Failure analysis and treatment of titanium tube condenser

By analyzing the failure causes of titanium tube condenser, the results showed that biological fouling was easily formed during the operation of titanium tube condenser, which led to the deterioration of heat transfer and vacuum drop of the condenser. The biological fouling on the surface of titanium tube condenser can be effectively cleaned by using high-pressure water jet technology; During the operation of the titanium tube condenser, the rubber ball cleaning device and the microbial control of the open circulating cooling water system can effectively avoid the formation of biological fouling in the titanium tube condenser.

The condenser is an important part of the steam turbine unit. Its main function is to establish and maintain vacuum at the steam turbine exhaust port to improve the thermal economy of the unit. At the same time, the condensed water from the exhaust steam is recycled as the boiler feed water to form a cycle. The operating condition of condenser directly affects the safety and economy of unit operation. At present, most condensers in power plants use copper alloy heat exchange tubes. Copper alloy tubes have good thermal conductivity and are affordable. However, serious corrosion and short service life have occurred in practical applications. In recent years, titanium materials have been gradually applied in the manufacture of condensers due to their excellent corrosion resistance, erosion resistance, high strength, light specific gravity and other characteristics.
In this paper, the failed condenser uses pure titanium TA2 (GB/T3625) heat exchange tube. During operation, a layer of biological fouling formed on the surface of the heat exchange tube, which affected the heat conduction performance of the titanium tube, leading to the deterioration of heat transfer and vacuum drop of the condenser. There are few reports on the failure of titanium tube condenser due to the formation of biological fouling. Therefore, the analysis of the reasons for the formation of biological fouling of titanium tube condenser and the corresponding treatment measures taken have reference value for the subsequent operation of titanium condenser.

1. Fault process

The condenser is a supporting equipment of a steam turbine generator unit. From May to December 2016, it was commissioned and operated with the unit, and began to operate continuously in December. See Table 1 for main technical parameters of condenser.

Table.1 Main technical parameters of condenser

Project	Parameter
Type	Horizontal shell and tube type
Pipe side inlet and outlet temperature/°C	28/41
Tube side medium	Circulating cooling water
Heat exchange tube	TA2,GB/T3625

During the period from March to March 2017, during the rated load operation of the unit, it was found that the condenser vacuum decreased slowly. The operators first checked and eliminated the tightness of the air extraction system and related pipelines, but the problem of condenser vacuum drop was not solved. In April 2017, the generator set was shut down for inspection, and the staff opened the cooling water inlet and outlet of the condenser to inspect the condenser. It was found that a layer of viscous soft dirt was attached to the surface of the circulating cooling water side of the condenser, as shown in Figure 1.

Figure.1 Viscous soft dirt is attached to the surface of the cooling water side pipe in the condenser

2. Fault analysis

2.1 Operation data analysis

The end difference of condenser is an important index to measure its heat transfer effect. The end difference is the difference between the saturated water temperature under the condenser exhaust pressure and the outlet temperature of circulating cooling water. Figure 2 shows the change of end difference with operation time under rated conditions since the titanium tube condenser was put into operation. It can be seen from the analysis of Figure 2 that from May 2016 to February 20, 2017, the end difference of the condenser operation was basically maintained at 10-13 ℃, while from February 20 to March 30, the end difference increased sharply, from 14.5 ℃ to more than 35 ℃, indicating that the heat transfer performance of the condenser continued to deteriorate during this period, which eventually led to the decline of vacuum.
When the cooling water volume is constant, the water resistance at the inlet and outlet of the condenser cooling water can directly reflect the scaling condition in the heat exchange tube of the condenser. According to the operation record data, the change of water resistance at the inlet and outlet of the condenser with time is obtained, as shown in Figure 3.

Figure.2 Change of condenser end temperature difference with operation time

Figure.3 Change of water resistance at cooling water inlet and outlet with operation time
It can be seen from the figure that from May 2016 to the end of February 2017, the water resistance was basically maintained at 45-55kPa, showing a slight growth trend. From February 24 to March 15, the water resistance increased sharply, from 54.6kPa to 113.7kpa, more than doubled. It shows that during this period, the fouling in the heat exchange tube of the condenser increases rapidly. Combined with the actual inspection pictures, it is confirmed that the fouling formed in the titanium tube condenser is the main reason for the deterioration of heat transfer and vacuum drop of the condenser.

