Abstract
Reliable circulation is one of the most critical requirements in high-pressure boiler operation. Even when circulation calculations satisfy conventional design criteria, improper distribution of downcomers and risers can create localized low-flow zones that lead to severe tube failures. This case study presents a detailed investigation of recurring bed evaporator tube failures in a 110 kg/cm² AFBC (Atmospheric Fluidized Bed Combustion) boiler.
Through circulation analysis and field investigation, the root cause was identified as weak circulation caused by improper distribution of downcomers and risers. Design modifications successfully restored circulation performance and eliminated the recurring failures.
Introduction
Modern high-pressure boilers are designed using advanced thermal and hydraulic modelling software capable of calculating circulation requirements with remarkable accuracy. However, while software can determine the required flow areas for downcomers and risers, successful circulation design depends equally on proper distribution throughout the boiler circuits.
In natural circulation boilers, water movement is driven by density differences between the downcomer and riser circuits. Any imbalance in flow distribution can significantly affect heat transfer performance and tube metal temperatures. If certain circuits receive insufficient flow, steam blanketing may occur, causing localized overheating and eventually tube failure.
This case study highlights a practical example where a boiler experienced repeated evaporator tube failures despite apparently acceptable circulation calculations. The investigation demonstrates why circulation distribution is often more important than circulation quantity alone.
Plant Background
The boiler under investigation was a high-pressure AFBC boiler operating at 110 kg/cm².
Within the first six months of operation, the plant reported repeated failures in bed evaporator tubes. These failures were causing:
- Frequent shutdowns
- Increased maintenance costs
- Reduced boiler availability
- Production interruptions
Initial inspections suggested that the failures were occurring predominantly in specific bed evaporator circuits rather than across the entire boiler, indicating a localized operating problem rather than a general materials issue.
Failure Investigation
A detailed metallurgical examination of the failed tubes was conducted.
The investigation revealed signs of:
- Heavy internal deposits
- Localized tube wall thinning
- Metal degradation beneath deposits
- Concentration of corrosive compounds
The failure mechanism was identified as under-deposit caustic gouging, a common but serious problem in high-pressure boilers.
Caustic gouging occurs when concentrated alkaline compounds accumulate beneath deposits on the tube surface. Under high heat flux conditions, water evaporates rapidly while dissolved solids remain behind. This creates highly concentrated caustic environments that aggressively attack the tube metal.
The damaged tubes exhibited characteristic signs of this phenomenon, confirming that localized overheating and poor circulation were contributing factors.
Understanding the Role of Circulation
In natural circulation boilers, adequate water flow must continuously remove heat from evaporator surfaces.
When circulation becomes weak:
- Steam bubbles remain attached to tube surfaces.
- Heat transfer efficiency decreases.
- Tube metal temperature increases.
- Water chemistry concentrates beneath deposits.
- Caustic attack accelerates.
- Tube failure becomes inevitable.
Sloped tubes are particularly vulnerable because steam separation can occur more easily when flow velocities fall below acceptable limits.
Although the boiler’s overall circulation ratio appeared satisfactory, the failures suggested that individual circuits were not receiving adequate water flow.
Design Review Findings
A comprehensive review of the circulation system was carried out. Interestingly, the total downcomer and riser cross-sectional areas were found to be adequate according to design calculations.
However, the investigation revealed significant issues in flow distribution.
Issue 1: Common Downcomer Arrangement
The boiler design utilized a common downcomer arrangement serving both front and rear water wall circuits.
While this configuration simplified the piping layout, it failed to account for differences in heat absorption between various evaporative circuits.
Each evaporator circuit experiences unique thermal loading conditions and therefore requires circulation tailored to its specific heat duty. Sharing a common downcomer reduced the ability of the system to distribute flow according to actual demand.
Issue 2: Uneven Heat Duty Across Compartments
The boiler was divided into five compartments.
The rear compartments contained bed superheater sections, which significantly altered heat absorption characteristics.
Specifically:
- Half of Compartment 3
- Entire Compartment 4
- Entire Compartment 5
were equipped with bed superheaters.
As a result, these compartments had reduced evaporator surface area and different heat loads compared with the front compartments.
Despite this variation, downcomers and risers were distributed equally across all compartments.
This equal distribution did not match the actual thermal requirements of the circuits, leading to circulation imbalances.
Issue 3: Inadequate Blowdown Arrangement
The bottom header drain line was designed with a diameter of only 1 inch.
