May-2020

Coke drum repair restores capacity (TIA)

The mechanical integrity of processing units is key for oil refineries to deliver smooth operations and maximum productivity. When structural damage was affecting the performance of a delayed coker unit in one of India’s largest refineries, Sulzer’s expertise proved fundamental to the repair of the equipment to full capacity.

Andrew Petticrew
Sulzer Limited

Article Summary

Delayed coker units and their key components, coke drums, are exposed to harsh environmental conditions that take a considerable toll on these pieces of equipment. A typical coker cycle is characterised by high variations in temperature and mechanical load, as the coke drum is heated to separate coke from lighter oils and subsequently cooled to quench the solid masses of coke obtained.

Cyclic heating, cooling, and loading expose the delayed coker units to recurring, short-term increases in mechanical hoop and axial stress. As a result, common deformations in these vessels include radial bulges and corrugations, as well as cracks on the unit’s shell, nozzle, and other areas.

When a major refinery and petrochemical complex in India noticed extensive bulging and cracking on one of its delayed coker units, it sought to restore its integrity. In this way, it could extend the equipment service life and minimise the risk of downtime while protecting the plant from costly equipment replacements and lengthy periods of reduced productivity. To repair the unit, the refinery turned to its preferred mass transfer specialist, Sulzer, which is also a provider of specialised field services for separation and mixing technologies.

Weld overlay solution
The operators of the oil processing plant had already installed a number of Sulzer’s components, such as column trays, as part of previous projects. Satisfied by the capabilities of these internals, the refinery asked for the support of Sulzer Tower Field Service (TFS) group. 

Expert teams started by conducting extensive onsite inspections and non-destructive testing. These allowed them to get a comprehensive understanding and mapping of the damage in the unit. The damaged surface, which extended for approximately 160 m2, featured bulges, cracks, as well as traces of erosion and corrosion.

Based on Sulzer’s findings, the two businesses agreed that weld overlay would be the fastest, most economical and long-lasting solution to extend the service life of the delayed coker unit. This repair technique consists of depositing a homogeneous layer of metal to fix losses of material on vessels. The weld overlay process would also be coupled with pre-welding activities, such as grit blasting and preheating, to maximise the effectiveness of the end result. 

The in-situ investigations were also crucial to closely simulate the physical entity and define a suitable course of action. In particular, by using finite element analysis (FEA), it was possible to get a computer-aided insight into how a particular weld overlay process would affect the thermal and mechanical behaviour of the structure when exposed to coker cycles.

These findings would then allow Sulzer’s engineers to fine-tune the process parameters accordingly. As a result of these simulations, the engineering teams decided to utilise a temper-bead process in order to apply a 9 mm double layer of an upgraded metallurgy of Inconel 625 alloy. The material would provide the shell with high corrosion and oxidation resistance, even in the harsh operating conditions of the coker cycles. 

Automated operations
As it was necessary to prepare the damaged area by preheating it at 150°C before the actual welding process, it was impossible to conduct manual weld overlay operations.

Therefore, Sulzer’s advanced automated technology, CladFuse, was selected to complete the repair. This uses a programmable logic controller (PLC) to define the movement of a carriage, which travels along a laser- levelled track system fixed to the wall that needs repairing. The PLC also controls a robotic index arm, located on the carriage, that moves the welding torch and the oscillator to create weld beads.

To cover the large worn area in a short amount of time, allowing the plant to reduce downtime and restore operations as soon as possible, Sulzer leveraged its global capabilities. The company was able to deploy teams from five different countries in order to repair various damaged areas simultaneously. 

Effective, fast repair
The repair of the delayed coker unit was completed in 25 days, during a planned shutdown of the plant. In this way, the refinery did not incur any unplanned downtime. As an additional benefit, the weld overlay has also reduced the potential impact of future thermal stresses and mechanical loads.

The delayed coker unit is now more robust and can run at peak performance again, processing 3.3 million t/y of crude oil.

Andrew Petticrew, Head of Global Sales for Automated Weld Overlay at Sulzer, concludes: “The feedback received from the customer was extremely positive. While delayed coker units and coke drums are regularly damaged by harsh operating conditions, advanced solutions, such as automated weld overlay, can extend service life while also improving their resistance to chemical, thermal and mechanical stress.”

This short case study originally appeared in PTQ's Technology In Action feature - Q2 2020 issue.

For more information: dorota.zoldosova@sulzer.com