Page review information
Edition covered
Water/wastewater controls practice — IEC 61511 · IEC 62443 · ISA-18.2 · AWWA/EPA context — exact governing revisions not yet recorded — verify on the linked standards pages
Content status
Reviewed
Coverage
Control patterns for intake screens, wet wells, and pump stations; illustrative sequencing, site design governs.
Last reviewed
August 2026
Primary reference
The official published standard — required for final decisions

Confirm the governing edition, jurisdiction, and project-specific requirements before applying anything on this page.

Water/Wastewater — System Reference

Intake and Raw Water Pumping

Scope: Raw water intake screens, wet wells, and pump stations that lift raw water to the treatment plant headworks. Covers pump start permissives, VFD speed control, multi-pump sequencing, and protection trips.

Standards Applicability

Standard Role in this system
IEC 61511 SIL assessment for wet well Low-Low shutdown (prevents pump damage, dry run)
ISA-18.2 Alarm rationalization — screen dP, suction pressure, motor temperature
NEC Art. 430 Motor branch circuit protection and overload sizing
AWWA M17 Pump station design reference (suction conditions, NPSH, pipe sizing)

Pump Station Control Architecture

flowchart TD
    A[Level Sensor LT-101\nWet Well] --> B[PLC]
    C[Pressure PT-101\nSuction] --> B
    D[VFD-101 Status] --> B
    E[ZSO-101\nDischarge Valve] --> B
    B -->|All permissives OK| F[Start Pump P-101\nRamp VFD to setpoint]
    B -->|Any permissive fail| G[Hold / Alarm to SCADA]
    F --> H[Monitor FT-101\nFlow Confirmation]
    H -->|Flow confirmed| I[Normal Operation]
    H -->|No flow 30s| J[Alarm: No Flow — Check Pump/Valve]

Start Permissive Chain

All permissives must be satisfied before any pump start command is accepted. Hardwired to motor control circuit AND mirrored in PLC for SCADA visibility.

flowchart LR
    A[Wet Well Level\nLT-101 > LL setpoint?] -->|Yes| B[Suction Pressure\nPT-101 > minimum?]
    A -->|No| X[HOLD — Wet Well Low-Low]
    B -->|Yes| C[VFD Ready?\nNo active fault]
    B -->|No| Y[HOLD — Suction Loss]
    C -->|Yes| D[Discharge Isolation\nZSO-101 open confirmed?]
    C -->|No| Z[HOLD — VFD Fault]
    D -->|Yes| E[Motor Temp OK?\nTE-101 < 120°C]
    D -->|No| W[HOLD — Valve Not Open]
    E -->|Yes| F[PERMIT — Start Pump]
    E -->|No| V[HOLD — Motor Overtemp]

VFD Speed Control

Raw water pumps run on a flow demand PID loop:

Parameter Value
Process variable (PV) Headworks flow FT-201 (m³/h)
Setpoint (SP) Operator-set target production flow
Manipulated variable (MV) VFD speed reference (0–60 Hz)
Minimum speed 20 Hz (prevents suction pipe sedimentation)
Acceleration ramp 5 Hz/s
Deceleration ramp 3 Hz/s

Key Engineering Decisions

Why hardwire the Low-Low level interlock? Dry running a vertical turbine pump destroys the pump within seconds. The Low-Low shutdown is a SIF per IEC 61511 — the logic must be in the safety layer (hardwired or Safety PLC), not relying on the process PLC to respond in time.

Multi-pump sequencing: Lead pump ramps to 58 Hz before assist pump starts. This prevents simultaneous starting inrush and avoids hunting between pumps. Lead/lag rotation occurs every 24 hours to equalize wear.

NPSH margin: Verify net positive suction head available (NPSHA) is ≥ NPSHR + 0.5 m margin at all wet well levels, including Low-Low. Document in pump data sheet.

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