A SIEMENS LOGO!8 PLC was installed into a refrigeration system for the:
- ammonia compressors sequential and parallel operation
- monitoring of critical and safety parameters in order to adjust and/or stop the system in case of limits excheeding
- recording of parameters values into its embedded data logger, for further user's processing.
The installation is comprised of three multistage piston compressors of 75KW each. The scope was to replace the old control system with a newer, more reliable and also to provide additional functions.
In order to select the appropriate PLC the following things were taken into consideration:
- the required number of Inputs-Outputs, both analog(AI/AQ) and digital(DI/DQ)
- its memory size and the ability to install expansion modules
- the communication protocol - the selected comes with an integrated web server and Ethernet port
- the programming software and the various functions it can process, FBD, LAD, User Defined Functions, Mathematical functions etc
- LOGO!POWER 230VAC/24VDC/1.3A
- LOGO!8 12/24RCE
- LOGO! AM2 2XINPUT(0..10V/0...20mA)
- LOGO! DM16 24R 8xDI/8xDO
The PLC monitors in real time the water level at the reservoir, the Power quality (overvoltage, assymetries etc) and the refrigeration's network HI and LOW Side Pressure values and limits. The compressors Start/Stop according to LIFO sequence order at preselected pressure levels. Further, The compressors priority sequense change every 24hours (1-2-3, 2-3-1, 3-1-2) in order to deliver similar Running Hours to each of them. In case of EMERGENCY the system restarts ONLY manually, after the operator has executed the required checks and repairs.
The PLC program was implemented by applying Function Blocks (FB) in order. What is really interested is the ability to create re-usable User Defined Functions (UDF), with specific I/O. This way provides better control of the entire program, simple and smaller routines and of course easier errors handling.These UDFs are stored into separate files and are inserted to the desired place within the main program. This application has a unique UDF for each compressor Loading/Unloading per stage, motor Starting/Stoping and entire compressor status monitoring.
Monitoring interface is achieved with the TDE Display. However, the user can display the PLC status through the Web Server's website or the LOGO App in the mobile phone. For safety reasons, for this application the PLC was not connected to the LAN, so monitoring and control is done only locally.
Text and images by george vakfaris.