Lap Joint Flange

ASME B16.5 Class 150 flange dimensions

Flange outside diameter, minimum flange thickness, bolt circle, bolting and raised face, in millimetres, for NPS 1/2 to 12 in. These dimensions are set by the size and the pressure class and are common to the flange types on this line; the bore, hub, socket or thread detail changes with the type and is confirmed on the drawing or the quotation. Thickness includes the 1.6 mm raised face.

Data to ASME B16.5; confirm against the project specification before fabrication. Sizes above 12 in and Class 300 to 2500 follow the same layout and are quoted against the specification. Send the size, class, facing and destination port and we will confirm availability and price within one business day.

What a Lap Joint Flange Is

A lap joint flange is a two piece connection. The pipe is butt welded to an ASME B16.9 stub end, and the flange itself is a plain ring that sits behind the flared lap of the stub end. The flange is not welded to anything – the bolting pulls it against the mating flange and the metal-to-metal contact between the flange face and the stub end lap provides the seal. Because the flange is loose, it can be rotated freely to line up the bolt holes, and because it never touches the fluid, it can be a different material from the pipe.

Why Use a Lap Joint Instead of a Weld Neck

Two practical reasons. First, bolt hole alignment: if a spool has to be dropped in between two existing flanges, a rotating flange removes the need to cut and re-weld anything to index the holes. Second, material cost: on a stainless or alloy line the flange is the heaviest single piece of material in the joint, and a lap joint flange is not wetted, so a carbon steel flange can be used behind a stainless stub end – the expensive material is confined to the part that actually touches the process fluid. Lap joints are also convenient where a section has to be opened for cleaning or inspection.

The Stub End Is Part of the Order

A lap joint flange cannot be installed without a matching stub end, so the two are normally ordered together. The stub end carries the pipe schedule and the butt weld to the pipe; the flange only carries the class, the facing and the bolt pattern, and the two must be the same size and class. Stub ends are dimensioned to ASME B16.9 and are made in two lengths, short and long, so the length has to be stated. Supplying the pair together avoids the common problem of a flange arriving without a stub end that fits it.

Where It Is Used and Its Limits

Lap joint flanges are common on stainless and corrosive service, on lines that have to be opened for cleaning, and on spools that must fit between fixed flanges. The usual limit is pressure: because the seal depends on the metal contact between the flange face and the stub end lap, lap joints are normally used at the lower classes, most often Class 150 and 300. They are not the choice for high pressure, for severe cyclic service, or for large sizes where the bolting load on the lap becomes heavy.

Lap joint against slip on: which one to order

Both types are used at the lower pressure classes, and on a spool list they look interchangeable. They are not: one is a single item with two fillet welds, the other is a pair of items with a butt weld.

Stub end: what has to be stated and what has to be measured

The stub end carries the pipe schedule and the butt weld to the pipe, so it is a pipe item rather than a flange item, and it is dimensioned to ASME B16.9.

• Pattern. Short and long pattern stub ends are both tabulated, and they are not interchangeable in a spool layout. The pattern has to be on the order.
• Schedule. The stub end follows the pipe schedule, not the flange class. A Class 300 flange and a Schedule 40 stub end are a normal combination.
• Lap dimensions. The lap outside diameter and thickness have to match the flange behind it. Mixing a stub end from one source with a flange from another is where mismatch is most often found.
• Replacement in an existing line. Measure the existing lap diameter, lap thickness and overall length before ordering. Nominal dimensions are tabulated, but a line that has been in service may have been built to a different pattern.
• Gasket. The gasket sits between the stub end lap and the mating flange face, so it is dimensioned by the facing and class of the mating flange rather than by the lap.

Grades supplied and minimum properties

The grade fixes the minimum strength and the chemistry. The table below gives the requirements that separate the forged flange material grades ordered most often; the full composition and mechanical table for each grade is on its own page.

The complete chemistry and mechanical table for each grade, with the notes that qualify it, is on the individual grade page – for example ASTM A105 flange. The stainless grade has the higher tensile requirement but the lower minimum yield strength, so the two are not interchangeable on the strength table alone. On a lap joint the flange and the stub end are separate items and may be ordered in different materials.

Source: specified minimum yield and tensile strength for each grade, per ASTM A105 / ASME SA105 and ASTM A182 / ASME SA182. The standard named against each grade is the governing document.

• A105
• A350 LF2
• F304
• F304L
• F316
• F316L
• F321
• F51

Download Lap Joint Flange data sheet (PDF) One page: supply range, grades and key sizes, what to state on the enquiry, and the inspection documents that come with the order. The standard is the governing document.

What decides the price

There is no list price for flanges: the same drawing can be quoted at different numbers depending on seven things. Send them and the quotation comes back within one business day.

Reply within one business day. Send the seven items above with the quantity and the destination port. Send the specification or WhatsApp the specification.