Real-World Fix: How We Overcame Unexpected Furnace Coking Downtime in Our Preheat Unit

Crude preheat furnaces are the silent workhorses of any refinery, critically impacting overall plant efficiency and profitability. Their optimal operation is paramount, yet they are constantly challenged by factors like coking, which can lead to sudden drops in heat transfer efficiency and, more critically, unexpected downtime. This article delves into a real-world scenario where a box-type crude preheat furnace faced severe internal coking, forcing an emergency shutdown. We’ll explore the diagnostic process, the accelerated decoking strategies employed, and crucial proactive measures to prevent such costly disruptions, ensuring stable and efficient refinery operations.

Understanding Box-Type Crude Preheat Furnaces: The Basics

Box-type crude preheat furnaces are critical components in refining processes, responsible for raising the temperature of crude oil before it enters the atmospheric or vacuum distillation columns. Their design is optimized for efficient radiant heat transfer. Crude oil flows through tubes arranged in the radiant and convection sections. In the radiant section, tubes are exposed directly to the flames from burners, absorbing heat primarily through radiation. In the convection section, tubes absorb heat from the hot flue gases through convection, maximizing overall thermal efficiency.

Design Principles and Heat Transfer Mechanisms

The design of box-type furnaces aims to provide uniform heating and minimize hot spots. Burners are strategically placed on the floor or walls to ensure even heat distribution. Heat transfer occurs via three primary mechanisms: radiation (from flames and refractory to the tubes), convection (from hot flue gases to tubes, especially in the convection section), and conduction (through the tube wall to the crude oil). Optimal furnace design balances these mechanisms to achieve the desired crude outlet temperature with minimal fuel consumption.

Common Operational Challenges and Failure Modes

Despite robust designs, furnaces face several operational challenges. Fouling, particularly coking, is a persistent issue. Coking reduces heat transfer efficiency, leading to higher fuel consumption, increased tube metal temperatures, and potential tube ruptures. Other challenges include flame impingement, leading to localized overheating; maldistribution of flow within parallel coils, causing some tubes to coke faster; and refractory degradation, affecting insulation and heat loss. Addressing these requires diligent monitoring and maintenance.

The Coking Conundrum: A Case Study in Unexpected Downtime

As detailed in the Pro-Tip, a sudden and unexpected drop in heat transfer efficiency within our box-type crude preheat furnace presented a significant challenge. This wasn't a gradual decline; it was a rapid deterioration that pointed to an acute problem. The most immediate symptom was the inability to maintain the crude oil outlet temperature at the desired set point, despite increasing the firing rate. This directly impacted the crude unit's throughput, signaling a major bottleneck.

Initial Symptoms, Diagnostic Process, and Immediate Actions

The operational team first noticed an abnormal increase in stack temperature, indicating more heat was leaving with the flue gases rather than being absorbed by the crude. Simultaneously, the tube skin temperatures in the radiant section, initially stable, began to show erratic spikes. A more detailed thermal imaging scan confirmed localized hot spots, especially on the top rows of the radiant tubes, strongly suggesting internal coking. The pressure drop across the coils also started to climb. Given the rapid decline in performance and the potential for tube damage, an emergency shutdown of the crude unit was initiated. This involved quickly bringing the unit offline, depressurizing, and cooling down to safely isolate the furnace. The immediate goal was to prevent tube rupture and prepare for the decoking operation to minimize lost production.

Root Cause Analysis: Why Severe Coking Occurs Unexpectedly

Investigating the root cause of unexpected severe coking is crucial to prevent recurrence. While routine coking is expected, sudden and extreme cases often point to specific triggers. In our incident, the detailed analysis pointed to a confluence of factors, highlighting the complexity of furnace operations.

Crude Quality Shifts, Operational Upsets, and Insufficient Monitoring

One primary suspect was a subtle, unannounced shift in crude quality. A batch of crude, ostensibly meeting specifications, likely contained a higher-than-usual concentration of asphaltenes or other heavy, unstable components that are prone to thermal decomposition at furnace temperatures. Even slight variations in crude composition can significantly alter coking propensity. Operational upsets, such as brief excursions into higher-than-design temperatures or reduced flow rates, can also create conditions conducive to accelerated coking, particularly if not immediately detected and corrected. Finally, our monitoring system, while robust for general operation, lacked the granularity to detect the early, subtle indicators of this specific type of rapid coking. For instance, localized high tube skin temperatures were not immediately flagged with the necessary urgency, allowing the situation to escalate quickly.

Accelerated Decoking Strategies: Restoring Performance Under Pressure

With an emergency shutdown in progress and significant production losses mounting, the decoking operation had to be executed with speed and precision. The strategy focused on a rapid and effective removal of coke to get the unit back online as quickly as possible.

