In industrial work settings, the accurate measure of flow is highly critical. Not only does this define profit or loss, but consumer and worker safety are at stake as well. Flow meters are vital in the precise and accurate measurement of flow rate (mass or volume), i.e. the amount of fluid that passes through a pipe.
Flow meters are used in a variety of applications. They measure the mass flow rate or volumetric flow rate. The application will depend on the capacity and type of the flow meter. As a result, each type of flow meter will have its own engineering requirements and constraints. A flow meter will often be referred to by its name. Common examples include flow indicator, flow gauge, liquid meter, etc. This depends upon the industry where it is being used; nevertheless, the function will be the same.
There are five essential types of flow meters. Although the list below is by no means exhaustive, it addresses the most well-known types of flow meters used in industries today:
1. Differential Pressure Flow Meters
his type of flow meter relies on Bernoulli’s equation to judge the flow of fluids through a pipe. Differential pressure flow meters present a constriction within the pipe which leads to a drop in pressure across the flow meter. As flow increases, the pressure drop increases. Impulse piping will route the downstream and upstream pressure of the flow meter to a transmitter that will measure the differential pressure to arrive at the fluid flow. Differential flow meters account for up to 21% of the world’s market for flow meters. They are commonly used in industries such as gas and oil, power, chemical, waste, water, mining, metals, paper and pulp, pharmaceutical, beverage, HVAC, and food.
2. Positive Displacement Flow Meters
These are also called mechanical flow meters. They measure the fluid volume, deliver it fast and refill it again. The amount of fluid transferred is calculated. Positive displacement flow meters measure the actual flow of fluid, whereas other flow meters measure other parameters and convert it into a flow rate. In these types of flow meters, the output is correlated to the fluid volume that passes via the flow meter. One can compare a PD flow meter to a stopwatch and a bucket. The stopwatch begins when the flow commences. It stops when the buckets have reached its limit. The flow rate is determined by dividing the volume by time. To obtain a continuous PD measurement, a system is required to continually fill and empty buckets and then divide the flow without letting it exit the pipe. These continually forming and subsiding volumetric displacements are represented by pistons responding in cylinders or gear teeth coupling against the internal wall of the meter.
Examples of positive displacement meters include oval gear meters, piston meters, rotary vane meters, rotating disk meters, etc.
Positive displacement flow meters are highly accurate. They are often used while transferring fluids or oils such as hydraulic fluids, gasoline, water and various gas applications.
3. Velocity Flow Meters
Velocity flow meters determine flow rate by measuring the velocity of a stream to arrive at the volumetric flow rate. These types of flow meters pursue a linear relationship to linear flow rates. Contrary to differential flow meters, no square-root relationship exists in such instruments. Therefore, velocity flow meters have a better range compared to other flow meters. Furthermore, they are less sensitive to alterations in viscosity, provided that Reynolds numbers (Re) of more than 10,000 is used. All velocity type of flow meter housings is fitted with special fittings called flanges. This enables them to be attached directly to pipelines.
The PV6000 series and V7000 series from Proteus Industries Inc. are examples of vortex flow meters. They both rely on the vortex concept to arrive at a reliable, accurate and cost-effective measurement of liquids that require heat transfers. An in-line sensor within the flow meter detects the frequency of the vortices shed by a bluff body within the flow stream. It then creates a pulse output signal which is proportional to flow rate of the fluid; thereby providing a highly accurate measurement.
4. Mass Flow Meters
Also called inertial flow meters, these devices measure the mass flow rate of fluids that travel through a tube. The calculations are mass of fluid traveling pass a static point per time unit. This type of flow meter is used in mass-related processes because they measure force which results from the speeding of mass. Two popular types of mass flow meters include the Coriolis mass meter and the thermal dispersion flow meter.
Coriolis flow meters rely on the Coriolis Effect. This occurs when a mass moves through a rotating system experiencing a special force which acts perpendicular to the motion’s direction and to the axis rotation. Here on earth, this effect deflects moving objects towards the right side in the northern hemispheres and in the southern, to the left. Coriolis flow meters provide a direct measurement of fluid density. These are highly accurate, irrespective of the liquid or gas being measured. The same measurement tube may be utilized for bitumen and hydrogen gas without recalibration. Coriolis flow meters are often used to measure the flow of natural gas. They are also used in industries such as power, mining, pharmaceuticals and waste water.
5. Open Channel Flow Meters
This type of flow meter measures the flow of liquids that are open in the atmosphere during the flow measurement path. The liquid being measured may either be contained in a pipe that is not filled with liquid and opened to the atmosphere during installation point of the flow meter or opens in the atmosphere the entire time. Level measurement and water velocity measurement meters are both required to arrive at an accurate flow rate. Typically, weirs, flumes, and v-notches are used to measure the flow liquids. These are dam-like arrangements which enable a limited or free-flow of fluids, based on the size and shape of the structure. Open channel flow meters are often used to measure free flowing fluids such as rivers, streams, sewer systems and irrigation channels.