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The condenser is a kind of heat exchanger that condenses the exhaust steam of the steam turbine into water, also known as the condenser. According to the different ways of steam condensation, condensers can be divided into two types: surface type (also known as wall type) and hybrid type (also known as contact type). In a surface condenser, the vapor separated from the cooling medium is condensed into a liquid on the cooling wall (usually a metal tube). The cooling medium can be water or air. This article focuses on surface (partition) cathodic protection.
Corrosion of condensers
The condenser pipes are mainly made of the following types: (1) copper alloy, the main condensation area of the condenser is mostly brass, and the air cooling area and the top layers are made of white copper; (2) Stainless steel, the austenitic stainless steel used in freshwater, brackish water and brackish water is mainly an alloy of Fe, Cr and Ni, such as the American AISI300series stainless steel; (3) Industrial pure titanium, high-purity titanium has low strength, good plasticity and corrosion resistance. Industrial pure titanium contains a small amount of iron, silicon, nitrogen, hydrogen, oxygen and other impurities, which increases the strength and reduces the plasticity.
In practical application, in addition to common chemical corrosion and the mixing of various different metals in the condenser, the direct contact between different metals can easily lead to serious galvanic corrosion. In addition, due to the limitation of operating environmental conditions, the quality of cooling water directly affects the degree of corrosion of condenser components. For example, if there is sewage discharge in the attachment, or the salt content is high, the corrosion of the cooling water pipe of the condenser is very large. Finally, due to corrosion, the equipment is scrapped early, production is stopped, materials are repurchased, and installation is installed, and the loss of time, cost, and manpower in the middle of the period is very amazing.
Condenser cathodic protection
At present, the traditional coating does not have a very ideal effect on the protection of the condenser, because it is difficult to achieve 100% of the coating in practical application, and the coating is easily damaged during the actual installation process. In addition, with the passage of time, the aging and cracking of the coating directly lead to the failure of the condenser to prevent corrosion. In ordinary coatings, there are also considerable pores between molecules, and the ions in the cooling water can easily penetrate through the coating into the pipe, and it is difficult to ensure the effectiveness of the condenser's anti-corrosion.
Cathodic protection solves the problem of anti-corrosion that is difficult to achieve by coating, because of its effective, economical, trouble-saving and other advantages, it has also achieved good application practice in the cathodic protection project of condenser at home and abroad.
The cathodic protection principle is a method of providing cathodic current based on the metal structure, so that the metal potential is reduced (cathodic polarization), so that the metal is not easy to lose electrons and become oxide. According to the different methods of cathodic protection, it is divided into two cathodic protection methods: impressed current and sacrificial anode. Both methods have their advantages and disadvantages, in fresh water, due to the higher resistivity of water, the driving voltage of the sacrificial anode is smaller, the output current is limited, and the output current can not be adjusted, with the generation of reaction products on the surface of the sacrificial anode, the reduction of the anode, etc., will cause the failure of anti-corrosion, and the life of the sacrificial anode is generally 3-5years, overdue use will also lead to the failure of cathodic protection, and the replacement of sacrificial anodes in some pipe fittings is also very complicated. The cathodic protection method of impressed current has many advantages, the potential can be adjusted automatically or manually, which is very convenient to operate, especially when the water quality in contact changes, the impressed current has incomparable advantages, and the output voltage can be changed to make the protected pipe fittings in a reasonable and normal cathodic protection. The service life of the impressed current generally depends on the life of the anode, and with the new MMO anode, the service life can be as long as 15-20 years.
Of course, cathodic protection also has its limitations, that is, its protective environment must be a liquid environment, and a dry environment can only use a coating as a corrosion protection.
