Lap Joint Flanges

Lap Joint Flanges

Lap Joint Flanges, also known as Loose Joints and simplified as LTF flange. It has a curved radius at the bore, which can connect with a stub end fittings and slide over the pipe. In this form, the pipe is usually welded to this stub end so the lap joint flange can rotate freely around the stub end. The main benefit of Lap joint flange is that when the flange is out of shape, it has no additional moment of force on the pipeline or equipment. The lap joint flanges does not directly tough the pipeline flow media, it mainly is' used on the cooper, aluminum and stainless steel pipeline.
Advantage of Lap joint flange is the flange can rotate for easier alignment of bolt holes, especially for big size pipeline. It also apply for pipeline that need frequent dismantling process for washing or inspection purpose. Its additional fittings materials shall be same as the pipe, however not a must for flange (Steel grade for flange often can be carbon steel). So it is suitable to transport corrosive media.
Lap joint flange can not bear high pressure, so it is mainly used on connection of low pressure pipeline.

Lap Joint Flanges Information Table


Alternative name L J Flanges, LJ flanges, Loose Joint Flange, LTF flange
Production Method Forged flange, Centrifugal Casting flange, Slicing flange 1.Casting flange advantage is in low cost and making complex form, but get casting defects 2. Forged flange advantage inner structure even, with no blowhole and harmful material, but in low production efficiency and slight higher cost 3.Slicing flange size is under the width of jobbing sheet. 4.Rolling up flange is mainly used for production of big size flange
Standard GB/T9122-2010, ANSI B16.5
Pressure ANSI Class 150-1500Lb
Size DN10-1200mm
Materials Carbon steel: ASTM A105, 20#, C22.8, ST37.2, A516 GR60/70
  Stainless steel: ASTM A182 F304, 304L, 316, 316L
  Alloy steel: Cr5Mo, 12Cr1MoV, 15CrMo, ASTM A182 F1, F5, F9, F11, F22
Surface Treatment Anti-rust oil, yellow or black painting, hot dipped galvanized
Packing In wood crate, Packed in Seaworthy Carton for Export
Usage Widely used on chemical, construction, watering, oil industry, water warming, electric power, ship building, extinguishing and protection
Advantage 1. flange can rotate for easier alignment of bolt holes 2. apply for pipeline that need frequent dismantling process for washing or inspection purpose 3. Flange material can be different from pipe material 4. Flange does not directly tough the media, so suitable to transport corrosive media. 5. Saving cost, When the pipe material is special and expensive, the flange welding cost for the same material is higher.
Disadvantage 1. Low pressure. 2. Low strength at the welding ring (especially when the thickness is less than 3mm)

Stub End - Key Fittings for Lap Joint Flange

A stub end is always used with a lap joint flange as a backing flange. There are three different types of stub ends, type A, B and C.

  •          Type A stub end can be machined to fit the standard lap joint support flange.

  •          Type B stub end is designed to be used with standard slip on flange.

  •          Either a lap joint flange or slip on flange, the type C stub end can be used


  • Stub end include long or short pattern length. The end of long pattern is also known as ASA Stub Ends. The short end is used with the larger flanges of ANSI 300 and 600. They are commonly used in most sizes ANSI 900 and above.

    Differences between lap joint flange and slip on flange

    Slip on flange is essentially a ring placed over the pipe end. And the flange face extending a sufficient distance from the pipe end, so as to apply a weld on the inner diameter. The outer diameter of slip on flange can also be welded to the rear side of flange. Comparing to weld neck flange, slip on flange has lower material cost and easier to align.

    The lap flange is very similar to a slip on flange. From the outside, it almost looks like the same. But you look on the backside, the holes and faces of lap joint flange has a curved radius to accommodate the lap joint stub end.

    If type B or type C stub end is used, the slip on flange can be used as the lap joint flange.