Flow expresses the quantity of fluid (liquid, gas or steam) that passes through a section in a unit of time. It can be expressed in volumetric (m3/h, l/min, l/h, etc.) or mass (kg/s, Kg/h, etc.) flow units. The main factor for flow measurement is the mean flow velocity that is used to calculate the Reynolds Number (Re), which subdivides the flow of the fluid into two categories: Laminar (typically with Re < 2000) and Turbulent (typically with Re > 10000).
All the flow meters offered by Riels Instruments (with the exception of the volumetric and mass flow meters) are mean or instantaneous velocity flow meters and are therefore sensitive to the velocity distribution in the section to be measured - the distribution changes with the Reynolds Number and thus with the operating conditions.
We list below the flow meters available according to the different principles of measurement:
-Electromagnetic flow meters
-Volumetric flow meters
Oval wheel litre counters
Oscillating piston litre counters
Nutating disc litre counters
-Turbine flow meter
-Vortex flow meter
-Mass or Coriolis flow meter
-Ultrasonic flow meter
-Open channel flow meter
-Thermal dispersion flow meter
Electromagnetic flow meters are based on Faraday's Law of Magnetic Induction, which states that a conductor (the liquid itself) immersed in a magnetic field produces at its ends an electromotive force proportional to the velocity with which the conductor crosses the flux lines of the magnetic field. Electromagnetic flow meters are ideal for precise and accurate measurements. As an electromagnetic flow meter is an empty tube, it is suitable for dirty liquids, solids suspended in liquids, or for liquid foods.
The conditions required for the correct operation of an electromagnetic flow meter are: full tube and electrically conductive liquid.
Volumetric flow meters indicate the flow on the basis of geometric volumes generated and calculated by a measuring device. Volumetric flow meters are comprised of a moving component that can be a disc, piston or a gear pair which, when thrust by the fluid, set in motion a drive shaft that transfers the movement of the mobile component to the exterior of the body where it is collected by a calculation device. Volumetric flow meters are used for fiscal readings given their high precision and are recommended for high viscosity fluids such as refined petroleum products like diesel, petrol, fuel oil, kerosene, naphta, LPG and solvents. One of the main disadvantages of volumetric flow meters is the wear and tear of the portable measuring device and this makes it unsuitable for use with dirty or abrasive fluids.
The turbine flow meters are comprised of a very light wheel, made up of a blade or rotor, that indicates the velocity of a fluid current and change it into a rotary motion, the velocity of which is dependent on the flow rate.
Turbine flow meters are suitable for industrial measuring of gas and liquids and are applied in turbulent regime (Re>10000). A turbine flow indicator is not suitable for high viscosity fluids as the blade is subject to wear and tear, making it necessary to filter the fluid to prevent solids in suspension in the fluid from damaging the instrument. A turbine flow meter is ideal for gases such as methane, nitrogen, air, hydrogen, etc., and fluids like water and neutral fluids.
Vortex flow meters are based on the Law of Karman, who studied and coded the formation of whirls in fluids in motion.
The whirls are caused by a suitably shaped body installed in the piping; it is, therefore, a phenomenon generated by the fluid itself and is theoretically consistent with the velocity of the fluid in a turbulent flow regime (Re>10000).
A Vortex flow meter is ideal both for liquids and gases in pipes of limited dimensions, but it is not suitable for viscous fluids which would need to be filtered.
Mass flow meters exploit the Coriolis effect that occurs when a fluid flows in a rotation system. Mass flow meters, therefore, are not subject to the variations in the physical parameters of the fluid such as temperature, pressure, density and viscosity.
A mass flow meter is comprised of a U-tube which, rather than made to rotate, is made to oscillate periodically at the natural frequency. A microprocessor ensures that the mass flow meter vibrates and to indicate and process the signals originating from the sensors. Coriolis flow meters can be used independently of the nature of the fluid, on condition that it is compressed if the fluid is a gas.
Ultrasonic flow meters exploit the capacity of materials to propagate sound, in the fluids in particular the propagation velocity of sound is a characteristic that depends on the mass density, pressure and temperature. Ultrasonic flow meters operate on the following principle: an electric impulse is applied to an electroacoustic transducer that turns it into a corresponding impulse of sound energy with a frequency of between 0,5 and 5 MHz. The impulse propagates itself in the fluid under test, with a velocity dependent on the velocity of the fluid itself, up to the receiver where it is turned into an electrical signal.
Ultrasonic flow meters can also be used for dirty or abrasive fluids and in piping of large dimensions - its precision, however, is affected by the fluid regime.
Open channel flow meters are applied where there is not a pipe but an open conduit (channel). In the open channel flow meters measurement of the level occurs in the narrow section where the flow increases in velocity. The maximum velocity allowed varies according to the dimensions of the channel from about 0,5m/s to 2 m/s. Open channel flow meters are usually used in “free flow” conditions with the level of the water that flows quite low so as not to place counter pressure on the high velocity jet that passes across the narrow section of the channel. An open channel flow meter can be used for medium and large flows and even with dirty fluids or those that have suspended solids.
Thermal dispersion flow meters are based on the principle of heat transfer according to which the flow being measured in a pipe is subjected, by means of an electrical heating element, to a heating power that will be absorbed by the fluid.
Thermal dispersion flow meters can be connected with internal or external heating of the piping or shunt type for small, medium and large piping. These latter are comprised of a capillary tube (suitably heated and measured) into which a part of the flow is thrust by a laminar measuring device placed in the flow piping.
A thermal dispersion flow meter is mainly used for gases but the thermal physical properties of the fluid must be known.
