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Flow Switches Information

Flow Switches Information

Flow switches are devices that monitor flow and send a trip signal to other devices, like a pump, for protection. These switches can be used for the measurement of gases, liquids, and steam. Flow switches can also be used to trigger an alarm and provide protection to a system if necessary. While all flow switches are flow meters, not all flow meters are flow switches because they are not all equipped with the ability to control flow rate. The switch consists of a valve body, sensing device, and a switching unit that is connected to a pipeline.  

Flow switches are used to sense the flow of a fluid or air through a valve contained within the switch.
A switching unit reads a signal sent by the valve to increase or decrease the electrical signal. The
switching unit can turn a pump or valve on/off to maintain the proper system flow. Flow switches can
be used to monitor flow characteristics such as volume, speed, and pressure and to adjust for changes
as they occur. These devices can also be used to help in identifying problems in the systems.  

Flow Switch Types

Flow switches are categorized either by the flowing media or by the measurement type. This means
that the types can be combined, such as volumetric liquid flow switch.

Gas flow switches are for media such as air and steam. They are commonly used in
HVAC applications.

Liquid flow switches are for media including water, lubricants, chemicals, and slurries.
They are used in a wide variety of industry applications.

Volumetric flow switches are used to measure the flow of liquids or gases. This
measurement is done based on volume per unit time (i.e. cubic feet per minute).

Velocity flow switches are used to measure the flow rate of moving media. This
measurement is done in terms of velocity (i.e. feet per minute). 

Flow Switch Applications

Flow switch applications include pump staging, pump or valve failure, flow or blockage detection, and flow protection. They are commonly used in reservoirs and tank storage systems to keep the tanks at the designated level, as well as in pipelines to ensure that the media flow through the pipe is continuous. Large flow switches can be found in commercial and large buildings for HVAC systems 

Flow Switch Specifications

Important operating and performance considerations include:  

Type of media- The type of media the flow switch will be exposed to is important to consider when
selecting a flow switch. A device for a water system is typically made of brass or bronze due to their
resistance to corrosion, rusting, or breakdown. Plastic can be used in applications that are not prone
to freezing or expanding in very hot conditions. Plastic is lightweight yet extremely durable and
resistant to rust.

Pipe diameter - The pipe diameter describes the size of the pipe(s) in the system. The flow switch
must fit securely over the pipe and it is therefore critical to know the pipe diameter when selecting a
flow switch.

Operating pressure - The operating pressure is the maximum head pressure of the process media
that the device can withstand. This factor should be considered when selecting the material of the
flow switch.

Media temperature range- Media temperature range is the maximum media temperature that can be
monitored. It is usually dependent on construction and liner materials.  

Flow rate

There are several types of flow rate. This is the most important specification to consider since the rate
is what triggers the movement of the switch.  

Mass flow - Mass flow rate is the mass of a substance which passes through a given surface per unit
of time. Mass flow rate can be calculated using the equations below  

For flat, plane areas:  

Velocity flow-

Velocity flow rate describes how much lateral distance along the system a fluid passes
per unit time. It can be calculated using the equation below. Velocity flow rate should be done for the
bulk fluid, instead of for the velocity of the fluid at a certain point. It is easy to measure and is
especially useful for liquids, which have a constant density.  

Volumetric flow - 

Volumetric flow rate is a relation of how much volume of a gas or liquid passes
through a fixed point system in a given amount of time. Volumetric flow rate can be calculated using
the equation below. It is especially useful for gas systems.  

Physical and Electrical Flow Switch Specifications

Physical specifications should also be considered when choosing flow switches. There are many different types of mounts and end fittings to choose from.  

Mounting types

In-line-  In-line mounting for flow switches is installed directly in the process line. They have a
variety of end fittings including flanges, in-line flow meters, and compression fittings. In-line
mounted flow switches typically require a straight run of pipe for installation.  

