These difficulties, however, can be overcome by taking a series of relatively simple measurements that provide high quality data.
Pressure transducers. The choice of the transducer depends on the operational range, whether gauge, absolute or differential pressure measurement is required. Accuracies of +/-0.25 percent are readily achievable.
For a sensor mounted inside a pneumatic conveying system, a distinction has to be made as to whether it will be used in the single-phase or two-phase section of the system. If there is the possibility of solid particles being in the pipeline, the gauge should be protected either by the choice of location or by the use of baffles or protection plugs. Another method is to use a wire in the port, the principle being that the mainstream flow will vibrate the wire and thus dislodge any clogging in the port.
Temperature measurement. Standard RTD and thermocouple sensors can be used. These provide an accuracy of 1 Degrees C. which is generally acceptable.
Gas flow. In pneumatic conveying systems, the product flow rate depends on the mass flow rate of gas into the system. Many techniques are currently used for measuring gas flow.
Differential pressure meters, which generally employ orifice plates, now are the most widely used and accepted method of measuring air flow. An overall system with density measurement can provide accuracies of between 2 and 5 percent. There is always a permanent pressure loss associated with these devices.
Vortex shedding meters offer a lower pressure drop and accuracies of about 1.5 percent and turndown ratio of 40:1. These meters have no moving parts and have a low pressure drop. However, they are very sensitive to swirl and to flow pulsations.
Thermal meters measure mass flow rate directly and can provide accuracies of 1 to 2 percent of full scale. They have a wide working range, with turndown ratios of around 50:1. Pressure drop is relatively low compared to differential pressure devices. Thermal meters, however, are sensitive to flow profile, and are relatively costly.
Proper handling of the data generated is crucial. Recent advances in portable data-acquisition hardware and associated software, along with the advent of smart sensors, mean that it is feasible to install a measurement system capable of recording a time history of plant operating conditions that can significantly enhance troubleshooting.
Elizabeth Knight is a senior consultant and Dr. Don McGlinchey is a consulting engineer at the Centre for Industrial Bulk Solids Handling of Glasgow Caledonian University (GCU), Cowcaddens Rd., Glasgow G4 0BA, U.K. The authors wish to recognize the contribution made to this article by their esteemed colleague Dr. Pedrag Marjanovic, who, the authors note with sadness, has since passed away.