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Introduction
This article will give detailed information on flow
switches.
The article will give details on:
What are flow switches?
Flow switches working principle
Types of flow switches
Installation of flow switches
Chapter 1: What are Flow Switches
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.
Construction
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.
Chapter 2: Flow Switches Working Principle
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).
Leading Manufacturers and Suppliers
Chapter 3: Types of Flow Switches
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.
Paddle Flow Switches
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.
Piston or Shuttle Flow Switches
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.
Solid-State Flow Switches
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.
Liquid Flow Switches
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
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.
Volumetric Flow Switches
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.
Velocity Flow Switches
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.
Types of Velocity Flow Switches
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.
Flapper Flow Switches
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.
Diaphragm Flow Switches
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.
Shuttle-Type Flow Switches
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.
Piston Flow Switches
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.
Flow Switch Circuit Diagram
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.
A flow indication is another name for the flow switch, while
the supervisory switch is another name for the tamper switch.
A flow switch's primary job is to identify and keep track of
the flow rate of process media like liquids, gases, and steam,
while a tamper switch's primary job is to determine whether or
not a sprinkler valve has been completely closed.
The weight of the flow switch is 569 grams, while the weight
of the tamper switch is 2 lbs.
A flow switch can be built using a hinged or spring-mounted
paddle, while this tamper switch can be built using an
actuator, often a lever or cable with a resting position.
Advantages and Disadvantages of Flow Switches
The following are some advantages of using a flow switch:
These switches are incredibly reliable tools for calculating
flow rates.
These switches provide precision, adaptability, and
affordability.
They are accurate.
The following are some drawbacks of the flow switch:
Gas inclusions may cause errors.
Flow switches provide decreased conductivity.
Costly upkeep is necessary after installation.
It is susceptible to rust caused by liquids with a water base.
It has an intricate design.
Stability is poor.
Low-density fluids are impossible to quantify.
Applications of Flow Switches
The following are some examples of flow switch applications:
The basic function of these switches is to measure flow rates.
These switches are essential for detecting fluid flow and
measuring fan speed.
This switch prevents the central heating system's heating
element from being boosted before the blower's airflow has
been detected.
These switches can be used for drain line flow monitoring,
safety spray nozzle monitoring, oil well system testing, pump
protection, and relief valve monitoring.
These switches are primarily utilized for fluid flow rate
monitoring and control in an industrial process system.
These switches are employed in pump staging, pump or valve
failure, flow protection, and flow or blockage detection.
These switches are typically utilized in reservoirs and tank
storage systems to keep the tanks at the desired level.
In big & commercial buildings, huge flow switches are employed
for HVAC systems.
Chapter 4: Installation of Flow Switches
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.
Types of Fluids Flow Switches Detect
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.
Flow Switch Specifications
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.
Physical and Electrical Flow Switch Specifications
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.
Mounting Types
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.
In-Line Flow Switch Diagram
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.
End Fittings Clamp
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 Output Options
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
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.
Flow Switches with extra functionality
There are flow switches with extra functionality available, such
as:
Audible or Visual Alarms
Instruments have visible or auditory alerts to warn users of
hazardous conditions.
Averaging / Multi-Insertion
Multi-insertion flow meters measure the flow rate at various
places along the flow stream to estimate the flow rate.
Controller Functions
Devices that have or take sensor input-output a control signal.
These controls may include limits, PID, logic, etc.
Programmable Meters
Usually, programmable meters have a microprocessor within them.
Electronic adjustments can be made for various materials,
ranges, outputs, etc.
Recorder / Totalizer Functions
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.
Sanitary
Devices are made for usage in hygienic settings, like those
found in the medical and food processing industries.
Suspended Solids / Slurries
Devices are capable of handling fluids with suspended particles
(slurries). Usually, the meter technology selected affects what
kinds of materials are used.
Leading Manufacturers and Suppliers
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