Gate valves become less reliable as they age. The stems of gate valves tend to break over time. This results in the valve being stuck in either the "On" or "Off" position. Gate valves are also known for sticking shut after an extended period of non-use. If a gate valve is broken or stuck, it may be necessary to remove it from the copper water line.
Find and turn off the water supply to the valve. If a valve cannot be found, turn off the main water valve to the house. Open a faucet on a lower level than the gate valve on which you are working to drain water from the pipes, if possible. If there isn't much water, open another faucet above the work area to break any vacuum that may have formed and allow water to drain out of the lines
Cut the pipe a few inches from one side of the valve, using a copper pipe cutter large enough to cut a 1-inch pipe. Turn the cutter around the pipe in a circular motion several times until it cuts through the copper pipe. Repeat the process on the other side of the gate valve. Remove the valve.
Sand both cut ends of the water line pipe, using plumber's sandpaper. Measure the distance between the pipes, subtracting 1/4 inch to leave room for the couplings. If you are replacing the gate valve with another gate valve or with a ball valve, place the valve on one end of the pipe before measuring. Cut a piece of 1-inch copper pipe the required length, using a pipe cutter. Clean both ends of the pipe, using sandpaper.
Clean two 1-inch couplings with a cleaning brush. Clean a new valve, if applicable. Apply soldering paste (also called flux) to all ends of copper pipe and to the insides of the couplings and the valve, using a brush. Assemble by sliding couplings onto each end of the water line. Insert the measured piece of copper. If adding a valve, place it on one end of the water line in place of a coupling. Solder the joints, using a propane torch. Hold the blue tip of the flame against the fitting until the flux melts. Gently hold the solder to the joint at the edge of the coupling until it melts and flows around the coupling’s edge. Repeat for both sides of each coupling or valve. Allow a minute for the joints to cool. Close all open faucets, then turn on the water supply and check for leaks.