How Many PLCs Are Required


Thread Starter


Requests for PLC in an oil/gas measuring station:

-measuring 50 Modbus analogic value;

-open close 6 electrical valves;

-emergency shut down the station (command some valves) after receiving an input from a smoke/fire system.

-transmit data into a SCADA system.

Please tell me how many PLCs are needed to fulfill all the above requests? Which are the proper architecture in terms of PLCs for a such station?

Thank you!
Based on your information, you will need at least 2 PLC :

- PLC for Process Control or for data acquisition system the will feed the data read to SCADA system for further process. Modbus functionality also covered by this PLC. But it is not clear for me
regarding "50 Modbus analogic value". is it meaning 50 Modbus register from one slave or means 50 slave register? If it is 50 registers from 1 slave, it should be ok to be implemented on 1 Modbus master in 1 PLC. but if this means 50 slaves, it will need multiple Modbus master in one or two PLC (depend on PLC capacity on Modbus master) PLC to get proper data speed, so the 50 nodes will be distributed on several Modbus masters.

- PLC for Shutdown System and Fire/Gas System that will take control of 6 electrical valves (I assume this will be shutdown valve). This PLC also can be connected to SCADA directly or through PLC for process control.

Your PLC system architecture will be similar like this:

Hope this help
A single medium sized PLC like a Schneidier M340 or M580 can fulfill this role. Make sure that communications ports, serial and Ethernet are available on the PLC you select and that they support Modbus protocols. Current IEC software (like Unity Pro) can handle all necessary logic and computations.

I assume that by "modbus analog values" that you are referring to values read via communications (Ethernet preferred) from something like a flow computer.

Just completed a pump station (2 input lines, 3 pumps, 1 outlet line, omni flow computer, massive communications with control centre, communications with a remote PLC site, communications with VFDs, communications with HMI, communications with micromotion flow transmitters) using a single rack M340.
Plllllllllllleeeeeeeeeeease perform some kind of hazard analysis to determine the required architecture.

The anecdotal "I used this or that" for "this application" may or may not be good for YOU.