APPLICATION OF AE TECHNIQUE AND THERMOGRAPHY FOR ASSESSMENT OF SLUDGE VOLUME INSIDE CRUDE OIL STORAGE TANKS

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1 More info about this article: APPLICATION OF AE TECHNIQUE AND THERMOGRAPHY FOR ASSESSMENT OF SLUDGE VOLUME INSIDE CRUDE OIL STORAGE TANKS Ireneusz Baran 1, Tomasz Dunaj 1, Marek Nowak 1 1 Office of Technical Inspection (UDT), ul. Szczesliwicka 34, Warsaw, Poland Central Laboratory of Technical Inspection (CLDT) Abstract: When crude oil is stored in large tanks, invariably high-molecular-weight organic sediments (paraffin) are deposited but the resulting sediments contains also solid particles (sand, silt, corrosion products, etc.) and also water. After some period of in-service of tank, these deposits build up to form a sludge, which causes a reduction in the storage capacity. These high in hydrocarbons a valuable raw material could be proper recovered and recycled back to the refining process. Therefore, it is significant to know the sludge characteristics shape and volume to define the best form of treatment (maintenance of tank for control of level of sludge) and removal method (in case of out of service and opening of tank). The knowledge about volume and shape of such sludge inside tanks is important for user of tank farm due to a reasons such as: maintenance of storage tanks and control/monitoring of level of sludge inside them, when conventional treatments are used to remove crude oil sludge, there is a potential for high environmental impact, so the choose of the method of cleaning is very essential, most of design of tanks is with floating roofs, what in case when inside tank is high level of sludge could cause problems and/or damage of roof for low level stored product. In this paper are presented AE and thermography techniques for assess of level, volume and shape of sludge inside storage tank. Thermography method is used to assess the level of sludge (heights) along the storage tank shell. AE technique is used in this case in different way than normally purposes. There is used specific layout of sensors on the wall and roof of the tank. The sensors generate of AE pulses according to established algorithm, which allows to record AE signals were caused the waves propagation and waves reflection (including from sludge) in storage product between sensors. Under measured differences in time of wave propagation are evaluated heights of sludge in various points inside on the bottom of tank. In 2017 Office of Technical Inspection (UDT) opened the project to develop a system for assessing of the level, volume and shape of sludge inside crude oil storage tanks, using AE technique and thermography. As part of the project, UDT has started cooperation with company PERN S.A. the national and regional leader in oil logistics. keywords: aboveground storage tank, sludge, crude oil, acoustic emission technique, thermography 1. Introduction When crude oil is stored in large tanks, invariably high-molecular-weight organic sediments (paraffin) are deposited but the resulting sediments contains also solid particles (sand, silt, corrosion products, etc.) and also water. After some period of inservice of tank, these deposits build up to form a sludge, which causes a reduction in 1

2 the storage capacity. These high in hydrocarbons a valuable raw material could be proper recovered and recycled back to the refining process. Therefore, it is significant to know the sludge characteristics shape and volume to define the best form of treatment (maintenance of tank for control of level of sludge) and removal method (in case of out of service and opening of tank). The knowledge about volume and shape of such sludge inside tanks is important for user of tank farm due to a reasons such as: maintenance of storage tanks and control/monitoring of level of sludge inside them, when conventional treatments are used to remove crude oil sludge, there is a potential for high environmental impact, so the choose of the method of cleaning is very essential, most of design of tanks is with floating roofs, what in case when inside tank is high level of sludge could cause problems and/or damage of roof for low level stored product. In this paper are presented AE and thermography techniques for assess of level, volume and shape of sludge inside storage tank. 2. Project of the system for assess of the sludge inside storage tanks The origin and the reasons of the work are based on the following needs: expectations from users/owners of tank farms (fuel bases with crude oil) storage tanks with crude oil require monitoring and good maintenance practice to keep sludge from becoming the problem, AE testing agency/laboratory need to have knowledge about level of sludge inside storage tank necessity to assess the level of sludge before starting AE testing of storage tank bottom according to procedures and standards. In 2017 Office of Technical Inspection (UDT) opened the project to develop a system for assessing of the level, volume and shape of sludge inside crude oil storage tanks, using AE technique and thermography. As part of the project, UDT has started cooperation with company PERN S.A. the national and regional leader in oil logistics. The goal of the project is to build up a concept and prototype system to assess the level and volume of sludge at the bottom of an crude oil storage tank. In principle, the system will use several research methods: acoustic emission in various measurement methods, evaluation of temperature field using thermography method and temperature measurement by non-contact method. So far in frame of the project, a number of measurement procedures have been developed, i.e. tests using combined measurement methods: acoustic emission methods and thermographic methods together with contactless temperature measurement. The purpose of the tests carried out according to these procedures is to collect measurement data of heights of sludge at points along the wall and on the bottom inside the tank. Collected data in the form of a measurement database allow 2

