A liquid flow meter measures the mass, volumetric, linear, and non-linear flow of fluid. To get the flow rate, the device measures fluid velocity. Like other devices used for process control, there are also different types of flow meters according to the application. The nature of the installation, as well as the properties of the fluid being measured, affect the selection of flow meters.
Different flow meter applications
There are five categories of liquid flow meters:
- Positive displacement
- Open channel
- Differential pressure
Details of these categories will be discussed further in this article.
Positive displacement flow meter
A positive displacement flow meter uses precise and fitted rotors for measuring fluid flow. Constant parameters such as a fixed volume move between these rotors. The movement or rotor spin equates to the fluid’s volume displacement. There is a corresponding electronic pulse transmitter which counts the number of rotor spins. All information gathered, including the fluid volume and rotor spin gets converted into the flow rate.
This type of flow meter is commonly used for measuring liquids such as vegetable and animal fat used in food processing, lubrication oil for machines, printing ink, heating oil, and polymer additives.
Specific examples of flow meters that fall under this category include rotary vane, reciprocating piston, and oval gear.
Mass flow meter
A conventional example of a mass flow meter is a Coriolis Effect flow meter. This type of flow meter was developed because there was a need for higher accuracy in measuring the fluid flow of processes related to mass. Examples of these processes include chemical reaction and heat transfer of fluids.
Mass flow meters are typically used in wastewater management, power plants, oil mining, and pharmaceutical laboratories.
Open channel flow meter
As the name suggests, an open channel flow meter refers to the device used in an open area where liquid passes through. Examples of this include sewers, rivers, streams, tunnels, and canals. To monitor the flow rate in open channels a flow meter is used to measure the flow rate by calculating the depth of the water.
Differential pressure flow meter
This type of flow meter is the most commonly used in measuring fluid flow rate. Around 50% of all flow meters used around the world fall under this category. The working principle of this flow meter applies the fluid’s drop in pressure as proportionate to the flow rate squared. To calculate the flow rate, you take the pressure difference, and then the square root of this measurement is made.
Some examples of a differential pressure flow meter include variable area flow meters, elbow-tap meters, and pilot tubes.
Velocity flow meter
To measure fluid flow rate using a velocity flow meter, you need to take the flow speed in one area of the flow and then integrate this with the total speed for the whole flow area.
A specific example of a velocity flow meter is a turbine flow meter used in petroleum pipelines. These flow meters detect leaks in the pipes and measure chemical and other liquids used in drilling oil.