Flanges by type
Lap joint flanges and stub ends for stainless cooling loops
A lap joint flange is a loose flange that sits behind a stub end welded to the pipe. Only the stub end touches the coolant, and the flange can turn to line up bolt holes. On stainless systems, that saves material and fitting time.
On stainless TCS and CDU piping, a 316L stub end with a lap joint flange gives a butt-welded, crevice-free bore like a weld neck, plus a loose flange that rotates for alignment and can be a lower cost material.
How the joint works
The stub end is a short pipe with a flared lap. It is butt welded to the pipe, so the bore is smooth and the weld can be purged and inspected like any stainless butt weld. The lap joint flange slides over the stub end and bears on the back of the lap. The gasket seals against the lap face, so the flange itself never contacts the fluid.
| Factor | Stub end + lap joint flange | Weld neck |
|---|---|---|
| Wetted parts | Stub end only | Whole flange |
| Bolt hole alignment | Flange rotates freely | Fixed once welded |
| Flange material | Can differ from the pipe | Must match the pipe |
| Bore | Smooth, butt welded | Smooth, butt welded |
| Best use | Skids, CDUs, frequently opened joints | Fixed headers, high stress joints |
Why it suits data center cooling
- CDU and skid connections: pre-fabricated skids rarely land with bolt holes perfectly aligned. A rotating flange takes up the difference without forcing the pipe.
- Clean loops: the butt-welded stub end keeps the inside of the TCS free of crevices.
- Cost: on large stainless lines, a carbon steel or lighter backing flange can reduce cost while the 316L stub end does the wetted work.
AWWA C228 stainless ring flanges are designed to be welded to the pipe and are not intended as loose backup flanges. For a loose flange on a stainless water main, specify a lap joint design.
Stub end patterns
ASME B16.9 long pattern stub ends have a longer barrel and suit heavier wall pipe. MSS SP-43 short pattern stub ends are made for thin-wall stainless pipe such as Sch 5S and 10S. We make both, together with the lap joint flanges, so one order covers the full joint.
Frequently asked questions
Which stub end should be used with a lap joint flange?
ASME B16.9 long pattern stub ends or MSS SP-43 short pattern stub ends, in the same grade as the pipe. MSS SP-43 stub ends are common on thin-wall Sch 5S and 10S stainless lines.
Can the lap joint flange be carbon steel on a stainless line?
Yes. Only the stub end is wetted, so the loose flange can be carbon steel if the specification allows it. Use a stainless flange where the joint may get wet from outside or where the specification calls for all-stainless construction.
Do you make the stub ends too?
Yes. RIC manufactures stub ends as part of its buttweld fittings range, so the stub end and lap joint flange can be supplied together on one certificate package.