PLCs for municipal water distribution telemetry

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This article about PLCs for wastewater telemetry is the seventh in the series
“All about municipal water supply telemetry“.

If you want to design and implement a telemetry system for water and wastewater reservoirs and pumping stations, water and wastewater treatment plants, pressure regulating stations, and macro-measurement points, you will find in this series of articles all the knowledge you need to design, build, and implement complete systems.

Along with the articles, links are provided to download complete electrical panel designs, as well as Ladder software for station automation and customizable SCADA software with screens for up to 10 tanks and 10 water lifts, all absolutely free of charge.

Read the article: ALL ABOUT MUNICIPAL WATER SUPPLY TELEMETRY

The solution we advocate and share for the construction of the telemetry system for municipal water distribution is the one based on PLCs. In this article we talk about:

  • Important factors when choosing a PLC
  • Telemetry panel architecture
  • PLC Sizing
  • Installation precautions

Factors to take into account when selecting the PLC

  • Technical support – The question is: when a question arises, do you have someone to call? When you call, does technical support help you think through and solve the problem? Look for brands with good technical support.
  • Cost-benefit – Do a good research and evaluate price x overall benefits of the product.
  • Cost of the programming tool– There are many brands that do not charge for the tool.
  • Processor performance – Look for the latest generation products with fast processors. Compare instruction execution times.
  • Real-time clock – It is important that the PLC has a real-time clock for collecting and storing data in historical tables.

HaiwellHappy Tool

  • Ability to simulate the program without having to connect to the PLC – This is a feature that only the most modern PLCs have, and they don’t necessarily cost more for this reason.
  • Communication ports – The number and type of communication ports is important, look for products that have the highest number of native communication ports.
  • Communication Protocols – For sanitation applications, we understand that the most important communication protocols are Modbus, TCP/IP, and Modbus TCP.
  • Remote programming capability – This is another key feature when you are looking to minimize maintenance costs because it allows you to diagnose and solve problems remotely, as well as perform software and firmware upgrades without the need to travel to the remote station.
  • Ease of maintenance – Detachable terminal blocks, easy replacement of modules, and good factory support are essential when choosing the brand of PLC that will be used for the municipal water distribution telemetry system.

Telemetry panel architecture

The PLC is the heart of the telemetry panel and the starting point of its design.

Components of the (remote) telemetry panel:

  • Power supply – Transforms alternating mains voltage to the usual DC voltages, usually 24 VDC, and manages the battery charge for operation in the absence of mains power;
  • CLP(Programmable Logic Controller) – Responsible for all local processing and automation of the remote;
  • Input Interfaces – Condition the field signals supplied by the sensors. They may be incorporated into the PLC or be external modules;
  • Output Interfaces – Condition the analog and digital signals produced by the PLC CLP for actuator control. They may be incorporated into the PLC or be external modules;
  • Radio modem – These can be radios spread spectrum. channeled or scrambled radios GPRS/GSM. They allow the remote to communicate with the CCOor with other remotes.

Example of a PLC installed in a telemetry panel.

PLC Sizing

One of the steps in defining an automation system is the PLC sizing. To do this, we must list the devices involved in the system and list for each one the number of analog and digital inputs and outputs needed for control and monitoring.

The table below is an example of how to format this information and get the input and output point totals. With these numbers, and adding a percentage of slack, usually between 10 and 20%, we can select the PLC model and modules that will compose it.Sizing of the treated water tank PLC

A typical tank can have the following sensors and actuators:

  • Reservoir level transmitter;
  • Macro flow meter;
  • Intrusion indicator;
  • Audible alarm.

The table of inputs and outputs of the PLC can be as follows:

The PLC for the tank automation panel that will serve this installation should have at least 3 analog inputs, 3 digital inputs, and 2 digital outputs. An RS232 or RS485 serial communication port is also required for radio communication with the CCO.

