Lap Joint Flange VS Slip On Flanges

What is a lap joint flange?

Lap Joint Flange (LJ flange/Loose 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. 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. The benefit of this is that there will not be any issues with bolt hole alignment. Lap Joint pipe flanges are often used for applications that require frequent dismantling.

Standards, Dimensions & Weight Of Lap Joint Flanges Details of Slip On flange How to measure slip-on flanges
Dimensional Tolerances of Lap Joint Flanges ASME B16.5 Material of Slip-on flange Lap Joint Flange VS Slip On Flange
What are slip on flanges Standards of Slip-on flange When to use slip-on flanges vs. lap joint flanges

flg lj det - Lap Joint Flange VS Slip On Flanges

LAP JOINT FLANGE’S DRAWS:

1. Lap Joint flange 2. Stub End 3. Butt weld 4. Pipe or Pipe Fitting

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.

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.

Dimensions on the lap joint flange are similar to weld neckslip on or socket weld flanges. The backing flange has the same number of bolt holes, size and thickness of a weld neck or slip on flange.

STANDARDS, DIMENSIONS & WEIGHT OF LAP JOINT FLANGES

lap joint flange with stub end banner - Lap Joint Flange VS Slip On Flanges

  • ASME/ANSI B16.5 Class 150 Lap Joint Flanges
    Nominal Pipe Size  Outside Diameter of Flange Thickness of Lap Joint Min. Diameter of Hub Length Through Hub Bore Corner Radius of Bore of Lapped Flange Diameter of Bolt Circle Diameter of Bolt Holes Number of Bolts Approxi-mate Weight kgs
    NPS O tf X Y B r W I n
    1/2 90 11.2 30 16 22.9 3 60.3 15.9 4 0.46
    3/4 100 12.7 38 16 28.2 3 69.9 15.9 4 0.66
    1 110 14.3 49 17 34.9 3 79.4 15.9 4 0.89
    1 1/4 115 15.9 59 21 43.7 5 88.9 15.9 4 1.06
    1 1/2 125 17.5 65 22 50.0 6 98.4 15.9 4 1.36
    2 150 19.1 78 25 62.5 8 120.7 19.1 4 2.10
    2 1/2 180 22.3 90 29 75.4 8 139.7 19.1 4 3.58
    3 190 23.9 108 30 91.4 10 152.4 19.1 4 4.00
    3 1/2 215 23.9 122 32 104.1 10 177.8 19.1 8 4.99
    4 230 23.9 135 33 116.8 11 190.5 19.1 8 5.62
    5 255 23.9 164 36 144.4 11 215.9 22.2 8 6.38
    6 280 25.4 192 40 171.4 13 241.3 22.2 8 7.74
    8 345 28.6 246 44 222.2 13 298.5 22.2 8 12.65
    10 405 30.2 305 49 277.4 13 362.0 25.4 12 16.65
    12 485 31.8 365 56 328.2 13 431.8 25.4 12 27.31
    14 535 35.0 400 79 360.2 13 476.3 28.6 12 39.89
    16 595 36.6 457 87 411.2 13 539.8 28.6 16 51.18
    18 635 39.7 505 97 462.3 13 577.9 31.8 16 57.07
    20 700 42.9 559 103 514.4 13 635.0 31.8 20 72.08
    24 815 47.7 663 111 616.0 13 749.3 34.9 20 100.11

