DP-flow measurement applications are made up of two constituent parts; a primary element, including orifice plates, flow nozzles, wedge meters, Pitot tubes and Venturi tubes, and a secondary element, such as a differential pressure transmitter or Multivariable transmitter with temperature measurement and integrated flow computer. The primary element is used to constrict the flow of a fluid in order to create a pressure drop, which in turn allows differential pressure to be measured to ascertain the flow rate.
Once the correct primary element is chosen it is placed in the pipe, creating an area of differential pressure and is connected directly to the secondary element. The secondary element or transmitter then takes a measurement and converts the reading into a useful quantitative value by using calculation methods within the multivariable transmitter such as ISO 5167 / AGA3 , Natural Gas compensation acc. AGA8 .
What benefits do they offer?
Multivariable transmitters offer a variety of benefits. By replacing separate dedicated instruments with a single device that can measure multiple values and calculate flow, multivariable transmitters can typically cut the cost of a mass flow installation by around 40 percent.
In addition to reducing the amount of devices, users can continue to save money due to minimising process connections, less cabling and fewer I/O modules used for recording and transferring measurement data to a centralised system. All these factors help reduce installation time, cutting costs further, without compromising on reliability or measurement accuracy.
Another advantage of choosing multivariable transmitters is the ability to limit the number of pipe penetrations. Installing just one instrument reduces the impact on flow and any negative effects on measurement accuracy. Furthermore, with the inclusion of integrated calculation functionality, the transmitter can measure any compensated levels of gas, liquid and steam, ensuring that any variations in temperature, pressure, density or viscosity are accounted for to maintain an accurate reading.
What factors should be considered when selecting a multivariable transmitter?
The main factor to consider is the choice of primary element. Due to different fluid properties, certain primary elements are better suited to different applications. Some examples include:
- Orifice plates
Suitable for clean liquids at low velocity and / or high viscosity, and low density gases.
- Venturi tubes
Suitable for clean liquids and gases.
- Flow nozzles
Suitable for high velocity fluids and flow testing for steam-raising plant.
- Pitot tubes
Suitable for liquids, gases and saturated and super-heated steam.
- Wedge meters
Suitable for fluids with high solid content, erosive and abrasive fluids and high viscosity fluids.
Another factor to consider is the need for temperature and pressure compensation. The density of a fluid can change relative to temperature and pressure changes. This density variation can then have an impact on the measurement accuracy if the flow rate is uncompensated. This adjustment occurs when flow rate is affected by temperature, pressure, density or viscosity.
Where can multivariable transmitters be used?
Multivariable transmitters are used to measure differential pressure flow in a variety of applications. One example of where they can be used is the standard volume flow value measurement of natural gas when filling or emptying underground storage caverns under high static pressure. Measurement of the gas flow during both filling and emptying must be as accurate as possible. To help take account of changes in temperature and pressure that can happen in this application, the density of the gas must also be factored in, with the measured value being specified as the standard volume flow of the natural gas.
The inherent benefits of multivariable pressure transmitters also make them ideal for level measurement applications. Differential pressure transmitters have long been used for hydrostatic level measurement, offering a cost-effective and easy to install solution, especially in applications with difficult operating conditions or where high pressures or temperatures need to be factored in.
Multivariable pressure transmitters are particularly ideal for handling changes in density and temperature that can affect the accuracy of a hydrostatic level measurement. By recording the process pressure and temperature in addition to the differential pressure, changes in the density of the measured medium can be corrected, significantly improving the quality of the measurement.
Where can I find out more?
For more information about the benefits of multivariable transmitters, call 0870 600 6122 or email email@example.com ref. ‘Multivariable pressure’. Alternatively, please visit www.abb.com/pressure.