3 next for spatial visualization (map of sediments) and assessment of the level and volume of sludge on the bottom of the storage tank. 3. Thermography and temperature measurement for evaluation of sludge level The use of thermography to assess the level of sludge along the storage tank shell is already a known method and described many times in various publications. Temperature differences are caused of differences in the density of mediums and, as a result, their thermal properties, which allows to determine the transition line from liquid crude oil to solidified sediments on the bottom. It should be noted that it is not always possible to use thermographic tests to determine the sludge line. For example, in the case of double-shell tanks, due to the specific thermal conditions in the interstitial space, it is sometimes necessary to use conventional methods of wall temperature measurement (eg, contactless method), combined with the appropriate method of evaluation of the temperature field. As part of the project, a series of thermographic and temperature measurements were carried out to determine the sludge line along the storage tank's wall. In fig. 1 below is presented the example of thermal image of tank with sludge on the bottom. Fig 1. Thermal image of tank wall with visible sludge line The measurement of the level of sludge along the wall on the circumference of the tank consists in assessing the temperature field every 3-5 m, as well as in the places where equipment (mainly pipelines and mixers) are present on the wall. After determining the heights of sediments at the specified points, a profile of sludge along the wall is drawn as shown in figure 2. Obtained measurement data of sludge heights are collected in the measurement database. 3

4 Fig 2. The line contour of sludge along circumference tank wall 4. AE technique for evaluation of sludge level AE technique is used in this case in different way than normal purposes. There is used specific layout of sensors on the wall and roof of the tank (fig.3). The sensors generate of AE pulses according to established algorithm, which allows to record AE signals were caused the waves propagation and waves reflection (including from sludge fig.4) in storage product between sensors. Under measured differences in time of wave propagation are evaluated heights of sludge in various points inside on the bottom of tank (fig.5). Obtained measurement data of sludge heights are collected in the measurement database. Fig. 3 shows an example of the layout of AE sensors when using AE technique for sludget heights measurement. Fig. 4 shows a waveform of the AE signal with echoes of waves from the bottom and roof of the tank. Fig. 5 presents the spatial visualization of the measurement data of sludge heights at specific points on the bottom of the storage tank. Fig 3. An example of the layout of AE sensors on the tested storage tank 4

5 Fig 4. The wave propagation and wave reflection in storage product between AE sensors Fig 5. Spatial visualization of the sludge heights at the measurement points on the bottom of the tank 5. Determination the volume of sludge on the bottom of the tank All measurement data of heights of sludge in many points, both on the wall and inside the tank are undergone mathematical polynomial interpolation, what finally give graph of shape of sludge (fig.6). Determination of the volume of sludge on the bottom of the tank, is carried out by the mesh of division into defined unit elements (fig.6) of the obtained spatial shape of the sludge and numerical counting (numerical integration using the rectangles method) volume of individual elements according to their heights. The result of the assessment of the sludge on the bottom of the storage tank is the volume of sludge in cube meters and spatial visualization of sludge. 5

6 Fig 6. Spatial visualization of the sludge shape on the bottom of the tank with division into defined unit elements 6. Conclusion The procedures and measurement methodologies developed at this stage of the project, as well as computational algorithms allow us to visualize the shape of sludge on the bottom of the tested storage tank and to determine the volume of sludge. The use of both AE technique and thermography method proved to be an effective tool for determining the heights of sludge at specific points inside the tested storage tank. References: 1. M.Monteiro, V.Svet, D.Sandilands, S.Tsysar: Experimental Investigations of Various Methods of Sludge Measurements in Storage Oil Tanks, Advances in Remote Sensing, Vol. 4, 2015; 2. H.Hooyberghs: Pictures the Emissions of Storage Tanks with the Use of Infrared Cameras. 6th Annual Aboveground Storage Tank Conference & Trade Show Houston, Houston, 2013; 3. I.Baran, G. Lackner: Analysis of Corrosion Processes and Leaks in Aboveground Storage Tanks with AE Monitoring, EWGAE 2012; 4. NDT Handbook - Third Edition Vol.6 - Acoustic Emission Testing, Editors: R.K.Miller, E.v.K.Hill and P.O.Moor, ASNT PN-EN : Non-destructive testing - Terminology - Part 9: Terms used in acoustic emission testing. 6. PN-EN 13554:2011 Non-destructive testing - Acoustic emission testing - General principles. 7. PN-EN 15856:2010 Non-destructive testing - Acoustic emission - General principles of AE testing for the detection of corrosion within metallic surrounding filled with liquid. 8. PN-EN ISO 18081: Non-destructive testing - Acoustic emission testing (AT) - Leak detection by means of acoustic emission. 6