New Tubes, Unexpected Findings: Ovality Discovered in Crude Heater Convection Zone

Last month Cokebusters was called in to perform mechanical cleaning and inspection services on a crude heater in Eastern Europe that had undergone a full tube replacement just two years ago. While the primary goal was to establish a reliable baseline for future inspections, the results revealed something unexpected: measurable ovality in several tubes within the convection zone. Though not always indicative of immediate concern, these findings offered valuable insights into tube geometry, likely stemming from manufacturing issues and early operational stress – underscoring the importance of post-installation integrity verification, even in new coils.

Identifying Ovality in the Convection Zone

During the inspection, ovality – a deviation from a tube’s perfect circular shape – was detected at several points within the Convection Zone. The most severe case showed an ovality of 14.29% above the nominal outside diameter (OD).

Ovality is an important structural consideration in fired heaters, as it can affect stress distribution, integrity under pressure, and long-term performance. The cross-sectional view of the most affected tube below, clearly shows the extent of the distortion.

This cross-sectional view of the most severe ovality detected within the Convection Zone of the Crude Heater highlights a deviation from true circular geometry, with ovality exceeding 14% in this tube. By combining internal diameter (ID), wall thickness (WT), and outer diameter (OD) measurements, Cokebusters is able to identify and quantify these subtle but important geometric distortions with high accuracy.

Using ultrasonic transducers and 360° scanning, our Smart Pig technology provides detailed insights into tube integrity – far beyond what conventional inspection techniques can offer. The result? Early detection of potential stress points and manufacturing anomalies, allowing asset owners to make informed decisions before performance or safety is compromised.

📊 Ovality = (Max OD – Min OD) / Average OD × 100%
This percentage highlights how far the tube geometry deviates from perfectly round – a key input for assessing tube performance and risk.

Further visualisation through section graphs and a 3D C-scan reinforced the findings and provided a comprehensive picture of geometric irregularities in the affected sections.

Root Cause Analysis: Why Did the Ovality Occur?

According to the client’s data, the Convection Box was fully replaced in 2023 and operational history showed no abnormal thermal exposure. This suggests the observed ovality likely stems from the tube manufacturing process, with further minor deformation possibly caused by thermal and mechanical stresses during normal refinery operation.

In such applications, ovality tolerance is an important design consideration. Permissible deviation depends on several variables including:

• Tube diameter and wall thickness
• Material properties
• Operating pressure and temperature
• Presence of other flaws or localised stresses

Integrity Assessment and Remaining Life Evaluation

Following the geometric inspection, the client requested a full integrity assessment of the Crude Heater coils. Cokebusters engineers used inspection data from June 2025 alongside design information, data sheets and operating parameters supplied by the client.

All analysis was conducted in accordance with API 579-1 / ASME FFS-1 (2016), specifically Parts 4, 5, 8, and 10. The assessment included:

• General and localised metal loss
• Potential creep damage
• Diametric growth (based on ovality and expansion)
• Remaining Life Calculations using both MAWP and Tmin methods

✅ Good news: No tubes were found to be at or below the minimum allowable wall thickness. All components were deemed fit for service and exceeded the required 2-year inspection interval.

Creep Assessment: Planning for the Unexpected

Although recorded operating temperatures remained below the creep threshold (~427°C) – peaking at just 392.2°C – Cokebusters performed a Level 1 Creep Assessment as a precaution. This included:

• Simulating a theoretical upset period (e.g. steam decoking)
• Assuming potential operation at the design maximum tube metal temperature (TMT) of 649°C

Even under these conservative assumptions, the combined creep damage fell well within acceptable limits.

Outcome: A Confident Baseline for Long-Term Integrity

This inspection and assessment exercise provided the client with:

• A solid baseline for future inspections
• Assurance that the newly installed tubes are performing as expected
• Detailed data to support informed planning for future shutdowns and budgeting
• Confirmation that no immediate action was required on any component

This case also highlighted the value of early post-installation inspections, helping operators identify fabrication-related anomalies before they become operational problems.

Cokebusters continues to support our clients across the energy sector with industry-leading mechanical cleaning and intelligent inspection solutions – delivering high-resolution data and actionable integrity insight for critical fired heater assets.

If you’d like to learn more about our patented Combined Service or Fitness-For-Service capabilities, get in touch with our team today.