Flowswitch Explained

Aflowswitch is a device that can be inserted in a pipe so that when liquid or air flows against a part of the device called a paddle, a switch is activated .

 This switch either closes or opens a set of electrical contacts. The contacts may be connected to energize motor starter coils, relays, or indicating lights.

 In general, a flow switch contains both normally open and normally closed electrical contacts.

Figure above shows a flow switch installed in a pipe line tee. Half couplings are welded into larger pipes for flow switch installations.

Typical applications of flow switches are shown above.These applications are commonly found in the chemical and petroleum industries. Vaporproof electrical connections must be used with vaporproof switches. The insulation of the wire leading to the switches must be adequate to withstand the high temperature of the liquid inside the pipe. (Consult the National Electrical Code® for insulation temperature ratings.)

Flow switches are used to detect liquid flowing through a pipe or air flowing through a duct. Flow switches, however, cannot detect the amount of liquid or air flow. To detect the amount of liquid or air flow, a transducer must be used. A transducer is a device that converts one form of energy into another. In this case, the kinetic energy of a moving liquid or gas is converted into electrical energy. Many flow sensors are designed to produce an output current of 4 to 20 mA. This current can be used as the input signal to a programmable controller or as the input to a meter designed to measure the flow rate of the liquid or gas being metered .

Liquid Flow Sensors

There are several methods that can be used to measure the flow rate of a liquid in a pipe. One method uses a turbine type sensor(Shown Above). The turbine sensor consists of a turbine blade which must be inserted inside the pipe containing the liquid. The moving liquid causes the turbine blade to turn. The speed at which the blade turns is proportional to the amount of flow in the pipe. The sensor’s electrical output is determined by the speed of the turbine blade. One disadvantage of the turbine type sensor is that the turbine blade offers some resistance to the flow of the liquid.

Electromagnetic Flow Sensors

 

Another type of flow sensor is the electromagnetic flow sensor. These sensors operate on the principle of Faraday’s Law concerning conductors moving through a magnetic field. This law states that when a conductor moves through a magnetic field, a voltage will be induced into the conductor. The amount of induced voltage is proportional to the strength of the magnetic field and the speed of the moving conductor. In the case of the electromagnetic flow sensor, the moving liquid is the conductor.
As a general rule, liquids should have a minimum conductivity of about 20 microhms per centimeter.
Flow rate is measured by small electrodes mounted inside the pipe of the sensor. The electrodes measure the amount of voltage induced in the liquid as it flows through the magnetic field produced by the sensor . Since the strength of the magnetic field is known, the induced voltage will be proportional to the flow rate of the liquid. A cut-away view of an electromagnetic flow sensor with a ceramic liner is shown .

Orifice Plate Flow Sensors

Orifice plate flow sensors operate by inserting a plate with an orifice of known size into the flow path . The plate is installed between two  special flanges . The flanges are constructed
to permit a differential pressure meter to be connected across the plate. When liquid flows through the orifice a difference of pressure is produced across the plate. Since the orifice is of known size, the pressure difference is proportional to flow rate. It is the same principle as measuring the voltage drop across a known resistance to determine the amount of current flow in a circuit. The disadvantage of the orifice plate sensor is that it does add restriction to the line. A differential pressure sensor is shown in Figure .

Vortex Flow Sensors

Vortex flow sensors operate on the principle that when a moving liquid strikes an object, a swirling current, called a vortex, is created. Vortex sensors insert a shedder bar in the line to produce a swirling current or vortex . This swirling current causes the shedder bar to alternately flex from side to side. 

The shedder bar is connected to a pressure sensor that can sense the amount of movement of the shedder bar . The amount of movement of the shedder bar is proportional to the flow rate.


Airflow Sensors

Large volumes of air flow can be sensed by propdriven devices similar to the liquid flow sensor . Solid state devices similar to the one  are commonly used to sense smaller amounts of air or gas flow. This device operates on the principle that air or gas flowing across a surface causes heat transfer. The sensor contains a thin film thermally isolated bridge with a heater and temperature sensors. The output voltage is dependent on the temperature of the sensor surface. Increased air flow through the inlet and outlet ports will cause a greater amount of heat transfer, reducing the surface temperature of the sensor.