This article will give detailed information on flow switches.
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A flow switch is a device that measures the flow rate and liquid pressure within a duct, loop, or system. Flow sensor and flow indication are other names for this switch. The primary purpose of a flow switch is to continually monitor total flow by tracking the movement of liquid, gas, or steam over a predetermined period. Below is a picture of the flow switch symbol.
A permanent magnet, reed contact, a second magnet, and a paddle mechanism are all used to construct the flow switch.
Paddle System: The paddle of the switch is propelled to move in the direction of fluid flow by the liquid flow within the pipeline. Therefore, the line's size significantly impacts the fluid flow needed to actuate the paddle.
Permanent Magnet: A permanent magnet is fastened to the paddle's top and activated to activate a reed switch, producing a potential-free output.
Reed Contact: Over the magnet, a reed contact is positioned away from the fluid flow.
Second Magnet: A reset force is produced using a second magnet with opposite polarity.
When the paddle system gets close to the liquid flow that needs to be monitored, it starts to move. The reed switch's contact will vary where the permanent magnet is concerning it. Depending on the type of contact, this switch's contact will either be ON or OFF.
The paddle in the switch will immediately return to its original position after the liquid flow is interrupted. The major purpose of this modification to the contact is to indicate the necessary output flow signal. These switches are typically used to protect a pump when it's necessary to keep an eye on the movement of air, liquid, or gas through a specific line.
The medium type is a major factor in how a flow switch operates. A system's fluid, air, or gas pressure and flow rate are primarily monitored by this switch.
Outlining the essential parts of a typical flow switch circuit will help one understand how this switch works. A magnetic trigger or a paddle is typically available with various flow switches (primary device). As a result, this paddle is always connected to a circuit and positioned in the channel where the liquid flows. The paddle turns once the gases or fluids are introduced, sending the signal to a secondary device like a transducer. This transducer will receive the signal and deliver it in readable form to the transmitter. The transmitter will then take readings after that.
Two things can happen when the liquid flow rate hits the switch's set point: either the circuit can be closed or opened by turning a pump ON or OFF, or it can generate an alert. These switches can be configured as either NO (Normally Open) or NC (Normally Closed), the switch's default state. The circuit is off when the switch is in the NO position (Open). Likewise, if the switch is in the NC position, the circuit is ON (Closed).
These switches can be divided into four categories, which are addressed below: paddle flow switches, piston or shuttle flow switches, solid-state switches, and switches for gas, liquid, volumetric, and velocity flow.
The paddle design flow switch is built with a hinged or spring-mounted paddle to make direct touch with the media flowing through the pipe. The paddle is maintained in place (or set point) when the media flows at the desired rate. A change in the flow rate will cause the paddle to deviate from its set point, which will then cause a tiny switch to be thrown, starting the desired action.
A free-floating magnetic piston in a piston (or shuttle) flow switch reacts to the flow rate in a pipe. Movement of the piston activates a hermetically sealed reed switch, which in turn causes the required action to be taken when the flow rate increases or decreases.
The notion of heat transfer underlies the operation of solid-state flow switches. Two temperature sensors make up a particular solid-state flow switch, a thermal dispersion flow switch. The temperature of the liquid in which the flow switch is submerged is measured by one sensor, which also serves as the reference. A built-in heating element is situated not far from the second temperature sensor. The media's increased flow rate cools the heated sensor as it heats. The number of cooling increases with the flow rate. The flow rate is above the user-adjustable set point when there is a decrease in the temperature difference between the two temperature sensors. Less cooling occurs when the flow rate decreases, which raises the temperature differential. The flow velocity and the temperature difference are inversely related. Greater heat disparities, for instance, will be produced by fast-moving liquids and vice versa.
These switches measure lubricants, slurries, chemicals, and water. As a result, there are several industrial applications for them. These switches are mostly used in automated industrial systems that process liquid media to guarantee safe & optimal flow rates.