Insertion- Insertion mounts are inserted perpendicular to the flow path. They typically require a
threaded hole in the process pipe, or another means of access.

Non-invasion- Non-invasive mounted flow switches do not require mounting directly in the process
flow, and can be used in closed piping systems.  



Clamp  Devices are inserted parallel to the flow path and clamped between two existing process pipes. External clamp-on flow meters are non-invasive. They do not require mounting directly in the process flow and can be used in closed piping systems. Ultrasonic, or Doppler, flow meters may use this type of mounting to read the flow through the pipe.
Compression - Compression fittings tighten down a sleeve or ferrule over a joint to prevent leakage.
Flanged Devices are inserted parallel to the flow path, usually between two pieces of existing, flanged process pipes. Circular or square flanges are used to connect the fitting, typically via bolting or welding.
Plain End Devices have a plain, straight-pipe end that can be inserted into the bell end of the connecting pipe.
Socket Weld /Union The end fitting is designed for welding or soldering and can be a weld neck.
Threaded Devices are inserted parallel to the flow path and threaded into two existing process pipes. National pipe thread (NPT) is the most common thread type.
Tube End /Hose Nipple Tube-end or hose-nipple is a common description for a short piece of pipe, usually with male threaded ends.
VCO® / VCR®- VCO® and VCR® connections are proprietary, threaded fittings for vacuum applications that use the Swagelok design. VCO® fittings have an O-ring face seal. VCR® fittings have a metal gasket face seal. VCO and VCR are registered trademarks of Cajon Company. Swagelok is a registered trademark of Swagelok Co.

Output Options

Flow switches can output:

Analog current- Analog current levels (transmitters) such as 4 - 20 mA are suitable for sending
signals over long distances. A current is imposed on the output circuit proportional to the
measurement. Feedback is used to provide the appropriate current regardless of line noise and
impedance

Analog voltage - Analog voltage outputs are simple, usually linear functions of the measurement

Frequency - Frequency or modulated frequency outputs include amplitude modulation (AM),
frequency modulation (FM), sine waves, and pulse trains.

Switch- The output is a change in state of a switch or a relay. If the process reaches a pre-established

threshold the flow switch will turn on or off to maintain proper function of the system.

Switch Specifications

Switch specifications include:  

Electro-mechanical flow switches use mechanical contacts like relays and reed switches.

Solid-state switches use electronic switches with no moving parts. There are two main types of solid state switches: field-effect transistors (FETs) and PIN diodes. FET switches create a channel that allows the current to flow from the drain to the source of the FET. PIN stands for highly doped positively (P) charged material in-between a highly resistive intrinsic (I) layer and negatively (N) charged material.  

Normal state options -  

Normally open (NO) switches do not allow current to pass through in the free position. They need to
"make" a contact to be activated.

Normally closed (NC) switches allow current to pass through in the free position and need to "break" contact (open) to be activated.

Number of poles and throws required - Most switches have one or two poles and one or two throws, but some manufacturers will produce custom level switches for special applications. The number of poles describes the number of separate circuits which can pass through the switch at the same time. The number of throws describes the number of circuits each pole can control. This is noted by the configuration of the circuit (NO/NC). Breaks are interruptions in the circuit caused by the separated contacts the switch introduces into each circuit it opens.

Flow Switch Features

Flow switches are available with additional features including:

Audible or Visual Alarms Instruments have audible or visual alarms to alert users to dangerous conditions.
Averaging / Multi-Insertion Multi-insertion flow meters determine the flow rate by taking flow rate measurements across several points in the flow path.
Controller Functions Devices have or receive sensor input, provide control, (limits, PID, logic, etc.) and output a control signal.
Programmable Typically, programmable meters include a built-in microprocessor. They can be adjusted electronically for different materials, ranges, outputs, etc.
Recorder / Totalizer Functions Totalizer functions totalize the amount of material, media, or process variable controlled. A recorder function may be a datalogger that logs system or process variables and/or control commands for later viewing or analysis. A chart recorder that can plot (chart) flow history or give total flow for a given unit of time may also be available.
Sanitary Devices are designed for use in sanitary environments, such as in medical or food processing applications.
Suspended Solids / Slurries Devices can accommodate liquids with suspended solids (slurries). Typically, the material types are determined by the meter technology chosen. For additional guidance, refer to information
Schematic of a flow switch

