Closed Loop Control for Pneumatic Conveying
Pneumatic conveying brings countless advantages. However, these types of systems require proper control to maintain efficiency and product quality. In this article, PIF spoke to Tony Brennan, Field Segment Manager, Gas and Micro for Bürkert, to find out more about the advantages of pneumatic conveying and how it can be maximised with a closed loop control system.
Closed Loop Control Systems
The process of transporting materials often requires the destination to change. For example, when a storage container is full and the product is diverted to the next container. The volume of air in the transport system is crucial to maintaining the product in suspension and making changes to the required volume must be matched by a suitable adjustment to the airflow in order to keep the product in suspension and avoid unnecessary wear to the pipework or degradation of the conveyed material.
The same applies to making a change to the product itself; this will also require an adjustment to the airflow to ensure the correct air velocity and product quality are maintained. However, the way in which the air is introduced to the conveying system can have a significant effect on the system design and the power requirements to operate it.
More traditional ‘blow pot’ solutions are giving way to systems that use multiple air injection points to provide a uniform air flow throughout the system. Increased levels of computer control and monitoring allows new installations to be delivered with much lower power requirements, which form a significant proportion of the operating costs.
One possible solution for delivering improved efficiency is a closed loop controller that features a flow sensor, a process controller and a control element, all in one unit. As such, a compact and flexible design would enable the flow controller to be installed easily and provide accurate management of the compressors.
Bürkert Valves for Closed Loop Systems
Bürkert specialises in developing solutions for customers in a diverse range of industries, delivering flow control systems for a wide variety of applications. The latest offering addresses the issues associated with creating a closed loop system for pneumatic conveying, all in one product. The Type 8750 flow rate controller provides a solution that can reduce operating costs and improve productivity through better management of the compressors.
The pneumatic seat valve compensates for the air leakage across the rotary valve that introduces the solids to the conveying air stream. The Type 8750 can store the flow leakage curve of each rotary valve so that for any given inlet pressure the 8750 knows how much additional air is required to compensate for the air lost from the system by the rotary valves.
The flow rate control system consists of an ELEMENT continuous control valve (Type 2301) with a compact process controller (Type 8693) and two pressure transmitters, (Type 8323). These components are supplied as an assembled system that negates the requirement for a separate flow meter.
Using the pressure difference across the valve and the given density and temperature of the medium, a nominal flow can be calculated, providing the flow characteristics of the valve to the process controller. The volume flow can then be adjusted by changing the stroke of the control valve. The control valve serves as an orifice plate to connect pipework so installation is straightforward and there is far less of a pressure drop compared to alternative arrangements.
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