4.3 SCADA & Instrumentation

4.3 SCADA & Instrumentation

Quick Answer: SCADA (Supervisory Control and Data Acquisition) is the system that lets an operator monitor and control remote sites — tanks, pumps, valves, and treatment — from a central screen. Remote RTUs or PLCs collect sensor data and send it over telemetry (radio, cellular, or hardwired) to a central HMI that displays levels, pressures, flows, and alarms and can start pumps or open valves. For small Class I systems, SCADA most often means watching tank level and pump status on a phone or laptop and getting an alarm when something is out of range.

What SCADA does

A distribution system is spread over miles of pipe, with tanks and pumps in places you cannot staff. SCADA exists to give one operator real-time visibility and control over all of it. The three jobs of SCADA are:

1. Data acquisition — read instruments at remote sites.
2. Supervisory control — send commands to pumps, valves, and other equipment.
3. Alarm and trending — warn the operator when a value is out of range and store history for analysis.

The pieces

In practice, RTU and PLC functions often live in one box. The key idea is that the field device is at the tank or pump station and the HMI is at the operator's desk (or phone).

What is monitored and controlled

A Class I operator should be able to list the common SCADA points:

• Tank level — high and low alarms; used to start/stop booster pumps.
• Suction and discharge pressure at pump stations — protects against deadhead and cavitation.
• Flow at master meters and well outputs.
• Chlorine residual at entry points (analyzers, not just grab samples).
• Pump status (running, stopped, fault) and run hours for maintenance.
• Power status (utility power, generator running, transfer switch position).
• Valve position for remote-controlled valves.
• Security / intrusion at remote sites.

On the control side, SCADA can start or stop pumps, open or close valves, and switch sources. Local PLC logic usually handles routine control (a pump turns on at low tank level and off at high), while the operator uses SCADA to override or adjust.

Telemetry options

• Radio — licensed or unlicensed UHF/VHF; common in rural systems with line of sight between sites.
• Cellular — easy to deploy at a single remote tank; a modem and a data plan; common for Class I small systems.
• Hardwired — fiber or leased line; high bandwidth; used in larger utilities.

For a small system, cellular modems at remote tanks reporting level and pump status to a cloud HMI is the typical Class I SCADA.

Alarms and how to respond

SCADA alarms are set with high and low limits. Common alarms a Class I operator handles:

• Low tank level — could mean a pump failed, a valve closed, or demand is outpacing supply. Check pump status and source flow.
• High tank level — could mean a fill valve stuck open or a pump not stopping. Check controls; verify overflow is not occurring.
• Low pressure at a pump station — possible main break, closed valve, or pump cavitation.
• High pressure — possible closed valve downstream or a pressure-control valve failed closed.
• Low chlorine residual — check feed, check analyzer, verify residual in the system.
• Power fail / generator running — confirm generator is carrying load and fuel is adequate.

The operator's job is to acknowledge the alarm, investigate the cause, take action, and log the event. Ignoring a recurring alarm is a common failure mode.

Cybersecurity for small systems

SCADA is a network, and a network can be attacked. AWWA publishes cybersecurity guidance for water systems; the core ideas are simple and apply even at Class I:

1. Segment the SCADA network from the business network and the internet. Do not let the HMI browse the web.
2. Change default passwords on RTUs, PLCs, and modems — a leading cause of intrusions.
3. Restrict access to the HMI; use individual accounts, not a shared login.
4. Patch and update firmware on field devices when patches are available.
5. Log activity — who logged in, what changed, and when.
6. Disable unused services (web servers, telnet) on field devices.

For a small system, the practical checklist is: separate the SCADA modem from the office Wi-Fi, change the vendor password on every device, and give each operator their own HMI login. If a vendor needs remote access, use a controlled, time-limited connection rather than an always-on port forward.

SCADA is not just monitoring

A common exam trap is treating SCADA as if it only displays data. SCADA includes control — the 'C' stands for Control. The operator can issue commands, and the system can run local control logic automatically. Know the difference between monitoring (read-only) and control (read and command), and know that SCADA does both.