The flow switch consists of a paddle system (1) which has a permanent magnet (2) located at its
upper end. A reed contact (3) is positioned outside the flow above this magnet. A second, magnet
(4) with opposite polarity is used to create a reset force.

The paddle system is moved once it comes into contact with the flow which is to be monitored.
The magnet (2) changes its position in relation to the reed switch contact (3). The contact
opens/closes depending on the contact type.

As soon as the flow is interrupted, the paddle returns to its original position and the reed switch contact opens/closes depending on the contact type. This change in contact (NO to NC or NC toNO) will be used to indicate the required flow signal output

Flow Switches with different Materials  

Flow Switches – Plastic Body

The plastic flow switches provide a high level of performance when it comes to ensuring any reduction in pressure is kept to a minimum, as well as other specific duties they are required to carry out.

The plastic flow switches is produced by teflon plastic material with a long paddle, combats the problems of corrosion and friction with all wetted parts of Teflon Plastic, suitable for small and big pipe, low cost, used in SPA Appliance, Sea Water Heating System, Water Purification Equipment, Welding Equipment, Chlorine Liquid, Laser System, to measure or control the liquid flow rate. International Protection could be IP68 (Optional). CE certificate for the flow switch, UL certificate for micro switch.

This Teflon Paddle flow switch could used in all kinds of corrosion conditions, e.g. Sea Water Pool, Paper mills, chemical plants, sewage treatment plant, washing plant, printing, printing and dyeing, electroplating plants, pharmaceutical plants.

Flow Switches – Metal Body

Metal flow switches can be found in a number of specific applications, including remote alarms
and indicators. They can be paddle flow, adjustable or inline. These products are useful at
operating the flow range of water, air, oil or gas in a process.

How to install and test a Flow Switch?

Because there are so many different kinds of flow switches, there’s also a very wide range of flow switch
installation methods and techniques. When looking for information on how to wire a flow switch, the most
important thing is to have a clear understanding of which type of switch you’re using and for what purpose.
There are some basic rules of thumb that apply to most flow switch installation methods, however. Broadly
speaking, these include:

■ Installing flow switches on a straight section of pipe or duct

■ Ensuring there’s a good length of straight pipe both in front of and behind the switch
- ideally, the equivalent of 10x the pipe’s diameter

■ Avoid fitting or wiring a flow switch near to bends, other fittings, valves, drains,
narrower or wider sections of pipe, and any other features that might cause obvious
fluctuations in flow rate through this area


Just as the exact methodology for installing a flow switch will depend on what kind of switch you’re using and
where, the same is true for testing the healthy function of any device already installed. To know how to test a
flow switch, you’ll need to know precisely what it is you’re testing for.

In many cases, you’ll be able to confirm a flow switch is working properly simply by observing it in action and
checking that the correct responses are triggered when flow starts or stops. In other cases, you may need to use
an ohmmeter to measure the resistance of the switch's electronics and confirm that there’s a completed circuit
(or ‘continuity’) being registered when the switch is activated.

If either of the above steps don’t produce the required result, it may be time to replace or repair the switch. For
certain types and locations of flow switches - including those attached to safety equipment such as fire sprinklers in public places - it’s typically a legal obligation to have them tested regularly.

Why does the Flow switch show the wrong indication ?  

The below are the possible causes for the false activation of flow switch :

● Entrapped air or gas.

● Turbulence in tubing or unit

● Mechanical sensor assembly damaged.

● Liquid is too viscous.

● Not using the fittings supplied by the manufacturer.

What is a fire sprinkler flow switch?

