In a wide variety of industrial applications, materials having diverse particle characteristics are pneumatically conveyed through enclosed pipelines. The main objective of pneumatic conveying is to transport reliably as much material as possible with minimal energy consumption. For abrasive bulk solids or fragile materials it is furthermore essential to maintain transportation velocities as low as possible to minimise damage to both the conveying system and the material. In dense phase flow, particles are transported in a non-suspended state. This type of flow is also known as plug flow. This flow regime consists of alternating sequences of dense material concentration (plug), followed by a section with no or very little material. In contrast to dilute phase flow, material velocities are comparably low in dense phase conveying, although higher mass transportation per time unit can be achieved. A stabile plug flow is characterised by a regular sequence of well shaped particle slugs with approximately the same lengths. Strong deviations from a periodical plug sequence and unstable material slugs are an indicator for conveying parameter maladjustment. For in-line monitoring of stability and sequence of material slugs in dense phase conveying, this paper presents two measurement principles. Both methods feature a non-invasive measurement technique from the outside of a closed conveyor pipe and are capable of performing quality inspection for the present material flow.

1. Optical measurement principle using a high speed camera

The high speed digital camera used for our investigations has the ability to record up to 3000 frames per second under optimal conditions. The camera was aimed at a sightglass in a horizontal section of the conveying pipeline. As a batch of product was conveyed, the high speed camera was used to record each slug of product as it passed through the sightglass. Since the conveyed material is illuminated by a spotlight through the sightglass, the contrast between 'material' and 'no material' in the camera images is easy to distinguish. Using only a small section of obtained camera images (cross-sectional measurement layer) and calculating the image histogram for the different recorded grey values, the fill level is easy to derive: for very dark images (histogram average close to zero) no or hardly any material is in the measurement layer while for bright images (histogram average close to 255) the pipe is filled in the section that has been recorded. When the grey values are determined for every recorded camera frame and plotted over time, the resulting signal shows the sequence of slug occurrence. The front face, the main body of the slug, the rear face, and the duration when no material is in the measurement layer can be detected in the grey value signal. This signal is used to monitor and rate the quality and stability of the flow.

2. Electric field based measurement principle using a fieldmeter

It is known that due to particle-particle and particle-wall collisions, pneumatically conveyed material carries electrical charge, which is also known as tribo-electric charge. This charge may cause high electric potentials in conveying pipelines. Electrostatic discharges present a major problem in many industrial applications. To avoid sparks and other discharge occurrences, proper grounding of pipes is crucial for safety reasons for certain conveyed materials. Especially on the inner surface of electrical isolators, potentials of several thousand volts can be measured during a conveying process. Those high potentials cause high electric field strengths that can be measured without risk even at half a meter from the pipeline. Practical measurements have been conducted using an influence-E-fieldmeter to determine the surface potential of a sightglass in dense phase conveying for different materials. The tribo-electric charge and hence the surface potential increases significantly when a material slug passes the measurement layer where the fieldmeter is installed. The measured surface potential signal clearly shows the sequence of bypassing slugs, each of them causing an increase of the signal of up to 8000 V followed by a decrease down to about –7000 V. Again, this signal is used to monitor and rate the quality and stability of the flow.

This paper presents both measurement conceptions in detail, discusses their applicability for flow monitoring in dense phase pneumatic conveying, and compares the advantages and disadvantages of each method.