A NEW AND NOVEL ABIOTIC-BIOTIC FOULING SENSOR FOR AQUEOUS SYSTEMS

Deposits on surfaces in water – bearing systems, also known as “fouling”, can lead to substantial losses in the performance of industrial processes as well as a decrease in product quality and asset life. Early detection and reduction of such deposits can, to a considerable extent, avoid such losses. However, most of the surfaces that become fouled, for example, in process water transport pipes, membrane systems, power plants, food and beverage industries to name a few, are difficult to access and the analysis of the water phase do not reveal the extent of the deposits. Furthermore, it is of interest to distinguish between biological and nonbiological deposits. Although they occur together, different counter measures are necessary. Therefore, sensors are required that indicate the development of surface fouling in real time, non-destructively, in-situ and can discriminate between abiotic and biotic based deposits. A new and novel sensor has been developed that provides said discriminate detection by utilizing conventional heat transfer reduction sensory coupled with ultrasonic detection of materials on the same surface concurrently. The technical aspects of the design, operation, and application will be discussed in the paper. Real time graphical detection followed by automated reduction control runs will also be presented as well as revealing if the deposit is biotic or abiotic. INTRODUCTION One of the main causes of performance loss, quality and runnability problems in industrial systems is related to contaminants and deposits. These deposits are composed of inorganic, organic or microbial matter, respectively. Most of the deposits contain various or even all types of these contaminants and form complex matrices. Of these, microbiological contaminations, also named biofouling, are one of the biggest issues and risks in water bearing industrial systems. They cannot only cause deposits that impact the function and efficiency of the systems. They often are the cause for health risks (e.g., Legionella). Fouling can be generalized into 4 forms, inorganic, suspended solids, organic, and microbiological. Of these forms of fouling, it is only inorganic crystallization fouling that does not lead to the worst form of corrosion, namely localized. This type of corrosion eventually transitions into high pitting penetration rates that drastically reduce the asset life. The majority of the fouling which occurs in aqueous systems are detected indirectly by means of reduced process side throughput, increased time to get to operating temperature and or pressure, pressure drop, approach temperature increase, or the use of extensive instrumentation to calculate “at that time” heat exchange U coefficients and or cleanliness factors. Under certain circumstances, some of these methods are not sufficiently accurate unless normalized. Or the measurements taken have not been corrected for cooling water or process flow changes, shear stress change and bulk cooling water change or surface temperature change. There may be a large lag time to foulant detection which can lead to foulant aging and dehydration to a point of being irreversible fouled, where-by chemistry and chemical adjustment in the water side environment would not provide cleansing of the surface and maintain a clean state. An example would be the comparative time for a side stream annular heat transfer test section to detect fouling of a well instrumented utility surface condenser, where in they were both operated at the same surface temperature and shear stress (velocity corrected for the geometry) on the same cooling water (Beardwood, 2011). The steam surface condenser heat transfer surface area for 175 MW would be 150,000 ft (13,935 X10 mm) would require a large quantity of foulant coverage to be detected compared to the annular test section which has 0.05 ft (4645 mm) of foulant detection surface. Heat Exchanger Fouling and Cleaning – 2017 ISBN: 978-0-9984188-0-3; Published online www.heatexchanger-fouling.com 83 CONCEPT DEVELOPMENT AND LABORATORY WORK Although methods had been developed to detect and identify the four forms of fouling and corrosion utilizing a grouping of on-line analyzers in conjunction with the annular heat transfer test section (Beardwood, 1996, 2000, 2003, 2011, 2016), a desire for more specificity from a singular analyzer was desired. An interest in ultrasonic measurements of deposit thickness began in late 2006, with a prototype being developed by 2008 which could detect down to 0.001 mm (1μm) thickness (Seida, Flocken, Pöschmann, Schultz, Klaua and Dietrich, 2009). By using a specific ultrasound frequency between 5 and 15 MHz at a speed of 1480 m/second @ 20°C, the lag time between transmission and reflection can be measure with a resolution of 120 picoseconds (< 1 nanosecond). Thickness determination was based upon time of flight of the sound between transmission and reflection. The transducer emits an