The Control Narrative Is the Contract

The plant is finished, the integrator is gone, and the operators have a question: what does the filter do when the influent turbidity spikes? The engineer says the specification covered it. The integrator says they built what they were told. The specification says the filters shall be automatically controlled. The program says something, in two thousand rungs, that only the person who wrote it can read. The question goes unanswered until the next turbidity spike answers it. This is the failure mode of a project with no control narrative, and it is the commonest failure mode there is.

What a narrative is

A control narrative, also called a functional description or a sequence of operations, is a document that says in ordinary language what the control system does, organized by process area and by piece of equipment. For a lift station it says: the station has two pumps, controlled by wet well level from a submersible transmitter, with floats as backup; in automatic, the lead pump starts at this level and stops at that one, the lag pump starts here; the lead alternates each cycle; a pump that fails to start within ten seconds of its command is faulted and the other is promoted; on loss of the level signal the station runs on floats and alarms; on loss of communication the station holds its setpoints and continues; these are the alarms, these are their priorities, these are the setpoints and the limits within which operators may change them. Ten pages for a lift station, a hundred for a plant, and every one of them readable by an operator.

Why before the program

Writing the narrative first forces the questions that otherwise get answered by the programmer alone at eleven at night: what happens when the transmitter fails, which pump starts after a power outage, whether the operator may raise the high level alarm and how far. Those are utility decisions and process decisions, and the people who should make them are in the room when the narrative is reviewed and not when the rung is written. The integrator then builds to a document that everyone agreed, and every ambiguity resolved in the review is a change order that did not happen.

The contract

Once agreed and signed, the narrative is what the integrator is obliged to deliver and what the utility is obliged to accept. The specification says the system shall be automatic; the narrative says what automatic means, and disputes about scope become questions of whether the narrative says so. It protects both sides. The integrator is not asked to build features that were never described, and the utility is not handed a program that does something nobody described. The engineer who writes a specification without a narrative has left the contract to be written by whoever writes the program.

The test plan

Commissioning without a narrative is a demonstration: the integrator shows the system working and everyone nods. Commissioning with one is a test: every sentence in the narrative becomes a check, performed with the equipment, witnessed, and recorded. The lag pump starts at the lag level; the fault on a pump promotes the other; the loss of the transmitter puts the station on floats; the communication loss holds the setpoints. A system passes when it does what the narrative says, and the signed test record is the proof that it did.

After startup

The narrative is the operator manual, because it says what the system does in the language operators use. It is the document a new engineer reads before opening the program. It is what every change is made against: a change request says which paragraph changes and how, the program is changed to match, the test is repeated for that paragraph, and the narrative is reissued. A plant that keeps its narrative current has a control system that can be understood without the person who built it. A plant that does not has a program and a rumor.

Who writes it

The engineer of record drafts it, from the process design and the utility standards, because the engineer owns the design intent. The utility reviews it line by line, because the utility owns the operation and knows what the last system did wrong. The integrator reviews it for what can be built and what the platform does differently, and proposes the alternatives. All three sign it. An integrator asked to write the narrative for a system they are also building will write one that describes what they were going to build anyway, which is not nothing, but it is not a contract.