Decoking Methods and Optimization for Emergency Response

For this specific incident, we opted for a highly optimized steam-air decoking procedure. This thermal method involves injecting a controlled mixture of steam and air into the radiant coils, where the oxygen in the air reacts with the coke at elevated temperatures, burning it off as carbon monoxide and carbon dioxide. To accelerate the process, the steam and air flows were carefully modulated, and the temperature profile was precisely controlled to maximize coke removal without damaging the tubes. Post-decoking, internal camera inspections were performed on critical tubes to verify coke removal completeness. While time-consuming, this step was deemed essential to ensure optimal performance upon restart and prevent immediate re-fouling from residual coke. Simultaneously, a mechanical pigging option was kept on standby, in case the thermal decoking proved insufficient for particularly stubborn deposits, though it was not ultimately required.

Proactive Measures: Preventing Future Furnace Coking Downtime

Learning from the unexpected downtime, our team implemented a series of proactive measures designed to prevent future severe coking incidents and enhance overall furnace reliability. This involved a multi-pronged approach combining advanced monitoring, operational best practices, and improved maintenance protocols.

Advanced Monitoring and Predictive Analytics for Furnace Coking

We upgraded our furnace monitoring system to incorporate real-time tube metal temperature (TMT) monitoring on a much finer grid, especially in critical radiant sections. This allows for immediate detection of localized hot spots, which are early indicators of coking. Furthermore, we integrated a predictive analytics model that uses historical data, crude quality variations, and real-time operational parameters to forecast coking rates. This model now provides early warnings, enabling us to anticipate when decoking might be necessary and schedule it during planned maintenance windows, avoiding emergency shutdowns.

Best Practices for Fuel Management and Burner Optimization

Optimized fuel management and burner tuning play a significant role in preventing coking. We implemented stricter control over fuel gas quality and pressure to ensure stable flame patterns. Regular burner inspections and adjustments are now standard practice to prevent flame impingement on tubes, which can lead to localized overheating and accelerated coke formation. We also introduced infrared thermography surveys during normal operation to identify and correct any burner-related issues causing uneven heating.

Routine Maintenance and Inspection Protocols

Our maintenance schedule for the furnace was revised to include more frequent, non-invasive inspections. This includes intelligent pigging during turnarounds to assess internal coke buildup and tube integrity. We also enhanced our thermal imaging surveys, conducting them more frequently and systematically to establish baseline temperature profiles and quickly identify deviations. Regular visual inspections of the radiant section for flame patterns, refractory condition, and any signs of tube bulging or discolouration are also now a priority.

Impact of Crude Oil Quality on Furnace Performance

Recognizing the significant impact of crude quality, we established a more rigorous incoming crude analysis protocol. This includes testing for asphaltene content, heavy metals, and other components known to contribute to coking. We also developed a robust crude blending strategy to mitigate the impact of coking-prone crudes when they cannot be avoided. This involves diluting problematic crude batches with lighter, less coking-prone feedstocks to reduce the overall risk to furnace operations.

Conclusion & Key Takeaways

The incident of unexpected furnace coking downtime served as a potent reminder of the delicate balance required for optimal refinery operations. While challenging, it provided invaluable lessons. The key takeaways emphasize the critical importance of a multi-faceted approach to furnace management: robust and intelligent monitoring systems for early detection, swift and decisive response protocols for emergency situations, and, most importantly, proactive maintenance and operational strategies to prevent future occurrences. By understanding the root causes, implementing advanced technologies, and fostering a culture of continuous improvement, refineries can significantly reduce the risk of costly furnace coking downtime, ensuring sustained efficiency and profitability.

Frequently Asked Questions

What is furnace coking?

Furnace coking refers to the formation of carbonaceous deposits, or coke, on the internal surfaces of furnace tubes. This occurs when hydrocarbons decompose at high temperatures, forming a hard, insulating layer. Coking reduces heat transfer efficiency, increases fuel consumption, and can lead to tube overheating and failure if not addressed.

How does coking affect furnace efficiency?

Coking acts as a thermal insulator, preventing efficient heat transfer from the flames to the crude oil flowing inside the tubes. This necessitates higher firing rates to achieve the desired outlet temperature, leading to increased fuel consumption and reduced thermal efficiency. It also increases the risk of tube rupture due to elevated metal temperatures.

What are common methods for decoking a furnace?

Common decoking methods include steam-air decoking (thermal decoking), where a mixture of steam and air is circulated through the tubes at high temperatures to burn off coke. Mechanical decoking, often involving specialized pigs or scrapers, and hydro-jet cleaning (high-pressure water jetting) are also used, especially for hard or localized deposits.

Can crude oil quality impact furnace coking?

Absolutely. Crude oil quality is a major factor. Crudes with higher concentrations of asphaltenes, heavy metals, or unstable components are more prone to coking. Even subtle shifts in crude composition can significantly impact the rate and severity of coke formation in furnace tubes.

How can unexpected furnace downtime due to coking be prevented?

Prevention involves a combination of strategies: advanced monitoring systems (e.g., real-time tube metal temperature monitoring, predictive analytics), strict control over crude oil quality and blending, optimized operational practices (e.g., stable flow rates, proper burner tuning), and robust routine maintenance protocols (e.g., intelligent pigging, thermal imaging surveys).