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Automatic Monitoring Systems of Hermetically Equipment's Condition in Chemical Plants L. Vadim - Nizny Novgorod State Technical University 'Russia Contact

Generic description of a system

Reliable diagnostics and warning of leaks of potentially dangerous mediums from the technological communications and devices on working manufactures is an actual scientific and technical task. The chemical enterprises operation practice displays a high percent of crashes caused by hermetically of binder apparatus elements of process flowsheets (butt joints, stuffing box seals etc.). Gas analyzing equipment inspecting air structure of an industrial zone is not capable to realize an exact leak localization at early stages of an imperfection development by virtue of low responsivity, unsatisfactory quick-action and absence of distributed monitoring. The author offers a construction concept of automized distributed systems of an on-line production equipment monitoring. The systems are based on extrasmall (2x2 mm), fast (~1 s) and high-sensitive (1¸ 10 ppm) microelectronic gas analyzing sensors (MOS-structure, resistive sensors, catalytic thermal, diodes with a Schottky barrier etc.), integrated by a local technological network with a defectoscopic information output to industrial digital computer of an anti-damage guard system and to operator station. Use of the extrasmall gas sensors with low energy consumption (less than 1 W) allows disposing them in immediate proximity from remote nodes of technological process, which are characterized by heightened danger (stuffing box seals, butt joints etc.). The low threshold sensitivity of the sensors allows determining concentrations of toxic and explosive substances, which are much below maximum allowable, due to what a prediction of an imperfection development of production equipment is carried out. The installation of the sensors in immediate proximity from prospective places of leaks allows operatively and with an adequate accuracy to determine emergency sites of manufacturing method. The information transmission mediums represent software and hardware allowing realizing a defectoscopic data exchange between sensors and monitoring station depending on a concrete technological control object. The realized transmission standards are: RS 485/422, 4...20 mA. The operator station software ensures a real-time handling of defectoscopic information, visualization and monitoring processes.control.

Sensitive elements of the automatic equipment monitoring systems

The existing ways of gas analysis are rested upon chemical, optical and chromatographic methods, which automation and miniaturization is hampered. Besides the development of different science and engineering branches advances a series of tasks on an environment monitoring, chemical-engineering processes control which successful decision is impossible without development of small-sized high-sensitive primary gas analyzing converters with low energy consumption.

Apart from classical requirements to responsivity, selectivity and speed of gas monitoring at construction of the distributed automatic detection systems of small concentrations of potentially dangerous mediums the special demands of gas sensors should be made:

• Absence of sample cutting systems, bringing in perturbation in a concentration field and lowering an opportunity of the further handling with the purpose of leak places localization - the sensor operation should be carried out in a diffusive condition
• Low cost ensuring an economic feasibility of multipoint monitoring - sensor manufacture should be mass, high-tech and ensure the low cost price
• Diminutiveness and low energy consumption enables installations of the sensors in out-of-the-way places of technological communications

To the listed above demands answer gas sensors, manufactured on integral chips technology (on the basis of resistors, diodes, MOS and hybrid structures):

• Resistive type (oxides of metals - SnO₂, ZnO; organic semiconductors; Pd/Pt catalytic sensors);
• Electrochemical (potentiometric electrodes, solid electrolytes);
• Metal- Oxide- Semiconductor Structure - MOS (capacitors and transistors - Pd-Pt/SiO₂/Si with different width of silicon oxide);
• Structures with a Shottky barrier (Schottky diodes, MOS-diodes, thyristor effect sensors with composition: metal - semiconductor Pd/ZnO);
• Masses - sensing sensors (quartz microweighing and sensors with usage of surface audio waves SAW- LiNbO₃ a quartz);
• Optical sensors (optoelectronic semiconducting laser + phototransistor; optic fibber sensors - laser + gas sensitive optic fibber + photodetector; photoaudio)

At the heart of microelectronic sensitive element action the series of physical-chemical processes causing an electrical sensor parameters change lays. The active gases detection mechanism influence on behavior of basic gas-sensitive structures indicators (adsorption and desorption times, threshold sensitivity, static characteristic fashion) was investigated in the work.

The detection process generally consists of four stages:

• Gas transportation to a gas-sensitive layer surface
• Physical adsorption and / or chemisorption
• Change registration of electrophysical parameters of gas-sensitive structure
• Desorption of detected gas from a surface of a sensitive element.

