Summary

Tomoflow Ltd has developed a new method of measuring multiphase flows in pipes. The Tomoflow system is completely non-intrusive and can be used in gas/solids, gas/liquid and liquid/solids flows. Tomoflow Ltd currently sells research versions of its system, and is in the process of developing stand-alone industrialised flowmeters for widespread commercial use.

Flowmeters

The output of a flowmeter tells the user how much material has passed through the meter in a certain time, either as a volume flowrate (for example litres per second) or as a mass flowrate (for example tonnes per hour). The measurement of flowrate may then be integrated to give the total amount of material delivered, for example the number of litres or tonnes. In the case of single liquids and gases the technology to measure flow is well understood and flowmeters are widespread, whether filling up a car on a garage forecourt or measuring gas consumed by a household, the amount of product is measured by a flowmeter and the indicated output of the flowmeter is accepted by both buyer and seller as the true amount delivered.

However, many industrial processes use mixtures of products pumped along pipes. For example, in manufacturing processes particulate materials (plastics, grain, catalysts...) are blown along pipes by compressed air or pumped in liquid, while in the oil business mixtures of gas, oil and water are common in long-distance transport. Such transport systems are very efficient, but the only way to measure the amount delivered is to separate the components and meter them individually or to fill some sort of tank or vessel and measure the volume or the weight of the product in the tank. This limitation is not inherent to the pumping mechanism, it is simply that the technology for measuring the flowrate of mixtures of materials in pipes by a flowmeter is not widely available. The limited flowmeter technology for these applications adds to the cost of the operation considerably (extra piping, valves, tanks and weighing mechanisms) and interferes with the process flow.

The technology

The Tomoflow system uses an imaging technique analogous to medical tomography ('scanners' - for example CAT, PET, MRI) where a cross-sectional image of an object is produced from external sensors. Such a cross-sectional image is known as a tomograph - hence tomography is the art of creating such pictures. Tomoflow uses electrical fields so that the imaging is faster and the sensor simpler than most medical scanners. Tomoflow uses an imaging system developed by our partner company, Process Tomography Ltd (PTL).

From the tomographic measurements of two cross sections with a known separation, images are computed. These images are segmented into zones. A statistical technique called correlation allows us to calculate the velocity in each zone from a rolling history of these images. With the sensor dimensions, we can then calculate the volumetric flow rate per zone. By integration (summing), we then derive volumetric flow and if the density is known, mass flow.

Figure 1. Technology overview

Unique advantages

By deriving velocity in each zone across the flow cross-section, it does not suffer from the gross errors in varying flow regimes characteristic of multiphase flowmeters that operate with an average velocity figure.

Figure 2. Summary of flow measurement results in gas/solids gravity-drop flow.

A summary of test results (above) for a range of flows indicates that the Tomoflow system calculates volumetric flow, converted here to mass flow rate, within typically a few percent of the true rate.