A butterfly valve in a 30", 80 psig main supply water line became inoperative. Water was critical to the plant's operation and the line could not be shut off to replace the valve. Team engineers were called to the plant. The piping system had several 30" x 30" inlets into the main header, connected with forged tees. After approval, Team engineers supplied plant personnel with a 30", 150 lb. SealTite^® fitting that was welded opposite the branch connection on the tee. A 30" hot tap was made through a 30" gate valve into the tee, and a special HTS line stop head was inserted across the line into the branch connection, thus isolating the inoperable valve from the main header. The valve was replaced, and HTS inserted a completion plug into the fitting so the 30" gate valve could be removed. A blind flange then was installed to cap off the line. Supply water to the plant was not interrupted during this operation. The job took 3½ days, and the plant avoided an expensive shutdown.