Application examples of condenser cathodic protection technology
Overview of the condenser of unit 7 of a power plant
Unit 7 of a power plant has a capacity of 100MW condenser thermal power generating unit, the condenser is a bisected double-flow surface type, the copper pipe grade used is HSn-70-1A, the number of copper pipes is 10336, and the cooling surface is6815m2, all replaced with copper pipes in 1992, pitting corrosion and perforation leakage began to occur after 2 years of use, and in 1997, it was shut down 9 times due to copper pipe leakage. At the same time, the tube sheet of the condenser of Unit 7 was painted in 1997, but only one year later, there was obvious local corrosion on the surface of the tube sheet, and the copper pipe leakage and corrosion of the tube sheet of Unit 7 made it difficult for the unit to continuously generate electricity at full load, which became a major obstacle to the completion of the power generation plan. In order to prevent the further development and expansion of corrosion, the combined anti-corrosion protection measures of impressed current cathodic protection technology and corrosion-resistant coating were adopted on the No. 7 condenser.
The condenser of Unit 7 of the power plant has a cathodic protection and anti-corrosion system
The cathodic protection system of the applied current of the condenser is mainly composed of an automatically controlled potentiostat, an auxiliary anode and a reference electrode.
The auxiliary anode is a new type of metal platinum niobium anode, which is an insoluble anode, which can ensure long-term use, and has the characteristics of large discharge rate, uniform current emission, large coverage area, low consumption, high reliability, light weight, easy installation, good mechanical properties and stable performance.
The reference electrode is made of high-purity zinc material with stable electrode potential and low potential fluctuation in fresh water. The electrode is designed as a straight rod type with a special sealing structure.
According to the structure of the condenser, the cooling water quality and the hydrodynamic conditions in the water chamber, the cathodic protection of the condenser of Unit 7 is designed as a multi-point decentralized control type.
In order to protect and enhance the protection of the end parts and bulges of the tube sheet and copper tube bundles, and at the same time reduce the power of the cathodic protection device, an indoor corrosion-resistant erosion coating was designed in the condenser of Unit 7. In fact, the anti-corrosion project of the condenser of Unit 7 is a combined anti-corrosion method of cathodic protection and coating protection.
The No. 7 condenser of the power plant has a cathodic protection and anti-corrosion effect
The No. 7 condenser cathodic protection and anti-corrosion system was put into operation on November 15, 1998
Corrosion of the top of the tube sheet and copper tube is suppressed
During the shutdown inspection of the system put into operation, it was found that the development of serious local corrosion on the original tubesheet, especially in the Tongguan area, was inhibited, and there was no obvious accumulation of yellow-brown corrosion products in the original pit and on the tubesheet. There is no development of corrosion such as pitting corrosion on the inner wall of the end of the tube sheet, and there are no traces of products such as alkaline copper carbonate on the surface.
Reduced the average corrosion rate of copper alloys
The inspection of the surface condition and weight of the monitoring test strips installed with copper tubes can also convincingly illustrate the effectiveness of the cathodic protection system in preventing condenser corrosion after it has been put into operation.
In the surface condition of the brass specimen that has not been protected in the condenser for 330 days, it can be seen that the yellow-brown basic copper carbonate and white zinc hydroxide corrosion products on the surface of the specimen are attached, and the copper alloy material has obvious corrosion in the cooling water of the unit when the surface has not received the impressed current cathodic protection. The average corrosion rate can be measured from the weight loss of the specimen to 0.022mm/a.
Surface condition of the copper sheet protected by an impressed cathodic current in a condenser for 330 days. As can be seen from the photo, the surface of the brass sheet is smooth and clean, and there is basically no deposition of corrosion products. The surface has a solid layer of red-black cuprous oxide and copper oxide. The average corrosion rate of brass is reduced to 0.0015 mm/a under cathodic protection.
The average corrosion rate of brass in the condenser of Unit 7 is reduced by more than 93% under cathodic protection conditions.
