Acid-Dosing Control for District-Heating Make-Up Water

The acid-handling system doses 92% concentrated sulphuric acid to prepare a solution of about 1% concentration. The solution regenerates H-cation filters in the district-heating make-up-water blocks and the chemical demineralisation unit. Metering pumps deliver acid from measuring tanks to a mixer, where it is combined with raw water. The system uses the acid concentration after the mixer as its feedback signal.

Automated process functions

• Protect acid measuring tanks and the low-point tank against high levels;
• Protect against excessive discharge pressure from the dosing pumps and excessive acid temperature upstream of check valves;
• Run sequential logic for supplying regeneration solution to the demineralisation and district-heating make-up-water filters, filling acid measuring tanks, and returning the low-point tank contents to the measuring tanks;
• Allow the operator to control actuators remotely from a computer;
• Regulate regeneration-solution concentration automatically;
• Provide warning and emergency alarms, archive process parameters, and display current and historical data.

Control-system architecture

The lower level uses a SIMATIC S7-315 2DP controller with SM-321, SM-322, SM-331 and SM-332 signal modules. The 15-module configuration requires an expansion rack and IM 365 interface modules. STEP 7 and S7-SCL implement the controller’s input/output, protection and control logic.

Two operator stations, configured to back each other up, use Pentium 4 computers with 19-inch LCD monitors running Windows and the TERMOCONT SCADA system. An MPI interface with an OPC server links the upper and lower control levels.