The mass flow rate of solids is considered as an important parameter of a pneumatic conveying system for ensuring product quality, efficient operational conditions and the control of the process.
In any transport system it is always desirable to measure the material transport rate in real time and without disturbing the flow. In the absence of any economically viable online mass flow measurement technology, system operators tend to use load cells at the receiving tank/silo to monitor the mass of material transported. These load cells are very costly if the size of the silo is very large.
POSTEC has developed a technique for mass flow measurement in pneumatic conveying pipelines, based on a scaling model, in which the gas-solids mixture is considered under a dynamically changing flow situation. The technique needs only a few measurements; pressure drop at a given location over a given length of pipeline and air flow rate during conveying. The technique also utilizes the predetermined pressure drop coefficient of the bulk materials, in a similar way as it is used in “PneuDesign” (the design and simulation software for pneumatic conveying systems).
Figure 1: Schematic diagram of the measurement principle
The two pressure gauges are placed approximately 2m apart and the pressure drop is measured when the gas-solids flow takes place, as shown in Figure 1. This can be used to calculate the mass flow of bulk solids in real time through the test section. The technique can be implemented in any type of conveying system. Figure 2 shows a typical set-up of mass flow meter in a blow tank system.
Figure 2: Practical implementation of mass flow meter
This technique has been tested in the semi industrial scale test rig at POSTEC with different bulk materials representing different groups in Geldart’s classification. Some of the results are shown in Figures 3 & 4.
Figure 3: Comparison of actual mass collected and calculated mass for cement
Figure 4: Weight-cell reading and mass flow prediction for Polyurethane pellets
The technique has been implemented in pneumatic conveying facilities in offshore oil platform supply vessels and the full scale validation has shown very good results.