Key decision points in reverse osmosis control system design
As one of the core technologies in the field of industrial water treatment, reverse osmosis (RO) technology has been widely used in high-purity water preparation, wastewater reuse and seawater desalination in the power, chemical, pharmaceutical, and electronics industries. The design of its core control system is directly related to the system's operating efficiency, energy consumption level and equipment life.
Cooperative Control of Preprocessing Systems
The reverse osmosis membrane is sensitive to the quality of the influent water and needs to be pre-treated at multiple stages (such as sand filtration, activated carbon adsorption, and ultrafiltration) to remove pollutants such as suspended solids, colloids, and organic matter. The control system needs to achieve:
- Multi-level process linkage
Dynamically adjust the coagulant dosage, backwash frequency and filtration accuracy according to the raw water quality;
- SDI (pollution index) online monitoring:
Real-time feedback of pretreatment effects through turbidity meters and particle counters to avoid clogging of membrane components;
- Optimization of pH Adjustment
Adjust the amount of acid added according to the alkalinity of the water source to control the risk of scaling and reduce reagent consumption.
Dynamic matching of high pressure pump and pressure regulation
The core driving force of the RO system comes from the high-pressure pump. The control system needs to solve:
- Frequency conversion speed control
The pump speed is adjusted by the frequency converter to match the water flow demand and reduce the impact of start-stop;
- Pressure balance between segments
In the multi-stage RO system, pressure sensor + electric regulating valve is used to optimize the pressure difference between stages (usually controlled at 0.2-0.3 MPa);
- Optimization of pH Adjustment
In seawater desalination scenarios, a turbocharger or pressure exchanger (PX) can be used to recover concentrated water pressure energy, reducing energy consumption by more than 30%.
Process requirements for instrumentation and program control during RO system operation
- Adjustment of dosage
The dosage is adjusted by a proportional adjustment mechanism, which automatically adjusts the metering pump to perform proportional dosing based on the signal from the water flow meter.
- Liquid level gauge
The metering box is equipped with an on-site liquid level gauge and a low liquid level alarm signal to ensure that the dosing process will not be interrupted due to lack of medicine in the liquid tank.
- Water supply dashboard
There is an on-site water supply instrument panel, which is equipped with flow indicator and flow accumulation meter, conductivity meter, pH value indicator. In addition, the parameter signals sent by the water supply pressure meter, flow meter, conductivity meter and pH meter are sent to the central control room for continuous recording; at the same time, the signal sent by the flow meter controls the metering pump to dosing in proportion; the high and low alarm signals sent by the pH meter are sent to the central control room for alarm.
- Security Filters
The inlet and outlet of the safety filter are equipped with pressure indicators. When the inlet and outlet pressure difference of the safety filter reaches the set value or runs for a certain period of time, the filter element needs to be replaced.
- High pressure pump switch
The inlet and outlet sides of the high-pressure pump are equipped with low-pressure and high-pressure switches respectively. When the inlet pressure of the high-pressure pump is lower than the limit value, the low-pressure switch is closed and sends a signal to the PLC, which alarms and automatically stops the operation of the high-pressure pump; when the outlet pressure of the high-pressure pump is higher than the limit value, the high-pressure switch is closed and sends a signal to the PLC. After a certain delay, if the pressure on the high-pressure side of the high-pressure pump is still higher than the limit value, the PLC outputs an alarm and automatically stops the operation of the high-pressure pump. If the high-pressure switch returns to the disconnected state within the delay range, the PLC automatically cancels the input signal.
- High pressure pump electric slow door opening
The outlet of the high-pressure pump is equipped with an electric slow-opening door. After the high-pressure pump is started, the slow-opening door automatically opens slowly to ensure that the RO membrane element is not damaged by water hammer. If the slow-opening door fails and fails to open within the specified time, the outlet pressure of the high-pressure pump increases, the pressure switch outputs an alarm signal and automatically stops the operation of the high-pressure pump through the PLC. Each RO device is equipped with an on-site instrument panel, on which are installed a flow meter for RO stage 1, stage 2 product water, and drainage (flow and cumulative flow value display), and a product water conductivity meter.
- Program control requirements
The parameter signals sent by the flow meter and conductivity meter are sent to the central control room for continuous recording, and have a high conductivity value alarm. The local panel is equipped with high-pressure pump start and stop buttons and indicator lights, system emergency stop buttons and indicator lights, and electric slow door opening and closing buttons and indicator lights.
- RO dial
Each RO device is equipped with an on-site pressure gauge panel, on which are installed the RO first-stage water inlet, second-stage water inlet and discharge pressure indicators.
- Central control panel
The central control panel is equipped with three-position operating switches (one automatic, one off, one manual) for the high-pressure pump, metering pump, and flushing water pump, as well as system program start and stop buttons, which can realize the automatic start control room remote control and local manual operation functions of the above devices. When the three-position switch is turned to the "automatic" position, the above devices cannot be operated locally.
RO device startup and operation process
- Start-up and operation of RO device program
First, place the control switches of the high-pressure pump and metering pump in the "automatic" position, and then operate the program start button of each RO device. At this time, the programmable logic controller (PLC) will automatically detect the liquid level of all metering boxes and the pressure on the inlet side of the high-pressure pump according to the established program. If a "low" liquid level or a "low" pressure on the inlet side of the high-pressure pump is detected, the PLC will execute an audible and visual alarm and interrupt the program operation.
After eliminating the alarm, press the program start button again, the program will resume running, and automatically start the dosing metering pump and high-pressure pump, and open the electric slow-opening door at the same time. If the high-pressure side pressure of the high-pressure pump is still higher than the set threshold after the preset delay, the PLC will send out an alarm signal and automatically stop the operation of the high-pressure pump and metering pump, and automatically close the electric slow-opening door;
If the high-voltage switch returns to the disconnected state within the delay range, the PLC will automatically cancel the high-voltage switch input signal, and the system will then enter the normal operation stage.
- Manual start-up and operation of RO device control room
When the control switches of the high-pressure pump, metering pump and flushing water pump are placed in the "manual" position, the above equipment can be operated in the control room.
- On-site manual start-up and operation of RO device
When the control switches of the high-pressure pump and metering pump are in the "off" position, the above equipment can be manually started and operated on site.
In any case, the operation of the RO device can be stopped immediately by pressing the system emergency stop button on the local instrument panel.
- RO device stopped
When the RO device stops automatically or the operator stops by pressing the program stop button, the high-pressure pump stops running, the metering pump stops running interlocked, and the electric slow-opening door at the high-pressure pump outlet is automatically closed.
RO device startup and operation process
The composition of the instrument and PLC control system is determined according to the requirements of the RO system for instrumentation and control.
- Liquid level switch
Gives a low level signal.
- Flow Meter
Instantaneous flow indication and flow accumulated value display.
- Conductivity meter
Digital display, with high conductivity value alarm output and 4~20mA current signal output.
- pH Meter
Digital display, with user-settable pH high and low alarm outputs and 4-20mA current signal output.