What does a flow meter do?
A flow meter is a flow rate measuring device used to determine the linear or nonlinear mass and volumetric flow of a liquid or a gas. The many names of flow meters include flow gauge, flow indicator, liquid meter, and flow rate sensor. How they are named depends on their industrial use. Their purpose is to improve the precision, accuracy, and resolution of fluid measure. They are a great investment to improve efficiency, have low maintenance, are easy to use, and are versatile and durable.
Flow meters can measure the volume of a material, its speed, or its mass. Using various calculations, they report mass flow, absolute pressure, differential pressure, viscosity, and temperature. Liquid flow is equal to the area and velocity of the flow or Q = A multiply v. Mass is calculated using the formula ṁ = Q multiply ρ, where Q equals flow rate and ρ equals density. With mass, the main concern is gases, chemical reactions, and combustion.
Applications of a flow meter
One of the considerations regarding the use of a flow meter is the type of flow, which can be open channel or closed conduit. Open channel is when the flow is open to the atmosphere, where closed conduit is when the flow is in a tube or pipe.
There are several features that need to be evaluated when determining the effectiveness of a flow meter. Remote monitoring, types of data, and the frequency of collection are a few of those factors. Below is a description of the applications for a flow meter.
Characteristics of a fluid and its flow
The list of things a flow meter measures are pressure, temperature, density, viscosity, and vapor pressure, which are displayed as a single reading. A major function of flow meters is to monitor safety conditions such as toxicity, bubbles, the presence of abrasives, and transmission qualities.
Ranges of pressure
Any mass needs force to move, which is part of Newton’s Second Law of Motion. In the case of fluids, in a confined pipe, the force that is applied to move the liquid is pressure. The density of the liquid determines the amount of necessary pressure, which indicates the flow rate. When a flow meter is measuring density and pressure, it uses that data to calculate flow rate.
Ranges of temperature
In terms of flowmeters, the measurement of temperature is referred to as thermal flow measurement, which measures heat transfer as a gas flows past a surface. A temperature sensor provides a measurement of the liquids or gases’ temperature, while a heated flow sensor measures the amount of heat transfer of the material’s flow.
The design of a flow meter is the primary device, a transducer, and a transmitter combined into a single instrument. Positive flow meters give precise real-time output and accurate measurements with a signal directly connected to the force of the gas or fluid. The output signal is connected to the flow meter system of a turbine or rotator wheel, plate, channel, nozzle, laminar, and pilot table system.
Flow meters are made from stainless steel plates, brass, aluminum, PVC, PVDF, and nylon. Their design depends on the viscosity of the measured substance, cleanliness of the flow, pressure, temperature, and pipe size. They can be custom manufactured to measure any form of flow.
Types of flow meters
There are two categories of flow meters: gas and fluid or liquid. Fluid flow meters have five subcategories: differential pressure, velocity, positive displacement, mass flow, and open channel.
Differential Pressure Flow Meters
Differential pressure flow meters use the Bernoulli Equation, which states as the speed of the flow of a fluid increases its pressure decreases. They report the difference between a primary and secondary measurement, where the first measurement causes a change in kinetic energy by directing the air through a hole in the flow meter measured by the second element. The sub-types of differential pressure flow meters are orifice plates, flow nozzles, venturi flow meters, and rotameters.
Orifice Plate Flow Meter Systems measure the change in the pressure of the flow, going from upstream to downstream with the flow being partly blocked in the pipe.
A Flow Nozzle is a simple and less expensive type of venturi meter. The nozzle is held between flanges of pipe carrying the material. A differential pressure sensor measures pressure drop as a measure of the flow rate.
Venturi Flow Meters measure liquid flow rate and check the change in pressure, which occurs due to a cross sectional flow area in the flow path.
Rotameters are variable flow meters that use a rotating float that moves vertically through a tapered tube and measures the pressure applied on a fluid by gravity. They are commonly used for measuring gases and liquids. Rotameters have linear output, simplicity, low cost, measure low pressure drop, and have a wide range of applications.
Velocity Flow Meter
Velocity flow meters calculate flow speed. They produce a reading by measuring the depth and average air velocity of the flow allowing engineers to maintain necessary flow throughout the stream. Velocity flow meters have a greater range than differential pressure devices. Types of velocity flow meters are pilot tube, calorimetric, turbine, and electromagnetic.
Pitot Tube Flow Meters check the velocity of flow by translating its kinetic energy into potential energy and are used in ventilation and HVAC systems.
Calorimetric Flow Meters, called thermal flow monitors, use the laws of heat transport by determining the distinction between continuous and regulated heating. They have two temperature sensors with one measuring the heating element, while the other measures the temperature of the fluid.