Four different on structure sensors were considered: catalytic thermal, resistive, MOS-structures and sensors on the basis of diodes with a Schottky barrier. Molecular hydrogen was used as active gas.

The catalytic thermal sensor is a film of platinum sputtered on a silicon substrate. The carried out experiments have shown a fast response time of catalytic sensors in microelectronic construction (adsorption and desorption times at effect of hydrogen with concentration of CH₂=100ppm have made t _a,d» 0.1c).

The resistive metal-oxide semiconductor sensor on the basis of SnO₂ becomes hydrogen sensitive at a temperature of T_op=520K. It allows assuming, that the sensor operation mechanism is based on a course of physical-chemical reactions on a semi-conductor surface such as H_v+O_s^2- Þ (HO^2-)_sÛ (HO)_s^-+e.

The operation mechanism of heterogeneous structures (MOS-sensors and sensors with a Schottky barrier) is defined by constitution and manufacturing technique. So for Pd-SiO₂-Si structures, it is supposed, that on a Pd surface the catalytic reaction of adsorbed H₂ molecules dissociation (HÛ H^-+e) to atoms is passing which diffuse to the Pd-SiO₂ interface and in a consequence of a reaction HÛ H^-+e develop a dipole layer changing electro-physical parameters of the sensor. At operating temperatures of T_op=370K the hydrogen threshold sensitivity has made CH₂^min=1ppm (MOS) and CH₂^min=0.5ppm (a Schottky barrier). The quick-action on forward and back front edges of gas feeding has made about t » 1c.

Fig 1: Dependence of response time on operating temperature of a sensor

The comparison of dynamic responses of the different microelectronic gas sensors has shown an essential advantage of the sensors with a Schottky barrier. However the greatest opportunities on construction multi-gas sensors have catalytic thermal and resistive structures.

It is obvious, that the development of methods and means of control by penetrating substances is connected directly to creation of new gas-sensitive sensors. Not all sensors are selective and high-sensitive, many do not meet the leak detection requirements, but intensive researches on improvement of parameters of sensors and new perspective materials and equipment continue. Use of modern integrated technique during sensor creation opens perspectives of creation on one substrate not only a system of different sensors allowing a leak determining of several gases, but also intermediate converters and microprocessor which is carrying out an information handling.

Signal processing in the distributed monitoring systems

The development of methods and means of the distributed automatic leak detection of potentially dangerous mediums requires a software development optimized on a precision and quick-action. A mathematical model synthesis of the distributed leak localization system is based on use of a block method of a complex system definition. Thus, the mathematical model of data paths represents a system of connected differential equations circumscribing separate stages of automatic localization process of leaks from production equipment (Figure2).

Fig 2: Sensor signal processing steps

During signal processing from the distributed sensors there are following problems:
• Different arranging conditions of sensors, distinction in parameters of detected mediums (temperature, pressure, damp...) and, as a consequence, different initial and boundary conditions
• Different detected mediums (and also sensors) in one technological system - use of different mathematical models of detection process
• Account of spatial distribution of sensors and compensation of convective streams.

Except for immediate leak places localization on working production equipment a task of distribution delimitation of gaseous medium leaks, and also spatial coordinates account of leak sources, which are not in a sensor coverage area, is contained in tasks of the distributed automatic equipment condition monitoring systems. The offered technique is based on distribution simulation of gaseous mediums with use of the differential equation circumscribing a convective diffusion in space:

(1)

where E- propagation medium concentration factor; C - detected gas concentration; T- time counted from a leak registration moment; W- convective component of a detected gas mass carry process in an examination zone; D- detected gas diffusivity in air under normal conditions; X - spatial coordinates of an examination zone (x, y, z); l (X) - distribution function of leak sources; Q_leak - leak magnitude. The boundary conditions are adjusted in real time in view of static and dynamic characteristics of sensors. Equal concentration lines are plotted with a help of the obtained data array and probable leak epicenter coordinates are determined using a gradient method. The given handling technique allows essentially reducing a number of used leak sensors without monitoring efficiency loss.

Conclusion

Use of the permanently installed extrasmall leak sensors distributed on a monitoring object allows realizing a reliable continuous automatic safety gauging of manufacturing methods. The detection of correlations between process variables and imperfection (leak) parameters will allow to realize in-process gauging functions with the purpose of lowering a danger level of production equipment having insignificant imperfections. The above mentioned package plan will essentially lower expenditures for nondestructive testing connected with a stopping and inspection of production equipment.

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