In fluid transportation systems in industries such as petrochemicals, water treatment, HVAC, and power, the stability and safety control of fluid flow are directly related to equipment operating efficiency and system safety. As an automated control component that can monitor fluid flow changes in real time and trigger switch signals based on preset thresholds, flow switches are important "guardians" to ensure the normal operation of fluid systems and prevent equipment from idling, damage, or overload operation.
Applicable scenarios and selection points
1. Typical applicable scenarios
The application scenarios of flow switches are extensive, covering fluid systems in multiple industries. Common scenarios include:
Petrochemical industry: used for transporting crude oil, refined oil, and chemical raw materials (such as methanol, ethanol, acid-base solutions) in pipelines, monitoring fluid flow to prevent abnormal flow caused by pump idling, pipeline blockage, or leakage; In the cooling system of the reaction vessel, monitor the flow rate of the coolant to ensure stable temperature of the reaction vessel and avoid safety accidents caused by overheating.
Water treatment industry: Monitor flow, control the start and stop of pumps, dosing equipment, and filtration equipment in the raw water, clean water, and sewage pipelines of water plants and sewage treatment plants; In reverse osmosis water treatment systems, monitor the inflow rate to prevent membrane components from dry burning and damage due to flow interruption or low flow rate.
HVAC industry: In air conditioning water systems (chilled water, cooling water) and heating systems, monitor water flow to ensure sufficient fluid circulation for equipment such as heat exchangers and fan coil units, and avoid equipment efficiency degradation or freezing due to insufficient flow; In the fresh air system, monitoring the fresh air flow rate, controlling the fan speed, and achieving energy-saving operation.
Power industry: In the boiler feedwater system and condenser cooling water system of thermal power plants, water flow is monitored to protect key equipment such as boilers and turbines. If the feedwater flow is insufficient, the boiler should be shut down in a timely manner to prevent dry burning; In the transformer cooling system, monitor the flow of cooling oil or cooling water to ensure normal heat dissipation of the transformer and avoid overheating damage.
Mechanical manufacturing industry: In the hydraulic and lubrication systems of machine tools, monitor the flow of hydraulic and lubricating oil to prevent wear and jamming of machine tool components caused by flow interruption; In the compressor cooling system, monitor the flow rate of cooling water to ensure the normal operation of the compressor and extend its service life.
2. Key selection points
The selection should take into account factors such as fluid characteristics, system parameters, installation conditions, etc., to avoid switch misoperation or inability to meet usage requirements due to improper selection. The core points are as follows:
Clarify fluid type and characteristics: Firstly, determine whether the monitored fluid is a liquid (such as water, oil, acid-base solution) or a gas (such as air, nitrogen, natural gas). Different types of flow switches are suitable for different fluids. At the same time, attention should be paid to the viscosity of the fluid (such as piston type for high viscosity hydraulic oil and thermal or blade type for low viscosity water), corrosiveness (corrosion-resistant materials such as 316L stainless steel and PVDF shell should be selected for corrosive fluids), impurity content (non blocking structures such as thermal or ultrasonic type should be selected for fluids with high impurity content, or used in conjunction with filters to avoid jamming of sensing elements of mechanical switches), temperature and pressure (flow switches that match the temperature and pressure resistance levels should be selected, such as thermal or metal shell flow switches for high-temperature fluids (>100 ℃), and high-pressure structures should be selected for high-pressure pipelines (>1MPa)).
Determine the flow range and threshold requirements: Based on the normal operating flow, minimum protection flow (lower threshold), and maximum allowable flow (upper threshold) of the system, select the flow switch with a flow measurement range that covers the actual needs, ensuring that the switch can be accurately triggered within the preset threshold range. For example, if the normal flow rate of the system is 5-20m ³/h and it is required to shut down when the flow rate is lower than 3m ³/h and alarm when it is higher than 25m ³/h, a flow switch with a measurement range of 0-30m ³/h and threshold values of 3m ³/h (lower limit) and 25m ³/h (upper limit) should be selected. At the same time, pay attention to the accuracy level of the flow switch. Generally, the accuracy of mechanical type is ± 5% - ± 10%, and the accuracy of electronic type is ± 1% - ± 5%. For high-precision requirements (such as measurement and precision control), electronic type should be selected.
Choose the appropriate installation method and interface: Determine the installation method (such as horizontal installation, vertical installation, side installation) and connection interface type based on the pipe diameter, installation space, and fluid flow direction. For example, blade type flow switches usually require horizontal installation and sufficient straight pipe sections (usually the first 5D and the last 3D, where D is the pipe diameter) inside the pipeline to avoid fluid disturbance affecting the measurement; Float type flow switches need to be installed vertically, with fluid flowing from bottom to top; The interface type should match the pipe diameter. For small pipe diameters (such as DN15-DN50), threaded interfaces can be selected, while for large pipe diameters (such as DN65 or above), flange interfaces or clamp interfaces can be selected.
Consider compatibility between output signals and systems: Select flow switches that match the output signal type based on the signal requirements of subsequent linkage devices such as PLCs, relays, and alarms. If only simple device start stop control is required, mechanical or basic electronic types that can output dry contact signals (such as SPDT, DPDT) can be selected; If communication with the central control system, data monitoring, or analog adjustment is required, an intelligent flow switch that outputs analog signals (such as 4-20mA) or digital signals (such as RS485) can be selected to ensure that the signal can be accurately recognized and processed by the system.
Pay attention to environmental adaptability and reliability: Select flow switches with corresponding protection levels and anti-interference capabilities based on the severity of the installation environment (such as dust, humidity, vibration, electromagnetic interference). For example, products with a protection level of IP65 or higher should be selected for outdoor or humid environments; Industrial sites with strong electromagnetic interference (such as near frequency converters) require the selection of flow switches with electromagnetic compatibility (EMC) certification; Mechanical switches with anti vibration design (such as blade type with buffer structure) should be selected for occasions with high vibration (such as pump outlet pipelines) to avoid misoperation caused by vibration.