Flow switches and flow meters are commonly used in industrial measurement. Many people are not clear about their differences. A flow meter can measure specific data. Such as vortex flow meters, electromagnetic flow meters, etc. Flow switches are used when the flow reaches the measured value. It can only play the role of interrupting the flow.
This article will introduce their differences from the aspects of working principle, classification, and application. I believe that after reading this article, you will have a general understanding of flow switches and flow meters.
Definitions of Flow Switch and Flow Meter
The main function of the flow switch is to open and close. It controls the switch of the pipeline fluid according to the actual flow size, which requires the role of relays to practice accurate control. Of course, some flow meters with alarm switch functions can also realize the function of flow opening and closing.
When the flow in the pipeline reaches a certain set switch point value, it will automatically issue a switch action. And the peripheral circuit is cut off or connected for switch quantity control, which is used to detect whether the flow reaches the set value. So compared with the flow meter, the main function of the flow switch is relatively simple, which is to instruct the flow to open and close. It is used to control the flow of air, steam or liquid.
A flow meter is a device used to measure the volume or mass flow of a liquid or gas in a fluid system. It is an instrument used to measure the flow rate or flow velocity of a fluid.It can reflect the flow rate or flow velocity of a fluid by measuring the amount of fluid flowing through a specific cross-sectional area per unit time.
It can be divided into insertion type, pipeline type and exposed type. Modern flow meters have gradually become intelligent, automated and networked, and can be remotely monitored and controlled through computer systems, greatly improving measurement efficiency and accuracy.
Types and Working Principles of Flow Switch and Flow Meter
Common types of flow switches
Mechanical flow switch: A mechanical flow switch uses the pressure difference or power generated when the fluid flows through the device to drive the mechanical structure and trigger the switch action. Common mechanical flow switches include turbine type, vortex type and piston type.
The turbine flow switch detects the flow velocity by the rotation of the rotor blades. The vortex flow switch detects the flow velocity by the vortex effect generated by the vortex generator. The piston flow switch uses the pressure difference generated when the fluid flows through the piston to complete the switch operation.
Electromagnetic flow switch: Electromagnetic flow switch detects the flow rate of fluid through the electromagnetic principle in the sensor. They usually have high sensitivity and accuracy. It is suitable for measuring the flow of various liquid media. Electromagnetic flow switches can also be divided into two types: magnetic induction and inductive.
Thermal flow switch: Thermal flow switch detects flow rate by measuring the heat exchange of fluid medium. Among them, common thermal flow switches include thermistor type, thermocouple type and thermistor type.
Optical flow switch: The Optical flow switch detects the flow of bubbles or particulate matter in the fluid medium through optical sensors. They are particularly suitable for scenes with bubbles or suspended particles in the liquid, such as water treatment, food processing and other fields.
Types and principles of various flow meters
Differential pressure flow meter
This is the most common flow technology, including orifice plates, venturi tubes and sonic nozzles. DP flow meter can be used to measure the flow rate of most liquids, gases and steam.
DP flow meter has no moving parts, is widely used and easy to use. But after clogging, it will produce pressure loss and affect accuracy. The accuracy of flow measurement depends on the accuracy of the pressure gauge.
Coriolis flow meter
This flow meter uses a vibrating fluid tube to produce a deflection corresponding to the mass flow rate for measurement. Coriolis flow meters can be used to measure the mass flow of liquids, slurries, gases or steam. High accuracy. However, the pipe wall must be regularly maintained to prevent corrosion.
Volumetric flow meter
The flow meter is used to measure the volume flow rate of liquids or gases. It introduces the fluid into the metering space and counts the number of rotations. Impellers, gears, pistons or orifices are used to divert the fluid. PD flowmeters are highly accurate and are one of the few methods for measuring viscous liquids. However, it also produces irrecoverable pressure errors and requires moving parts.
Thermal mass flow meter
The fluid velocity is measured by measuring the increase in the temperature of the fluid or the decrease in the thermal sensor. Thermal mass flowmeters have no moving parts or holes and can accurately measure the flow of gases. Thermal mass flowmeters are one of the few technologies that can measure mass flow and are also one of the few technologies used to measure large-caliber gas flow.
Turbine flow meter
When the fluid flows through the turbine flow meter, the fluid causes the rotor to rotate. The rotation speed of the rotor is related to the velocity of the fluid. The average velocity of the fluid sensed by the rotor is used to deduce the flow rate or total amount. Turbine flowmeters can accurately measure clean liquids and gases. Like PD flowmeters, turbine flowmeters also produce irrecoverable pressure errors and require moving parts.
