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Panshi Case Database Case Sharing | Distillation Unit Top-End Corrosion Loop – Online Corrosion Monitoring, Risk Early Warning, and Hidden Danger Identification
2025-08-26
Basic Information
Corrosion Investigation Status
1. On-site Investigation Status
In April 2022, the corrosion rate of the inductive probe on the top-of‑tower atmospheric leg pipeline at a certain plant exceeded the standard, reaching as high as 0.88 mm/year. Pulse eddy current inspection with thermal insulation removed revealed a suspected thinning signal at the bend located downstream of the shell-side outlet header of the top-of‑tower condenser. The pipeline has specifications of DN200 × 6.5 mm; the thinned area is situated slightly to the left of the bend’s centerline. The minimum wall thickness within the inspected thinning zone was 5.1 mm, and the eddy current inspection indicated a thinning rate of 20.2%. In accordance with the Guiding Opinions on the Implementation of Pulse Eddy Current Inspection for Refining Units, the workshop was advised to remove the thermal insulation for re‑inspection.
Eddy Current Inspection Map of the First Bend After the Shell-Side Outlet Header
After identifying a suspected location during the thermal insulation inspection, a detailed inspection of the thinned bend was carried out after removing the insulation. The inspection proceeded from right to left. Upon re‑inspection, it was confirmed that a thinning signal still existed on the left side of the bend’s centerline—specifically on the north side of the bend, near the support pipe. The minimum wall thickness in the thinned area, as measured by bare‑pipe inspection, was 5.31 mm, while eddy current testing revealed a thinning rate of 21%. The thinned area measures approximately 20 × 40 mm.
Eddy Current Inspection Map of the Bare Pipe at the First Bend After the Outlet Manifold of the Shell Side in E2150AB
Two eddy current inspections confirmed thinning at the same location on the bend. Ultrasonic verification was conducted, and the minimum ultrasonic thickness measurement in the thinned area was 5.11 mm, resulting in a relative wall thickness reduction of 21.4% compared to the design wall thickness.
Process Flow Status
After the overhead oil and gas from the vacuum tower are cooled by the top-of-vacuum condenser, the liquid phase flows to the top-of-vacuum oil–water separator.
Corrosion Analysis
1. Corrosion Mechanism Identification
The primary form of corrosion in the low-temperature corrosion loop at the top of the vacuum tower is liquid-phase corrosion caused by the condensation of acidic water (HCl) within the condensation system, with ammonium salt deposition potentially occurring as well. The operating temperature at the tower top is 70°C, and condensation may occur in localized areas at the tower top as well as in certain pipeline sections—such as the top reflux distributor, the points where the tower top directly interfaces with the top reflux inlet piping, the inlet and outlet of the top draw condenser, and the inlet and outlet pipelines of the vacuum tower oil–water separation tank. Additionally, connections like relief valves, vents, and the tower top vapor lines can also lead to condensation-induced erosion.
2. Corrosion Circuit Analysis
The plant employs a corrosion prevention and control approach that combines process-based corrosion monitoring with online inductance probe monitoring for the top-of-column corrosion loop.
In the first half of 2022, the monthly corrosion rate at the outlet of the shell side of the top condenser in this circuit showed significant fluctuations. The highest monthly corrosion rate was recorded in April 2022, indicating that the top oil and gas from the atmospheric‑vacuum distillation unit are highly corrosive. Based on the early warning provided by the inductance monitoring system, a plan was formulated to conduct a pulse eddy current inspection for potential hazards in the corrosion loop at the top of the vacuum tower, with the primary focus on the pipeline running from the shell-side outlet of the top condenser to the oil‑water separation tank at the top of the vacuum tower.
On-site photo of the probe connecting the outlet of the topping condenser to the inlet of the oil–water separation tank at the top of the pressure reduction tower.
Trend Chart of Inductive Probe Corrosion Rates at the Shell-Side Outlet Header of the Top-Reduction Condenser – First Half of 2022
Conclusions and Recommendations
1. Conclusion
By monitoring changes in the corrosion trend of online corrosion monitoring inductive probes, potential corrosion risks within the equipment can be identified in a timely manner. Using eddy current scanning for inspection and verification allows for the prompt detection of hidden corrosion hazards, thereby preventing corrosion-related leakage incidents.
2. Recommendation
(1) During operation, this potential hazard area shall be designated as a fixed monitoring point and included in the key pipeline monitoring ledger. It shall be inspected every six months, and extended inspections and investigations shall be conducted on downstream sections of this loop pipeline that are prone to liquid accumulation and erosion, so as to promptly identify other potential hazards and prevent corrosion and leakage.
(2) Pay close attention to changes in crude oil properties, properly manage electrostatic desalination operations, ensure qualified desalination rates and desalination efficiency, and reduce the content of corrosive media in the overhead system at the source to mitigate corrosion in the overhead system.
(3) Monitor the corrosion probe data at the top of the tower on a daily basis to track trends in corrosion loss. Based on the probe data, promptly adjust process anti-corrosion parameters such as the injection dosage and injection volume at the tower top to reduce the corrosion rate in the tower top system. If the corrosion rate exceeds the specified threshold over a certain period, promptly conduct a thorough investigation to identify potential hazards.
Shenyang Zhongke Weier Anti-Corrosion Technology Center
2015.08.26
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