A flow control valve is a valve that will allow the flow of liquids to be controlled or stopped. Control valves have a wide spread use throughout the industry. A common type of flow control valve is the needle valve. A needle valve consists of a valve stem and a seat. The end of the stem is tapered, and when lowered into the seat, restricts flow. If the stem is lowered all the way down, flow is stopped. When specifying flow control valves, it is important to know the correct diameter required, the system pressure and flow, and the operating temperature that the valves are to be subjected to.