Soda Ash Control
Soda ash, or sodium carbonate (NA2CO3) is a water-soluble salt of Carbonic acid. It is often used in municipal water treatment plants to raise the pH of the water. “Historically, sodium carbonate was extracted from the ashes of plants growing in sodium-rich soils, such as vegetation from the Middle East, kelp from Scotland, and seaweed from Spain. Because the ashes of these sodium-rich plants were noticeably different from ashes of timber (used to create potash), they became known as soda ash” – source Wikipedia. Soda ash control is an important aspect of the use of soda ash in water treatment facilities.
In this soda ash control application, Sterling Systems & Controls worked with the construction contractor and end-user (municipality) to provide a control system and automation for the soda ash processing and handling equipment. The control system consists of a stainless steel NEMA4X PLC Control Panel that contains a Main Disconnect, E-Stop button, Alarm Horn and Beacon, Alarm Silence button, and a 6” PanelView Compact Color Touchscreen HMI. A second control panel is also included, which is a stainless steel NEMA4X Silo Fill Control Panel containing Manual/Off/Auto (MOA) switches, indicator lights, and an Alarm Horn. Indicator lights notify the operator of silo levels and the status of selected equipment. An alarm horn notifies the operator of all urgent or abnormal conditions. The horn can be silenced at any time via an alarm silence pushbutton and from the HMI.
MOA switches on the Silo Fill Control Panel provide manual-off-auto operation for selected equipment. A switch must be placed in the automatic position for the PLC to control the corresponding equipment automatically. When MOA switches are placed in the A (automatic) mode, the system is operated by setpoints and the system parameters are entered into the HMI. Pressing the on-screen Start pushbutton will enable automatic operation of the selected process. When the system is enabled and in automatic mode, it will control the Slurry Refill, Silo Filling, and Slurry Injection processes as described below.
Slurry Refill Startup Process
The soda ash control system is started as follows:
- The Operator ensures that the Emergency Stop (E-Stop) is pulled out and the system is powered up and ready. Using the Touchscreen HMI, the operator places all equipment in the automatic mode using each item’s MOA switch.
- As long as the E-Stop is pulled out and the Agitator is not in the O (off) Mode, the Agitator will run continuously.
- The operator verifies and adjusts the initial Screw Feeder speed setpoint (feed rate) on the Touchscreen HMI as necessary.
- The operator presses the on-screen Slurry Refill Start push button.
- The System will evaluate the Anti-Blockage switch (AB) and confirm no blockage. An alarm will be generated if a blockage is detected, and no equipment will be started.
- If the Silo Security Level (low level) indicates a security level, the system will generate an alarm, and no equipment will be started.
- If the Slurry Tank High Level indicates a high-level condition, the system will generate an alarm, and no equipment will be started.
- If the Slurry Tank Batch Level indicates that the tank level is at or below the batch level but not at or below the Slurry Tank Low Level, the system will start the Injector Motor and confirm that it is running. A short programmable Spin-Up timer will also be started.
- The system will open the Water Supply Solenoid Valve when the Spin-Up Timer expires.
- The system will simultaneously start the Screw Feeder and Arch Breaker.
- The speed of the Screw Feeder is determined by an initial setpoint on the Touchscreen HMI, which can be changed in real-time at any time from the process screen. System Parameters are provided to define the Maximum and Minimum allowable speed setpoints.
- The Touchscreen HMI displays the status of the equipment on the screen and indicates that the system is running.
Slurry Refill Shutdown Process
The equipment can be stopped in one of 8 ways:
- By Pressing the E-Stop button. This will electrically turn off all equipment immediately and stop the system.
- By Pressing the Slurry Refill Stop pushbutton on the Touchscreen HMI. This causes the system to shut down as described below and Disables the system until the Start pushbutton is pressed again.
- By Detecting the Silo Security Level (low level). The system will attempt to resolve any product bridging by running the silo vibrators for a programmable amount of time. If the SSL condition still exists, equipment will be shut down as described below, and the system will be disabled until the condition has been corrected and the Start pushbutton is pressed again.
- By Detecting the Slurry Tank High Level. Equipment is shut down as described below, and the system remains enabled but idle until the Slurry Tank Batch Level is detected.
- Detecting the Slurry Tank’s Low Level will shut down the equipment immediately. The system will be disabled until the condition has been corrected and the Start pushbutton is pressed again. Note: The process can start when there is a Slurry Tank Low-Level condition, but once the slurry surpasses this level and falls back below, the alarm will be triggered, and the equipment will be shut down immediately.
- By Detecting a signal on the Anti-Blockage switch. Equipment is shut down as described below. The system is Disabled until the condition has been corrected and the Start pushbutton is pressed again.
- By Detecting that the Agitator is not running. Equipment is shut down immediately. The system is Disabled until the condition has been corrected and the Start pushbutton is pressed again.
- By Detecting that the Screw Feeder is not rotating from the Rotation Sensor. Equipment is stopped immediately.
Once the system determines it is time to shut down the equipment, based on one of the conditions listed above, it turns off the Arch Breaker and Screw Feeder immediately.
- The Water Supply Solenoid Valve is also closed, and the system starts a programmable Purge Timer for the Injector Screw.
- Once the Screw Feeder Purge Timer expires, the system will turn off the Injector Screw.
Other Processes Automated
Automation of the following processes also exists in this system:
- Silo Filling Startup
- Silo Filling Shutdown
- Slurry Injection Startup
- Slurry Injection Shutdown
- System Alarms
- Manual Control
- Manual Interlocking Rules
- System Parameters