Printed carbon steel local corrosion tendency, reducing the average corrosion rate of carbon steel
It can be clearly seen from the surface condition of the carbon steel specimen that has not been protected by the impressed current cathodic in the condenser for 330 days that the surface of the carbon steel specimen is seriously corroded, showing an obvious local corrosion-ulcerative corrosion crater form. After the corrosion products attached to the falling surface are removed, a deep corrosion pit can be seen, and it is already larger than 1mm. In addition, it can be seen that the edges of the specimen have been corroded and damaged. From the weight loss of the specimen, it can be calculated that the average corrosion rate of carbon steel under this working condition is 0.57mm/a.
There is no ulcer-like local corrosion on the surface of the specimen, and a loose yellow-brown precipitate is attached. After removing the surface attachments, the surface of the specimen is smooth and free of pits. In addition, there are no defects in the edges of the specimens. The average corrosion rate of carbon steel is reduced to 0.0144 mm/a under cathodic protection conditions.
The average corrosion rate of carbon steel was reduced by 74.8% under the condition of cathodic protection of the condenser of Unit 7.
The average corrosion rate of carbon steel in fresh water cooling water is not large, but it is common to have localized porosity and ulcer-like corrosion, which is the most dangerous form of corrosion for metal structures. In the condenser, the galvanic action formed by the direct expansion of the carbon steel tube plate and the copper tube will aggravate the local corrosion development of the carbon steel tube sheet. The average corrosion rate obtained from the specimen does not directly reflect the degree and hazard of such local pit erosion or ulcer corrosion. In actual engineering, if the metal only corrodes uniformly at a small acceptable corrosion rate, the protective measures adopted can reduce or prevent the metal from occurring this dangerous local corrosion form, which indicates that the anti-corrosion measures are effective. From the corrosion inspection of the condenser tube sheet of Unit 7 and the evaluation of the surface condition of the specimen, it is shown that the cathodic protection has achieved obvious results in preventing the corrosion of the tube sheet. Therefore, the use of cathodic protection and anti-corrosion in No. 7 condenser is very effective, and the control of the cathodic protection system is also suitable.
Analysis of the economic benefits of cathodic protection in the condenser of unit 7 of the power plant
Cathodic protection is not a surface treatment method, it is a technology that fundamentally stabilizes the metal and reduces the tendency of the metal to corrode in the cooling water, it will bring long-term results, and it is a very effective, trouble-saving and economical method.
In the short period of time when the cathodic protection and anti-corrosion measures implemented by the condenser of Unit 7 have inhibited the pitting, dezincification and galvanic corrosion near the expansion of the tube sheet and at the end of the copper tube, which will delay the leakage of the copper pipe, prolong the power generation and increase the amount of labor lost by the unit due to the plugging of the copper tube of the condenser, and also improve the quality of the water due to the reduction or non-leakage of cooling water in the condenser, ensure the quality of the feed water, and reduce the green scale of the boiler. The steam quality is improved, which improves the safety, economy and reliability of the unit operation, extends the overhaul interval, and reduces the number of temporary repairs.
At present, the copper pipe value of a 100MW unit condenser and the cost of replacing copper pipes are about 2.4 million. If the life of the copper tube is extended by one year, it is equivalent to an annual economic benefit of about 120,000 yuan. If the condenser copper tube leak detection and plugging times are reduced, the economic benefits are obvious. If we take into account that a 100MW unit generates nearly 3 million kWh of electricity a day, a power outage will cause nearly 40% of the direct economic losses to the power plant for one dayabout 10,000, and the indirect loss is even greater. If the copper pipe is damaged beyond repair due to corrosion, the damage is difficult to estimate. In addition, improving the safety and reliability of unit operation and reducing unit maintenance will also bring obvious economic benefits.
The one-time investment and maintenance cost of the condenser cathodic protection and anti-corrosion system of Unit 7 is less than 18,000 yuan a year, which is relatively small and economical compared with the cost of other anti-corrosion measures.
Therefore, the use of cathodic protection system in the condenser to prevent corrosion can obtain a high input-output return, and the production management is simple, and the maintenance workload is very small, so this technology has great economic and social benefits, and it is very valuable for popularization and application.
