Gate valves are named after the gate-like disk that slides across the path of the flow. Gates are formed by machining both sides of the metal disk and both faces of the vertical valve seat.

Gate valves are classified as either Rising-Stem or Non-Rising-Stem valves.

In the non-rising stem design, the stem is stationary and the gate rises on a spindle. As the hand wheel on the stem is rotated, the gate travels up or down the stem on the threads while the stem remains vertically stationary. This type of valve will almost always have a pointer indicator threaded onto the upper end of the stem to indicate the position of the gate.

In a rising stem design the gate is affixed to the stem and the stem itself rises along with the gate. Valves with rising stems are used when it is important to know at a glance whether the valve is open or closed and when the stem and gate could become damaged by fluid contaminants if exposed to the fluid. In this type of valve, the stem rises out of the valve when the valve is opened. The non-rising stem type is more common in residential pool systems.

Gate Valves have long been the choice for main shutoff valves in residential plumbing systems, but are now often being supplanted by ball valves. Most pools built after 1980 or so have used Jandy type valves. The main weakness of a gate valve is the gate itself, which sometimes detaches from the stem and fails after long-term use. And in an emergency, they require several turns of the handle to close whereas a ball valve closes in a quarter turn.

My Gate Valve won't open (or it won't close)?!? As mentioned above, a major weakness of the gate valve is the stripping of the threaded stem which results in the gate being detached. These valves may be called "spinners". Turn and turn though you may,  the valve gate stays in one place. Another possibility is that the gate has develped a hole or a crack that is allowing water to pass through, even though the gate is closed. In either case, you should be able to loosen the nut on top of the valve, pull out the stem and gate, and replace the "guts" of the old valve with the stem and gate pulled out of a new, identical valve. Or, it may be better just to cut the valve out and replace it with a ball valve or Jandy type valve.

My Gate Valve is leaking water out of the top?!? Inside of the nut, under the handle, there once existed an intact o-ring or other sealing device designed to keep the flow of water inside of the valve. After many years, these sealants may fail. To repair, most plumbers will simply loosen the nut and wrap plumber's tape (teflon tape or packing string) around the stem. Use a small screwdriver to pack it in between the stem and valve body.