Gas flow switches are instruments that are primarily used for detecting the velocity of the gas. These could be standalone sensors or sensor systems with displays. The flow rate that will be measured is one of these switches' crucial parameters. A revolving vane inside the device gauges how much fluid moves through the flow switch. When the flow reaches a specific pace, the vane activates a set of contacts that complete an electrical circuit. The flow of air and vapors can also be measured using these switches. HVAC (Heating, Ventilation, and Air Conditioning) applications typically use these switches. However, these switches' technology diverges significantly.
These switches are mostly employed to gauge the flow of gases or liquids. The volume for each unit of time can determine how this measurement is carried out. Therefore, these switches measure the volume flow or amount of fluid in motion in terms of a volume unit each time.
Pumping speeds are often described and measured using volume flow. An upper limit determines the volume flow rate. The equation below represents the fluid flow through a surface in a unit of time. The amount of flow that crosses the border after some time determines the volume change.
This formula was created using the ideal gas law PV = nRT and the gas density formula = m/V. The mass flow equation is then changed to reflect these formulae.
This fundamental equation is derived from a straightforward equation that only holds for cross sections with flat, plane surfaces. For curved surfaces, one uses the surface integral.
Where:
v = the material element is flowing velocity field
A = area/surface cross-section vector
Since mass and volume are density-related, volumetric and mass flow rates are related. The mass or weight of the media passing through a system is measured by mass flow. Usually, the units are expressed as weight per unit of time. While mass is unaffected by changes in pressure or temperature, volume does. Therefore it is crucial to consider these variations. When considering these variations, one must always be aware of the media's density. Using the equation below, one may determine the mass flow rate from the volumetric flow rate.
These switches are typically employed to gauge the rate at which moving media flow. Therefore, this measurement can be made in terms of speed or feet per minute.
Inferential flow measurement, positive displacement, velocity meters, and real mass flow meters are the most popular varieties. By directly measuring another value and extrapolating the flow based on established correlations between the directly measured value and flow, inferential measurement refers to the indirect measurement of flow. The most popular unit used nowadays is differential pressure, which is used to infer the rate of flow of a liquid. Orifice plates, Venturi tubes, flow nozzles, cone types, Pitot tubes, target meters, elbow tap meters, and rotameters are a few differential pressure meters.
Positive displacement meters detect liquid flows directly. This machinery separates and advances the fluid in predetermined intervals. The measured increments, which can be counted using mechanical or electrical methods, add together to form the overall flow. They frequently work with fluids with high viscosities. Indicators of positive displacement feature rotating discs, oval gears, and pistons.
Devices that work linearly concerning volume flow rate include velocity-type gas flow switches and liquid flow switches. They have a wider operating range because there isn't a square-foot relationship like there is with differential pressure devices. Turbines, electromagnetic, vortex, and ultrasonic meters are a few types of velocity meters. True mass flow meters, such as thermal meters or Coriolis meters, measure the mass rate of flow direction.
Current, voltage, frequency, and switched output are typical analog electrical outputs for velocity gas and liquid flow switches. Serial and parallel output methods are also available for computers. These sensors can be installed as insertion or inline devices. Flanges, threaded connectors, and clamps can all hold inline sensors in place. Insertion-style sensors often stick in the process flow after threading through a pipe wall.
A flapper switch is a flow switch with a flapper device. This switch allows the flapper to be attached to a flat surface and faces the direction of flow through a hinge. This end may have a permanent magnet attached to it, and above the magnet, outside the fluid flow, a reed contact may be connected.
These switches can monitor the liquid flow within pipelines in several chemical industries. The key characteristics of the paddle flow switch are excellent performance, sturdy construction, compact design, and longer useful life.
The fundamental distinction between diaphragm flow switches and differential pressure switches is that there is a space that permits liquid to flow during the transition between the intake & the outlet port.
