Interpreting P&IDs can often be very challenging especially for beginners. In this piece, I shall be elaborating on some commonly misunderstood terms used in P&IDs to enable the beginner better understand how to interpret the P&ID drawings of their respective plants.
Pilot light
A pilot light indicates which number of normal conditions of a system or device exists. It is unlike an alarm light, which indicates an abnormal condition. The pilot light is also known as a monitor light.
Sensor
A sensor is that part of a loop or instrument that first senses the value of a process variable, and assumes a corresponding, predetermined, and intelligible state or output. The sensor may be separate from or integral with another functional element of a loop. The sensor is also known as a detector or primary element.
Set point
The set point is an input variable that sets the desired value of the controlled variable. The set point may be manually set, automatically set, or programmed. Its value is expressed in the same units as the controlled variable.
Shared controller
This is a controller, containing pre-programmed algorithms that are usually accessible, configurable, and assignable. It permits a number of process variables to be controlled by a single device.
Shared display
This is the operator interface device (usually a video screen) used to display process control information from a number of sources at the command of the operator.
Transducer
Transducer is a general term for a device that receives information in the form of one or more physical quantities, modifies the information and/or its form, if required, and produces a resultant output signal. Depending on the application, the transducer can be a primary element, transmitter, relay, converter or other device. Because the term "transducer" is not specific, its use for specific applications is not recommended
Transmitter
This is a device that senses a process variable through the medium of a sensor and has an output whose steady-state value varies only as a predetermined function of the process variable. The sensor may or may not be integral with the transmitter. A transmitter is often required where the instrument signal needs to be sent to a central control room or transmitted through some distance.
