Water-cooled wall crawling robot

Boiler is an important production equipment in power plants. The water-cooled wall tubes inside the boiler are easily eroded by flue gas, coal ash, and flames during long-term service, which can cause wear and corrosion, resulting in local thinning of the tube wall. Under the action of high-pressure and high-temperature steam inside the tube, serious accidents such as tube bursting and leakage may occur. Boiler leakage is different from general pipeline leakage, which cannot be repaired during production operation and often requires shutdown for repair, resulting in huge economic losses. Therefore, various power plants attach great importance to effectively reducing and avoiding boiler tube bursting. It is of great significance to use crawling robots to inspect the water-cooled wall without having to fully cover the furnace frame during short-term shutdown.

1、 Types of corrosion on water-cooled walls of power plant boilers

1. Corrosion under the scale on the inner wall

When the oxide produced by corrosion under the scale in the heating surface of the water-cooled wall tube causes steam stagnation or a decrease in flow rate, a reaction will occur and a magnetic iron oxide film (Fe3O4) will be formed on the metal surface, which is called "steam corrosion". If the hydrogen gas generated by the steam corrosion cannot be quickly carried away by the airflow, it will interact with the surface of the steel pipe, causing decarburization and making the steel brittle, so it is also called "hydrogen corrosion".

The corrosion of the inner wall is essentially a chemical reaction process, and the higher the local temperature, the stronger the reaction. The residual hydroxides generated during the reaction will slow down the local steam flow rate, further promoting the progress of the above reaction. Under scale corrosion generally occurs on the inner wall of the boiler water-cooled wall tube towards the fire side, and the damage form is like shells. Corrosion under the scale develops to a deeper level, causing perforation and rupture of the pipe wall. In addition, the steel surface in the corroded area is covered with a loose rust layer, causing poor thermal conductivity of the pipe wall, resulting in local overheating of the pipe wall, material creep, and even outward bulging of the pipe wall, ultimately leading to rupture.

In multiple boiler inspections at different power plants, the following main characteristics of defects in the inner wall of water-cooled wall tubes were found:

（1）Defects are mostly corrosion pits, with very few cracks.

（2）All corrosion pits occur on the fire side, mostly in the middle.

（3）The size and depth of corrosion pits vary and develop over time of service.

（4）The distribution on the furnace wall is irregular. If there is a pipe burst in a certain area, and several nearby pipes are cut open for inspection and found to be intact, running for a period of time may cause pipe bursts in other locations.

2. High temperature corrosion of outer wall

The main reasons affecting the external corrosion of water-cooled walls are the composition of flue gas near the water-cooled wall and the temperature of the pipe wall. Specifically, due to the high flame temperature near the burner, which can reach around 1400 ℃, the mineral components in coal emit more corrosive gases, creating conditions for the corrosion of the heating surface. In addition, due to the high heat flux density and temperature gradient of the water-cooled wall tubes near the burner area, the tube wall temperature often reaches 400-450 ℃, which also plays a significant role in the high-temperature corrosion of the tube wall.

The metal of boiler water-cooled wall tubes undergoes oxidation reactions under the action of oxidants such as oxygen and sulfur, resulting in high-temperature corrosion. When the smoke and ash layer contain corrosive components, the pipe will corrode and even cause pipe bursting.

The high-temperature corrosion that occurs on the water-cooled wall of coal-fired boilers usually belongs to sulfide type high-temperature corrosion, mainly caused by H2S gas in the flue gas. When the oxygen content in the combustion zone flue gas is low and there are reducing gases (such as CO, H2, etc.) present, sulfur and chlorine in coal will generate H2S and HCI gases, which will react with water vapor to form highly corrosive acidic gases. These gases will corrode the metal oxide film on the pipe wall, causing damage to the protective film on the metal surface.

In addition, low nitrogen and oxygen deficient combustion under current environmental requirements is an important cause of high-temperature corrosion.

2、 The hazards of water wall corrosion

The water-cooled walls of power plants operate in extremely harsh environments, with issues such as wear, tear, high-temperature corrosion, and scaling. Failure to detect these problems in a timely manner can ultimately lead to accidents.