  • ASME/ANSI B16.5 Class 300 Lap Joint Flanges
    Nominal Pipe Size  Outside Diameter of Flange Thickness of Lap Joint Min. Diameter of Hub Length Through Hub Bore Corner Radius of Bore of Lapped Flange Diameter of Bolt Circle Diameter of Bolt Holes Number of Bolts Approxi-mate Weight kgs
    NPS O tf X Y B r W I n
    1/2 95 14.3 38 22 22.9 3 66.7 15.9 4 0.70
    3/4 115 15.9 48 25 28.2 3 82.6 19.1 4 1.16
    1 125 17.5 54 27 34.9 3 88.9 19.1 4 1.50
    1 1/4 135 19.1 64 27 43.7 5 98.4 19.1 4 1.86
    1 1/2 155 20.7 70 30 50.0 6 114.3 22.2 4 2.63
    2 165 22.3 84 33 62.5 8 127.0 19.1 8 3.02
    2 1/2 190 25.4 100 38 75.4 8 149.2 22.2 8 4.48
    3 210 28.6 117 43 91.4 10 168.3 22.2 8 6.08
    3 1/2 230 30.2 133 44 104.1 10 184.2 22.2 8 7.68
    4 255 31.8 146 48 116.8 11 200.0 22.2 8 10.07
    5 280 35.0 178 51 144.4 11 235.0 22.2 8 12.64
    6 320 36.6 206 52 171.4 13 269.9 22.2 12 16.39
    8 380 41.3 260 62 222.2 13 330.2 25.4 12 24.57
    10 445 47.7 321 95 277.4 13 387.4 28.6 16 39.40
    12 520 50.8 375 102 328.2 13 450.8 31.8 16 56.33
    14 585 54.0 425 111 360.2 13 514.4 31.8 20 81.97
    16 650 57.2 483 121 411.2 13 571.5 34.9 20 106.10
    18 710 60.4 533 130 462.3 13 628.6 34.9 24 127.52
    20 775 63.5 587 140 514.4 13 685.8 34.9 24 157.92
    24 915 69.9 702 152 616.0 13 812.8 41.3 24 237.21

  • ASME/ANSI B16.5 Class 600 Lap Joint Flanges
    Nominal Pipe Size  Outside Diameter of Flange Thickness of Lap Joint Min. Diameter of Hub Length Through Hub Bore Corner Radius of Bore of Lapped Flange Diameter of Bolt Circle Diameter of Bolt Holes Number of Bolts Approxi-mate Weight kgs
    NPS O tf X Y B r W I n
    1/2 95 14.3 38 22 22.9 3 66.7 15.9 4 0.70
    3/4 115 15.9 48 25 28.2 3 82.6 19.1 4 1.16
    1 125 17.5 54 27 34.9 3 88.9 19.1 4 1.50
    1 1/4 135 20.7 64 29 43.7 5 98.4 19.1 4 2.01
    1 1/2 155 22.3 70 32 50.0 6 114.3 22.2 4 2.83
    2 165 25.4 84 37 62.5 8 127.0 19.1 8 3.42
    2 1/2 190 28.6 100 41 75.4 8 149.2 22.2 8 5.00
    3 210 31.8 117 46 91.4 10 168.3 22.2 8 6.71
    3 1/2 230 35.0 133 49 104.1 10 184.2 25.4 8 8.56
    4 275 38.1 152 54 116.8 11 215.9 25.4 8 14.29
    5 330 44.5 189 60 144.4 11 266.7 28.6 8 23.80
    6 355 47.7 222 67 171.4 13 292.1 28.6 12 27.93
    8 420 55.6 273 76 222.2 13 349.2 31.8 12 42.59
    10 510 63.5 343 111 277.4 13 431.8 34.9 16 76.04
    12 560 66.7 400 117 328.2 13 489.0 34.9 20 90.92
    14 605 69.9 432 127 360.2 13 527.0 38.1 20 109.42
    16 685 76.2 495 140 411.2 13 603.2 41.3 20 154.99
    18 745 82.6 546 152 462.3 13 654.0 44.5 20 189.94
    20 815 88.9 610 165 514.4 13 723.9 44.5 24 243.66
    24 940 101.6 718 184 616.0 13 838.2 50.8 24 346.39