ultrasonic signal to the surface to be measured (either the reflector plate or a pipe wall) at a predefined interval and then receives the reflected echo. The time interval between the emitted signal and its echo is used to calculate the distance between the sensor and reflector surfaces. If scale forms on the surface of the reflector, then the time required for the ultrasound wave to complete its circuit is reduced. This result is measured as the difference in value in relation to the “zero point” set at the beginning of a measuring cycle. Figure 1 depicts the change in time for the ultrasound signal to pulse and echo on a clean versus a scaled surface. The time data are converted to scale build-up or removal, as the time either decreases or increases, respectively. This allows the analyzer to continuously monitor the thickness of any build-up on the target surface, with positive values indicating a scale forming environment and negative values showing the removal of scale. The accuracy of measurement and resolution of the value is high enough (1 μm) to be applicable for monitoring and controlling the effectiveness of a scale control chemical program in real time. The sensor is illustrated in Figure 2 and results of a field application is provided in Figure 3. A number of challenges needed to be overcome in order to have accurate results. These were completed prior to 2010. Onboard algorithms were developed to correct/compensate for changes in bulk fluid temperature, pressure and ionic strength; which affect the speed of sound. An example of the influence of temperature and conductivity on the sound velocity is illustrated in Figure 4. For example at 20°C the error in thickness calculations is about 30 μm per 1°C and 7 μm per 1 mS of conductivity at 10 mS conductivity water. Fig. 1 Ultrasound impulse and reflection for a clean and scaled surface, with a lower time value (ts) for a scaled surface. Fig. 2 Analyzer probe showing the transmitter and removable reflector plate. Fig. 3 Analyzer output showing the effect of an antiscalant on reducing the rate of scale build-up, and removing scale (negative rate of build-up) if dosed at a sufficient level. Heat Exchanger Fouling and Cleaning – 2017 ISBN: 978-0-9984188-0-3; Published online www.heatexchanger-fouling.com 84 Fig. 4 Sound Velocity Dependency of Temperature and Conductivity in Water. Self-Scaling Monitor These sensors are used in fluids that are “self-scaling” or fouling; that is, the system has suspended solids or the scalant concentration has exceeded the solubility limit in the bulk fluid. Also known as a homogenous exsitu crystallization process in which the fouling is mass transfer dominant. It was further postulated that heat exchanger fouling could be simulated provided the sensor reflector cap was heated. This was found to be misleading as the results only validated that much of the deposition was on the backside from reverse eddies as the water past the probe. Heat exchanger fouling that are not associated with suspended solids are insitu, attachment dominate and scales are from heterogeneous crystallization. Additional information regarding the self-scaling monitor can be found in the associated patent (Seida et al., 2009). Heated-Scaling Monitor Obviously the corrective actions in the self-scaling unit could be transferred to a heated unit, however there were many technical/scientific design hurdles that needed to be addressed in order to generate data and graphical trends that were informative, aligned to known real world outcomes and in real time. The individual items that were required to be engineered and designed in order to achieve a functional heat transfer test section device in combination with thickness measurements were as follows:  A sample conditioning conduit with flow normalization and test section placement determination.  Equivalent shear stress development from flow velocity for the geometry compared to the known calculated shear stress simulation values of the annular heat transfer test section as associated to other heat exchanger cooling water channel geometries (Beardwood, 2011).  Constant flow maintenance.  Design and construction of the ultrasonic sensor probe and associated heated target surface test section. Polyether ether ketone (PEEK) insulation and thermal conduction material of construction required the use of a Copper Nickel Alloy.  Design and mounting of the heat transfer test section, over 7 iterations.  Heater cartridge sizing for specified bulk water, and heater surface temperatures and flow conditions, and a supply of constant clean electrical power (CFD modelling).  Core material selection to achieve the upper limit surface temperature required and repeatable thermal couple placements (T2; near the wetted end surface and T3; near the heater cartridge) for calculating fouling factors.  Bulk water temperature correction (T1) for calculating fouling factors and fluid density and kinematic viscosity of the water. Without these correction