Flow Switches

A flow switch is simply a device that monitors the flow of liquid, gas, or steam. When the flow reaches a certain pre-set flow rate, a switch is triggered that has the capability to signal an action, such as shutting down the flow entirely. This is done through electrical signals created by the switching mechanism. In principle, flow switches are similar to a light switch. They just happen to be in a stream of flow and do not require that a person be physically present to turn them off and on.

Flow switches can handle a wide variety of different media running through them, but by far, the most common media for flow switches are water and air.

Flow switches can either be simple stand-alone units that do nothing but monitor flow rates and trigger at a specified flow rate, or they can also be found as an integral element of a flow meter. Flow meters provide a continuous measurement of the flow rate and can also provide other functions essential to certain applications through integral electronics, like batching and totalizing.

Not all flow meters offer switching capacity, but it is important to note that for certain application areas, a flow meter with a switch may be the best choice because the flow meter can offer certain advantages for application specifications. For example, a simple flow switch may not offer high enough temperature specifications, but there may be a flow meter with an integral switch that can handle the high temperature.

Because flow meters are generally more expensive than simple flow switches, it is usually best to investigate stand alone flow switches first to see if there is one that can meet your application specifications. If there is not, then a flow meter that offers an integral switch may be the answer.

A flow switch monitors the flow rate of the media moving through a process line and a pressure switch monitors the pressure within a process line or pipe or vessel. Both pressure switches and flow switches essentially perform the same function, acting as a trigger for a function. They just monitoring different elements of an application. Pressure switches are used in many common places, like the air compressor you may have in your garage. They are also commonly used in pneumatics and hydraulics where force to move something is created by pressurizing a liquid or a gas.

Flow switches are primarily classified by the media that they are monitoring (like air or water) the principle of operation, their installation method, and particular application areas. Many flow switches are versatile in that they can meet the needs of many applications. Below are some explanations of different types of flow switches.

Each type of flow switch will offer different advantages and drawbacks. Avoid costly errors down the road by making sure you have chosen the right flow switch for the job. Make use of our free expertise and let us do the work of choosing the right flow switch for your application and your budget.

Water flow switches are plentiful and used in many different places. Most switches that are built for liquids can be used as water flow switches. KOBOLD offers a wide variety of water flow switches, including all metal versions for high pressures, low flow water switches, and plastic flow switches. We offer both inline and insertion water flow switches. Our available technology types for water include variable area, thermal, and paddle flow switches.

Air flow switches monitor the flow of air and most any flow switch marketed for gases is compatible with air. Our air flow switches are available in variable area, thermal, and paddle technologies. There are also inline and insertion models to choose from. It is worth noting that some flow switches like our KSR/SVN and S-Series are capable of monitoring both water and air. This is not true across the board as many switches are only able to monitor water or air.

Inline flow switches are installed by placing them via integral connections into a piping system. They have a “flow body” where the mechanism of monitoring is located, and this is placed between two sections of pipe. Inline flow switches require that the installation either be a new piping system or if not, then the system must be shut down, the pipes must be cut, and the switch then installed. Existing processes requiring a new flow switch are sometimes easier served by an insertion style flow switch.

Insertion flow switches offer ease of maintenance and replacement over an inline flow switch. Generally, the switch electronics will be in a housing that remains affixed outside the pipe, while the paddle or measurement probe is placed within the flow path inside the pipe.

Paddle flow switches come in models for liquid (water) and models for gas (air). They are not as likely to be able to monitor both liquids and gases with one model. The monitoring mechanism is simple. A paddle is affixed via a hinge onto the housing where the electronical components reside. The paddle is placed into the flow stream and the force of the flow pushes the paddle downstream. The faster the flow, the more force is created and the more the paddle is pushed. The switch engages when the flow reaches a predetermined rate. Most paddle flow switches are insertion type, although there are inline models as well like our PSR.

Thermal dispersion flow switches and calorimetric flow switches are different terms for the same technology. A heated probe is inserted into the pipe or duct via a hole in the piping and the housing while the electronics are always outside of the pipe. The flow of the liquid (water) or gas (air) passing by the heated probe dissipates the heat, much like when you stand outside without a coat and the wind removes your body heat and chills you. The temperature drop is then converted by the electronics and used to create a switch-point value. Visit our article on Thermal Flow Technology to learn more about this principle of measurement.

Thermal dispersion flow switches offer an advantage over variable area flow switches and paddle switches because there are not mechanical parts that experience wear and tear, so they require less maintenance and offer a longer service life than their counterpart technologies. Our KAL-Series thermal dispersion switches are best sellers and well known in the industry. They are in use around the globe in a variety of applications. They are available for both liquids and gas. They are also available in 3A models for sanitary applications.

Variable area flow switches operate on a simple principle. They consist of a glass or plastic or metal tube that contains a float inside it. The flow of the media moves the float as the flow increases. When the float reaches the pre-set limit, the switch is triggered.

Choosing a flow switch can be quick and painless if you speak to one of our expert engineers.