  • ASME/ANSI B16.5 Class 900 Lap Joint Flanges
    Nominal Pipe Size  Outside Diameter of Flange Thickness of Lap Joint Min. Diameter of Hub Length Through Hub Bore Corner Radius of Bore of Lapped Flange Diameter of Bolt Circle Diameter of Bolt Holes Number of Bolts Approxi-mate Weight kgs
    NPS O tf X Y B r W I n
    1/2 120 22.3 38 32 22.9 3 82.6 22.2 4 1.69
    3/4 130 25.4 44 35 28.2 3 88.9 22.2 4 2.28
    1 150 28.6 52 41 34.9 3 101.6 25.4 4 3.41
    1 1/4 160 28.6 64 41 43.7 5 111.1 25.4 4 3.89
    1 1/2 180 31.8 70 44 50.0 6 123.8 28.6 4 5.41
    2 215 38.1 105 57 62.5 8 165.1 25.4 8 9.57
    2 1/2 245 41.3 124 64 75.4 8 190.5 28.6 8 13.54
    3 240 38.1 127 54 91.4 10 190.5 25.4 8 11.13
    4 290 44.5 159 70 116.8 11 235.0 31.8 8 18.96
    5 350 50.8 190 79 144.4 11 279.4 34.9 8 31.45
    6 380 55.6 235 86 171.4 13 317.5 31.8 12 40.16
    8 470 63.5 298 114 222.2 13 393.7 38.1 12 72.66
    10 545 69.9 368 127 277.4 13 469.9 38.1 16 105.50
    12 610 79.4 419 143 328.2 13 533.4 38.1 20 141.92
    14 640 85.8 451 156 360.2 13 558.8 41.3 20 162.02
    16 705 88.9 508 165 411.2 13 616.0 44.5 20 199.98
    18 785 101.6 565 190 462.3 13 685.8 50.8 20 277.51
    20 855 108.0 622 210 514.4 13 749.3 54.0 20 348.93
    24 1040 139.7 749 267 616.0 13 901.7 66.7 20 671.17

  • ASME/ANSI B16.5 Class 1500 Lap Joint Flanges
    Nominal Pipe Size  Outside Diameter of Flange Thickness of Lap Joint Min. Diameter of Hub Length Through Hub Bore Corner Radius of Bore of Lapped Flange Diameter of Bolt Circle Diameter of Bolt Holes Number of Bolts Approxi-mate Weight kgs
    NPS O tf X Y B r W I n
    1/2 120 22.3 38 32 22.9 3 82.6 22.2 4 1.69
    3/4 130 25.4 44 35 28.2 3 88.9 22.2 4 2.28
    1 150 28.6 52 41 34.9 3 101.6 25.4 4 3.41
    1 1/4 160 28.6 64 41 43.7 5 111.1 25.4 4 3.89
    1 1/2 180 31.8 70 44 50.0 6 123.8 28.6 4 5.41
    2 215 38.1 105 57 62.5 8 165.1 25.4 8 9.57
    2 1/2 245 41.3 124 64 75.4 8 190.5 28.6 8 13.54
    3 265 47.7 133 73 91.4 10 203.2 31.8 8 17.29
    4 310 54.0 162 90 116.8 11 241.3 34.9 8 27.02
    5 375 73.1 197 105 144.4 11 292.1 41.3 8 51.41
    6 395 82.6 229 119 171.4 13 317.5 38.1 12 60.85
    8 485 92.1 292 143 222.2 13 393.7 44.5 12 103.41
    10 585 108.0 368 178 277.4 13 482.6 50.8 12 181.39
    12 675 123.9 451 219 328.2 13 571.5 54.0 16 286.48
    14 750 133.4 495 241 360.2 13 635.0 60.3 16 384.80
    16 825 146.1 552 260 411.2 13 704.8 66.7 16 492.31
    18 915 162.0 597 276 462.3 13 774.7 73.0 16 638.30
    20 985 177.8 641 292 514.4 13 831.8 79.4 16 766.56
    24 1170 203.2 762 330 616.0 13 990.6 92.1 16 1227.85

  • ASME/ANSI B16.5 Class 2500 Lap Joint Flanges
    Nominal Pipe Size  Outside Diameter of Flange Thickness of Lap Joint Min. Diameter of Hub Length Through Hub Bore Corner Radius of Bore of Lapped Flange Diameter of Bolt Circle Diameter of Bolt Holes Number of Bolts Approxi-mate Weight kgs
    NPS O tf X Y B r W I n
    1/2 135 30.2 43 40 22.9 3 88.9 22.2 4 3.01
    3/4 140 31.8 51 43 28.2 3 95.2 22.2 4 3.43
    1 160 35.0 57 48 34.9 3 108.0 25.4 4 4.87
    1 1/4 185 38.1 73 52 43.7 5 130.2 28.6 4 7.12
    1 1/2 205 44.5 79 60 50.0 6 146.0 31.8 4 10.10
    2 235 50.9 95 70 62.5 8 171.4 28.6 8 14.67
    2 1/2 265 57.2 114 79 75.4 8 196.8 31.8 8 20.91
    3 305 66.7 133 92 91.4 10 228.6 34.9 8 32.29
    4 355 76.2 165 108 116.8 11 273.0 41.3 8 49.09
    5 420 92.1 203 130 144.4 11 323.8 47.6 8 82.84
    6 485 108.0 235 152 171.4 13 368.3 54.0 8 128.65
    8 550 127.0 305 178 222.2 13 438.2 54.0 12 184.69
    10 675 165.1 375 229 277.4 13 539.8 66.7 12 356.50
    12 760 184.2 441 254 328.2 13 619.1 73.0 12 498.67