2.2 Cause analysis of dirt formation

• (1) Formation of biological slime. The dirt on the surface of titanium tube condenser is soft dirt, which is sticky and slippery. According to relevant data, the dirt should be biological slime, which is propagated by microorganisms in the circulating water, and is mainly composed of these microorganisms, mixed with sediment, inorganic substances and dust to form attached soft muddy sediment. The cooling water used for the titanium tube condenser is provided by the open circulating cooling water system, which is easy to bring in a lot of microorganisms and dust during the operation of the system. According to the above analysis, the fault occurred from the end of February to March. The temperature began to rise, and the temperature of cooling water became suitable for the growth and propagation of microorganisms. At this time, the unit and the supporting titanium tube condenser were in continuous operation. A large number of microorganisms in the cooling water entered the condenser with the water flow and attached to the tube wall, gradually forming a film of viscous substances, so that small sludge and other impurities in the water remained on the tube wall, gradually forming a biological sludge.
• (2) Titanium tubes are prone to microbial attachment. With the more and more extensive application of titanium materials in condensers, their existing problems and solutions have also been widely studied.
• At present, the application and research of titanium condenser in relevant literature mainly focus on the sealing welding between titanium tube and tube sheet, vibration wear of titanium tube and chemical cleaning of iron pollution and oil stains. At the same time, some studies have pointed out that the generation of biological fouling is closely related to the performance of heat exchange tube materials. Since titanium does not have the same intrinsic toxicity and corresponding bactericidal effect as copper alloy, it is necessary to prevent microorganism adhesion and take measures to control the formation of biological fouling during the application of titanium tube condenser. During the use of the titanium tube condenser, no control measures have been taken for the formation of biological fouling in advance. Therefore, the titanium tube condenser will fail in a short time under the condition that biological fouling is easy to form.

3. Treatment measures and effects

3.1 Condenser cleaning

High pressure water jet technology is to use the device to generate high pressure water, and use the special nozzle to generate a small water flow with highly concentrated energy to impact, grind, smash and peel the dirt on the pipe surface, so as to achieve the purpose of cleaning the dirt. High pressure water jet technology was used to clean the titanium tube condenser. After cleaning, the soft dirt in the condenser tube was basically removed, and the titanium tube recovered its metallic luster. After cleaning, the condenser was subjected to a tube side hydrostatic test, with a pressure of 0.3MPa. The pressure did not drop after 30 minutes of pressure holding. After the completion of condenser cleaning, cooperate with the unit for operation and commissioning. Under rated working conditions, the unit vacuum, condenser end difference, cooling water inlet and outlet water resistance and other parameters are restored to normal state, and the titanium tube condenser cleaning work achieves the expected purpose.

3.2 Add rubber ball cleaning device

According to the relevant technical parameters of the titanium tube condenser, a rubber ball cleaning device is configured for it. During the later operation of the condenser, the rubber ball device is regularly used for cleaning, so as to prevent and eliminate the microorganism adhesion in the titanium tube.

3.3 Microbial control of circulating cooling water system

According to the previous analysis on the causes of biological slime formation, the open circulating cooling water system is prone to produce and bring in a large number of microorganisms and dust. At present, using chemical methods to control microorganisms in circulating cooling water system is a common and effective means. According to the operation characteristics of the circulating cooling water system, the oxidizing and non oxidizing biocides shall be put into use alternately on a regular basis, and the circulating water quality shall be monitored to ensure that the indicators such as heterotrophic bacteria and biological sludge amount meet the specifications and standards. After taking targeted measures to control microorganisms in the circulating cooling water system, the formation of biological fouling in the titanium tube condenser was effectively avoided, and the safe and normal operation of the titanium tube condenser was ensured.

4. Conclusion

This paper analyzes the problem of vacuum drop during the operation of titanium tube condenser, and provides corresponding treatment measures, which has certain reference value for the operation of titanium tube condenser:

• (1) Biological fouling is easy to form during the operation of titanium tube condenser, which leads to the deterioration of heat transfer and vacuum drop of the condenser.
• (2) High pressure water jet technology is used to clean the biological fouling on the surface of titanium tube condenser.
• (3) During the operation of the titanium tube condenser, the rubber ball cleaning device and the microbial control of the open circulating cooling water system can effectively avoid the formation of biological fouling in the titanium tube condenser.

Author: Chen Dingqian, Luo Jian, Gong Shenjun
Source: China Titanium Tubes Manufacturer: www.titaniuminfogroup.com
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