If you prefer to make the selection without any assistance, here are the application variables you should consider in flow switch selection:

• Maximum and minimum flow rate
• Maximum liquid temperature and maximum system pressure
• Pipe diameter and desired connection type
• Available installation space and switch orientation in the piping (vertical/horizontal)
• Media compatibility with the materials of the switch that will have contact with the liquid
• The exact type of switch (like proximity or reed)

Many flow switches, usually thermal and paddle type switches, are installed using an insertion method where a hole is created in the pipe and a connection is created/added to the hole that is compatible with the connection type on the switch. The paddle of the paddle flow switch or the thermal probe of the thermal dispersion switch are then inserted into the pipe to where they are in the stream of the flow. The housing with the electronics generally sits affixed to the outside the pipeline. We offer one paddle flow switch that already comes mounted in a section of pipe that is then connected to an incoming and an outgoing pipe, the PSR Paddle Flow Switch.

Because of the inherent monitoring/measuring principle, most variable area flow switches will be inline flow switches. These are installed as described above where the switch is part of a device that is inserted into the piping with a line coming in and a line going out that are affixed via connections on the pipes and connections on either side of the flow switch body.

Depending on the technology type of the flow switch, and to ensure accuracy, it is best to make sure that the flow is not turbulent around the flow switch and that it is kept as laminar as possible, without surges of flow. The way to protect against this is to make sure there is a length of “straight pipe run” before and after the flow switch where there are no bends, devices, or other elements that will agitate the flow. Visit our article to learn more about straight pipe runs.

• Remove all packing materials and transport retainers and ensure that no such materials remain in the device.
• Make sure that the maximum operating pressure and temperature of the device are not exceeded.
• Install the flow switch in the piping system, ensuring that the piping is rigidly supported at the inlet and outlet of the switch with pipe clamps.
• Ensure that the piping is aligned with the inlet and outlet fittings and that no piping induced stresses are exerted on the switch. This stress can result in damage to the switch during system operation.
• Protect the measuring tube from external damage.
• Avoid pressure surges in the measuring tube, such as those arising from fast start up/shut down or pulsating flow.
• If possible, after completing the mechanical installation, check the threaded connection between the device and the piping for leakage immediately.
• Make sure any electrical supply lines are de-energized when connecting the device!
• Connect the power supply line to the electrical connections per the user manual for your device.

For any variable area float switches that are measuring media with contaminants, lime or dirt deposits on the housing and the internal parts should be removed regularly. To clean a variable area flow switch, proceed in the following manner:

• Make sure that the electrical supply lines are de-energized.
• Unscrew the flow switch from the line.
• To disassemble the flow switch, remove the stud screws.
• Clean the flow switch in a cleaning bath or in an ultrasonic bath.
• When reassembling the flow switch, be sure that the gaskets are seated properly.

If thermal or paddle flow switches experience build-up on their probes or paddles, they will also need to be removed and cleaned.

Testing a flow switch is typically done by introducing flow into the system and then seeing if it triggers at the preset switch points. If you are not sure that it is switching at the correct flow rate, you can verify the flow rates easily if you are using a flow switch that is integral to a flow meter that will show you the exact flow rate at any given time. You can adjust the flow rate to the desired switch points and make sure the electrical output signals at those values.

Testing a standalone flow switch that is not connected to a flow meter can be a little less straightforward if you want to check the exact switch points. You can verify that it is switching though increasing and decreasing the flow to see if it is triggered or not. Flow could also be verified through another flow meter near the switch or through a rented clamp-on flow meter that shows continuous flow rate. But most switching applications aren’t going to require that degree of accuracy if the application did not require a flow meter in the first place. What is desired is a repeatable switching point.

If it does not seem that the flow switch is working, you can also test to verify that the electrical components are still functioning and that the switch has the power it needs to function.

Resetting a flow switch depends on the application that the flow switch is being used in. For example, in residential hot tubs, sometimes resetting the integral flow switch is as easy as turning the power off and on. For our purposes however, we are speaking about resetting a flow switch in an industrial process setting.

To reset or adjust the switch point of an industrial flow switch, consult your product manual as the method will vary based on both the technology type and whether or not it has a digital display/controller. Some variable area flow switches are as easy as sliding the switch contact mounted on the outside of the flow tube up or down to the desired flow rate on the scale printed on the flow body. Others, like a digital thermal switch, will likely require following the integral menu programming in the device.

If your flow switch is no longer triggering at your specified flow rate or it is not switching at all, even though it is clear that the set flow rate has been reached, then the switch is probably bad. The first thing to verify is that the switch has power. If that can be confirmed, it may be time to repair or replace your switch. For paddle switches, you can also manually verify that the paddle is still correctly attached to the housing and is still moving freely.

Flow switches can vary in price depending mostly on the technology type and materials. For example, a metal flow switch will cost more than a plastic flow switch. Flow switches with digital menus and no moving measurement parts, like thermal dispersion switches, will also generally cost more. Switches that come as an integral part on a full flow meter that also provides continuous rate measurement and not just monitoring, will generally cost more.