A SAMA diagram documents a control strategy as a chain of functional blocks, showing how a measurement becomes a controller output and what happens to the signal along the way. It does not show pipework, physical location or wiring. Where a P&ID draws a controller as one bubble, a SAMA diagram opens that bubble and draws the arithmetic inside it. The notation comes from the Scientific Apparatus Makers Association, and it survives because power generation control strategies are too involved for a P&ID bubble to describe.
In short
- SAMA documents the strategy, the P&ID documents the plant. Both describe the same loop and neither contains the other's information.
- The symbol shape tells you the function class. Circles measure or display, squares process a signal automatically, and a block with a diamond corner means a human can intervene.
- The letter or operator inside a block tells you the computation, such as a summer, a multiplier, a square root extractor or a high select.
- Line style is information. Solid carries an analog value and dashed carries a discrete state, so you can tell computation from decision before reading a single symbol.
- Every sheet is read against its own legend. Owner-to-owner variation is normal because the notation is carried by convention rather than by a maintained standard.
Why the notation exists beside the P&ID
Consider a drum level controller. On a P&ID it is one bubble, LIC-301, with a line to a
control valve. That drawing is complete and correct for its purpose, which is to say what is
installed and how it is piped.
What the P&ID cannot say is that the controller does not act on drum level alone. On any boiler above a modest size, the controller sums drum level with steam flow and feedwater flow, so that a sudden steam demand moves the feedwater valve before the level has had time to fall. That is three measurements, a subtraction, a summation and a cascade. A bubble has nowhere to put it.
The control narrative describes the same behaviour in prose, and prose is readable but not checkable. A SAMA diagram is the middle document, precise enough to configure from and visual enough to argue about in a design review.
The symbol families
Shape carries the first layer of meaning. The families below are consistent across the published references, though individual owners add their own variants.
| Shape | Family | What it represents | Typical examples |
|---|---|---|---|
| Circle | Measuring and readout | A process measurement entering the strategy, or a value displayed to an operator | Transmitter input, indicator, recorder |
| Square or rectangle | Automatic signal processing | A computation the system performs without a human | Summer, multiplier, controller, function generator |
| Block with a diamond corner | Manual signal processing | A point where an operator can act on the signal | Auto and manual station, manual loading station, bias setter |
| Trapezoid or arrow form | Final controlling element | The output leaving the strategy for a device | Valve output, drive speed reference |
The practical use of the shape layer is triage. Scanning a sheet for diamond-cornered blocks finds every place a human can intervene, which is usually the first question asked during a review or an incident investigation.
The operators inside the blocks
The second layer of meaning is the symbol printed inside an automatic processing block. These are the ones worth memorising, because they are what makes the sheet a computation rather than a flowchart.
| Symbol | Function | What it does to the signal |
|---|---|---|
Σ | Summer | Adds inputs, with a sign shown per input where one is subtracted |
Δ | Difference | Subtracts one input from another |
× | Multiplier | Multiplies two signals, commonly used for ratio control |
÷ | Divider | Divides one signal by another |
√ | Square root extractor | Linearises a differential pressure flow measurement |
f(x) | Function generator | Applies a characterised curve, such as a valve or a fuel-air relationship |
> | High select | Passes the higher of two or more inputs |
< | Low select | Passes the lower of two or more inputs |
A/M | Auto and manual station | Transfers between computed output and operator-set output |
K | Gain or bias | Scales or offsets the signal by a fixed amount |
P, I, D | Control modes | Shown singly or combined to state which modes the controller uses |
T | Transfer | Switches the path between two sources on a discrete command |
A controller drawn as a square containing P I D is a three-mode controller. The same square
containing P I is a two-mode controller, and the difference is visible without opening the
configuration.
A worked decode
Three-element drum level control is the strategy the notation is most often used for, so it is the one worth walking. Read it as a chain.
| Step | Block | What enters | What leaves |
|---|---|---|---|
| 1 | Circle, steam flow | Differential pressure from the steam flow element | A raw flow signal |
| 2 | Square, √ | The raw differential pressure signal | Linearised steam flow |
| 3 | Circle, feedwater flow | Differential pressure from the feedwater element | A raw flow signal |
| 4 | Square, √ | The raw differential pressure signal | Linearised feedwater flow |
| 5 | Square, Δ | Steam flow and feedwater flow | The mismatch between what leaves and what enters |
| 6 | Circle, drum level | Level transmitter | Measured drum level |
| 7 | Square, P I D | Drum level against its setpoint | A level correction demand |
| 8 | Square, Σ | The flow mismatch and the level correction | A feedwater demand |
| 9 | Square, P I | Feedwater demand against measured feedwater flow | A valve demand |
| 10 | Diamond-cornered block, A/M | The valve demand | Either the computed demand or an operator value |
| 11 | Trapezoid | The selected demand | Feedwater valve position |
Ten of those eleven steps are invisible on the P&ID, which shows a level controller and a valve. Reading the SAMA sheet tells you why the feedwater valve moves when nothing has happened to the level, which is exactly the question an operator asks and the P&ID cannot answer.
Two details in that chain carry disproportionate weight. The Δ at step 5 is why the strategy
is called three-element rather than single-element, and removing it collapses the scheme to
ordinary level control. The A/M block at step 10 is the only place a human enters, so it is
the block a commissioning engineer proves first.
How SAMA sits against the other drawings
| Question | Answered by |
|---|---|
| What is installed and how is it piped | The P&ID |
| Which tag is which, and what does it measure | The P&ID and the instrument index |
| How does the control computation work | The SAMA diagram |
| Why does it work that way | The control narrative |
| What is wired to what, terminal by terminal | The loop diagram |
| What shuts the plant down, and on what | The cause and effect matrix |
A project that has the P&ID and the narrative but no SAMA sheet has described the strategy twice in words and never once in a form that can be checked block by block. A project that has SAMA sheets and no narrative has the arithmetic without the intent, which is harder to review than it sounds because a correct-looking computation can still be the wrong strategy.
Reading a sheet you did not draw
Legacy SAMA sheets are common on retrofits and control system migrations, and they arrive without the person who drew them. Four habits get a reader through one.
Start with the legend on the sheet itself rather than a general reference, because owner variation is real and a symbol borrowed from another utility's convention is the usual cause of a misread. Trace the line styles before the blocks, so the analog spine of the strategy separates from the discrete permissives around it. Find the diamond-cornered blocks next, since they mark every operator intervention point and therefore every place the diagram stops describing automatic behaviour. Then work backwards from the final element, because a strategy read output-first tends to expose its purpose faster than one read input-first.
On power plant sheets, the tag identifiers will usually be in the plant's own designation system rather than an ISA-style loop tag, which is worth knowing before you go looking for a tag that does not exist in that form. For the coding most commonly met on these sheets, see KKS coding explained.
For the drawing that shows the same loop as wiring, see loop diagrams and how they differ from P&IDs. For the document that states the intent the strategy implements, see writing a control narrative.