AAP Flow Measurement Devices
What is a Flow Measurement?
Differential pressure sensors operate on the general principle that the pressure drop across the meter is proportional to the square of the flow rate. A pressure transducer used to measure this pressure differential. The mechanical part of the sensor is used to produce the pressure differential, while the electrical part of the sensor measures the pressure difference.
How Do They Work?
A pitot tube differential pressure meter uses two tubes or split tubes to measure a differential pressure. The first tube measures the static pressure and is usually mounted into the pipe wall. The second or central tube measures the impact pressure. The faster the flow, the higher the impact pressure. The differential pressure between the impact and static pressure is measured with this sensor. A silicon pressure sensor diaphragm or thin steel diaphragm with a strain gauge is generally used in this device. The flow rate is proportional to the square root of the difference between the static and impact pressure divided by the air density. This sensor does not work well at low velocities, since a differential pressure is being monitored. It is best for high air flow measurement.
Different Types of Flow Measurement:
Thermal flowmeters use the thermal properties of the fluid or gas to measure the flow in a pipe or duct. In a typical thermal flowmeter, a measured amount of heat is applied to the heater of the sensor. Some of this heat is lost (dispersion) to the flowing fluid. As flow increases, more heat is lost. This type of measurement can create very accurate readings of low flowing air or gas. It can also be used to measure caustic liquids.
In Line Turbine Meters
Turbine meters monitor the revolutions of a turbine that is inserted into the liquid to determine flow rate. Turbine meters require a clean liquid so the turbine does not jam. They can be made to also work in caustic environments.
Vortex Shedding Meters
Vortex flow meters operate under the vortex shedding principle, where an oscillating vortexes occur when a fluid such as water flow past a bluff (as opposed to streamlined) body. The frequency that the vortexes are shed depend on the size and shape of the body. It is ideal for applications where low maintenance costs are important. Industrial size vortex meters are custom built and require appropriate sizing for specific applications.
Electromagnetic flow meters detect flow by using Faraday’s Law of induction. Under Faraday’s law of induction, moving conductive liquids inside of a magnetic field generates an electromotive force (voltage) in which the pipe inner diameter, magnetic field strength, and average flow velocity are all proportional.
Inside an electromagnetic flow meter, there is an electromagnetic coil that generates a magnetic field, and electrodes that capture electromotive force(voltage). Due to this, although it may appear as if there is nothing inside the flow pipe of an electromagnetic flow meter, flow can be measured. As the flow changes, the electromotive force (voltage) captured by the electrodes changes as follows. This is a very accurate meter with no pressure loss and is unaffected by temperature, pressure, density or viscosity of the liquid. It cannot be used in gasses or liquids that have no electrical conductivity.
Ultrasonic Flow Meters
Ultrasonic flowmeters use sound waves to determine the velocity of a fluid flowing in a pipe. At no flow conditions, the frequencies of an ultrasonic wave transmitted into a pipe and its reflections from the fluid are the same. Under flowing conditions, the frequency of the reflected wave is different due to the Doppler effect. When the fluid moves faster, the frequency shift increases linearly. The transmitter processes signals from the transmitted wave and its reflections to determine the flow rate.
Transit time ultrasonic flowmeters send and receive ultrasonic waves between transducers in both the upstream and downstream directions in the pipe. At no flow conditions, it takes the same time to travel upstream and downstream between the transducers. Under flowing conditions, the upstream wave will travel slower and take more time than the (faster) downstream wave. When the fluid moves faster, the difference between the upstream and downstream times increases. The transmitter processes upstream and downstream times to determine the flow rate.
This technology can be very accurate and is used for custody transfer (meaning accounting accurately for an expensive fluid) of natural gas and petroleum liquids. High turndown (can read low as a percentage of the full scale or top reading), handles high pressures, is repeatable (consistent), handles extreme temperatures, can be used clamped to the outside of a pipe without penetration, is low maintenance, highly reliable and self –diagnosing.