Butt Weld Cap

Cap, Plug and Blind Flange

These three all close an opening, but they are not interchangeable. A cap welds onto the outside of the pipe end and is permanent; removing it requires cutting. A plug is a threaded or socket weld fitting that goes inside a fitting opening. A blind flange bolts onto a flange face and can be removed and reinstalled with hand tools, making it the choice where access is needed later, for example at the end of a header that may be extended. The ASME B16.9 cap is a butt weld item designed to give a smooth, full strength closure with the same pressure rating as the adjoining pipe, and it is used where a permanent or long term closure is required. Where the closure may need to be opened for inspection or pigging, consider a blind flange or a closure with a flanged end instead.

Wall Thickness and Pressure Rating

The pressure rating of a cap is determined by its wall thickness and geometry, in the same way as a length of pipe. ASME B16.9 requires the wall thickness of any point on the cap to be not less than 87.5 percent of the nominal wall thickness ordered. Where the cap terminates a line that will be subjected to full design pressure, the cap should be ordered in the same schedule as the pipe so the pressure rating is consistent. For hydrostatic test closures, the cap must be rated for the test pressure, which is typically 1.5 times the design pressure, so this should be stated at the time of order rather than assumed.