surface of the gap between the two pieces).
Socket welding describes when a slip-on fitting, usually used for small diameters, is inserted into the fitting until it bottoms out. The pipe is then pulled back from the bottom and welded to the fitting. Failure to pull the pipe back can cause weld failure due to stress.
Select the flange size that is appropriate to the steel pipe. The fit needs to be snug and not loose. The slip-on flange consists of a bored hole with a diameter just slightly larger than the steel pipe.
1. Slip the flange over the end of the pipe base.
2. Thread the flange pins to tighten the flange’s grasp on the pipe. The pins will keep the pipe securely in place inside the flange. There are usually six to eight pins in place. Use a screwdriver to tighten the pins all the way.
3. Try to pull the pipe out of the flange to make sure that the pins are efficiently tightened. If they aren’t, re-tighten them with the screwdriver. Inspect the pins to make sure they are down all the way and that the pipe cannot move inside the flange.
4. Use a hammer to slightly tap the flange in place. The flange face should be flush with the end of the steel pipe.
5. Use a 12-inch wrench with 85 pounds force to tighten the pins. Apply 80 to 85 foot-per-pounds torque on the pins, or until the pin heads break off.
6. Lubricate the gasket of the pipe and stretch it over the pipe end, with the beveled edge positioned in the field flange.