Vortex flow meter
The vortex flow meter is a non-streamlined vortex generator placed in the fluid. The velocity of the vortex is proportional to the velocity of the fluid, so the volume flow rate is calculated. Vortex flowmeters are suitable for measuring liquids, gases or steam. It has no moving parts and no fouling problems. Vortex flowmeters generate noise. And the fluid is required to have a high flow rate to generate vortices.
The propagation time method and the Doppler effect method are commonly used methods for ultrasonic flowmeters. It is used to measure the average velocity of the fluid. Like other velocity meters, it is an instrument for measuring volume flow. It is an unobstructed flowmeter. If the ultrasonic transmitter is installed outside the pipeline, it does not need to be inserted. It is suitable for almost all liquids, including slurries, with high accuracy. However, the pollution of the pipeline will affect the accuracy.
Electromagnetic flow meter
Measurement principle: Faraday’s law of electromagnetic induction proves that a conductor moving in a magnetic field will induce an electric potential. Using the electromagnetic measurement principle, the fluid is a moving conductor. The induced potential is proportional to the flow rate and is detected by two measuring electrodes, and then the transmitter amplifies it and calculates the flow rate based on the cross-sectional area of the pipeline.
When a conductive fluid flows through an electromagnetic field, the velocity of the fluid can be obtained by measuring the voltage. The electromagnetic flow meter has no moving parts and is not affected by the fluid. It is very accurate to measure conductive liquids when the pipe is full.Flow meters can be used to measure the flow rate of slurry fluids.
Applications of Flow Switch and Flow Meter
What does a Flow Switch do?
The application range of flow switches is very wide:
- Water treatment system
- Water cooling equipment
- lLiquid transportation system
- Pump system
- Industrial process control
- lHVAC
- lPharmaceutical
- lFood
- Chemical processing
What does a Flow Meter do?
Flow meters have many different applications and can be equipped with integrated flow control valves to control the output flow.
- lPetrochemical
- Power Industry
- Food and Beverage
- Pharmaceutical Industry
- Water Resource Management
- Wastewater Treatment
- Water Supply System
- Heating System
- Irrigation Management
- Transportation
- Medical Equipment
Flow meters are designed to measure flow. On the other hand, flow transmitters not only measure flow but also help control and monitor flow.
Flow meters are standard mechanical systems such as orifice plates, venturi meters, etc. On the other hand, flow transmitters have a mechanical body and electronic functional circuits.
Flow meters are designed for manual field applications, however, flow transmitters can be operated remotely.
We generally install flow switches in the following three locations:
- Near the water outlet of the water pump: Installing a flow switch at the outlet of the water pump can easily monitor the output flow of the water pump and ensure that it is working within the normal range. Once an abnormal flow occurs, the flow switch can respond in time to protect the water pump and the entire pipeline system.
- At the entrance of key water-using equipment: For some important water-using equipment, such as large cooling equipment or heating systems. The flow switch can be installed at the entrance of the equipment. In this way, when the water flow of the equipment is insufficient or too large, the flow switch can send a signal in time. It can prevent the equipment from being damaged due to abnormal flow.
- Branch of the trunk pipeline: In a complex pipeline system, the branch of the trunk pipeline is also an important location for installing a flow switch. By setting a flow switch here, the flow distribution of each branch can be monitored and controlled.
The flow switch controls the water supply of the pump room and the start and stop of the fire pump by detecting the water flow. The pressure switch determines the normal operation of the system and whether the water source is sufficient by monitoring the pipeline pressure.
Specifically, they can be distinguished from the following points.
Control object: The flow switch is mainly used to monitor the flow rate and flow of liquid or gas in the pipeline. The pressure switch is mainly used to monitor the pressure change in the pipeline.
Working principle: The flow switch determines whether the liquid or gas in the pipeline reaches a certain flow rate by measuring the fluid velocity or flow rate in the pipeline. The pressure switch senses the normal operation status of the fire protection system by measuring the pressure change in the pipeline and detects abnormal conditions in time.
There are many differences between flow switches and flow meters. Both flow meters and flow switches need to be selected according to actual conditions. If you need to measure the flow, choose a flow meter. If you need to monitor the flow and control it, choose a flow switch.
The flow meter is mainly used to measure the flow in real time. It can respond to flow changes in real time. If the flow meter is connected to the control system, an alarm point can also be set in the system. The flow switch sends a signal when the flow in the pipeline reaches the set switch point value, and cuts off or connects the peripheral circuit. It is only used to detect whether the flow reaches the set value.
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