Source: Dianlian Intelligent Manufacturing
Recommended unit: China Resources Power (Heze) Co., Ltd
Authors: Yang Wenyi, Wang Yanbo
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The condenser is an important equipment in the thermal system of the power plant, and it is an indispensable equipment to ensure the smooth thermal cycle of the unit. Condenser leak monitoring is also a crucial part of the day-to-day chemical supervision process, and in the event of a leak, proper and efficient disposal can minimize the damage.
Zero, overview
On March 18, #2机组热力系统水汽指标突然全线升高, the water vapor quality deteriorated rapidly, and the preliminary judgment was that the water vapor quality deteriorated due to the leakage of circulating water from the condenser. On the 19th, the company quickly organized professional and technical personnel such as chemistry, metal, and steam turbines to find out the cause of condenser leakage and carry out emergency treatment through data analysis, on-site inspection, endoscopic inspection, tube inspection, material test analysis and other means.
First, the course of the incident
At 21:18 on March 18, the #2机凝结水泵出口比电导率5.88μs/cm, the specific conductivity of the outlet mother tube of the polishing system was 0.08μs/cm, and the pH was 6.06. At 21:19, the specific conductivity of the condensate pump outlet began to increase rapidly and reached the full scale of 9.99μs/cm of the online chemical instrument, and the conductivity change curve is shown in Figure 1-1. The laboratory personnel immediately took samples for manual testing, and the instrument maintenance personnel cooperated to check the status of the online chemical instrument, and the on-site manual laboratory confirmed that the value of the linear chemical instrument was correct.
Figure 1-1 Specific conductivity curve of condensate pump outlet
At 21:19, the specific conductivity of the outlet mother pipe of the condensate polishing system was 0.10μs/cm, and the water quality was qualified. At 21:48, the specific conductivity of the outlet mother tube of the condensate polishing system reached 1.0μs/cm of the full scale of the online chemistry instrument, and the specific conductivity curve of the outlet mother tube of the polishing system is shown in Figure 1-2.
Figure 1-2 Specific conductivity curve of the outlet mother tube of the condensate polishing system
#2锅炉省煤器入口氢电导率自21时30分开始升高; At 21:43, the hydrogen conductivity at the inlet of the economizer reached 0.30μs/cm, and at 22:09, the hydrogen conductivity at the inlet of the economizer reached 1.0μs/cm at the full scale. At 23:11, at #2锅炉省煤器入口pH降至7.0, the pH of the economizer inlet was as low as 5.52, the water vapor quality deteriorated across the board, and the unit was forced to shut down at 00:26 on the 19th.
When the quality of water vapor deteriorates, the steam turbine professional takes measures such as adding sawdust and load reduction to the circulating water to try to reverse the deterioration of water vapor.
Second, the on-site inspection
2.1 Condenser leak detection
On March 19, the circulating water pump of unit #2 was kept running, and the internal inspection of the steam side of the condenser was carried out, and the leak detection work was carried out. After the internal inspection of the steam side, sawdust was found on the steam side of the 2B condenser, and a leakage point was found on the return side of the outer ring tube bundle of the 2B condenser, and the leakage point was preliminarily judged to be about 0.5 meters away from the end tube plate of the outer ring return chamber and about 0.5 meters away from the bottom of the pipe bundle.
Stop the circulating water pump and drain the water storage on the water side of the condenser, close the manhole door on the steam side and fill the water to check for leakage, and further clarify the location and number of leaking pipes. On March 20, the leakage of the steam side irrigation of the condenser of unit #2 confirmed that a stainless steel pipe was full of water leakage, as shown in Figure 2-1. Irrigation continued to reach a high level, and no other significant leaks were found.
Figure 2-1 Photos of the leaking stainless steel pipe
After on-site inspection, the stainless steel end tube plate and stainless steel nozzle of the return chamber and outlet chamber of 2B condenser were not obviously adhered to the sludge, and no obvious scaling and corrosion were found, and no obvious slime, scaling and corrosion were found in the stainless steel nozzle and the surrounding area where the leakage occurred, and the internal photos are shown in Figure 2-2.
Figure 2-2 Inside the condenser
2.2 On-site endoscopy
The inside of the leaking stainless steel pipe was inspected from the return side of the outer ring of the 2B condenser using an industrial endoscope, and it was found that the leaking pipe was fractured at a distance of about 0.7m from the end tube sheet of the return chamber, the fracture edge was irregular, the edge was suspected to be corrosive, and the fracture was not new. Cone-shaped objects and brush debris were found outside the fracture, and the on-site endoscopic examination is shown in Figure 2-3.
Figure 2-3 Characteristics of leaking pipe fractures
Remove the leaking stainless steel pipe, conical object, brush and other debris as shown in Figure 2-4. It was confirmed that the conical object and the brush were integrated, which was the guide cone used in the installation of the stainless steel pipe of the condenser, and the conical head was made of metal aluminum, which may be the guide cone left over from the installation of the stainless steel pipe of the condenser during the construction period of the unit.
Figure 2-4 Photos of leaking pipes and debris
On the return side of the condenser, the endoscope was continuously pushed forward along the leaking pipe for inspection, and the endoscope was advanced to a depth of about 6 meters, and no remaining pipes and residues were found, as shown in Figure 2-5. On the outlet side of the leaking stainless steel pipe, the internal condition of the pipe was checked, and the endoscope was advanced about 0.7m, and it was found that the pipeline was impassable, and the end was blocked by metal welding, as shown in Figure 2-6.