According to relevant statistics, 60% of non-stop accidents in thermal power units are caused by boiler "four tube" bursts, of which 75% are caused by corrosion and wear of water-cooled walls.

Nearly 80% of large power plant boilers in our country are affected by high-temperature corrosion of water-cooled walls. High temperature corrosion can cause the thickness of water-cooled walls to continuously thin, material strength to decrease, and water-cooled walls may experience tube bursting and leakage, endangering the normal and safe operation of the entire boiler. Once an unplanned shutdown accident occurs, it will cause huge economic losses to the power plant.

Therefore, the prevention and control of high-temperature corrosion in boilers is essential for their safe and stable operation. Regular inspections of water-cooled walls can detect and prevent accidents from occurring in a timely manner.

3、 Traditional detection of water wall corrosion

At present, the detection method for boiler water-cooled walls in China is mainly manual scaffolding or lifting platforms. The outer wall detection mainly uses the naked eye and hand feel, as well as ultrasonic thickness gauges to measure the thickness of the pipe wall and identify the location of corrosion; Use an ultrasonic thickness gauge or a cutting tube endoscope to inspect the corrosion inside the tube.

The dust inside the boiler furnace is relatively large, and it is necessary to wait for the furnace to cool down to room temperature before building the scaffolding. Manual inspection of water-cooled walls requires harsh working conditions, heavy workload, slow progress, and high costs. Moreover, human factors and incomplete data can easily lead to missed detections. And the maintenance of boiler water-cooled walls belongs to high-altitude operations in confined spaces, with a high degree of risk. Once a high-altitude fall occurs, it will cause serious consequences for the parties involved and the enterprise.

4Water-cooled wall crawling robotThe detection

4.1 Introduction to Testing

Our company is conducting research on the issue of manual inspection of water-cooled walls. The R&D team has successfully developed a water-cooled wall crawling robot through on-site inspections at the power plant and multiple practical trials. The water-cooled wall crawling robot can replace manual inspection of water-cooled walls. The robot adopts permanent magnet technology to adsorb on the water-cooled wall and can move on the water-cooled wall. Through real-time high-definition video, it completes functions such as flushing, cleaning, inspection of the appearance of the water-cooled wall pipes, automatic detection of pipe wall thickness, and defect positioning. The intelligent all-round detection of wear and explosion prevention of water-cooled walls in power plants has been achieved, which is of great significance for promoting safe operation of power plants, saving construction time and costs.

4.2 Composition and Principle

The robot adopts a magnetic adsorption structure, which can reliably climb within the furnace range. The vertical tension of the robot is greater than 200kg. It can be assembled into modules according to different tasks, including driving walking mechanism, detection mechanism, cleaning device, and display controller.

The driving walking mechanism includes a vehicle body, a reduction motor, a frame, and magnetic wheels, with the reduction motor installed inside the vehicle body.

The testing institutions include front and rear high-definition cameras, electromagnetic ultrasonic thickness gauges, and positioning devices, which are respectively installed on the vehicle body.

The cleaning device consists of a cleaning brush and two high-pressure water spray devices, distributed on both sides of the vehicle body.

The display controller adopts a 7-inch high-definition touch screen Android system control terminal to control the walking, detection, cleaning, and thickness display of the water wall crawling robot.

4.3 Water-cooled wall crawling robotCharacteristics and technical parameters of

(1) High definition video

The camera adopts a 2-megapixel CMOS image sensor, a 150 ° wide field of view, LED lighting, real-time 1080P high-definition camera and storage. It can detect and record the high-temperature wear of the on-site environment, robot walking, and pipe wall appearance.

(2) High pressure flushing

The two high-pressure water cleaning systems equipped can achieve a flushing force of 5 megapascals when cleaning the water-cooled wall, which can efficiently wash away some slag and coke on the water-cooled wall, with a flushing water consumption of no more than 10L/min.

(3) High precision thickness measurement

Using an electromagnetic ultrasonic thickness measurement device, without the need for coupling agents, the wall thickness of water-cooled walls can be measured with an accuracy of 0.1mm. Eliminating the inconvenience of traditional ultrasonic thickness measurement methods that require the application of coupling agents for thickness measurement.