Flow switch

Image Credit: Rotameters | Grainger | Dwyer Instruments

 

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.

 

how to select flow switches

Flow switch cut-away. Image credit: Orange Research

 

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.  

 

how to select flow switches

Parts of a flow switch. Image Credit: Myron L

 

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 installation. Video Credit: PoolGuySupply

 

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:

 

Where:

 = flow rate

ρ = mass density of the fluid

  = velocity field of the mass elements flowing

 = cross-sectional vector area/surface

Q = volumetric flow rate

 = mass flux

 

For areas that are curved:

 

 

  • 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.

how to select flow switch

In-line flow switch. Image Credit: Orange Research

  • 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. 

End Fittings

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.  

how to select flow switches

FET transistor. PIN diodes

Image Credit: Circuits Today | Microwaves 101

  • 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

 

Resources

Flow Switches

Agilent Solid Switches