Figure 2-5 Checking the depth of the leaking pipe
Photo
Figure 2-6 Internal inspection of the outlet side of the leaking pipe
3. Tube sample testing
The stainless steel pipe that leaked was extracted, and there were suspected traces of discoloration on the outer wall of the fracture at the leaking end, and the cross-sectional edge of the fracture at the leaking end was irregularly sawtooth. The inner wall of the leaking stainless steel pipe leakage end is attached to rust, and the detailed photos of the leaking pipe fracture are shown in Figure 3-1. There is a thin layer of scale on the inner wall of the pipe, and there are no obvious corrosion pits and other corrosion characteristics.
Figure 3-1 Appearance and nozzle of the leaking pipe
Metallographic detection is carried out on the sample after grinding and polishing, as shown in Figure 3-2.
Figure 3-2 Metallographic detection of tubular samples
From the test results, the following conclusions can be drawn:
(1) The alloying elements of leaking pipes and normal operation pipes meet the requirements of the standard.
(2) The structure near the fracture of the leaking pipe is coarse and there is a slip line structure caused by deformation, and the leaking pipe is far away from the fracture area, and the metallographic structure of the pipe sample is normal during normal operation. The test results show that the part has undergone thermal processing with abnormal processes, such as flame cutting, welding fuse, etc., and there are a large number of slip lines in the austenite structure, indicating that the part has undergone cold deformation, and a large number of slip lines have been generated in the structure under stress. There is a slip line structure in the austenite microstructure, which is not good for the corrosion resistance of the material.
(3) The hardness near the fracture of the leaking pipe exceeds the upper limit of the standard, which is related to the existence of a large number of slip lines in the tissue. The hardness values of other normal tissue areas tested meet the requirements of the standard.
Fourth, the cause of the leakage analysis
According to the analysis of the above results, the leaking condenser pipe has been plugged abnormally, and the plugging end is difficult to achieve a complete fit between the plugging and the stainless steel pipe, and there is a slight gap between the plugging and the stainless steel pipe, and the short blind pipe is filled with circulating water during the operation of the unit, and the circulating water seeps into the gap between the plugging and the stainless steel pipe, forming a certain corrosive environment. At the same time, during the operation of the unit, the plugging material is subjected to two codirectional forces: the positive pressure of the circulating water and the negative pressure on the steam side of the condenser. During the long-term operation of the unit, under the action of multiple factors such as corrosion, water vapor disturbance or stainless steel pipe vibration, the plugging gradually loosened and fell off suddenly, resulting in the sudden leakage of the condenser.
5. Analysis of circulating water quality
Check the circulating water quality supervision report of the unit from January 2018 to January 2021, and the chloride ion concentration in the circulating water of unit #2 has been maintained below 1600mg/L in the past three years. According to DL/T712-2010 "Guidelines for the Selection of Condenser and Auxiliary Cooling Pipes of Power Plants", the source of circulating water is reclaimed water, and when the condenser pipe is 317L stainless steel pipe, the chloride concentration in the circulating water should be less than 2000mg/L. #2机组循环水中氯离子浓度低于DL/T712 required value, the circulating water quality does not have the corrosion ability of 317L stainless steel pipe.
6. Emergency response and planning
When the water vapor quality deteriorates, the chemical supervision immediately conducts water quality analysis and confirmation, and then the steam turbine professional adopts the methods of adding sawdust to the circulating water and reducing the load of the unit to try to reverse the deterioration of the water vapor situation, and when it is found that the disposal measures are ineffective, the three-level treatment in the strict GB/T 12145-2016 "Water Vapor Quality of Thermal Power Generating Units and Steam Power Equipment" is decisively shut down, which lasts about 3 hours and 8 minutes, and quickly takes countermeasures to avoid the expansion of the accident. The deterioration of water vapor quality minimizes the impact of water vapor deterioration on the boiler and steam turbine system.
Due to the tight downtime processing time, only the leaking pipe sample was plugged in. Failed to conduct a comprehensive inspection of the stainless steel pipe of the condenser of the unit, and planned to extract the stainless steel pipe on the outlet side of the leakage pipe for inspection and analysis when the conditions for the next shutdown were met, and conduct a comprehensive inspection of the stainless steel pipe of the condenser.
In the follow-up unit start-up process, the chemical supervision adopts the extension of the start-up flushing time, strengthens the water quality supervision in the flushing stage, and then carries out the follow-up operation after the flushing water quality is qualified to ensure that the system is clean and reduce the risk of corrosion.
It is planned that during the next unit overhaul, the focus will be on the corrosion of the four pipes. Increase the position and number of boiler economizer and water wall pipe cutting to detect the scale and corrosion, increase the number of superheater and reheater pipe cutting to detect the thickness of oxide scale, and check the salt accumulation and corrosion of steam turbine blades when the next steam turbine removal.
In view of the condenser leakage incident, the relevant professions should learn a lesson, strengthen the project quality supervision and acceptance during the follow-up maintenance work, attach great importance to the condenser leakage detection work, and improve the reliability of the equipment.