In fire protection, flow switches, or water flow detectors, are exactly that: devices that sense
when water flows through a fire sprinkler system. This change in how fast water moves
activates the sensor. In turn, it sends a signal to a fire alarm control panel, a notification device (like a bell), or both

What is the difference between flow switch and flow meter?

Flow meters simply monitor flow rates and the user has to check the flow rate themselves, as
and when rather than at a predetermined set point, to detect if there are any issues. Whereas flow
switches detect the presence of the flow, at a predetermined set point, alert the user, and perform
a designated action.  

What are the advantages of flow switches and flow meters?  

Flow switches and flow meters are very reliable devices for determining flow rates. A couple of
advantages worth mentioning are that they can be used for a number of different applications,
including industrial and biomedical to name but a few. Flow switches also offer versatility,
economy and accuracy. Some switches can be used to actuate audible or visual alarms, as well as
relays or other controls.

Like flow switches, flow meters have a number of advantages in their favour. Firstly, they can
give you a true mass flow measurement and very high accuracy readings. They are also unaffected by pressure, temperature and viscosity with self-draining characteristics.

Flow switches and flow meters available at Fluid Controls  

Fluid Controls can supply a complete range of flow switches, flow meters and excess flow valves. Malema’s range
of flow switches, flow meters and excess flow valves are recognised by the leading certifying bodies around the
world. Widely used in industries ranging from offshore to semi-conductors, including harsh environments, their
products are UL and ATEX approved as well as CE compliant. Available in a choice of materials, these units are
suitable for almost all measuring and monitoring applications.

How to Test a Flow Switch?

1. Put the cover on the flow switch and turn the power on.

2. Start flow through the designated system.

3. Observe flow in operation to determine if the flow switch is working properly.

4. Repeat this process many times to ensure the flow switch and designated system is in fact working accurately. 

Is a flow switch normally open or closed?

Flow switch is used to activate a device when flow starts. When flow starts and the increasing
operating flow is reached or exceeded, the common (black wire) and normally open (red wire)
contacts are closed, while the common (black wire) and the normally closed (blue wire) are open

How do I know if my flow switch is bad? 

”No Flow” light coming on probably means the cell's flow switch has gone bad. Verify that the pump is running first (water is actually flowing through the line). If "No Flow" stays on the flow switch needs replacing.

Flow Switch Cleaning Procedure

The flow switch is designed to provide years of trouble free operation; however it can become
clogged preventing proper operation. There will be two distinct symptoms when the flow switch
is clogged.

1) When the hot water fixture is turned on (at a flow rate above .5 gpm) the boiler will not
respond and the DHW Temp light will not come on.

2) The Boiler will lock out with a code ER5, or the DHW Temp light will remain lit up when no
domestic hot water is being used.

In either of these cases the flow switch may be clogged and should be cleaned and checked prior
to replacement. To clean the flow switch, remove it from the water line and remove NUT “A”
(see diagram below). A light spring sits on the inside of the nut and a magnetic plunger sits
inside the spring. Remove these components and clean any debris that may be caught in them.
Clean the bore of the switch using a small brush or cloth. For switches that have lime or hard
water scale build up, use a scale remover that is compatible with brass. When cleaning is
complete, replace the plunger then the spring and reinsert the nut fitting and torque to 25lbft. To
test the switch use a thin object such as a pen or screw driver to depress the plunger in the
direction of the flow and check continuity across the two red wires. With the switch depressed
there should be continuity, and when the switch is released the circuit should open. Reinstall the
switch keeping it pointed with the arrow in the direction of water flow.

Note: If the switch has debris other than hard water scale a filter with a 100 micron screen or
smaller should be installed prior to the switch to prevent clogging.

*NOTE: the section of the switch where the wires exit must not be dented or crushed as this will
destroy the switch. Place wrenches so that no strain is placed on this portion of the unit.  

6th Sep 2021 Syed Furqan Ali

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