Managing Pipeline Scaling
Molly Bragg, Marketing Associate, Flowrox Inc.

Scaling is a common problem in the Minerals & Metallurgy, Oil & Gas, Power Plants, Pulp & Paper and Municipal Waste Water industries, where production rates can be adversely affected by the hardening of iron, salts and other minerals in pipes and valves. The article highlights the new technology Electrical Capacitance Tomography (ECT), which allows operators to see inside piping systems without stopping the process or opening up the pipeline, and enables 3D-imaging and measurement of non-conductive media inside process pipelines and tanks.

In process industry, it is usually beneficial to have comprehensive information on the state of a process. First of all, this information may be essential for process control in various situations, since it typically provides means for keeping the process running in its optimal operating range, and thus increasing the overall productivity of a plant. Accurate knowledge on the state of the process helps operators to predict possible problems, and on the other hand, makes it possible to plan maintenance operations in a cost-effective way. Secondly, process measurements are needed for acquiring a better understanding and a deeper insight into process behavior in general, which can open ways for developing new solutions in improving process efficiency.

Process Measurements
Process measurements can be categorised into point-like measurements and volume measurements. Point-like measurements give process information from a single point in a fixed location, and typically such quantities are temperature, salinity and pH. In many situations point-like measurements are sufficient for efficient process monitoring and control. However, in the presence of inhomogeneities and/or multiple material phases, point-like measurements do not necessarily provide appropriate information for operators since spatial variations cannot be identified. In such cases, it would be useful to know how certain material or quantity is distributed over certain volume instead of getting information from a single point.

'Process tomography' is a general term for volume measurement techniques intended for cross sectional or 3D imaging of material properties and distributions in various industrial positions such as pipes, vessels and reactors. Process tomography covers several tomographic techniques, e.g. electrical, X-ray and ultrasound methods, and typical applications include monitoring of different mixing and sedimentation processes as well as determination of material concentrations and distributions in multiphase flows. An important feature of tomographic techniques is that the measurements are typically carried out from the periphery of the target using a specially designed sensor in order to avoid disturbing the process. Tomographic measurements enable non-invasive process monitoring, which can give comprehensive understanding on the behavior of the process system and provide valuable information for process optimisation and control.

Electrical Capacitance Tomography (ECT)
The fundamental principle in tomographic measurements is to expose the target of interest to an appropriate stimulus and then measure the response that depends on the material properties within the target volume. For instance, in X-ray tomography, the target is exposed to X-rays from multiple directions and the attenuation of intensity is measured along the beam lines to obtain information on the distribution of the attenuation coefficient. In electrical tomographic techniques, excitation signals are applied to the volume of interest via a suitable measurement setup and the objective is to measure output signals that depend on the electrical properties of the material, such as conductivity or permittivity distribution. A special algorithm is then used to find an estimate for the actual material property distribution.

ECT is an imaging technique that can be used for the determination of the permittivity distribution of dielectric medium within a region of interest. Several organizations have contributed to the development of different sectors of ECT technology. Important pioneering work in ECT was carried out at the US Department of Energy, Morgantown Energy Technology Center, USA, and at the University of Manchester Institute of Science and Technology, UK .

ECT measurement is based on the use of an ECT sensor which consists of a set of electrodes mounted around the periphery of the target region. In fixed sensor geometry, the capacitance of each electrode pair depends on the permittivity distribution of the material in the sensor. The basic procedure for capacitance measurements is to apply an excitation voltage (AC) to one of the electrodes while other electrodes and other physical components are grounded. A potential distribution and thus an electric field are formed within the sensor, and the underlying permittivity distribution affects the shape of the field.

This is because permittivity is a measure for a material’s ability to 'resist' external electric fields. In other words, the higher the material permittivity the weaker the electric field within the material when placed in an external electric field. The effect of spatial permittivity changes on the potential distribution is visualised in figure 1. Due to the electric field, electric charges are attracted to the grounded electrodes, and they are measured indirectly from electric currents. The capacitance data is obtained from the fundamental definition of capacitance Ci=Qi/V, where Qi is the charge on the ith electrode and 'V' is the excitation voltage. A complete set of measurements is obtained by using each electrode as the excitation electrode in turn and the rest as the measurement electrodes.

Capacitance data as such does not give a very clear view on the actual permittivity distribution, even though some qualitative interpretations can be made by experienced persons in certain situations. Therefore, for full utilisation of ECT measurements, it is important to determine an estimate for the permittivity distribution in question. This necessitates the availability of a realistic mathematical model that simulates capacitance measurements given a permittivity distribution. Such a model can be derived from the famous Maxwell’s equations that form the basis of electromagnetic field theory. The basic idea in image reconstruction is to find a permittivity distribution for which the observations predicted by the model are in good agreement with actual ECT measurement data. In general, such an image reconstruction problem is an ill-posed inverse problem since it has, due to the very limited number of capacitance measurements, an infinite number of solutions that produce exactly the same capacitances as the ECT measurements. In order to enable the determination of a stable solution for this problem, it is necessary to impose additional constraints for the solution. These constraints can be of qualitative and/or quantitative nature depending on how much is known about the permittivity distribution in advance. It is important to notice that different scaling conditions may require different types of additional constraints to obtain optimal results. All in all, ECT imaging leads to a nonlinear problem that is solved iteratively by starting from an appropriate initial guess for the permittivity distribution and by gradually proceeding in the direction where better agreement with model data and measurements is reached. This procedure is illustrated in figure 2.

ECT is typically used for the imaging of electrically insulating materials. However, the ECT measurement technique can also be used in cases where the conductivity is not negligible. In such situations, material permittivity is a complex-valued quantity, and in the image reconstruction, it is essential to solve the distributions of real and imaginary parts of the permittivity distribution. For this purpose the measurement system must be capable of measuring the electrical charges on the electrodes, and in addition, the phase shift between the charges and excitation voltage.

Scaling Watch System
The formation of scaling in pipelines and tanks can be a major problem and inconvenience in the process industry. The formation of layers of unwanted substances on the surfaces of process equipment may restrict fluid flow, decrease process efficiency or limit heat transfer, and therefore cause adverse effects on profit earning capacity. Scaling Watch System are used for online monitoring of process pipes for identifying possible process scale issues. Having adequate information on the state of scaling in pipelines can help optimise cleansing operations, avoid unexpected shutdowns and optimise the use of antiscalant chemicals.

The Scaling Watch system uses the ECT technique to measure pipe volume and to characterise scale condition in pipelines. The applicability of ECT measurements is based on the differences in the electrical properties between the process fluid and unwanted scale material. Materials of different electrical properties affect the ECT measurements, and therefore reconstructed ECT images reveal how materials are distributed within the Scaling Watch sensor. The estimated permittivity distribution provides information on scaling thickness and how it is distributed on the pipe walls . In addition, it is possible to determine estimates for scaling growth rate and free volume percentage.

The components of the Scaling Watch system are the ECT sensor and electrical cabinet. The sensor consists of a tubular metallic body and typically 8 electrodes that are mounted on to the body so that they are in contact with the process fluid. Some measurement electronics are brought to the sensor, which affects the sensor dimensions. The electrical cabinet contains a power source, a computer for data processing and an equipment for enabling remote connections. In the installation of the scaling watch system, a segment of the process pipe is replaced by a sensor with an appropriate inner diameter and the electrical cabinet is connected to the sensor with a bus cable. After connecting the cabinet to the power grid the system starts automatically and is ready for use. The scale situation in the sensor can be monitored with a web-based application through which measurement settings can also be changed.