Discussion on the erosion of pipelines caused by steam condensate after pressure reduction via a control valve
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Dear colleagues: A similar situation has arisen in my workplace: 1. The elbows and tees in the condensate pipelines at the outlet of the heat exchanger often leak, and there is a noticeable thinning of the pipe walls. 2. When 19 bar of steam condensate is depressurized to 8 bar steam via a control valve, the elbows and splices of the control valve, as well as the valve itself, often become extremely thin, almost like sheet metal. 3. Leaks occurred at the elbows and tees of the condensate lines at 8 bar and 19 bar. My humble thoughts: 1. I suspect there is an issue with the steam; perhaps the quality of the water used to fill the boiler is problematic. What tests are generally conducted on the water used in boilers to check for corrosion? 2. Is it a design issue? Typically, leaks and thinning occur in areas where vapor-liquid two-phase flow takes place. The facility has only been in operation for three years – could such problems arise in the steam and condensate pipes? Thank you to all for your answers; let’s learn together and improve together!
When the 19 bar steam condensate is depressurized to 8 bar through a control valve, some of it flashes into steam (the amount of vaporized steam can be calculated using heat balance principles). A two-phase flow of steam condensate and steam thus forms in the pipeline. Pipe elbows and tees are most severely affected by the flow dynamics, resulting in higher local resistance; this leads to thinning of the pipe walls and leaks. It’s not a problem with the water quality ； Leakage issues also occurred at the elbows and tees of the 19 bar condensate pipeline, again due to the presence of steam in the condensate. It must be noted, however, that the reason for the thinning of the tube wall is not the presence of steam, but rather the erosive effect of water droplets carried in the steam on the tube wall (which constitutes mechanical erosion corrosion). Solution: 1. Control the medium flow rate ； 2. Or increase the pipe diameter ； 3. Increase the wall thickness of elbows and tees. 4. Improve the insulation of the steam condensate transfer pipelines (to reduce heat loss) ； 5. The 19 bar steam condensate, after being depressurized via a control valve, does not all turn into 8 bar steam; therefore, vapor-liquid separation should be carried out promptly before it is sent for separate use. The above opinions are for reference only.
This post was last edited by zpg on 2011-3-22 at 16:50. If, as you say, the control valves “frequently” end up with their surfaces worn down to a very thin layer of metal, it’s certain that in addition to the mechanical corrosion caused by water hammer, as mentioned by the person above, corrosion caused by CO2 and dissolved oxygen is also a significant factor. If the condensate isn’t properly treated, corrosion can be quite severe; it’s likely that the Fe ion content in your plant’s condensate is high as well. You should check this and see if similar corrosion occurs on the heating surfaces of your boilers. Corrosion doesn’t occur in isolation – it affects not only pipes but also equipment. Think more broadly and gather more information and evidence, such as comprehensive water quality analysis reports, which will help you make an accurate assessment.
Thank you to the two people above; I’d like to know the specific items included in a comprehensive water quality analysis, as it seems that another department is responsible for handling water quality matters