TABLE OF CONTENTS Regulators and valves are the last, most expensive, and least understood element in a process control loop. They are used to control the process variable by regulating gas flow, liquid flow, and pressure. In many processes, this involves control of many thousands of cubic meters of a liquid, using low-level analog, digital, or pneumatic signals. Regulating gas and/or liquid flow also can be used to control temperature. Control loops can be local self-regulating loops under pneumatic, hydraulic, or electrical control; or the loops can be processor controlled, with additional position feedback loops. Electrical signals from a controller are either low-level signals, which require the use of relays for power control, or amplification and power switching devices, and possibly optoisolators for signal isolation. These power control devices are normally at the point of use, so that electrically controlled actuators and motors can be supplied directly from the power lines.
Gases used in industrial processing, such as oxygen, nitrogen, hydrogen, and propane, are stored in high-pressure containers in liquid form. The high-pressure gases from above the liquid are reduced in pressure and regulated with gas regulators. Before they can be distributed through the facility, the gas lines may have additional regulators at the point of use. Other types of regulators are used for release of excessive pressures and control of liquid levels.
A spring controlled regulator is an internally controlled pressure regulator, as shown in Figure 13.1(a). Initially, the spring holds the inlet...
