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 its 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 disassembly and assembly during the operation of a plant. The ability to spin that backing flange compensates for the misalignment of the bolt holes during reassembly.

Well, I’ve seen Slip-on flanges used in a chemical processing plant, all low-pressure steel lines. I imagine that the cost savings over weld necks would have been substantial. The same plant had (loose) lap joint flanges on the SS lines. Again, all mostly low-pressure Sch10 stuff and weld necks would have considerably added to the cost.

Not sure about the statement in Neill's post “Lap joint flanges have very low fatigue resistance and should not be used in cyclic or vibrating service.” The stub end is butt-welded to the pipe end, so that’s OK for cyclic service unless the reference was to Slip-on type flange connections where fillet welds exist.

Certainly, any oil and gas work I’ve been involved in has all been weld neck flanges.

Generally, if you consider the additional welding time for the two fillet welds on a slip-on flange, while the purchase price is lower, there is usually no savings in installation cost. Consequently, my comment about the only real advantage being to make up for misaligned flanges.

Regarding the Sch10 lap joint flanges, weld necks are available in Sch10 as well but I do agree there can be cost savings on lap joints since the hub is usually only carbon steel.

The ring on the lap joint flange is going to impose mechanical loading into the stub end on the pipe which is going to oscillate based upon the cyclic loads that are present. This will to an extent cause the stub end to act like a coat hanger being bent back and forth. It’s not the weld that is subject to fatigue but the base material in the stub end of the lap joint flange.