The main items of process equipment have automatic instrumentation that controls the pressure and/or liquid level and sometimes temperature within the equipment. Figure 1-25 shows a typical pressure controller and control valve. In the black box (the controller) is a device that sends a signal to the actuator, which opens/closes the control valve to control pressure. Figure 1-26 shows a self-contained pressure controller, which has an internal mechanism that senses the pressure and opens/closes the valve as required.
Figure 1-27 shows two types of level controllers that use floats to monitor the level. The one on the left is an on/off switch, and the two on the right send an ever-increasing or decreasing signal as the level changes. These floats are mounted in the chambers outside the vessel. It is also possible to mount the float inside. Capacitance and inductance
probes and pressure differential measuring devices are also commonly used to measure level. Figure 1-28 shows a pneumatic level control valve that accepts the signal from the level controller and opens/closes to allow liquid into or out of the vessel. In older leases it is common to attach the valve to a controller float directly through a mechanical linkage. Some low-pressure installations use a lever-balanced valve such as shown in Figure 1-29. The weight on the lever is adjusted until the force it exerts to keep the valve closed is balanced by the opening force caused by the head of liquid in the vessel. Temperature controllers send signals to control valves in the same manner as pressure and level controllers.
