Field-mounted instrument

How to read it

Instruments on a P&ID are drawn as circles, called bubbles or balloons, and the lines across a bubble say where the instrument is. A plain circle with nothing across it is field-mounted. The DOE’s handbook calls it a locally mounted instrument; course notes by Peter Woolf and others define a field-mounted instrument as one physically in the field, or the plant, and not accessible to an operator in a control room. A chemical engineering course text by Jonathan Verret and others puts it as near what it is measuring.

Kuphaldt’s Lessons In Industrial Instrumentation explains that the type of bubble tells something about an instrument’s location, under the ISA standard. A bubble with a line across it is a control room instrument, and a bubble inside a square is a shared display.

Also searched as: balloon symbol, field-mounted instrument p&id symbol, field-mounted instrument symbol, instrument bubble, locally mounted instrument symbol.

How to read the tag letters

The letters in a bubble are the instrument’s tag, and the number under them is its loop. In Kuphaldt’s account of the ISA 5.1 system, the first letter is the variable the loop measures or controls, whatever the device itself senses: every instrument in a level loop starts with L, even a transmitter that infers level from pressure. The letters after it are the functions. So FIC is a flow indicating controller, PT a pressure transmitter, TE a temperature element and LT a level transmitter, and FC-135 is a flow controller in loop 135. The DOE’s handbook builds tags the same way, from the sensed parameter, then the indicator or controller type, then the type of component.

Look-alikes

A circle with a zigzag through it is equipment, a heat exchanger, and a circle with a line leaving it at a tangent is a centrifugal pump.