Dimensional Tolerances of Lap Joint Flanges ASME B16.5

20180118105733 82057 - Lap Joint Flange VS Slip On Flanges

Lap joint flange drawing

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

What are slip on flanges?

Slip On flanges or SO flanges are designed to slip over the outside of pipe, long-tangent elbows, reducers, and swages. The flange has poor resistance to shock and vibration. It is easier to align than a weld neck flange. This flange is ideal for low pressure applications since the strength when under internal pressure is about one third that of a weld neck flange. This flange has a raised face. Slip On flanges or SO flanges are commonly lower in price than weld-neck flanges, and to this effect are a popular choice for our customers. However, customers should bear in mind that this initial cost saving may be diminished by the additional cost of the two fillet welds required for proper installation. Moreover, weld-neck flanges have a higher life expentancy than slip-on flanges under duress.
The slip on flange is positioned so the inserted end of the pipe or fitting is set short of the flange face by the thickness of the pipe wall plus 1/8 of an inch, which thus allows for a fillet weld inside the SO flange equal without doing any damage to the flange face. The back or outside of the slip-on flange or SO flange is also welded with a fillet weld.

Slip-On flanges or SO flanges are commonly lower in price than weld-neck flanges, and to this effect are a popular choice for our customers.
slip on flanges banner - Lap Joint Flange VS Slip On Flanges
It is welded both inside and out to provide suffcient strength and prevent leakage.

Slip-On Pipe Flanges. As made obvious by their name, these pipe flanges slip over the pipe. They’re manufactured with an inside diameter that is slightly bigger than the pipe’s outside diameter. These attachments are connected to the pipe via fillet weld at the top and bottom of the flange.

However, customers should bear in mind that this initial cost saving may be diminished by the additional cost of the two fillet welds required for proper installation. Moreover, weld-neck flanges have a higher life expentancy than slip-on flanges under duress.

Slip-on flanges are all bored slightly larger than the O.D. of the pipe. They are preferred over welding neck flanges bu many users due to their lower intial cost, but final intallation cost is probably not much less than that of the welding neck flange because of the additional welding involved.

Weldneck and Slip-On Orifice Runs

Weldneck and Slip On Orifice Runs 2 - Lap Joint Flange VS Slip On Flanges

Standards, Dimensions & Weight

Slip-on flange is slipped over the pipe and then fillet welded. Slip-on flanges are easy to use in fabricated applications.

Details of Slip On flange

A combination of flange and elbow or flange and tee is not possible, because named fittings have not a straight end, that complete slid in the Slip On flange.

flg so det - Lap Joint Flange VS Slip On Flanges
1. Slip On flange 2. Filled weld outside 3. Filled weld inside 4. Pipe
  • Size: 1/2” to 60”, DN 12mm to DN 1500mm .
  • Pressure class: Class 150 to Class 2,500, PN 2.5 to PN 250
  • Facing: RF / RTJ

Material of Slip-on flange:

  • Carbon steel: ASTM A105,ASTM A105N,GB 20,C22.8.
  • Alloy steel: ASTM/ASME A182 F1-F12-F11-F22- F5-F9- F91
  • Stainless steel: ASTM/ASME A182 F304-304L-304H-304LN-304N
  • ASTM/ASME A182 F316-316L-316H-316LN-316N-316Ti
  • ASTM/ASME A182 F321-321H, F347-347H
  • Low temperature steel: ASTM/ASME A350 LF2.
  • High performance steel: ASTM/ASME A694 F42 , F52, F56, F60, F65, F70

Standards of Slip-on flange:

