Reasons for corrosion at the elbow behind the steam trap
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Corrosion and perforation occurred at the second elbow after the steam trap in the steam drain pipe, as well as in the vertical section immediately adjacent to it; there was no significant corrosion at the elbows located before or after that elbow. Has anyone encountered a similar problem? What is the reason?
I’ve never encountered it! Could it be that poor welding quality is causing local problems?
It belongs to multiphase flow corrosion: In condensate pipelines, due to the low concentration of other ions, the main types of electrochemical corrosion that occur are CO2 corrosion and oxygen corrosion. The flow of fluid inside the pipes at the bends accelerates these electrochemical reactions. In addition to the erosion of the pipe walls by the gas-liquid mixture, the turbulence caused by the change in flow pattern at the bends removes the protective layer formed as a result of oxygen corrosion, thereby increasing the rate of corrosion of the metal surface
The leakage occurs in the straight section below the elbow (the thinned area extends 200 mm from the weld); there is also thinning beneath the elbow. What is puzzling is that the elbows upstream and downstream of the leakage site are intact.
This is also the first reason that comes to mind, but there is no sign of corrosion at either the upstream or downstream end.
Is it a quality issue with the elbow itself?
We are in the same situation: first, it’s the impact of water when the drain valve releases water; second, sometimes when the drainage system is operated in bypass mode, the high pressure of water vapor exerts force on this section of piping.
What’s unclear is that the impact should affect each elbow; only the third one shows a reduction, while the first, second, and fourth ones show almost no thinning (from 5.6 mm to 5.1 mm)
Connecting the elbow 6 to the atmosphere or linking it to the air in the condensate collection tank might help reduce corrosion. In the situation I encountered, there were water hammer vibrations in this pipe; the intermittent production process resolved the issue by opening and sealing the pipe.
Drainage on the shell side of the heat exchanger, for continuous operation.
That’s just thinning due to erosion; it’s not corrosion! When water at high temperature is depressurized, some of it vaporizes, which increases the scouring force!