Gate valves that are infrequently operated have a tendency to freeze up; these are uncomplicated and typically well-made machines and seldom actually break. If you encounter a broken gate valve, it is very likely that removal and replacement will be far cheaper than contacting the original manufacturer for internal parts. There are a number of measures that will probably free up a seized gate valve.
Gate valves are so named because they close off the flow of water through a pipe just as gates in fences prevent passage from one side to the other. Gate valves have a wedge-shaped internal plunger that is raised and lowered into a housing, properly called a "valve bonnet," by the twisting of a stem. This design ensures that, when fully open, the valves have the same bore as the pipe they interrupt; they do not constrict flow at all. Gate valves typically have a round handle at the end of the stem that looks like a small boat helm; these handles are often painted red.
Before Working on the Valve
First check that there is water in the pipe: If the utility or well pump is experiencing an issue or the pipe is frozen, the gate valve is not to blame. If the supply is otherwise fine, isolate the gate valve if possible. Closing other valves, or stop-taps, up- and downstream could save mess and irritation later. Position a bowl or bucket beneath the valve if possible; otherwise, stuff towels around the area.
Penetrating oil usually works to free gate valves that are seized for no other reason than lack of use. The lubricating oil found in almost every home, garage and workshop in the U.S. is not -- contrary to popular opinion -- a penetrating oil. Its use will do little more than make the gate valve slippery and more difficult to work on. True penetrating oils can be found in auto parts stores and, occasionally, in the auto aisles of department stores. Clean away any verdigris or atmospheric deposits from the gate valve's visible threads, follow the oil manufacturer's directions for use and wait time, then tap the valve gently but firmly with a lightweight hammer to jar free any foreign matter gluing up the threads and try to open it as normal. Use a non-flammable penetrating oil in case the subsequent application of heat is necessary.
If the gate valve is still stuck after using the penetrating oil at least three times, try extending the leverage you have on the handle. Slip a sturdy screwdriver blade through the tines of the handle or use a pair of vise grips on the stem. Support the valve with your free hand so as to prevent the force you apply with the lever from being transferred to the pipe and very gradually increase pressure until the seize breaks or you fear continuing will bend the pipes on either side of the valve.
Heat is often used to free up two metal components that have become bonded; nuts locked onto bolts by rust are almost certainly the most common use. The idea is to make one component of the valve heat up much faster than the other so that one expands while the other doesn't, causing the unwanted bond to crack. Heating slowly, therefore, does not work. Only an extremely hot flame will work, such as a soldering blow-torch or a welding or cutting torch. Caution must be used to avoid scorching or setting fire to the gate valve's surroundings. Use the heat source judiciously -- do not leave the flame in contact with the valve for so long that any washers catch fire or liquefy and begin to leak -- then apply leveraged pressure immediately.