How do I add PLC and SCADA monitoring to water pump stations?

Both plants run without a controller. Level relays start and stop the lift pumps, and a mechanical time clock cycles the aeration blowers. A PC has nothing to read yet, so the work is a PLC retrofit first and a SCADA layer second. Commission it so the pumps keep running on the existing relays until the new system is proven.

The checks below follow commissioning order. Each one names the reading to take, what the outcome means, and which check comes next.

Existing controller in each pump panel

1. Open each control panel and trace what energizes the contactor coils. Reading for this installation: level relay contacts drive the pump contactors, time-clock contacts drive the blower contactors, and there is no programmable device.
2. If a PLC or smart relay turns up, record the make and model from its rating label and skip to the communication and SCADA checks. In that case SCADA only needs a driver for the existing controller.
3. If only relays and a clock are present, continue to the I/O count. Every status the PC will show has to come from a new PLC input wired to a physical contact.

SCADA is a PC application that reads and writes memory inside a controller over a network or serial link. It does not connect to field devices directly. Without a PLC or remote I/O between the motors and the PC, there is no data path.

I/O count from the existing field devices

Before counting, get panel drawings or do a field walk-down that lists every motor starter, level device and selector switch. The table uses a typical per-motor point set. Treat it as an assumption and adjust it to what is actually in each panel.

Estimated totals under these assumptions: Plant 1 has about 16-17 DI and 4 DO. Plant 2 has about 13-15 DI and 3 DO. Add 20-30% spare so there is room to grow. If you replace the level switches with a continuous level transmitter, add one analog input per wet well or tank. That gives you level trending and lets operators adjust start/stop setpoints from SCADA without moving floats.

Choose the architecture from the site layout:

• Separate sites: give each plant its own PLC.
• Same compound: one PLC with remote I/O is possible. Separate PLCs still keep each plant running independently if the other one faults.

Compact versus modular PLC selection

Any mainstream PLC handles two to three alternating pumps and a pair of timed blowers. The real requirement is expandability, because more points and plants are expected later. Within the Omron range, the compact CP1L and CP1H fit this scale. The modular CJ1 and CJ2 are larger and more costly.

Take these readings before ordering:

1. Built-in I/O. Compare the CPU's built-in DI/DO count from the datasheet against your I/O total plus spare. If it falls short, confirm that expansion units cover the gap.
2. Communication port. Confirm the port type, built-in Ethernet or an option board, matches the link chosen in the next check.
3. Electrical compatibility. Check the CPU supply voltage against the panel control voltage. Check the input type (sink/source) and voltage against the auxiliary contacts you will wire.
4. Local HMI. For operation at the panel, an Omron NP3 (3.8 in) or NP5 (5.7 in) terminal is an inexpensive option. It lets operators see pump status and change timer settings when the PC link is down.

Communication path from each plant to the PC

Measure the physical distance and note the existing infrastructure between each plant and the PC.

• Same building or short cable run: use Ethernet from the PLC port to a switch and on to the PC. Stay within copper segment length limits, or use fiber for longer runs.
• Separate sites with no cable: use a radio modem, cellular router or leased line. Set the SCADA polling rate to suit the link bandwidth, and poll status less often than alarms.
• CPU with serial port only: use a serial-to-Ethernet gateway or a PC serial port, and accept slower update rates.

Before moving on, confirm that the SCADA package lists a driver for the exact CPU and port. SCADA polls the PLC. If the link drops, the PLC must keep sequencing pumps on its own. Keep all pump and blower logic in the PLC and use the PC only for supervision and operator commands.

SCADA tag licence against the real point count

SCADA licences are sized by tag count, and packages define a tag differently. Some count every tag, including internal calculations and alarms. Others count only tags linked to PLC addresses.

AFCON PCIM is one package suited to this job. It includes drivers for common PLC brands at no extra cost and comes in 100, 300, 800, 2000 and Unlimited I/O versions. It counts only I/O tags to the PLC, not internal tags, so derived values do not consume licence points. Examples of derived values are run-hour totals, alarm summaries and pump-cycle counts computed in SCADA.

