Developing and Applying of Real- time On-line Monitoring Technology
for Cooling Water Corrosion and Fouling
Liqun ZHENG, Zixu WANG, Jin ZUO, * Chunshu LI, * Zhigang WANG, * Jinguo ZHANG
(Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016,China)
(*Tianjin Petrochemical Machinery Research Institute Tianjin 300271,China)
Abstract
Since corrosion and fouling is the major factor affecting the heat exchange efficiency and life, and corrosion inhibitor is applied in most of the cooling circuit water equipment. So we have developed an on-line corrosion and fouling monitoring system applied in cooling circuit water, based upon the theory of weak polarization and chemical heat transfer. Its advantages are embodied in the following aspects: first, it could meter accurately at a certain flow speed; second, it can provide a general analysis and evaluation of corrosion and fouling; third, it has a capability of visible data control and real-time computer monitoring. The system has a good effect on the production at TPCC, which has provided instruction and direction to the production and the monitoring result of which is helpful to know about the state of equipment processing.
Key words: corrosion; fouling resistance; on-line monitoring
I. Preface
Cooling System is the central part of petrochemical production. The cooling water corrosion and fouling is the main factor causing the damage of cooling equipment and inefficiency. To achieve the scientific and effective management in cooling water system, ensuring low corrosion rate and low fouling rate, it is absolutely necessary to build up correspondingly a set of real- time, on-line monitoring system of corrosion and fouling to control the operation and instruct the production. The CMB Instant Corrosion Speed Meter developed by IMR, CAS, is based on electrochemical weak polarization. Fouling Resistance Technology emerged in the 1980s. It is Nanjing University of Chemical Technology and Lanzhou Petrochemical Machinery Research Institute that successfully developed fouling resistance monitoring meter on the basis of chemical heat transmission theory.
However, the addressed above cannot meet the practical need. The principal reasons are identified below. First, most of chemicals used in the current industry is anti-incrustation and corrosion inhibitor. Therefore, corrosion and fouling monitoring is of great help to analyse and properly instruct the dose of chemicals added. Second, the fouling resistance meter cannot be used for remote control at the present time. So people usually carry heavy instrument to the work field. Third, there is a lack of the experimental research on instant corrosion rate in the medium with relatively high flow speed. IMR, CAS cooperates with Tianjing Machinery Institute on developing a bound on-line monitoring system consisting of cooling water corrosion and fouling. Through one-year field application and adjustment, the system works in a stable state. We have obtained the result corresponding with practice.
II. the Theory of Corrosion Fouling Resistance Monitoring System
1. Corrosion testing principle: the principle of this system is based upon weak polarization testing method, which can quickly and consistently test instant corrosion speed. Judge the effect of chemicals either by monitoring the cooling water corrosivity or regulating chemical period.
2．the Fouling resistance testing principle
III. the Principle Frame
IV. Composition and Installation of the System
1． Composition of the system
This system consists of PC, monitoring meter, probe and data processing software. The monitoring meter under the control of PC collects data periodically and transmits it to the PC simultaneously. Due to the application RS232-RS485, the data can be transport to 1200 meters. Furthermore, it can drive 255 monitoring meters. As a result, this system is easily developed into a DCS network and enlarge the scope of its application.
2． Installation of the System
This system had been completed and installed at the Chemical Plant, Tianjin Petrochemical Company, monitoring corrosion and fouling of two water cooling heat exchanger. The implementing process is that install a corrosion probe on both the inlet and outlet, fix the temperature probes at the inlet and outlet of cooling water and hot fluid. And then, the corrosion fouling resistance monitoring meter is fixed at the field of plant. The computer and RS-485 are located in the equipment monitoring center, connected to the monitoring meter by cable.
V. the Monitoring Result and Analysis
1．the Research in Lab on Corrosion Rate Monitoring
A set of experimental facility is set up, especially providing evidence for process cooling water corrosion monitoring. It is for the use of the research on regulation of variable corrosion rate at 2~4 m/s.
In fig.1, the flow speed is 3.3m/s and the temperature is 52℃. Within the first 120 minutes, the corrosion medium is running water without chemicals. Afterwards, under the same condition of flow speed and temperature, corrosion inhibitor is added. The result tells that when without any corrosion inhibitor, the corrosion rate is 0.05-0.2mm/a, while when adding chemicals, it is 0.002-0.004mm/a. Seen from the curve, the corrosion rate with corrosion inhibitor is dramatically decreasing, compared with that without any chemicals, as well as the variable range is smaller. It also shows that the Corrosion & Fouling Resistance Monitoring System can monitor the corrosive state of industrial cooling water system at 2m/s or even higher.
2． the Real-time On-line Monitoring Result at Workshop
Fig.5 and 6 demonstrates that in one acid cleaning process, corrosion rate and heat transfer coefficient k change with time. From the diagrams, we know corrosion rate increases from 0.04mm/a to 0.8mm/a in the acid cleaning process. After it, the rate has a slow decline and then tends to be stable. The growth of corrosion rate results from added acid in acid cleaning. Before acid cleaning, the corrosion rate is 0.02-0.03mm/a through weight lost coupon method, which is close to the result by monitoring. The heat transfer coefficient was 800WK-1 om-2 at the beginning. But it waved enormously in acid cleaning, causing the coefficient increased to 1000 WK-1 om-2. The whole acid cleaning process last 8 hours. We can work out the fouling resistance before and after acid cleaning according to the formula above by K, which can be calculated by computer. The bigger the heat transfer system is, the smaller Rt is. Due to it, the transfer effect is better with more efficiency.
VI. Conclusion
Over long-time processing, the conclusion is drawn that the Corrosion & Fouling Resistance Monitoring System can make it come true to test instant corrosion rate at a certain flow speed and reflect how the corrosion rate of cooling water changes. Also, the monitoring of fouling heat resistance corresponds with the circumstances of the heat exchangers. To summarize, the development and application of the Corrosion & Fouling Resistance Monitoring System is worthy of being applied and widespread.
Reference
Xiaoming Kuai, Liqun Zheng, the R&D on Intelligent Corrosion Current Meter, Corrosion Science and Protection Technology, 1995, 7(2): 32
Introduction to author: Liqun Zheng, born in 1964, graduated from Northeast University in 1985, in China. She is titled senior engineer.
