A control valve serves as a variable orifice. The variable orifice includes a port and a closure member such as a plug, disk, or ball. In some applications, the closure member of the control valve is in a constant state of motion, responding to continuous changes in the signal from the control system. To ensure proper operation and long life in a throttling application, a control valve must be ruggedly built. The requirement for constant throttling and the heavy-duty construction that is necessary to withstand the rigors of throttling service distinguish control valves from many other valve types. Refer to Figure 2 and note the distinctions between control valves, block valves, and emergency isolation valves.