This article will give detailed information on flow switches.
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A flow switch is a device used to monitor the flow rate and pressure of liquids or gases within a duct, pipeline, or system. Also known as a flow sensor or flow indicator, its main function is to continuously track the flow of fluids—be it liquid, gas, or steam—over a set period. The illustration below shows the symbol commonly used to represent a flow switch.
A flow switch is composed of several key components, including a permanent magnet, a reed contact, a second magnet, and a paddle mechanism.
Paddle System: The paddle within the switch moves in response to the fluid flow in the pipeline. The size of the pipeline influences the amount of fluid required to activate the paddle.
Permanent Magnet: Attached to the top of the paddle, the permanent magnet triggers a reed switch to produce a potential-free output when activated.
Reed Contact: Positioned adjacent to the permanent magnet but away from the fluid flow, the reed contact detects changes in the magnet's position.
Second Magnet: This component generates a reset force with opposite polarity to the permanent magnet.
As the paddle system approaches the monitored liquid flow, it begins to move. This movement causes the reed switch's contact to change based on the position of the permanent magnet. Depending on the reed contact type, the switch will either be in an ON or OFF state.
When the liquid flow ceases, the paddle in the switch promptly returns to its initial position. This change in position signals the required output flow status. Flow switches are commonly employed to safeguard pumps by monitoring the flow of air, liquid, or gas through a particular line.
The type of medium being monitored plays a crucial role in the operation of a flow switch. This switch is designed to primarily track the pressure and flow rate of fluids, air, or gases within a system.
Understanding the essential components of a typical flow switch circuit can clarify how it functions. Flow switches often include a magnetic trigger or a paddle, which is placed in the flow path of the liquid. When the flow introduces gases or fluids, the paddle moves, sending a signal to a secondary device, such as a transducer. This transducer processes the signal and sends it in a readable format to a transmitter, which then records the readings.
When the flow rate reaches the set point of the switch, two actions are possible: the switch can either open or close the circuit to control a pump or generate an alert. Flow switches can be configured as NO (Normally Open) or NC (Normally Closed). In the NO configuration, the circuit is open when the switch is in its default state, while in the NC configuration, the circuit is closed by default.
Flow switches can be categorized into four main types: paddle flow switches, piston or shuttle flow switches, solid-state switches, and those designed for specific applications such as gas, liquid, volumetric, and velocity flow monitoring.
The paddle flow switch features a hinged or spring-mounted paddle that comes into direct contact with the flowing media inside the pipe. When the flow rate is at the set point, the paddle remains in its designated position. If there is a deviation in the flow rate, the paddle shifts from its set point, activating a small switch to trigger the intended action.
In a piston (or shuttle) flow switch, a free-floating magnetic piston responds to changes in the flow rate within a pipe. As the piston moves, it activates a hermetically sealed reed switch, which then triggers the necessary action based on whether the flow rate increases or decreases.
Solid-state flow switches operate on the principle of heat transfer. A specific type, known as a thermal dispersion flow switch, features two temperature sensors. One sensor measures the temperature of the surrounding liquid and serves as a reference, while the second sensor is positioned near a built-in heating element. As the flow rate increases, the heated sensor is cooled more rapidly by the flowing media. This increased cooling results in a reduced temperature difference between the two sensors. Conversely, when the flow rate decreases, less cooling occurs, leading to a greater temperature differential. Therefore, the temperature difference inversely correlates with the flow velocity, with higher flow rates resulting in larger temperature disparities and lower flow rates causing smaller differences.
Flow switches are used to measure various substances, including lubricants, slurries, chemicals, and water. They find extensive application in industrial settings, particularly in automated systems that handle liquid media. These switches play a crucial role in ensuring the safe and efficient flow rates of these fluids within processing systems.
Gas flow switches are designed to measure the velocity of gases, including air and vapors. These devices may function as standalone sensors or as part of a sensor system with integrated displays. A key parameter for these switches is the flow rate, which is determined by a rotating vane within the device. As gas flows through, the vane rotates and, upon reaching a certain speed, triggers contacts to close an electrical circuit. These switches are commonly utilized in HVAC (Heating, Ventilation, and Air Conditioning) systems, although their technology can vary significantly.
