Sensor/transmitter action

Depending on the degree of potential catastrophe, there usually is more than one safety interlock for a potential catastrophic event. Each of these safety interlocks including the sensor/transmitter, control function, and final control element are usually independent of the other safety interlocks for the same event. For maximum protection, each sensor for the same event should be unique to eliminate the potential of a common failure. The safety interlock must be fail-safe. This means that any loss of interlock power—electricity, air, hydraulics, etc.— loss of signal, must produce the same action as the safety interlock produces when it is activated (tripped). 

For the components shown in Figure 3.15, assume the action shown in Figure 3.16, and also assume an air to open actuator (I/I) for the valve. Note that the valve, process, and sensor/transmitter are all direct acting. Therefore, in order to get the desired negative feedback action (I/D overall loop action) the controller must be set to reverse action. 

Process-variable feedback for the controller is achieved by one of two methods. The process variable can (I) be measured and transmitted to the controller by using a separate measurement transmitter with a 0.2-I.0-bar (3-15-psi pneumatic output, or (2) be sensed directly by the controller, which contains the measurement sensor within its enclosure. Controllers with integral sensing elements are available that sense pressure, differential pressure, temperature, and level. Some controller designs have the set point adjustment knob in the controller, making set point adjustment a local and manual operation. Other types receive a set point from a remotely located pneumatic source, such as a manual air set regulator or another controller, to achieve set point adjustment. There are versions of the pneumatic controller that support the useful one-, two-, and three-mode combinations of proportional, integral, and derivative actions. Other options include auto/manual transfer stations, antireset windup circuitry, on/off control, and process-variable and set point indicators. 

Instrumentation is the critical link between the manufacturing process and the control system. Instruments are the eyes (i.e., transmitters, sensors) and limbs (i.e., actuators, positioners) of a process control system and enable it to perform the actions that were once performed by operators and laboratory technicians. If an instrament should malfunction, data integrity and the predefined control actions will be affected. Indeed unsatisfactory instrumentation can cause significant operational problems. It is essential that an instrument is carefully chosen to be fit for purpose (i.e., correct type, size, materials, accuracy, repeatability, reliability, documentation, etc.) to enable confidence to be gained in its ability to perform its intended function. ... 

Measurement devices (sensors and transmitters) and actuation equipment (for example, control valves) are used to measure process variables and implement the calculated control actions. These devices are interfaced to the control system, usually digital control equipment such as a digital computer. Clearly, the... 

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