Turbine Flow Meters use flow to measure the speed of the turbine and are used with clean and viscous liquids with an accuracy of 0.5% of the reading. They have a multi-blade rotor mounted at right angles to the flow. Outputs are a sine wave or square wave frequency. Signal conditioners can be mounted on them but are only found on explosion proof classifications.
Electromagnetic Flow Meters, or magnetic flow meters, are volumetric and use Faraday's law of electromagnetic induction.
Vortex Flow Meters measure flow rate by placing an obstruction directly in the flow path forcing liquids or gases to move around it. The flow around the obstruction produces two symmetrical vortices on the opposite side, which changes the pressure of the flow. A flow sensor, between the vortices, measures pressure changes.
Ultrasonic Flow Meters measure fluid velocity using ultrasound to calculate flow volume. There are two types of ultrasonic flow meters: in-line and clamp-on flow meters. In-line ultrasonic meters are inserted into the flow pipe and consist of two sets of ultrasonic devices aligned opposite each other. Clamp-on devices use the same two devices but are not inserted into the pipe.
Hydraulic Flow Meters measure the flow rate or volume of a liquid inside a hydraulic system and are used for testing, troubleshooting and maintaining hydraulic systems. They measure how efficiently and effectively the system is running and address any problems.
Air Flow Meters measure air velocity and pressure. They are ideal for taking quick constant measurements of ventilation installations and process checking.
Positive Displacement Flow Meter
Positive displacement flow meters, or PD flow meters, measure viscous liquid flow using rotors as sensors. Airflow is displaced by vanes, gears, pistons, or diaphragms. Rotation of the rotors is related to the volume of the flow. They are used for measurements when straight pipe is not available or as a replacement for turbine meters and paddlewheel sensors, when there is too much turbulence in the flow.
Mass flow meters
A mass flow meter measures the volumetric flow rate by dividing the mass flow rate by the density of the fluid. It uses Coriolis Effect, which is an inertial force that acts on objects in motion within a reference point.
Thermal Mass Flow Meter uses two sensors to take temperature measurements with an active heat sensor to measure heat loss in the liquid and a flow meter to determine the flow rate.
Mass Gas Flow Meter, or Coriolis flow meters, work on the basis of the Coriolis Effect, motion mechanics. When the fluid enters the sensor, it is split and the tubes of the sensor oscillate creating a sine wave. The time delay between the oscillation of the two tubes produces a mass flow rate. They are used for leak testing and low flow measurements.
Open channel flow meters
Open Channel Flow Meters check the height of the liquid and are used with flows that are exposed to the open air.
Spring and Piston Flow Meters measure the annular flow through an orifice that is formed by a tapered cone and piston. Scales are based on gravities of fluids, where oil is 0.84, and water is 1.0. They have a simple design and are an alternative to rotameters since they can be easily installed to transmit electrical signals.
Flow Switches are not flow meters but simple mechanical instruments for monitoring the flow of air and fluids. They are set to a predesignated level and activate when there is a fluctuation in the level. Once activated, they remain on until the error in the system is corrected.
Digital Flow Meters are defined as any flow measurement device with a digital display. A common digital meter is used by power companies to measure the flow of electricity to your home and sends your usage data to the electrical company, which uses the data to bill you.
Additional flow meter types
Water Flow Meters measure the volume of slurries, water, or other closed pipe fluids. The flow rate is measured in cubic meters or liters.
Fuel Flow Meters measure the amount of fluid being transferred. They have a digital or mechanical visual display that lets the user know how much fuel has been transferred during a transaction.
Peak Flow Meters are used for breathing disorders and measure the lungs capacity. They are handheld and an inexpensive device that measures lung capacity by blowing into the mouthpiece, which measures, in liters, the amount of air taken in each minute.
Flow Indicators allow an operator to view a liquid as it moves through a pipe. They are often called sight flow indicators and are a part of an industrial process that requires an instant and inside look at the flow inside a pipe. Flow indicators do not take calculations or measurements regarding mass or speed. Also known as plain sight indicators, they are the simplest form of flow meter and have no moving parts.
Flow meter installation
Flow measurement is a constant task for any industry and is the reason that flow meters have become so essential. For a flow meter to be able to supply accurate and reliable data, it has to be installed correctly.
Listed below are some of the critical steps to follow when installing a flow meter:
- Know the area where it will be installed.
- It cannot be installed where there are vibrations or magnetic fields.
- Know the direction of the flow.
- Avoid downward flow in liquid applications.
- Must be installed on a straight pipe.
- Ensure it is completely filled with fluid.
- Check for vapors or air in liquids and droplets in gas lines.
- If possible, put a filter upstream of the meter to eliminate solids.
- Allow for expansion of the pipe.
- Have a by-pass line for times when repair is necessary.