Flow measurement devices are primarily used to measure the movement of media (gases, liquids, or slurries) within piping systems or channels. The primary purpose is to quantify the amount of the media being moved in order to meet a process demand or a usage requirement.
There are two types of flow measurement devices. These are categorized as volumetric flow and mass flow measurement devices. Volumetric flow makes up 95+% of the flow measurement devices in the market and range in accuracy from a flow indicating device that can be very accurate, depending on the design and type.
Within the Volumetric Flow Measurement Devices you have the following measurement technologies:
1) Differential Pressure Flow Measurement Devices:
Differential pressure flow meters measure the differential pressure across an orifice where flow is directly related to the square root of the differential pressure produced. There are also primary and secondary elements in differential flow meters. The primary element produces change in kinetic energy using either flow nozzle, pitot tube, orifice plate, or venturi. The secondary element measures the differential pressure and provides the signal.
Differential pressure meters represent around 20% of all flow meters around the world. They are commonly used in industrial applications, such as the oil & gas industry, as well as beverage, water, pharmaceutical, mining, paper and chemical applications.
2) Positive Displacement Flow Measurement Devices:
Positive displacement (PD) flow meters measure the volume filled with fluid, deliver it ahead, and fill it again. This calculates the amount of fluid transferred. It measures actual flow of any fluid while all other types of flow meters measure some other parameter and convert the values into flow rate. In PD flow meters, output is directly related to the volume passing through the flow meter. PD flow meters include piston meters, oval-gear meters, nutating disc meters, rotary vane type meters, etc.
Positive displacement flow meters are known for their accuracy. They are commonly used in the custody transfer of oils and fluids, like gasoline, hydraulic fluids, as well as in-home use for water and gas applications.
3) Velocity Flow Measurement Devices:
Velocity flow meters measure velocity of the media, then taking the cross sectional area of the meter flow body or piping to calculate the volumetric flow rate. Because there is no square-root relationship (as with differential pressure devices), their rangeability is significantly greater. Velocity meters have minimum sensitivity to viscosity changes when used at Reynolds numbers above 10,000. Most velocity-type meter housings are equipped with flanges or fittings to permit them to be connected directly into pipelines. Velocity Flow Measurement devices include turbine, paddlewheel, vortex shedding, electromagnetic and sonic/ultrasonic flow meters.
4) Open Channel Flow Measurement Devices:
Open Channel Flow Measurement of liquid in channels include v-notch, weirs and flumes. These dam-like structures, or overflows, allow for a limited or concentrated free-flow of liquids based on the unique shape and size of the structure. This type of flow meter allows for a reading of the volumetric flow rate to be calculated.
Common applications of open channel meters include free flowing liquids like streams, rivers, irrigation channels and sewer/waste water systems.
Mass flow meters are more effective in mass related processes as they measure the force that results from the acceleration of mass. More specifically, the force is measured as the mass moving per unit of time, instead of the volume per unit of time.
Within the Mass Flow Measurement Devices you have the following measurement technologies:
1) Thermal-type mass Flow measurement Devices:
Thermal-type mass flow measurement devices have traditionally been used for gas measurements, but designs for liquid flow measurements are available. These mass meters also operate independent of density, pressure, and viscosity. Thermal meters use a heated sensing element isolated from the fluid flow path. The flow stream conducts heat from the sensing element. The conducted heat is directly proportional to the mass flow rate. The sensor never comes into direct contact with the liquid. The electronics package includes the flow analyzer, temperature compensator, and a signal conditioner that provides a linear output directly proportional to mass flow.
2) Coriolis mass Flow Measurement Devices:
Coriolis meters are true mass meters that measure the mass rate of flow directly as opposed to volumetric flow. Because mass does not change, the meter is linear without having to be adjusted for variations in liquid properties. It also eliminates the need to compensate for changing temperature and pressure conditions. The meter is especially useful for measuring liquids whose viscosity varies with velocity at given temperatures and pressures.
Typical applications for mass flow meters are tied to chemical processes. In addition to the chemical and gas industries, typical industries using mass meters include pharmaceutical, power, mining and wastewater.
