What aspects should be considered when designing a reverse osmosis control system?

The operation and monitoring of the reverse osmosis desalination system is performed by a PLC, instruments, computer system, and process flow simulation screen, with manual operation buttons and control room operation buttons; the system has interlock protection and alarm indication functions.

1. Process requirements for instruments and programmable control in the RO system operation

(1) Chemical dosing uses proportional control, automatically adjusting the metering pump according to the signal from the feed water flow meter.

(2) The metering tank is equipped with a local level gauge and a low-level signal alarm to ensure that chemical dosing is not interrupted due to an empty chemical tank.

(3) A local feed water instrument panel is provided, equipped with a flow indicator and totalizer, conductivity meter, and pH indicator. A feed water pressure gauge is also provided. Parameter signals from the flow meter, conductivity meter, and pH meter are sent to the central control room for continuous recording; simultaneously, the signal from the flow meter controls the metering pump for proportional dosing; high and low alarm signals from the pH meter are sent to the central control room for alarming.

(4) Pressure gauges are installed at the inlet and outlet of the security filter. When the pressure difference between the inlet and outlet of the security filter reaches a certain value or after a certain operating time, the filter element needs to be replaced.

(5) Low and high-pressure switches are installed on the inlet and outlet sides of the high-pressure pump, respectively. When the inlet pressure of the high-pressure pump is lower than the set value, the low-pressure switch closes and sends a signal to the PLC, which alarms and automatically stops the high-pressure pump; when the outlet pressure of the high-pressure pump is higher than the set value, the high-pressure switch closes and sends a signal to the PLC. After a certain delay, if the high-pressure pump outlet pressure is still higher than the set value, the PLC outputs an alarm and automatically stops the high-pressure pump. If the high-pressure switch returns to the open state within the delay time, the PLC automatically cancels the input signal.

(6) An electric slow-opening valve is installed at the outlet of the high-pressure pump. After the high-pressure pump starts, the slow-opening valve automatically opens slowly to ensure that the RO membrane elements are not damaged by water hammer. If the slow-opening valve malfunctions and fails to open within the specified time, the outlet pressure of the high-pressure pump increases, and the pressure switch outputs an alarm signal, and the PLC automatically stops the high-pressure pump.

(7) Each RO unit is equipped with a local instrument panel, which includes flow meters for the first and second stages of RO product water and drainage (displaying flow rate and cumulative flow), and a product water conductivity meter. Parameter signals from the flow meter and conductivity meter are sent to the central control room for continuous recording, and have a high conductivity alarm. The local panel is equipped with start/stop buttons and indicator lights for the high-pressure pump, a system emergency stop button and indicator light, and open/close buttons and indicator lights for the electric slow-opening valve.

(8) Each RO unit is equipped with a local pressure gauge panel, which includes pressure indicators for the first stage inlet water, second stage inlet water, and drainage of the RO.

(9) The central control panel is equipped with three-position switches (automatic-off-manual) for the high-pressure pump, metering pump, and rinsing water pump, system program start/stop buttons, which can realize the automatic start, control room remote operation, and local manual operation functions of the above devices. When the three-position switch is in the "automatic" position, the above devices cannot be operated locally.

(10) RO unit startup and operation process

① RO unit program startup and operation. First, switch the "automatic-off-manual" three-position switches of the high-pressure pump and metering pump to the "automatic" position, then press the program start button of each RO unit. At this time, the PLC automatically detects the liquid level of all metering tanks and the inlet pressure of the high-pressure pump according to the program. When there is a "low" liquid level or "low" pressure alarm at the inlet of the high-pressure pump, the PLC gives an audible and visual alarm and stops the program operation. After eliminating the alarm, press the program start button, the program resumes operation, and automatically starts the chemical dosing metering pump, high-pressure pump, and opens the electric slow-opening valve. After a certain delay, if the high-pressure side pressure of the high-pressure pump is still higher than the set value, the PLC outputs an alarm and automatically stops the high-pressure pump and metering pump, and automatically closes the electric slow-opening valve; if the high-pressure switch returns to the open state within the delay time, the PLC automatically cancels the high-pressure switch input signal, and the system enters the normal operation stage.

② RO unit manual startup and operation from the control room. When the "automatic-off-manual" three-position switches of the high-pressure pump, metering pump, and rinsing water pump are switched to the "manual" position, the above equipment can be operated in the control room.

③ RO unit local manual startup and operation. When the "automatic-off-manual" three-position switches of the high-pressure pump and metering pump are switched to the "off" position, the above equipment can be manually started and operated locally. In any case, the operation of the RO unit can be stopped by pressing the system emergency stop button on the local instrument panel.

(11) When the RO unit automatically stops running or is stopped by the operator pressing the program stop button, the high-pressure pump stops running, the metering pump interlocks and stops running, and the electric slow-opening valve at the outlet of the high-pressure pump automatically closes.

(12) Interlock between metering pump and high-pressure pump

The reverse osmosis system includes two RO units and a chemical dosing system, each RO unit equipped with a high-pressure pump. When one high-pressure pump starts, the metering pump of the chemical dosing system interlocks and starts; when both high-pressure pumps stop, the metering pump of the chemical dosing system interlocks and stops; when one high-pressure pump is running and the other is stopped, the metering pump works normally.

(13) The RO unit is equipped with a rinsing system. After the RO unit stops running for a certain period of time, the rinsing water pump can be automatically started, and the rinsing water inlet and discharge valves can be opened to rinse the RO membrane elements at low pressure.

(14) The central control panel is equipped with a digital display and an audio device, which can display alarms visually and audibly; it is equipped with a system simulation screen, which can display the operation of the RO system; it can continuously record various parameters that need to be recorded. An ammeter displays the current of the high-pressure pump motor.

2. Composition of instruments and PLC system

The composition of the instrument and PLC control system is determined according to the requirements of the RO system for instruments and control.

Level switch. Provides a low-level signal.

Flow meter. Displays instantaneous flow rate and cumulative flow.

Conductivity meter. Digital display with high conductivity alarm output and 4~20mA current signal output.

pH meter. Digital display with user-settable high and low pH alarm outputs and 4~20mA current signal output