For high-pressure service, a larger drain arrangement is generally preferred to ensure effective sludge and deposit removal.
The investigation recommended increasing the drain size to 2 inches to improve blowdown efficiency and reduce deposit accumulation within the system.
Circulation Analysis
To verify the hydraulic performance of the boiler, a detailed circulation analysis was performed.
The overall circulation number was calculated as approximately 15. From a conventional design perspective, this value appeared acceptable. However, overall circulation ratio alone does not guarantee proper flow distribution throughout individual circuits.
The investigation therefore focused on local circuit velocities.
Calculated Bed Coil Velocities
Compartment 2 & 3 bed coil velocities were calculated to be of 1.03 & 0.9 as compared with other compartment velocities of 1.34, 1.60, 2.03 in compartment 01, 04 & 05 respectively. Compartment 4 & 5 velocities were more due to less heat load and higher downcomer sizing for rear side of boiler.
The results clearly showed that Compartments 2 and 3 experienced substantially lower circulation velocities compared with the remaining compartments.
Root Cause Identification
The circulation study revealed a critical imbalance.
Because the rear compartments had reduced evaporative heat duty but received the same downcomer allocation, they attracted a disproportionate share of circulation flow.
Consequently:
- Compartments 4 and 5 received excess circulation.
- Compartments 2 and 3 received insufficient circulation.
- Steam generation exceeded local water replenishment.
- Steam blanketing developed in vulnerable regions.
- Tube metal temperatures increased.
- Caustic concentration occurred beneath deposits.
- Tube failures followed.
This finding explained why failures were concentrated in specific circuits rather than distributed throughout the boiler.
The root cause was therefore identified as improper circulation distribution rather than insufficient total circulation capacity.
Engineering Solution
After identifying the hydraulic imbalance, a design modification was proposed.
The primary recommendation was to install a separate downcomer arrangement serving Compartment 3 and associated circuits.
The objectives were:
- Improve water availability in low-flow regions
- Balance circulation between compartments
- Eliminate steam blanketing
- Restore acceptable evaporator velocities
- Reduce thermal stress on tubes
Additional improvements included optimization of riser distribution and enhancement of blowdown capability.
Results After Modification
Following implementation of the revised circulation arrangement, the boiler’s hydraulic performance improved significantly.
Key improvements included:
Improved Flow Distribution – The additional downcomer successfully redirected circulation toward previously under-supplied circuits.
Higher Evaporator Velocities – Flow velocities within the affected bed evaporator circuits increased to acceptable levels.
Elimination of Steam Blanketing – Improved circulation prevented localized steam accumulation on tube surfaces.
Reduced Risk of Caustic Concentration – Better water movement minimized deposit-related concentration mechanisms responsible for caustic gouging.
Improved Reliability – The recurring evaporator tube failures were eliminated following the circulation modification.
Lessons Learned
This case study provides several important lessons for boiler designers and operators.
1. Overall Circulation Ratio Is Not Enough
A satisfactory circulation number does not guarantee proper flow distribution throughout all circuits.
2. Heat Duty Must Drive Distribution Design
Downcomer and riser allocation should be based on actual thermal loading rather than geometric symmetry.
3. Separate Circuits Require Separate Evaluation
Each evaporative circuit should be evaluated individually for velocity, circulation ratio, and steam generation characteristics.
4. Early Warning Signs Should Not Be Ignored
Repeated tube failures within a short operating period often indicate a fundamental design issue rather than an operational problem.
5. Detailed Hydraulic Analysis Saves Cost
Advanced circulation analysis can identify hidden flow imbalances before they result in catastrophic failures.
Conclusion
The investigation of recurring bed evaporator tube failures in a 110 kg/cm² AFBC boiler demonstrated the critical importance of proper circulation distribution in high-pressure boiler design.
Although the overall circulation number was acceptable, significant hydraulic imbalances existed between individual compartments. Unequal flow distribution resulted in weak circulation within specific evaporator circuits, leading to steam blanketing, under-deposit caustic gouging, and premature tube failure.
By introducing a dedicated downcomer arrangement and optimizing circulation distribution, flow velocities were restored to acceptable levels, steam separation was eliminated, and boiler reliability improved substantially.
This case highlights a fundamental engineering principle: successful boiler circulation design depends not only on achieving sufficient circulation quantity but also on ensuring proper circulation quality and distribution throughout every evaporative circuit.
Author:
R.Nagarajaprasath
Director
Boilertech Energy Services, Erode, Tamil Nadu
