The selection of sewage treatment PLC control cabinet requires comprehensive consideration of multiple factors. The following are the specific points:
Control requirements
Clear control objects: Determine the number, type and operation requirements of the equipment to be controlled, such as water pumps, fans, agitators, valves, etc. The control logic and parameters of different equipment are different. For example, water pumps may need to control start and stop, variable frequency speed regulation, and valves need to control the opening and closing degree.
Determine the control function: According to the sewage treatment process, clarify the functions that the PLC control cabinet needs to achieve, such as liquid level control, flow regulation, pH value regulation, timing control, sequence control, fault alarm, etc.
PLC selection
Brand and reliability: Choose well-known and well-reputed brands in the market, such as Siemens, Mitsubishi, Omron, etc. These brands of PLC products usually have high reliability, stability and rich product lines, which can meet the needs of sewage treatment projects of different complexity.
I/O points: Count the number of input and output points according to the control requirements, including analog input and output (AI/AO) and digital input and output (DI/DO). Considering a certain margin, generally reserve 10%-20% of the I/O points for subsequent expansion or possible changes.
Processing capacity: According to the complexity and control requirements of the sewage treatment system, select a PLC with appropriate processing speed and storage capacity. For systems that include a large number of analog processing, complex calculations or high-speed counting functions, the PLC needs to have strong computing power and large storage capacity.
Communication capability: PLC should have a variety of communication interfaces, such as RS485, RS232, Ethernet, etc., so as to communicate with the host computer monitoring system, other PLCs or intelligent instruments and other equipment to achieve remote monitoring and data transmission.
Electrical component selection
Circuit breaker: According to the total power of the PLC control cabinet and the current of each branch, select a circuit breaker with appropriate rated current and short-circuit breaking capacity to cut off the circuit and protect the equipment from overload, short circuit and other faults.
Contactor: According to the power and working voltage of the controlled equipment, select a contactor with appropriate rated current and coil voltage to control the start, stop and on/off of the equipment.
Relay: used for signal conversion, isolation and expansion of I/O points, etc. Select the appropriate type and number of contacts of relays according to the control requirements.
Inverter: If there are water pumps, fans and other equipment that require variable frequency speed regulation, select the appropriate inverter according to the power, voltage level and speed regulation range of the equipment.
Cabinet design and manufacturing
Cabinet material: Common cabinet materials include cold-rolled steel plate, stainless steel, etc. In the sewage treatment environment, if there are corrosive gases or liquids, it is recommended to choose stainless steel, which has good corrosion resistance and protection performance.
Cabinet size: According to the number and size of PLC and other electrical components, the size of the cabinet should be reasonably designed to ensure a reasonable layout of the internal space for easy installation, wiring and maintenance. At the same time, the ventilation, heat dissipation and protection level of the cabinet should be considered.
Protection level: There may be dust, water vapor, corrosive substances, etc. at the sewage treatment site. The cabinet with a suitable protection level should be selected according to the actual environment. Generally, it is required to reach IP54 or higher to ensure the normal operation and service life of the PLC control cabinet.
