technical problem

The high temperature corrosion and abrasion of the “water wall”, “superheater tube”, “reheater tube” and “economy tube” of power station boilers are the main causes of pipeline leakage and are also common technical problems. The safe operation of the company poses a great threat and often leads to accidents. The power plant is referred to as the “four tubes” of the power plant boiler.

Causes

“Safety Technology of Power Station Boiler, Pressure Vessel and Pressure Piping”

There are two main types of blasting for small pipes outside the furnace: weld blasting and erosion and corrosion thinning blasting. Lack of quality control in all aspects of design, construction, operation and maintenance is the main cause of blasting accidents of small pipes outside the furnace.

1. Design reasons

According to design conventions, boiler plants and design institutes do not provide construction layout drawings for pipelines with DN≤80 mm, and construction units often do not carry out secondary design. The installation location of pipelines can only be determined according to the actual conditions on site. According to this construction, two defects may occur: one is that the pipe support is unreasonable, or the straight pipe section is too long, which causes the expansion of the pipe when heated, causing stress cracks in the weld; the other is that there are too many elbows and the pipe is too long. The flow resistance is large, the temperature drop of the medium in the pipe is too large, and the confluence of the medium at different temperatures is likely to cause thermal fatigue cracks in the high-temperature main pipe or equipment. In addition, when designing drainage, sewage and other pipelines, the pipeline material behind the valve should be considered for cooling and pressure reduction, resulting in inconsistent materials for the front and rear pipe sections of the valve, which cannot meet the needs of hot start and sharp drop in main steam temperature. Claim.

2. Construction reasons

(1) Poor pipeline installation process

The joints of the pipes are misaligned, and the weld is blasted after scouring and thinning; the welding of the stainless steel sampling tube of the superheater generally adopts flame welding butt joint method, resulting in poor weld quality.

(2) Poor quality of pipe processing

After the pipe is cold-bent, the bending radius is too small, the ellipticity and the wave are too large, resulting in the reduction of the effective diameter, and the elbow is easy to blast due to erosion and thinning.

(3) Lax quality inspection requirements

The DL5007-92 welding section of the construction specification clearly defines the drainage, drainage, sewage, and sampling pipes below *76 mm within the boiler water pressure range as Category III joints, that is, the construction welds behind the primary or secondary doors are only for visual inspection. No flaw detection is performed; the drain pipe connected to the main steam pipe is mistaken for starting draining, and it is classified as a type II welded joint, and the inspection is controlled at only 5% of the inspection volume, resulting in an inspection rate much lower than 50%. The quality of the seam cannot guarantee the use of the pipeline under high temperature and high pressure.

3. Reasons for operation and maintenance

(1) Lax supervision and management

Operation and maintenance units have insufficient understanding of the importance of safe operation of small pipes outside the furnace, and have not included small pipes outside the furnace into the scope of supervision and inspection, resulting in serious pipeline corrosion, erosion thinning and valve leakage for a long time.

(2) The quality of maintenance is not high

The maintenance unit has insufficient power, and the welding and metal inspection level is not high. During major and minor repairs, only the boiler body and pressure vessel are emphasized, and the small pipes outside the furnace are ignored. The quality of the maintenance is difficult to guarantee.

(3) Overdue service is common

According to the “Inspection Regulations for Power Industry Boiler and Pressure Vessels” (DL647-1998), the small-diameter pipes, elbows, tees and valves connected to the superheater outlet header, steam header, and main steam pipe, run for 105 hours All of the above should be replaced.