Waterworks City Leipzig
Assessment of the digestion tower of a biogas plant
The subject of the investigation was the digestion tower of the biogas plant at the Leipzig-Rosental sewage treatment plant. There are corroded pipes inside the digestion tower, which we analyzed and evaluated extensively.
Task:
After emptying the digester, holes were found next to the weld seam of a T-joint of the extraction pipe. Damage was to be documented in order to draw up a renovation concept.
Basics:
The damage was recorded and documented on site based on a drawing and the information provided by the employees. According to this, the pipe is used to remove the sewage sludge. The sewage sludge is pressed from below into the riser pipe and from there into the horizontal pipe by the load of the sewage sludge. The consistency at the extraction point is compared to thin apple pulp. The sewage sludge contains sand.
Analyses carried out and results
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Inspection of the documents provided
A nominal diameter was available for the analysed pipeline. No details of the wall thickness could be found.
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Visual inspection
The visual inspection is documented on images. The damage is limited to the area of the horizontal extraction pipe. A sample was taken for a possible later inspection.
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Wall thickness measurements
Wall thickness measurements were carried out on various areas of the damaged pipe using the wall thickness gauge type DM4 and the probe type DA 412 from Krautkrämer GmbH&Co, Hürth. The wall thickness of a neighbouring pipe was also measured for comparison.
Evaluation and summary of the results
A damaged area was documented around the weld seam connection of a vertical pipe to a horizontal pipe with a nominal diameter of DN=300 mm. The wall thickness of the vertical pipe was measured at approx. 6 mm. The measured wall thickness of a neighbouring vertical pipe was 2.2 mm. The wall thickness of the horizontal pipe was approx. 0 to 2.6 mm in the connection area with the vertical pipe. In the area of the outer wall, wall thicknesses of 4.5 to 4.7 mm were measured. The investigation revealed that the damage was caused by corrosion. The walls of the vertical pipe protruding into the pipe cross-section of the horizontal pipe favoured turbulence. This leads to increased abrasion in the area of the pipe walls.
Notes
During repairs, edges that obstruct the flow should be avoided as far as possible.
The suitability for welding and the material strength can be analysed using the sample taken. A spectral analysis to determine the chemical composition of the steel and hardness measurements according to Vickers, including the standardised conversion of hardness to strength, can be used to draw conclusions about the material and its suitability for repair welding. Based on these results, a repair concept can be created by defining the dimensions and the material from which the replacement piece is to be manufactured as well as the welding filler material to be used, the shielding gas and the welding process.