These switches are primarily used to measure the flow of gases or liquids. The measurement is typically based on the volume of fluid moving through the switch per unit of time. Consequently, these switches quantify the flow rate by determining the volume of fluid passing through the system over each time interval.
Pumping speeds are commonly described and measured using volume flow rates. The volume flow rate is determined by an upper limit and represents the fluid flow through a surface over a unit of time. The volume change is calculated based on the amount of flow that crosses the boundary over a given period.
This formula is derived from the ideal gas law, PV = nRT, in conjunction with the gas density formula, ρ = m/V. The mass flow equation is adapted to reflect these principles.
The fundamental equation is based on a simple formula applicable to flat, planar cross-sections. For curved surfaces, a surface integral is used instead.
Where:
v = the velocity field of the material element flowing through the surface
A = the area or surface cross-section vector
Since mass and volume are related through density, volumetric and mass flow rates are interconnected. Mass flow measures the mass or weight of the media passing through a system, typically expressed in units of weight per unit of time. While mass remains constant regardless of pressure or temperature changes, volume does not. Therefore, it is important to account for these variations. To determine the mass flow rate from the volumetric flow rate, one must consider the density of the media, as shown in the equation below.
These switches are typically used to measure the rate at which moving media flows. Consequently, the measurement can be expressed in terms of speed, such as feet per minute.
Inferential flow measurement, positive displacement, velocity meters, and real mass flow meters are the most popular varieties. Inferential measurement refers to the indirect measurement of flow by directly measuring another value and extrapolating the flow based on established correlations between the directly measured value and flow. The most popular unit used today is differential pressure, which is used to infer the rate of flow of a liquid. Some examples of differential pressure meters include orifice plates, Venturi tubes, flow nozzles, cone types, Pitot tubes, target meters, elbow tap meters, and rotameters.
Positive displacement meters detect liquid flows directly by separating and advancing the fluid in predetermined intervals. The measured increments, which can be counted using mechanical or electrical methods, are added together to form the overall flow. These meters frequently work with fluids of high viscosity and often feature rotating discs, oval gears, or pistons.
Devices that work linearly with volume flow rate include velocity-type gas flow switches and liquid flow switches. They offer a wider operating range compared to differential pressure devices, which have a square-foot relationship. Examples of velocity meters include turbines, electromagnetic meters, vortex meters, and ultrasonic meters. True mass flow meters, such as thermal meters or Coriolis meters, measure the mass rate of flow directly.
Typical analog electrical outputs for velocity gas and liquid flow switches include current, voltage, frequency, and switched output. Computers also use serial and parallel output methods. These sensors can be installed as insertion or inline devices. Inline sensors are held in place by flanges, threaded connectors, or clamps, while insertion-style sensors are threaded through a pipe wall and protrude into the process flow.
A flapper switch is a flow switch equipped with a flapper device. This switch features a flapper that is attached to a flat surface and positioned to face the direction of flow through a hinge. The end of the flapper may have a permanent magnet attached, while a reed contact is positioned above the magnet, outside the fluid flow.
Flapper switches are used to monitor liquid flow within pipelines in various chemical industries. Key characteristics of flapper flow switches include excellent performance, sturdy construction, a compact design, and a longer useful life.
The main difference between diaphragm flow switches and differential pressure switches is that diaphragm flow switches include a compartment that allows the liquid to move through the space between the inlet and outlet ports during operation.
After this process, the liquid typically moves in a serpentine or zigzag course within the switch housing. This movement causes a significant pressure drop as the liquid flows through the flow switch.
Shuttle-type flow switches operate based on the concept of a moving force, which is influenced by the fluid's velocity or pressure changes against a disc. A shuttle is attached at the base of the switch, with a magnet positioned above the disc. The upper part of the shuttle, which resembles a spindle, counteracts gravity. When the liquid flow reaches the designated actuation area, the shuttle is displaced. As the flow diminishes, the shuttle returns to its initial position, thereby controlling the reed switch within the unit.
