Lap Joint Flange

Lap joint flange, abbreviated as LJF, is a flange that consists of two parts, a stub end and the backing flange.  On drawings, it can be abbreviated as LJF.  On a Lap Joint, the flange slides over the pipe and the stub end is the connection where the gasket sits.  The end where the gasket sits is the same outside diameter as the face of a raised face flange.  In general, the thickness of the hub is between ¼” to 3/8”.

The 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.

Dimensions on the lap joint flange are similar to weld neck, slip 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.

Stub Ends

There are three different types of stub ends, Type A, B and C.  Type A stub ends are machined to fit in a standard lap joint backing flange.  The mating surfaces are shaped with the same profile to allow for uniform loading of the flare face.  Type B stub ends are ends that are designed to be used with a standard slip on flange.  Both Type A & B stub ends are either cast or forged.

Type C ends are designed to be used with either a lap joint flange or a slip on flange.  Type C ends are fabricated from pipe.  The most common type of fabrication is through the use of a machine flare.  This is done by using a machine to flare the end of the pipe and then cutting it to length.

The stub end is available in two separate lengths, a short pattern and a long pattern.  Long pattern ends are also known as ASA Stub Ends. Short pattern ends are used with larger flanges for ANSI 300 & 600.  They generally are used for most sizes ANSI 900 & above.

 

Lap Joint Flange Advantages and Disadvantages
Advantages	Disadvantages
Lap joint flanges are often more cost effective than other types of flanges, making them an attractive option for certain applications. The two components of lap joint flanges, the stub end, and the backing flange, allow for easy alignment during installation. This can simplify the welding process. Lap joint flanges offer the advantage of swiveling around the stub end, which allows for easy alignment adjustments.  This can be beneficial during installation or in situations where alignment may change over time. The stub end and the backing flange can be made from different materials, allowing for flexibility in material selection based on the specific requirements of the application. If a section of the pipe needs to be replaced, the stub end can remain welded to the pipe, while the backing flange can be easily removed and replaced without cutting or altering the pipe.	Lap joint flanges are generally not suitable for high-pressure applications compared to some other types of flanges.  The joint design and the use of stub ends may limit their pressure carrying capacity. The design of lap joint flanges may result in a less rigid connection compared to other flange types.  This can be a concern in high stress or high vibration environments. The swiveling capability that makes alignment easier also means that lap joint flanges may require more space than some other types of flanges.  This can be a consideration in tight or confined spaces. If not properly installed or if the joint is subjected to movement, there is a risk of leakage at the gasket interface.  Proper gasket selection and installation are crucial to minimize this risk. Lap joint flanges may not be suitable for extreme temperature conditions, as the joint design and materials may have limitations in terms of temperature resistance.

  

Lap Joint Flange Datasheets The datasheets we have on this site are shown below.  For simplicity sake, only datasheets that adhere to B16.5 are shown.  ASME B16.5 covers flange dimensions from ½” to 24”.  For sizes larger than this (ASME B16.47 Series A & B), please visit our flange datasheets page.
Class	Flat Face
ANSI 150	Lap Joint, ANSI Class 150 (in)
ANSI 300	Lap Joint, ANSI Class 300 (in)
ANSI 400	Lap Joint, ANSI Class 400 (in)
ANSI 600	Lap Joint, ANSI Class 600 (in)
ANSI 900	Lap Joint, ANSI Class 900 (in)
ANSI 1500	Lap Joint, ANSI Class 1500 (in)
ANSI 2500	Lap Joint, ANSI Class 2500 (in)