Water valves comes in a variety of designs, with some specialized functions as well. From the outside, most types of valves probably look the same, but the blocking mechanism can be quite different. Some of the most common types of domestic water valves are as follows:
Some valves are specifically designed to automatically open to release the gas or water pressure inside of it; these are called safety valves, or pressure relief valves. When the valve contains dangerous liquids or gas, including toxic chemicals and flammable petroleum, it must be perfectly secure. Once the valve is closed, it should not allow the liquid or gas to pass through it under any circumstance.
An perfect example of this kind of valve is one fitted into hot water outlet. When you turn the handle clockwise or counterclockwise, the valve will move up and down. As shown by the image on below, water will flow from the horizontal pipe and come out of the vertical pipe. You can turn the handle to allow different amounts of flow, allowing you to control the amount of water you will get. The green colored component at the bottom is the safety item. When hot water pressure builds up, it will push the safety valve down; the hot water will escape through the bottom and release the pressure. In other words, the designer intentionally lets the valve fail through a safety relief. When the pressure gets too high, it is better to let the valve open and release the pressure, rather than letting the pressure keep building up and destroy your pipe, boiler, or hot water heater.