A piston flow switch includes a permanent magnet placed within the liquid flow path inside the unit housing. When the piston is displaced by the pressure differential from the liquid flow, it triggers a hermetically sealed reed switch through magnetic activation.
A piston flow switch contains a permanent magnet located in the path of the liquid flow within the housing. As the liquid flow creates a pressure differential, the piston moves, which in turn activates a hermetically sealed reed switch through magnetic interaction.
The circuit diagram for a flow switch is shown below. This circuit controls the flow of fluids within a system. A flow switch can be installed within a pipe, where it activates in response to the movement of liquid or air against a paddle component. The switch will either open or close a set of electrical connections linked to relays, indicator lights, or motor starters. Usually, the switch features both normally open (NO) and normally closed (NC) electrical contacts.
As depicted in the circuit diagram above, the motor will activate when coil "M" is energized, causing the switch contact to close in response to adequate liquid or air flow. This setup employs a single-phase circuit with a flow switch, ensuring the motor turns on with liquid flow and stops when flow ceases. Overcurrent protection for the motor is provided by overload heaters in this configuration.
Below is a comparison between a flow switch and a tamper switch.
Here are several benefits of utilizing a flow switch:
Here are some disadvantages of using a flow switch:
Below are some examples of applications for flow switches:
Flow switches are commonly installed within industrial piping systems and typically feature two ports on either side, with shapes ranging from cylindrical to rectangular. These ports facilitate straightforward installation into the piping network. To identify which liquid has reached its destination when two different liquids are present at the system's ends, the ports should be connected to separate pipes carrying each liquid. Due to the differing viscosities of the liquids, an electrical current will flow through one port but not the other, based on the thickness of the fluids. This variation in viscosity allows for the detection of the first fluid, enabling the switch to turn on or off depending on whether the arrival of both fluids was anticipated.
A flow switch is capable of detecting a range of liquids, including both clear liquids like water and opaque or turbid liquids.
The primary consideration for utilizing a flow switch involves understanding its interaction with various applications. These switches monitor the flow within a conduit and provide a signal to activate or control system components such as pumps. Key factors to consider when selecting a flow switch include the type of media being monitored, the diameter of the pipe, the temperature range, and the operating pressure. Flow switches find applications in diverse systems, including blending or additive processes, duct-based heating, and air supply systems.
Key considerations for operation and performance include:
Material Used: It's important to select a flow switch based on the type of media it will encounter. For water systems, materials like brass and bronze are commonly chosen due to their resistance to corrosion, rust, and wear. Plastic is an option when the media won’t experience freezing or extreme temperatures, as it is lightweight and resistant to rust.
Pipe Diameter: The diameter of the pipe is crucial for selecting a flow switch, as the switch must fit appropriately over the pipe's size. Accurate measurement ensures proper functionality and installation.
Operating Pressure: Consider the maximum head pressure that the device can endure from the media. This factor influences the choice of material for the flow switch.
Media Temperature Range: The temperature range indicates the highest temperature the media can reach. This is usually determined by the construction and lining materials of the switch.
Flow Rate: The flow rate, which can vary, is a critical specification because it directly affects the switch’s activation. Ensure this is appropriately matched to the switch's capability.
Mass Flow Rate: The mass flow rate refers to the amount of material passing through a specific area per unit of time. The following formulas can be used to calculate the mass flow rate:
For flat, plane surfaces:
Where:
ρ = mass density of the fluid
v = velocity field of the mass elements flowing
A = cross-sectional vector area/surface
Q = volumetric flow rate
m = mass flux
For curved areas:
Velocity Flow Rate: The "velocity flow rate" measures how far a fluid moves laterally through a system in a given unit of time. This can be calculated using the equation below. Unlike calculating the fluid's velocity at a particular point, the velocity flow rate provides a broader measure of the fluid's movement. This measure is straightforward to assess and is especially useful for liquids due to their constant density.
For curved areas, "velocity flow" describes the rate at which a fluid moves laterally through a system. To calculate this, use the equation provided below. Instead of measuring the fluid's velocity at a single point, the velocity flow rate assesses the movement of the fluid throughout the entire system. This measurement is easy to obtain and is particularly useful for liquids due to their consistent density.
