Identify Process Variables in P&ID

Identify Process Variables in P&ID as they apply to the level controller shown in this P&ID (LIC 135), controlling the level of liquid in the horizontal receiver vessel:

  • Process Variable (PV)
  • Setpoint (SP)
  • Manipulated Variable (MV)
  • Process alarm

Identify Process Variables in P&ID


Setpoint (SP): The point at which the controller tries to maintain the liquid level inside the vessel

Manipulated Variable (MV): The controller’s output signal, which tells the control valve how far to open or close, thus influencing the amount of liquid exiting the vessel at the bottom.

Process alarm: level indicator (LI) does double-duty as a high- and low-alarm unit in addition to being an indicator for the operators. We know this from the “LAL” and “LAH” labels near the bubble.

Incidentally, the “LG” instrument on the left-hand side of the receiver vessel is a level gauge, also known as a sightglass. It is used for manual inspection of vessel level.

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Credits: Tony R. Kuphaldt

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