Sequencers

What a sequencer is for

Some operations always happen in the same order. A filter backwash closes the influent valve, drains to a level, air scours, washes, settles, filters to waste, and returns to service. A high-service pump start pre-lubricates, opens the suction valve, starts the motor, and opens the discharge valve slowly. A polymer batch fills to a level, adds neat polymer for a time, mixes, and ages. Each of these is a list of steps, and the controller code that runs them is a sequencer. The alternative, a tangle of timers and latches that happens to produce the right order, works until someone has to change step four.

What every step needs

Number and name

Numbered by tens so a step can be inserted later; named in the words the operators use. Step 30, Air Scour.

Actions

The outputs held while the step is active: valves open, blower running. Outputs not named are in their default state.

Completion condition

Feedback, not assumption: a valve limit switch, a level, a flow, an analyzer reading, or a timer where time is the only measure. A step that waits for a valve to open watches the open limit, not a five-second timer.

Timeout

The longest the step may take before something is wrong, with an alarm that names the step and the condition it was waiting for.

Fault response

Hold, abort, or retry, decided per step. A timeout on the drain step may hold and wait for the operator; a timeout on the discharge valve during a pump start aborts and stops the pump.

Manual advance

A supervisor-level button that forces the transition, for commissioning and for the day a limit switch is broken, logged when used.

Three ways to build one

Example: filter backwash

Coding pattern

CASE BW_Step OF
  10: (* Isolate *)
      FV_Influent_Close := TRUE;  FV_Effluent_Close := TRUE;
      IF ZSC_Influent AND ZSC_Effluent THEN BW_Step := 20; END_IF;
      IF BW_StepTimer.ET > T#2m THEN BW_Fault := TRUE; BW_Step := 900; END_IF;

  20: (* Drain down *)
      FV_Waste_Open := TRUE;
      IF LT_Filter_PV < BW_ScourLevel THEN BW_Step := 30; END_IF;
      IF BW_StepTimer.ET > T#30m THEN BW_Hold := TRUE; END_IF;  (* hold, alarm *)

  30: (* Air scour *)
      FV_Air_Open := TRUE;  Blower_RunCmd := TRUE;
      IF BW_StepTimer.ET > BW_ScourTime THEN BW_Step := 40; END_IF;

  (* ... *)

  900: (* Abort: safe state *)
      Blower_RunCmd := FALSE;  WashPump_RunCmd := FALSE;
      FV_Air_Open := FALSE;    FV_Waste_Open := FALSE;
      IF BW_Reset THEN BW_Fault := FALSE; BW_Step := 0; END_IF;
END_CASE;

(* Step timer restarts on any step change; step number written only inside the CASE *)

Two disciplines keep this readable for years. The step number is written only inside the case structure, so there is exactly one place to look for how the sequence advances. And outputs are set inside the step that uses them and cleared in a common section that runs when the step changes, so a step cannot leave a valve open by forgetting to close it.

Faults, holds, and power loss

A hold freezes the sequence in its current step with outputs as they are, and waits for an operator to resume, advance, or abort. It fits steps where waiting is safe, such as a drain that is slow because the waste line is restricted. An abort drives every output the sequence controls to a defined safe state and goes to an abort step that waits for a reset. It fits steps where waiting is not safe, such as a blower running against a closed valve. The functional description says which applies to each step; the code should not guess.

Power loss in the middle of a sequence is the case that gets forgotten. If the step number is retentive, the controller powers up in step 50 with the wash pump commanded to run and the valves in whatever positions they drifted to. If it is not retentive, the sequence restarts at idle with the filter half drained and the influent valve closed, and the filter sits there. The robust answer is a recovery step: on first scan, the sequence goes to a step that reads the valve positions and level, alarms that a sequence was interrupted, and either returns to idle safely or lets the operator choose where to resume.

What the HMI shows

• The current step number and name, in the same words as the functional description.
• The step timer and the timeout, so a slow step is visible before it alarms.
• The condition the step is waiting for, as text: Waiting for influent valve closed limit.
• Start, hold, resume, abort, and the supervisor-level advance, each logged.
• The last completed sequence with its total time and any holds, and a trend of step durations over time; a wash that takes longer every week is telling you something.