Estimate of PLC-linked tags for both plants (labeled estimate):

• About 39 field points from the I/O table.
• About 14 command and mode bits (remote start/stop and Remote enable for 7 motors).
• 2 blower time setpoints.
• Up to 7-14 runtime and start counters, if they are held in the PLC.

That totals roughly 60-70 tags, which fits a 100 I/O licence with margin. Move up to 300 if you add analog levels, flow meters or further plants. If your package lets you read a PLC word as one tag and split out the status bits with internal tags, packing bits into words lowers the licensed count. Confirm how the package defines a licensed tag in its documentation before you design the tag map.

PLC logic replacing the level relay and time clock

Plant 1's two pumps swap lead each fill cycle. Plant 2's three pumps rotate the lead. Pumping is permitted only when the well is not low and the tank is not full.

• Case A: blower 1 runs 15 min while blower 2 rests, then they swap, so one blower is always running.
• Case B: blower 1 runs 15 min, both stop for 15 min, then blower 2 runs 15 min.

Watch the existing time clock through one full hour and record which case it follows. Make run and stop times PLC parameters so the same code covers both cases.

Set the feedback-fail time T_FbkFail from observed contactor pull-in and motor start behavior on site. Keep the level start/stop points as physical switch inputs until a transmitter is installed.

Monitor-first retrofit and control cutover

Phase 1 wires the PLC for monitoring only while the level relays and clock keep control. Lock out each panel before wiring.

1. Run feedback: wire a spare normally-open auxiliary contact from each pump and blower contactor to its PLC input. Add an auxiliary contact block where none is free. Confirm the PLC input LED follows the contactor pull-in before wiring the next motor.
2. Overload trip: wire each overload relay's auxiliary contact to its input. Confirm the input changes state when you operate the overload's trip test.
3. Level status: take it from a spare output contact on the level relay or from an added float switch. Do not parallel conductive level probes onto PLC inputs, because the probe relay's sensing circuit and a PLC DC input do not share a circuit cleanly. Confirm the input tracks the relay's own indication through a fill and draw cycle.
4. Parallel run: connect SCADA and run in parallel for several days. Compare the logged starts, stops and blower cycles against what operators see at the plant. Move to Phase 2 only when the records match.

Phase 2 transfers control to the PLC, one plant at a time:

5. Output wiring: drive each contactor coil from a PLC output through an interposing relay.
6. Hardwired protection: keep the overload contact and a low-level dry-run cutoff in series with the coil, independent of the PLC. Confirm the pump cannot start with the low float down, even with the PLC output forced on.
7. Hand-Off-Auto selector: wire it so Hand bypasses the PLC but never bypasses the overload or dry-run cutoff.
8. Remote permission: accept SCADA commands only when the selector reads Auto and a PLC Remote-enable bit is set.
9. Retire old controls: disconnect the level relay and time clock from the coils only after the tests below pass on that plant.

Proving the PLC and SCADA at each plant

1. Point-to-point check: toggle every field input and confirm the PLC input and matching SCADA tag change. Force every output and confirm the correct contactor pulls in.
2. Alternation: run at least four fill cycles and confirm the lead pump rotates in order, two pumps at Plant 1 and three at Plant 2.
3. Fault transfer: trip the overload on the running lead pump. Confirm the next available pump starts and SCADA shows the alarm.
4. Dry-run cutoff: with the PLC in Auto, drop the well level below the low guard. Confirm the pump stops while the PLC output LED is still on, which proves the cutoff is hardwired.
5. Blower timing: time one full blower cycle with a stopwatch and confirm the run and rest periods match the configured times for the case identified above.
6. Communication loss: unplug the PLC network link. Confirm pumps and blowers continue on PLC logic and SCADA raises a communication alarm. Reconnect and confirm the live values recover.
7. Remote command: issue start and stop from the PC with the selector in Auto and Remote enabled, and confirm the contactor follows. Repeat with the selector in Hand and in Off, and confirm the PLC rejects the SCADA command in both positions.

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