Once this is done, the liquid normally flows in a zigzag pattern inside the switch body. There will be a large pressure drop whenever a liquid passes through the flow switch because of this action.
Based on the idea of a moving force, shuttle-type flow switches primarily function because of the fluid's velocity or variation in pressure on a disk. A shuttle is linked at the switch's base, and a magnet is connected on top of the disk. The spindle-like upper portion of the shuttle body resists the pull of gravity. The shuttle is moved once the liquid flow reaches the actuation area. When the fluid flow decreases, the shuttle returns to its original position, controlling the reed switch in the unit stem.
A piston flow switch features a permanent magnet positioned in the unit housing's liquid flow route. A hermetically sealed reed switch in the device is magnetically activated once the piston is displaced due to the pressure differential caused by liquid flow.
A piston flow switch features a permanent magnet positioned in the unit housing's liquid flow route. A hermetically sealed reed switch in the device is magnetically activated once the piston is displaced due to the pressure differential caused by liquid flow.
The flow switch circuit's circuit schematic is displayed below. This circuit manages the fluid flow. A flow switch can be set up inside a pipe to engage when liquid or air is supplied against a paddle that is a component of the device. This switch either opens or closes a group of electrical connections connected to reinforce relays, indicator lights, and motor starter coils. Typically, this switch has electrical contacts that are both normally open (NO) and typically closed (NC).
According to the circuit schematic for the flow switch shown above, the motor can start when coil "M" is energized, and the contact of the switch closes due to sufficient liquid or air movement. In this case, a single-phase circuit is used with a flow switch. Therefore, the motor will turn ON when there is a liquid flow. Similarly, the motor stops operating if there is no liquid flow. Motors are protected from overcurrent in this circuit by overload heaters.
A comparison between a flow switch and a tamper switch includes the following.
The following are some advantages of using a flow switch:
The following are some drawbacks of the flow switch:
The following are some examples of flow switch applications:
Flow switches are often placed in a piping system for industrial purposes. They typically have two ports on either side and are cylinder- or rectangle-shaped. Thanks to these ports, the switch may be easily installed into any piping system. However, to distinguish which liquid has arrived at its destination when two liquids arrive simultaneously at their respective ends of the piping system, the ports must be connected to two separate pipes that carry two different liquids. When this occurs, the different viscosities of the fluids cause an electrical current to flow through one port, not through another port (thickness). This difference in fluid viscosity enables the first fluid to be detected, turning it on or off based on whether it was planned for them to arrive concurrently or not.
A flow switch can detect any liquid, whether a transparent liquid like water or an opaque (turbid) liquid.
So, the main focus is an overview of how a flow switch interacts with applications. These switches monitor the flow in a channel and provide a trip signal to various system components, such as pumps. When choosing this switch, several factors must be considered including the media type, pipe diameter, temperature range, and operating pressure. These switches are employed in blending or additive systems, duct-type heating, air supply systems, etc.
Important operating and performance considerations include:
Material Used:The sort of media the flow switch will be exposed to must be considered when choosing a switch. Due to their corrosion resistance, rusting, and disintegration, brass, and bronze are frequently used to make equipment for water systems. Plastic can be used when it won't freeze or expand under extreme heat. Plastic is incredibly strong and rust-resistant while being lightweight.
Pipe Diameter: The pipe diameter identifies the system's pipes' size. Knowing the pipe diameter when choosing a flow switch is crucial since it must fit snugly over the pipe.
Operating Pressure: The device can withstand the highest head pressure from the process media. When deciding on the flow switch's material, take this into account.
Media Temperature Range: The greatest media temperature that can be seen is known as the "media temperature range." Typically, it is based on the structure and lining materials.
Flow Rate: The flow rate might take many different forms. However, given that the rate causes the switch to move, this specification should be given priority.
Mass Flow Rate: The mass of a material that traverses a specific surface in a unit of time is known as its mass flow rate. The following formulae can be used to determine the mass flow rate.
