Fishmouth
Fishmouth turns up in 3 fields: valves, geosynthetics, waterproofing. Here is what it means to the people in each one.
Fishmouth in valves
A saddle-shaped cut on the end of a branch pipe so it sits flush against the curve of a larger pipe.
For a stub-in branch, the fitter marks and cuts the branch end into a curved saddle that follows the header's outside diameter, then bevels it for welding. The cut looks like an open fish's mouth. It is also a verb: to fishmouth a pipe. Fitters lay it out with a wraparound and template, a torch and a grinder, or a dedicated saddle-cutting machine.
"Fishmouth that 2 inch for the stub-in and keep your bevel even all the way around or it'll be a nightmare to weld."
Fishmouth in geosynthetics
Wrinkle or bulge in the top sheet of a seam overlap that stops the two sheets lying flat for welding.
Fishmouths form when one panel has more slack or expansion than its neighbor, leaving an open, mouth-shaped gap at the overlap edge. A wedge cannot weld through one, so the bulge is cut, laid flat and patched with an extrusion weld. A seam full of fishmouths usually means panels were laid in the heat without enough attention to slack.
"Cut the fishmouth at station 120, lay it flat and patch it, don't try to run the wedge through."
Fishmouth in waterproofing
An open wrinkle at the edge of a membrane lap that forms a channel water can run into.
When a sheet is laid with a ripple, or not rolled firmly at a lap, the edge lifts into a small tunnel shaped like a fish's open mouth. Water follows the channel under the lap. The fix is to slit the wrinkle, lay it flat, and patch over it with a target piece extending a few inches all round. Inspectors look for fishmouths at every end lap and corner.
"Got a fishmouth at every end lap on the north wall, slit them and target-patch before the board goes on."