A US refinery identified a problem with its second stage desalter and their overhead accumulator vessels during a routine inspection.
Anickel-copper thermal spray coating had been applied in both of these vessels approximately 20 years ago. Initially, the coating performed as expected, however after some time, localised damage was evident on the bottom third of both vessels. Initially, various organic coatings were chosen for patch repairs to the bottom of the shell. These coatings needed to be applied repeatedly every 4-5 years. In 2014, the inspection revealed areas where both the historical nickel-copper thermal spray (TS) coating and organic patch repairs had worn away completely, leading to deep pitting and metal wastage beyond existing corrosion allowance.
Moreover, when weld repairs were attempted adjacent to the failing TS, a crack had formed on its heat affected zone. In 2017, the refinery’s engineers decided they needed a more permanent solution.
IGS inspected the vessels and proposed a High Velocity Thermal Spray (HVTS) cladding solution to stop corrosion for the expected life of the asset, without any further maintenance anticipated for at least the next 15+ years. HVTS technology utilises next generation alloy materials, which offer erosion-corrosion protection, even in HT/HP service up to 1371°C/2500°F. Both organics and weld overlay were evaluated as potential options by the refinery and, given the upgraded metallurgy and the time savings of HVTS compared with overlay, the refinery project manager welcomed this proposal and HVTS work was scheduled for the spring of 2019.
IGS started by clean blasting each unit, allowing the refinery’s QC to inspect and address any issues with the vessels’ substrate.
The Overhead Accumulator required no mechanical work, whereas the First Stage Desalter needed to be welded due to the deep pitting that occurred due to the poor quality of the previously applied coating.
TS COATING FAILURE
Typical Thermal Spray (TS) coatings are not suitable for internal protection of mission critical process equipment due to their permeability, weaker bond strength and propensity to cracking. TS systems cannot produce flat and tightly-packed particle sizes or nano-scale grain structures, leading to the coating’s failure due to corrosion and/ or permeation. In addition, TS materials’ cladding particles form surface oxides in flight. These oxides then form part of the applied TS structure and are permeable, creating a pathway for corrosive media to penetrate through the coating leading to subsequent substrate corrosion, and premature TS failure. Moreover, being applied at high thicknesses, TS structures are inherently highly stressed, and such highly stressed coatings have a high propensity to crack and subsequently fail in service.
HVTS application was completed on time and within schedule, even accounting for unexpected delays. The bottom third of the overhead accumulator, including the stem pipe with a vortex breaker and a flange, were protected with HVTS. With regards to the crude desalter, HVTS was applied from the five o’clock to the seven o’clock position of the vessel and the manway.
Final inspection of the work scope involved the generation of a cladding thickness record, with mapped electromagnetic stand-off gauge (MLO) readings on a defined reference grid across the area protected. This record will be used for future inspection and verification of the cladding’s integrity. HVTS cladding systems can be readily inspected visually for any signs of deterioration, and thickness measurements taken with a magnetic liftoff gauge. IGS has invested heavily in the development and optimisation of its High Velocity Thermal Spray (HVTS) process technology, materials technology, application procedures and field personnel capability. Decades of successful cladding applications, coupled with fast mobilisation and turnkey applications, are helping position IGS as a go-to service provider for oil, gas and petrochemical sites worldwide. ■