Volumetric Flow Rate: This measures the volume of gas or liquid passing through a fixed point in a system within a specific time frame. The equation below is used to determine the volumetric flow rate. This measurement is particularly advantageous for gas systems.
When choosing flow switches, physical criteria must be considered, as there are various mounting and end-fitting options available.
Mounting options for flow switches are designed for integration with the process line. They include inline flow meters, compression fittings, and flanges. Inline-mounted flow switches generally require a straight pipe section for installation.
Insertion mounts are placed parallel to the flow direction, often requiring additional access methods, such as a threaded hole in the process pipe.
Non-Invasive: Non-invasive flow switches can be used in closed pipe systems without needing direct installation in the flow path.
Clamp: Devices are clamped between two existing process pipes and installed parallel to the flow channel. External attachment flow meters are non-intrusive and can be used in closed piping systems without direct mounting in the flow path. Doppler or ultrasonic flow meters often use this type of mounting to measure flow through the pipe.
Compression Fittings: Compression fittings use a sleeve or ferrule to secure a junction and prevent leaks.
Flanged: Devices are fitted between two existing flanged sections of process pipes, usually aligned parallel to the flow direction. The connection is made using circular or square flanges, typically secured by bolting or welding.
Plain End: Devices come with a straight, plain pipe end designed to fit into the bell end of the connecting pipe.
Socket Weld / Union: This end fitting, which may include a weld neck, is intended for welding or soldering.
Threaded: Devices are screwed into two existing process pipes and positioned parallel to the flow channel. The most common thread type used is the National Pipe Thread (NPT).
Flow switches can generate signals that are transmitted over long distances using analog current levels (transmitters), such as those ranging from 4 to 20 mA. The output circuit carries a current proportional to the measurement, ensuring accurate transmission despite line noise and resistance through feedback.
Analog Voltage: Outputs based on analog voltage measurements are typically straightforward linear functions.
Frequency: Outputs can use frequency modulation (FM), amplitude modulation (AM), sine waves, or pulse trains, where frequency or its variations are involved.
Switch: The output involves a switch or relay that changes state. For instance, a flow switch will activate or deactivate when the process reaches a set threshold, maintaining the proper operation of the system.
Switch specifications include:
Mechanical contacts in electro-mechanical flow switches often use relays and reed switches.
Solid-state switches, which have no moving parts, utilize electronic components. The main types are Field Effect Transistors (FETs) and PIN diodes. FET switches create a channel for current to flow from the FET's drain to the source. PIN diodes consist of a highly doped positive (P) material sandwiched between a negative (N) material and a very resistive intrinsic layer.
Normal State Options:
Normally Open (NO) switches do not allow current to flow when open. They require contact to be made to activate.
Normally Closed (NC) switches allow current to pass when in their default state and require breaking contact (opening) to operate.
Required Number of Poles and Throws: Some manufacturers offer custom switches with varying poles and throws for specific needs. The number of poles denotes how many separate circuits can be controlled simultaneously, while the number of throws indicates how many circuits each pole can manage. This setup is reflected in the switch’s NO/NC configuration, with breaks (interrupts) being introduced into each circuit as the switch operates.
There are flow switches with extra functionality available, such as:
Instruments have visible or auditory alerts to warn users of hazardous conditions.
Multi-insertion flow meters measure the flow rate at various places along the flow stream to estimate the flow rate.
Devices that have or take sensor input-output a control signal. These controls may include limits, PID, logic, etc.
Usually, programmable meters have a microprocessor within them. Electronic adjustments can be made for various materials, ranges, outputs, etc.
The amount of the controlled material, media, or process variable is totaled by totalizer functions. For example, a data logger that records system or process variables and controls commands for subsequent viewing or analysis performs a recorder function. There may also be a chart recorder that may plot (chart) flow history or provide total flow for a specified period.
Devices are made for usage in hygienic settings, like those found in the medical and food processing industries.
Devices are capable of handling fluids with suspended particles (slurries). Usually, the meter technology selected affects what kinds of materials are used.
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