Pump with flow control loop (P&ID)
Classic centrifugal pump pulling from an atmospheric tank, with a flow transmitter (FT-101), flow indicating controller (FIC-101), and a fail-closed pneumatic control valve — the minimum viable P&ID that every process engineer recognises at a glance.
For the process engineer
The pump-with-flow-control loop is to P&ID what the "hello world" program is to software: every process engineer has drawn one, every piping textbook uses it as chapter one, and once you understand it you understand the grammar of every more complex diagram.
What the diagram shows. T-101 is an atmospheric tank (dome-roof symbol, open to atmosphere). The centrifugal pump P-101 pulls fluid from the bottom nozzle and pushes it downstream through a 4-inch water service line. The flow transmitter FT-101 — a field-mounted discrete instrument, drawn as a plain circle per ISA-5.1 §4.1 — sits on the discharge line L2 and measures the volumetric flow rate. Its 4–20 mA electric signal travels to FIC-101, the flow indicating controller in the main control room (drawn as a circle with a horizontal line through it, indicating it is panel-mounted and has a display). FIC-101 computes the error between the setpoint and the measured flow and drives V-101, a diaphragm-actuated control valve, open or closed via a pneumatic signal. V-100 is a manual gate valve upstream — the isolation valve you close when you need to pull the pump for maintenance.
ISA-5.1 signal line types. The two non-process lines in this diagram are drawn differently from the main pipe: the electric signal between FT-101 and FIC-101 is a dashed line (stroke-dasharray: 6 4), and the pneumatic signal between FIC-101 and V-101 is a solid line with small perpendicular slash marks at regular intervals. These are not stylistic choices — they are ISA-5.1 §5.2 mandatory line type codes. An instrument technician reading the diagram needs to know, at a glance, whether a run of wire failed or an air supply pressure dropped. The line type tells them which.
Fail-safe position. V-101 is marked fail: "FC" — fail-closed. When the air supply to the diaphragm actuator is lost (instrument air header trips, tubing rupture), the spring return pushes the valve shut. This is the correct failure mode for a pump discharge valve: loss of control should stop flow, not open it fully. If this were a cooling-water valve on a reactor, you'd want fail-open instead. The choice of fail position is one of the first decisions in a safety instrumented system review (HAZOP, SIL assessment) and it belongs on the P&ID from day one.
Why not draw.io. Process engineers currently draw P&IDs in AutoCAD P&ID, SmartPlant P&ID, or draw.io with stencil libraries — all of which require manual symbol placement and wiring. The Schematex DSL lets you describe the topology and generate a standards-compliant SVG. This is particularly useful when a process is described in prose (from a process description document or a licensor's PFD) and needs to be turned into a P&ID for HAZOP review — the conversion becomes a text transformation task that LLMs can assist with, producing a reviewable first draft rather than a blank canvas.