Crossbridge Energy's Fredericia Refinery in Denmark, one of the most energy-efficient refineries in the world, experienced integrity concerns in its vacuum flash tower in the visbreaker unit. Excessive fouling had led to accelerated sulphidic corrosion in a section of the vacuum flash tower, which needed to be mitigated.
The visbreaker unit converts long residue, which is the bottom product from atmospheric distillation into lighter and more valuable products such as gasoil, kerosene and naphtha. The Fredericia refinery produces fuels such as heavy marine fuels, light and heavy gasoil, diesel, gasoline and LPG, and the facility is integrated into the local district heating network producing hot water to the grid. It is the ambition of the refinery to become CO2 neutral by 2035.
Challenge
The vacuum flash tower was experiencing several issues:
- Fouling of stacked PAL/RACHID rings leading to irregular flow patterns
- High-temperature sulphidation corrosion as a result of turbulent flow
- Internal corrosion and erosion damage in the original SS 410 material
- Pitting in the top section despite being above the dewpoint
- Need for structural integrity improvements through localized welding
Technical Assessment
The visbreaker unit, also known as a thermal gasoil unit, involves:
- Long residue cracking and distillation
- Gasoil distillation and drying
- Kerosine distillation
- Vacuum distillation
- Heat recovery in the form of hot water to the domestic grid, as opposed to using air coolers.
Well known corrosion mechanisms in this type of unit are:
- Sulphidation, sulphidic corrosion
- Naphthenic acid corrosion
- Aqueous acidic overhead corrosion
- Sour water corrosion
In this case, sulphidation was the problem. This degradation mode usually produced an iron sulphide scale on the corroding surface limiting the corrosion rate somewhat. Low alloy chromium and chromium nickel steels produce more tenacious, protective sulphide scales on the surface that result in enhanced resistance to sulphidic corrosion. At high flow velocity this scale can be broken down continuously, leading to increased corrosion. In this particular column a AISI 400 grade steel was believed to offer sufficient corrosion protection at the operating temperatures between 180 and 350°C. However, the increased fouling in a section of the column caused preferential flow along the column wall, leading to breakdown of the protective sulphide scale and consequential increased corrosion.
In addition, increased pitting corrosion was found in an area immediately above the cladded section. This corrosion is believed to be from acidic sour water corrosion.
IGS HVTS® (High Velocity Thermal Spray)
HVTS was technically approved with the following specifications:
- Internal alloy upgrade covering approximately 30 m² of surface area
- Complete cladding scope was then extended to 55 m² due to additional corrosion identified at the start of the project
- Implementation during a planned outage window starting September 2nd, 2024
Project Execution
The project was carefully planned with several key considerations:
- Timeline: 8 shifts total, operating 2 shifts per day over 4 days
- Resource allocation: IGS increased resources to meet the extended scope needs while not increasing critical path time
- Coordination with other maintenance activities, particularly flare repair work
- Integration with regular 6-month inspection and cleaning cycle
Results
The implementation proved successful with multiple positive outcomes:
- Zero safety incidents during execution
- Reduced operational expenses (OPEX)
- Lower life cycle costs (LCC)
- Improved equipment reliability
- Integration with existing contractor qualification system (aligned with Shell standards)
Testimonial
"IGS's HVTS® solution proved to be an ideal solution for our vacuum flash tower. The technical execution was impeccable, completing the work well within our outage window with zero safety incidents. The importance of executing on time during a very busy turnaround is key to success. IGS kept their promises and finished on time despite a scope increase going from 30m2 to 55m2.”
- Lars Jung, Senior Materials & Corrosion Engineer at Crossbridge Energy Fredericia Refinery.
This case study demonstrates the successful implementation of modern corrosion protection technology in a critical refinery operation, balancing technical requirements with operational constraints while maintaining high safety standards and efficiency goals.
