Two factors have lead to an increase in importance for the need to achieve accurate liquid mass, as opposed to volumetric, measurement in process industries in recent years. These are the emergence of enterprise resourse planning (ERP) systems, and the advances made in process measurement instrumentation. Both have worked to increase reliability and reduce costs of inventory monitoring.
Historically, the measurement of liquid mass storage and a company’s computerised management systems have seen little, if any, integration. Companies traditionally utilised MRP systems to handle the back end of the business, ie bills of material, work in progress, inventory management etc. The weight of the product manufactured in the plant and the weights of the component products in storage vessels was factored in, but rarely took cognisance of the factors which can affect the accuracy of a process measurement.Now ERP systems expect to access realtime measurements in the plant, not merely historical accounting figures derived from ‘value of opening stock + stock added – stock sold = current stock value’. The development of fieldbus has made the technology available for ERP systems to directly access the information from the measuring devices. Further demands of the ERP philosophy are that, in terms of inventory control, liquids need to be accounted for by weight and not volume, especially in custody transfer applications where product is sold by the kilogram. The reason for this is simply because environmental changes such as temperature and pressure can affect volume but not mass. Thus, the task has fallen on instrument manufacturers to find better ways to measure mass. For solids, this has not been a problem since it is still acceptable to convert a level measurement into weight by assuming an average density of the product. Ultimately, the accuracy of solids weight measurements is not as important as that of liquids. Liquid mass has historically been measured using loadcells or, more recently, strain gauges. Both of these measuring methods, which weigh the vessel or tank in which the liquid is contained, have limitations that generally render their measurements less reliable and repeatable than required. For example, they require frequent re-calibration, their installation costs are high and environmental factors such as wind, rain and plant vibration can affect their accuracy. It has proved far more practical to measure the mass of liquid as the vessel is being filled or emptied and this has been achieved through the use of the Coriolis mass flowmeter such as Endress+Hauser’s Promass. In its basic form, the meter consists of two metering tubes, which are oscillated at their natural resonant frequency (Figure 1). One sensor is mounted at the inlet side of the tubes and one at the outlet. Under no-flow conditions, the frequency signals monitored by the sensors are exactly in phase with each other. As the mass of the liquid starts to flow through the meter, a distortion of the meter geometry occurs, resulting in a phase difference between inlet and outlet sensor. This phase difference is directly proportional to the mass flow rate of the liquid. Accuracies to within 0,5% of reading are achievable and, in special cases, to within 0,1%. Installing a Coriolis flowmeter on the inlet, outlet or both, can enable management to accurately determine, in realtime, the mass of material in stock and, therefore, its value. In certain applications with high flow rates or large diameter pipes, it may not be possible to use a Coriolis flowmeter. Instead, a radiometric measuring system together with an electromagnetic flowmeter can be installed (Figure 2). The radiometric system consists of a radioactive source, such as caesium (137Cs) or cobalt (60Co) and source container, a detector (or scintillator tube) and transmitter. Extremely low levels of gamma radiation are emitted from the source; these rays are attenuated as they pass through the material being measured. The highly sensitive scintillator tube measures the relative degree of attenuation and converts the signal into a density measurement. This measurement, combined with the volumetric flow measurement obtained from the electromagnetic flowmeters, provides a mass value that is accurate to within 2% of reading, which is generally acceptable in the mining and chemical industries. It is these advances in technology and ERP systems, combined with the growing popularity in digital fieldbus technology which will give users complete access to and control of all plant and process data from the shop floor to the top floor. This will lead to improved control over stock levels, allowing plant operators to optimise their operations and deliveries, reduce product losses through leakage or theft and ultimately lower their operating costs.Endress+Hauser (011) 444 1386
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