For flat, plane areas:
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 term "velocity flow rate" refers to how far a fluid travels laterally along a system in one unit of time. The equation below can be used to compute it. Instead of using the fluid's velocity at a specific spot, the velocity flow rate should be calculated for the fluid. It is simple to measure and particularly helpful for liquids because their density is constant.
For curved areas, the term "velocity flow" refers to the rate at which a fluid moves laterally through a system. The equation below can be used to compute it. However, instead of using the fluid's velocity at a specific spot, the velocity flow rate should be calculated for the fluid in its entirety. It is simple to measure and particularly helpful for liquids because their density is constant.
Volumetric Flow Rate: A gas or liquid volumetric flow rate measures how much it moves through a fixed point system in a specific time. The following equation determines the volumetric flow rate. Gas systems can notably benefit from it.
When selecting flow switches, physical criteria should also be taken into account. As a result, there are numerous mount and end-fitting varieties from which to select.
The process line has mounting built for flow switches. Inline flow meters, compression fittings, and flanges are just a few of the end fittings they have. Installation of inline-mounted flow switches normally involves a straight pipe run.
Mounts for insertion are inserted parallel to the flow path. Therefore, they often need another access method, such as a threaded hole in the process pipe.
Non-Invasive: Non-invasive installed flow switches can be utilized in closed pipe systems and do not need to be mounted directly in the process flow.
Devices are clamped between two already-existing process pipes and installed parallel to the flow channel. Flow meters that attach externally are non-intrusive. They can be employed in closed piping systems and do not need to be mounted directly in the process flow. This kind of mounting may be used by Doppler or ultrasonic flow meters to measure the flow through the pipe.
Compression Fittings: Compression fittings compress a sleeve or ferrule over a junction to stop leaks.
Flanged: Devices are installed between two sections of already-existing, flanged process pipes, usually parallel to the flow channel. The fitting is connected using circular or square flanges, usually by bolting or welding.
Plain End: Devices feature a straight, plain pipe end that fits into the connecting pipe's bell end.
Socket Weld / Union: This end fitting, which can be a weld neck, is made to be welded or soldered.
Threaded: Devices are threaded into two existing process pipes and inserted parallel to the flow channel. The most typical thread type is the national pipe thread (NPT).
Flow switches can output signals that can be sent over great distances using analog current levels (transmitters), such as those between 4 and 20 mA. The output circuit is subjected to a current proportional to the measurement. Despite line noise and resistance, the proper current is provided through feedback.
Analog Voltage: Analog voltage measurement outputs are typically straightforward linear functions.
Frequency: Amplitude modulation (AM), frequency modulation (FM), sine waves, and pulse trains are examples of outputs that use frequency or are modified by frequency.
Switch- A switch or relay's state changes as the output. For example, the flow switch will turn on or off whenever the process hits a predetermined threshold to preserve the system's correct operation.
Switch specifications include:
Relays and reed switches are used as mechanical contacts in electro-mechanical flow switches.
Electronic switches without any moving parts are used in solid-state switches. Field effect transistors (FETs) and PIN diodes are the primary categories of solid-state switches. FET switches build a channel that enables current to move from the FET's drain to the source. PIN stands for highly doped positively (P) charged material sandwiched between a negatively (N) charged material and an intrinsic layer that is very resistant.
Normal state options:
Switches normally open (NO) do not let current flow in the open position. Therefore, to be activated, they must "make" a contact.
Normally closed (NC) switches need to "break" contact (open) to operate; current can pass through them in the free state.
Required Number of Poles and Throws: Custom-level switches can be made by some manufacturers for unique purposes; these switches typically have one or two poles and one or two throws. The number of poles indicates how many different circuits can go through the switch simultaneously. The number of throws indicates how many circuits each pole can control. The circuit's (NO/NC) configuration is a reminder of this. The separated contacts the switch introduces into each circuit it opens cause breaks, which are interrupts.
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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