Selected setting for the example:

  • CPU Model Haiwell T16S0T: 8 DI + 8 SD + COM RS232 + COM RS485
  • Haiwell S04AI analog input module: 4 AI

Dimensioning the CLP of the treated water elevator

We consider in this example a lift station consisting of:

  • Pressure transmitter at the lift entrance;
  • Pressure transmitter at the outlet of the elevator (booster);
  • Two groups of direct start pumps;
  • Measurement of voltage, current, and power factor of the motor-pump groups by multimeters of electrical quantities, one per group, communicating by RS485 with the PLC;
  • Intrusion indicator;
  • Audible alarm.

Each pump drive panel provides the following digital signals for PLC control:

  • Drive command (digital output from the PLC);
  • Local Manual/Automatic switch (PLC digital input);
  • Confirmation that the pump is armed and running (digital input from the PLC).

Thus, the table of PLC inputs and outputs is as follows:The PLC for the tank automation panel that will serve this installation should have at least 3 analog inputs, 6 digital inputs, and 4 digital outputs. It also requires an RS232 serial communication port to communicate by radio with the CCO, and an RS485 port to communicate with the electrical quantities multimeters.

  • CPU Model Haiwell T16S0T: 8 DI + 8 SD + COM RS232 + COM RS485
  • Haiwell S04AI analog input module: 4 AI

Other Settings

Other station configurations, such as reservoir centers with more than one reservoir, pumping stations with more than two pump groups, motors driven by inverters or soft starters, boosters, macro measurement points, etc., follow the same PLC sizing idea. This article presents how to dimension the PLC and should not be understood in a limited way, but as a practical procedure to define the number of IOs and communication ports.

PLC installation precautions

The PLC must always be mounted on control panels, in our case it is the telemetry panel itself, properly grounded and protected against surges and installed in ventilated places and as far away from the corrosive vapors of gases such as chlorine used in water treatment.

Electrical grounding and protection systems are extensively covered by the NBR5410 standard, which is easily obtainable on the Internet.

Surge protection on AC power input

Surge Protective Devices must be installed on the AC power input of the telemetry panel. The module

S


W3300

 is an example of an SPD designed to compose electrical command and automation panels and integrates the following functions:

  • Sectioning
  • Over-current protection by means of fuses
  • Over voltage protection by varistors
  • Bipolar socket with ground
  • Power-on Signaling

By including several functions in a single module, the device simplifies frame assembly and therefore contributes to more compact lay-outs.

Analog input protection against surges

Most of the 4 to 20 mA inputs of
PLC
s have a resistor of about 150 to 200 ohms at their input.

What happens when the sensor shorts out and supplies the 24 V, with no current limit, to the 4 to 20 mA analog input? The resistors used in the analog inputs of the
CLP
 are not sized to support this power and will inevitably burn out.

O

circuit shown alongside

 protects not only the analog channel, but also the 24 V supply that is provided to the field sensor. The protection is in three stages, by means of the three types of surge suppressors:

  • Gas spark gap;
  • Metal oxide varistor;
  • TVS diode.

Digital Output Protection

ID2908 - Relay isolator for 8 digital outputs

We always suggest using PLCs with transistor outputs and external isolator relays. Why do we suggest this? Because in the case of a current overload that can happen when you drive a solenoid or contactor coil in progress, this damages the relay. If the relay is internal to the PLC it will be necessary to change the digital output module, while if the relay is external to the PLC it will be sufficient to replace the relay.

If you are looking for a way to reduce the space taken up by relays in your automation panel, here is a simple, functional, and cost-effective solution.

This device

 is designed to create 8 isolated relay outputs for use with 24 VDC transistor output PLCs. The vertical mounting of the isolator module allows us to have 8 relays on only 23 mm of DIN rail.

Surge protection on RF (radio frequency) connection

Always use surge protectors when connecting the antenna cable. Also called RF spark gaps, these devices protect the radio and make it easier to connect the panel to the external RF cable.

 

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