Common Terms Used to describe P&ID Drawings

Interpreting or describing P&ID may be a very challenging task for beginners. In this article, we will show some common terms used in P&ID that make beginners or freshers understand how to describe the P&ID diagrams of their respective industrial projects.

Computing Device:

• This device is commonly known as an arithmetic and logic unit.
• This device performs all required arithmetic calculations and logic operations and displays the computed output valve to output devices.
• This computing device is known as a computing relay.

Converter:

•  It is an electronic device that receives a measured valve from the sensor as its input.
• This device receives a sensor’s output signal in one form and translates the same information into proportional another form of signal designated as a transmitter.
• Basically, the physical quantity of the signal is converted to a proportional electrical quantity. Such as temperature in degrees to mill volts or in ohms in case of T/C or RTDs.
• A converter is also known as a transducer which is a completely general term.
• Generally, a Flow Element (FE) is connected to a Flow Transmitter (FT), but not to a converter (FY).
• Though a device called a transducer sometimes may not be recommended for signal conversion.
• Current (I) to Pneumatic (P) converter is the most recommended converter that we often come across it in most P&ID drawings.

Sensor:

• A sensor is the input device called detector or primary element it is a part of the instrument loop.
• This sensor initially senses or detects the process value of a process variable, and assumes a corresponding, predetermined, and intelligible state or output.
• The sensor is an integral element within the functional loop.

Transducer and Transmitter:

• The term transducer is a general device that converts or modifies the information received by the sensor.
• The transducer produces a resultant output signal by sensing a process variable through the medium of a sensor in the form of a physical quantity and this signal is converted into proportional electrical quantity.
• Depending upon the application type, the transducer can be considered as a primary element, transmitter, relay, converter, or another device.
• This device has an output whose steady state value varies only as a predetermined function of the process variable.
• The sensor may be an integral element of the transmitter.
• A transmitter is a device that is required when the measured instrument signal needs to be sent to some distance or to a central control room.

Monitor:

• The term monitor in instrumentation is used to sense, measure, and control the status or magnitude of more than one process variable to derive some useful information.
• The monitor can also be used as a verb and is unspecific when used in P&ID drawings sometimes meaning analyzer, indicator, or alarm.

Panel:

• A panel is a structure or a rack that consists of a group of instruments mounted in it.
• This panel houses the operator process interface and has a unique designation.
• The panel includes one or more sections, cubicles, consoles, or desks.
• The panel is a synonym for the board on Piping & Instrumentation Drawing.

Local:

• The term local defines the instrument type and installed location for particular process control in industry.
• The instrument is neither in a panel nor neither console nor it is mounted in the control room.
• A local instrument is commonly in the sector or in area of a primary element or a final control element.
• The word FIELD is often used synonymously with the term local.

Local Panel:

• The term local panel is not a central panel or main panel.
• Local panels are commonly seen in the sector or in the area of plant subsystems or sub areas of plant.
• Note that the term Local Panel Instrument must not arise confusion with another term Local Instrument.
• The word local instrument implies an instrument in the field.

Panel Mounted:

• The term panel mounted is applicable to an instrument that is mounted in a panel or in a console.
• It makes it easy to access for normal use for an operator.
• Generally, a function that is accessible by an operator in a shared display system is the equivalent of a discrete panel-mounted device.

Shared Display:

• The term shared display is the operator interface device usually a video screen called a monitor.
• This displays the process and control information from various sources of that control system at the command of the operator.

Shared Controller:

• The term shared controller is a type of electronic controller that includes pre-programmed algorithms.
• These algorithms are usually accessible, configurable, and assignable.
• This controller allows various process variables to be controlled by a single device.

Set Point:

• For every instrument, the term set point is an input variable for setting a desired fixed value of the controlled variable.
• The set point is set manually, automatically, or through the program.
• Both set point and controlled variable values are expressed in the same units.
• The term set point is used in closed loop system and is most important for controlling a process within the design or operating parameter of the process.

Pilot Light:

• A pilot light indicates the existence of number of normal conditions a system or device.
• It is different from an alarm light, used to indicate an abnormal condition
• In other words the monitor light is noted as pilot light.