Vortex flowmeters, operating according to the Karman vortex street principle, have proved effective for measuring liquids, gases and steam. The Krohne Optiswirl is an enhancement of this device type. Temperature and pressure measurement and a correction factor calculator are incorporated in the Opti-swirl as standard.
Michael Möller
In the Karman measuring process, vortices form on a bluff body placed in the meter. As soon as the flow velocity increases, vortex shedding occurs and a more or less periodic vortex system, referred to as a vortex shedding street, is created. The vortices can be observed on the left and right sides of the bluff body as the product circumflows it. They rotate in opposite directions. Since the frequency of the separated vortices is proportional to the flow velocity, the user can calculate the flow rate.
Vortex flowmeters primarily measure volumetric flow; however, if the density is entered in the converter, mass flows or standard volume flows can also be displayed. Such applications assume constant pressure and/or temperature conditions. However, in most processes the pressure and temperature fluctuate, often leading to significant measuring errors. It is precisely here that accurate measurement is essential, as energy sources like steam and gas are expensive. In the case of overheated steam for example, a pressure change of 1 bar causes a measuring error of 17.43 % and results in corresponding extra costs. A similar scenario occurs when measuring compressed air, which is typical of many gas measurements.
Complicated compensation
In applications such as these, incorrect readings are usually redressed using a separate correction factor calculator and additional pressure and temperature sensors. Installations of this kind are very complex and expensive, as pres-sure and temperature sensors have to be built into the pipe and wired accordingly. In addition, when installing the pressure and/or temperature sensors, defined gaps have to be maintained behind the vortex flowmeter so as not to affect the measurement. The temperature and/or pressure cannot therefore be measured at the same location as the volumetric flow. This results in further inaccuracies when determining the readings.
Some manufacturers have already incorporated a correction calculator and temperature sensor into their flowmeters, although a separate pressure sensor is still essential. In order to determine the accuracy of these systems, the measuring errors of the individual components have to be added together. Depending on the design, an overall measuring system accuracy of at best 4 to 5 % can be achieved.
Two-wire meter
Temperature and pressure measurement and a correction factor calculator are already incorporated in the Optiswirl. There is no need for additional pressure and temperature sensors or for extra wiring. The meter is thus suitable for almost any application – it is capable of measuring the operating, standard volumetric and mass flows of conductive and non-conductive liquids, gases or vapours. The Optiswirl features the ISP (Intelligent Signal Processing) technology developed by Krohne. This smart system analysis ensures accurate evaluation of readings from which all external perturbations have been eliminated. ISP is based on a notch filter that allows the user to track only the actual signal. The filter system first analyses the measurement signal and then searches for the vortex signal in the complete spectrum. All other frequencies are filtered out. Users are consequently assured of an accurate and stable measurement.
The Optiswirl is pre-calibrated in the factory, making it very easy to install in the piping. In accordance with the plug & play principle, the meter performs an automatic self-test after commissioning and is ready for use immediately. The Optiswirl features a human machine interface with a simple operating concept characterised by intuitive operator prompts. The high-contrast display guarantees optimum readability and is controlled using bar magnets. Pactware is included as standard at no extra cost. The Optiswirl electronics are easy to replace on site because all data is saved and transferred accordingly.
The accuracy of the vortex flowmeter is +0.75 % for liquids and +1 % for gases and vapours. In each of the cases described here, repeatability is +0.1 %. The standard Optiswirl model is designed for product temperatures between -40 and +240 °C and pressures up to 100 bar. Higher pressures are possible on request. Various approvals, including Atex and FM, are available for hazardous area applications.
cpp 448
Namur
Vortex flowmeters
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