  • ASME/ANSI B16.5,
  • MSS SP 44
  • CSA Z245.12
  • EN1092-1,EN1759-1.
  • DIN2573,DIN2576.
  • BS1560,BS 4504,BS 10.
  • AFNOR NF E29-200-1
  • ISO7005-1
  • AS2129
  • JIS B2220
  • UNI 2276. UNI 2277.UNI 2278 .UNI 6089 .UNI 6090

Buying Tips

Factors to consider before buying slip-on flanges are as follows:

  • Size
  • Design Standard
  • Material
  • Normal Pressure
  • Face Type
  • Flange Diameter
  • Flange Thickness
  • Durability
  • Corrosion resistant

Why slip on flanges are preferred to welding neck flanges?

For many users, slip on flanges continue to be preferred to welding neck flanges because of the following reasons:

  • On account of their initially lower cost.
  • The reduced accuracy needed in cutting the pipe to length.
  • The greater ease of alignment of the assembly.
  • The calculated strength of slip-on flanges under internal pressure is approximately two-thirds that of welding neck flanges.

How to measure slip-on flanges?

slip on flange - Lap Joint Flange VS Slip On Flanges

Take the measurements of:

  • OD: Outside Diameter
  • ID: Inside Diameter
  • BC: Bolt Circle
  • HD: Hole diameter

Key Features:

Some important features are as follows:

  • One size fits all pipe schedules.
  • Fabricators can more easily cut pipe to length for slip-on flanges.
  • The smaller thickness of this flange allows for easier alignment of bolting holes.
  • They are generally not preferred for high pressure temperature environments.

Lap Joint Flanges VS Slip On Flanges

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.
  • 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.
  • 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.
  • 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. 

Disadvantages of lap Joint flanges:

  • Understand low pressure.
  • The strength of the weld ring is low (especially when the thickness is less than 3mm).
  • Loose flanges are mainly used in medium and low pressure applications. The precision requirements (dimensional accuracy, tolerance requirements, roughness requirements, flatness requirements) during manufacturing are not high, so the sealing requirements are not high.

Advantages of slip on flanges:

  • Low cost installation
  • Less time needed to spent on ensuring the accuracy of the cut pipe
  • They are somewhat easier to align
  • The slip-on flanges have low hub because the pipe slips into the flange before welding
  • The flange is welded both inside and outside to provide sufficient strength
  • They prevent leakage

When to use slip-on flanges vs. lap joint flanges

Currently we are using lap-joint flanges and welding them directly to the piping.  Is this an appropriate way to use lapped flanges, or should I change to using slip-on flanges.
I understand that lapped flanges are generally used with but ends, so as to make it possible to rotate the flange when needed, but do not understand wether or not a lapped flange should be welded or not.
Please advise as to the correct use of flanges.
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.

When to use slip on flanges?

Slip-On (SO) Flanges are preferred by some contractors, over the Weld-neck, because of the lower initial cost. However, this may be offset by the added cost of the two fillet welds required for proper installation. The strength of the slip-on flange is ample for it’s rating, but its life under fatigue conditions is considered to be only one-third that of the weld-neck flange.
The slip-on flange may be attached to the end of a piece of pipe or to one or more ends of a pipe fitting. The slip-on flange is positioned so the inserted end of the pipe or fitting is set back or short of the flange face by the thickness of the pipe wall plus 1/8 of an inch. This allows for a fillet weld inside the SO flange equal to the thickness of the pipe wall without doing any damage to the flange face. The back or outside of the flange is also welded with a fillet weld.
A variation of the Slip-On flange also exists. This is the Slip-On Reducing Flange. This is simply a larger (say a 14″) Slip-On flange blank that, instead of the Center (pipe) hole being cut out (or drilled out) for 14″ pipe it is cut out for a 6″ (or some other size) pipe. The SO Reducing flange is basically used for reducing the line size where space limitations will not allow the length of a weld neck flange and reducer combination. The use of the Slip-On Reducing Flange should only be used where the flow direction is from the smaller size into the larger size.

When to use lap joint flanges?

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 work simplification comes into the picture where there are cases 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 reassembly.

Source: China Flange Manufacturer – Yaang Pipe Industry (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.)

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

  • https://www.yaang.com/when-to-use-slip-on-flanges-vs-lap-joint-flanges.html

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