When to use slip-on vs. lap-joint flanges 4

The following is an excerpt from my article on the the pipingdesigners.com web site.

Weld Neck Flanges:
Weld Neck Flanges are distinguished from other flange types by their long tapered hub and gentle transition of thickness in the region of the butt weld that joins them to pipe or a fitting. A weld-neck flange is attached to a pipe or a fitting with a single full penetration, "V" bevel weld. The long tapered hub provides an important reinforcement of the flange proper from the standpoint of strength and resistance to dishing. The smooth transition from the flange thickness to the pipe wall thickness by the taper is extremely beneficial under conditions of repeated bending caused by line expansion or other variable forces, and produces an endurance strength of welding neck flanged assemblies equivalent to that of a butt-welded joint. This type of flange is preferred for severe service conditions, whether loading conditions are substantially constant or fluctuate between wide limits.

The weld neck flange is used in each of the seven flange ratings and has the advantage of requiring only one weld to attach it to the adjacent pipe or fitting.
The key dimension for a weld neck flange is the length through the hub from the beveled end to the contact face of the flange. This "length" includes the bevel, the tapered hub, and the thickness of the plate part of the flange and the raised face. To obtain the correct dimension you must look at a correctly constructed flange dimension chart (see the "Tools" button on this website) or a flange manufacturers catalog. Electronic piping design software will normally already have the correct dimension built-in.

It is important to understand and remember that the (1/16") raised face on the Class 150 raised face and on the Class 300 raised face flanges is normally included in the length dimension. However, the ¼" raised face is not included in the chart or catalog length dimension for the Class 400 and higher pressure rated flanges. The raised face dimension for Class 400 flanges (and up) normally must be added to the chart or catalog length to arrive at the true total length of these higher-pressure flanges.

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.

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.