Engineering Guide · 6 min read

SAMA Diagrams Explained: How to Read Boiler and Turbine Control Logic

A P&ID shows the hardware of a plant. A SAMA diagram shows its control logic: how signals are measured, combined and turned into valve and motor commands. Power-plant control documentation has long used the SAMA style, which comes from the Scientific Apparatus Makers Association functional-diagramming standard (PMC 22.1).

Function-oriented, not hardware-oriented

The key idea is that each symbol represents a function, independent of whether it is implemented in a DCS, a PLC or analog electronics. The diagram reads left to right: measurements enter on the left, they pass through blocks that process them, and commands leave on the right toward the final control element.

The basic symbol set

Circle — a measurement or readout: transmitter inputs, indicators.
Rectangle / square — an automatic signal-processing function: summer, gain, bias, lag, lead/lag, integrator, high or low select, function generator.
Diamond — a manual function: a hand/auto station or manual setpoint adjustment.
Trapezoid — the final controlling element, such as a control valve or damper actuator.

Inside each rectangle a short label states the function: Σ for a summer, K for a gain, f(x) for a function generator, ∫ for an integrator, d/dt for a derivative, and a logic word such as AND, OR or NOT for logic gates. Signal lines carry arrows, and every block also shows which signal enters which input, which matters for blocks such as a subtractor where the order changes the sign.

Example: three-element drum-level control

Boiler drum level is hard to control because of shrink and swell: when steam demand rises suddenly, pressure falls, bubbles in the water expand and the level momentarily rises even though the water inventory is dropping. A controller that only watches level would first close the feedwater valve, which is exactly the wrong action.

The standard remedy is three-element control. The drum-level measurement is compared with its setpoint and passed through a PI controller. The steam-flow signal is added as a feedforward, so a demand change moves the feedwater valve right away in the correct direction. Feedwater flow is then subtracted, so the final command corrects any mismatch between steam out and water in. A SAMA diagram shows this as a few circles for the three measurements, a summing block, a PI block, a hand/auto diamond and a trapezoid for the feedwater valve. Reading it left to right, you can see what happens when any signal moves.

Why practising matters

Logic that looks right on paper can hide timing problems: lags, dead time and limits change how a loop behaves. The SAMA Logic Simulator on this site lets you place blocks, wire them with the standard shapes and watch signals propagate, so you can test a scheme before it is built into a control system.

Reading tips

Start from the final element and work backwards to see what drives it. Note every high or low selector, since they decide which signal is in control at any moment. Check every limit and rate-of-change block, because they define what happens during start-up and trips. And always confirm the tag names against the P&ID and the loop sheets, because the diagram is only useful if its signals are the plant’s signals.

This guide is general engineering information for education and estimation. Numerical examples come from this site’s calculators; always confirm design values against the applicable standard, the equipment manufacturer’s data and a qualified engineer’s review.

Related calculators

SAMA Logic Simulator Control Loops PID Tuning