Power plant boiler water wall tube corrosion detection

Power plant boiler water wall tube corrosion detection

Boiler is one of the most important production equipment in thermal power plant ,the furnace water wall tube in the long-term service by flue gas,fly ash and flame,such as erosion,abrasion,corrosion,appear easily cause local thinning wall ,in the pipe under the action of high pressure and high temperature steam,finally produce to continue to repair in the operation of production,often have to stop to repair,the huge economic loss,thus the power plant to reduce and avoid the bursting of boiler tube leakage is very seriously,to strengthen the water wall tube running material in service monitoring and inspection has very important practical significance。

The water wall tube inner surface corrosion mechanism

The cause of water wall tube thinning are mainly outer wall soot blowing the corrosion and wall corrosion under the scale.the former in the outer wall of pipe is related to the flow of furnace flue gas,there are rules ,general uniform thinning can with the naked eye or feel is to find a location,using ultrasonic thickness meter can accurately measure the remaining thickness。While the latter on the inner wall of the tube ,the position is random,now can only rely on ultrasonic thickness meter to local tube corrosion in selecting examination,but as a result of the measurement before descaling,daub the preparation process of coupling agent,to limit the number of the measurement,in thousands of root dozens of meters high in the water wall tube of find out specific corrosion pit,like a needle in a haystack。

Or the furnace caused by the feed line formed when water containing iron oxide and cupric oxide slag deposits on the inner wall of the heating surface of boiler water wall tube,when this medial wall meter contact with the oxide,produces the following reaction

4Fe2O3+Fe→3Fe3O4

4CuO+3Fe→Fe3O4 + Cu

Reaction is chemical way,ferric oxide and cupric oxide as the anode,tube wall metal as the cathode ,cathode corrosion in the reaction continuously,this is teh first stage of corrosion under the scale,inner steam iron contact with above 400° and the4H2O+3 Fe→Fe3O4 +4H2↑

following reaction will occur 

When in the water wall tube of heating surface caused by corrosion under the scale oxide steam stagnation or reduced    velocity.the above reaction will occur and the form magnetic iron oxide film on metal surface,called “steam corrosion”

After the steam generated by the corrosion generated hydrogen away if not taken away quickly by air。Decarburization causes steel brittle,so it is known as “hydrogen corrosion”.

Internal corrosion is essentially a chemical reaction process,the more local temperature the intensive reaction will be ，the    reaction of hydroxide residue will make steam slows down  。More to promote the reaction,scale corrosion usually occurs under the boiler water wall tube side of this wall,under-deposit  corrosion develops into deeper,the pipe wall perforated burst,in addition,Corrosion area of the steel surface with loose rust layer,generates pipe wall has poor heat conductivity,appear local wall overheating,Produce the material creep,there are even  wall bulge outward,finally fracture。

· In several different power plant boiler inspection, found that the water wall tube inner surface defects have the following main features

· （1） flaw most are corrosion pit,crack is little

·   (2) all corrosion pit are occur at the side with face to fire

·   (3) area size of corrosion,depth are different,

·   (4)no regulation

Detecting principle

For nondestructive testing of the pipeline in service, usually by ultrasonic, fluorescence, magnetic powder, eddy current and magnetic flux leakage method, ultrasonic measurement precision, is a kind of need the single point measuring method of coupling agent, low detection efficiency. Fluorescence, magnetic powder, high sensitivity, but is only suitable for detecting the surface crack, cannot detect pipe wall corrosion defects. Conventional eddy current method is only suitable for surface testing, the further development of the far field eddy current method can check the pipeline corrosion, probe need through the pipe internally, however, that for many a closed container line is not allowed. Of pipeline magnetic flux leakage method of internal defects with high detection sensitivity, detection speed, low cost, simple operation and visual clear signal。

Magnetic flux leakage method to detect the basic principle is to use an appropriate excitation circuit of magnetic field in the pipe,Make wall partial saturation magnetization,When checked the pipe corrosion pit or cracking of the region,The partial wall magnetic resistance increase,The area of magnetic field to produce distortion,Part of the magnetic field from the wall surface leaked,Form local area leakage magnetic field, the nature of the leakage magnetic field distribution and the defect that there is a connection and geometry size. With a magnetic sensor for leakage magnetic field distribution and translated into can observe the electrical signals, can be obtained and reflect the graphics pipeline defects。

The simplest magnetic flux leakage detection principle describinggraph。

For different excitation way different, magnetic flux leakage method is divided into two kinds: permanent magnetic magnetization magnetization and communication. The former with dc magnetization or permanent magnet magnetization to generate the excitation, the latter to motivate alternating magnetic field with alternating current. Ac magnetic flux leakage method in addition to a constant magnetic method in the detection device, a lightweight also increased in signal acquisition phase, frequency, etc, the information content, can collect more defect feature, but detection device is relatively complicated and expensive。

In order to reduce the alternating magnetic field produced by the skin effect, eddy current ac magnetic flux leakage method working frequency was lower than those of conventional eddy current testing many, generally in 5 ~ 100 hz, depending on the pipe material and wall thickness. Due to this characteristic, this method is also known as low frequency electromagnetic detection, hereinafter referred to as LFET。

3   Instruments and signal processing

ET-1Steel pipe corrosion scanning apparatus using low frequency alternating current electromagnetic field technology ,saddle probe attached on the outer wall of steel pipe,To scan the pipe wall corrosion condition,This scan method detecting speed, high accuracy, save work time saving, and easy to implement, and no oxide layer on the surface of tube and dirt removal process, also do not need to apply the coupling agent. Scan speed can be up to 3 ~ 5 m/min. Check the defect types including the corrosion pits, flush fluid dust thinned, crack, oxygen corrosion, hydrogen corrosion, etc. Typical steel pipe to be tested is the boiler water wall tube, heat pipe, and so on.。

Instrument signal processing system by scanning probe, signal amplifier, filter, phase processing circuit and computer, etc。

Scanning probe from outside wall defect signal, after amplification into filter, based on the interference signals and real signals of different defect characteristics of filter from the testing site space electromagnetic interference and probe in the beating. Phase shift is used to offset due to the tube wall thickness, material quality and different probe gap produce phase difference。

Graph 2    signal process circuit flow chart

4.instrument features:

1.Touch screen operation mode and shuttle digital knob,direct and simple。

2.Chinese friendly man-machine dialogue, easy to operate

3.Microprocessor control, digital test circuit, high reliability and stability.

4.100 Instrument built-in applications,nonprofessional operators can get optimized set

5.High contrast liquid crystal display。No matter in the dark environment or direct sunlight can be observed clearly

6.Built-in smart lithium-ion rechargeable batteries, full of electricity can work continuously for 5 hours

7.A specially designed can restrain noise filtering software maintenance on-site electromagnetic interference

8.Cast aluminum chassis structure, shockproof anti-collision, strong and durable.

9.Scanning probe cable is 8 meters long, up to 50 meters max

10.In line with the on-site detection probe roller design, light and easy to move

11.Lithium battery or ac 100 v ~ 240 v adaptive power supply or batteries

12.Just weight 0.9kg,even long term operation ,will noe tired

13.90*240**55mm Slim and thin body,Contain the battery only 2 kg weight,Suitable for field and high altitude work under the harsh environment.