Fusible plug loop

Offshore F&G layout practice (ISA 5.1-style letter codes) / ISO 7240 / NFPA 72 / API RP 14C

Drawn as

An instrument-air (pneumatic //) loop with small fusible plug elements (small filled squares marked FP) placed around the wellhead or equipment.

Meaning

Pneumatic fire detection: a plug melts in a fire, the loop pressure falls, and the wellhead or package shuts in. Classic on wellhead platforms. Not modelled; note it on the trees.

Variants

API RP 14C tags them TSE
Some legends draw a circle with "FP" on the loop