Advanced Decoking & Cleanliness Verification (ADCV)
By: Tim Haugen, Quest Integrity
As seen in Inspectioneering Journal. Download the PDF version.
Mechanical cleaning otherwise known as mechanical decoking or descaling of fired heaters helps ensure the optimal performance of many heaters found in the refining and petrochemical industry. Internal fouling can reduce production throughput by restricting the flow of product through a coil. It can also prevent the proper transfer of heat to the feedstock, which can lead to elevated tube temperatures and eventual tube failures. As such, it is imperative that all fouling is removed to achieve target run times and throughput.
The manner in which mechanical cleaning is carried out has changed very little since its inception in the early 1990s. Hard-bodied tools, typically referred to as a pig, with metal cleaning appendages or studs are placed into a coil flooded with water (typical medium) and pushed from one end of the coil to the other via a pump (see Figure 1). The studded cleaning pig travels through the coil scrapping the fouling off the walls. Additional cleaning is achieved by employing a method of an incremental increase in cleaning pig sizes based on the coil’s nominal internal diameter and the estimated thickness of the fouling.
The cleaning process continues until fouling is no longer removed from the coil, as evident by the color of the water and lack of particulates at the coil output. As further evidence that a coil is clean, some companies may also run an oversized light-colored foam pig through the coil assuming any leftover fouling would discolor and potentially damage the foam pig.
Comparing the pre-cleaning coil pressure and flow rate to the post cleaning pressure and flow rate is another common method for verifying cleanliness. A decrease in pressure and increase in flow after the final cleaning runs indicates a reduction in fouling, although whether the coil is completely clean is unknown.
The Mechanical Decoking Challenge
While the mechanical cleaning process can be quite effective, the methods to confirm the overall cleanliness of a coil do not provide adequate assurances that all of the fouling has been removed. In fact, smart pigging inspections have discovered varying amounts of leftover fouling that have followed decoking activities in a vast majority of heaters since 2001.* In some instances, owner/operators noted elevated tube temperatures following a startup, leading to either immediate and unexpected shutdowns for additional cleaning or reduced run times when additional cleaning was not possible. In these instances, leftover fouling resulted in lost production and significant revenue losses.
A New Approach to an Old Problem
Quest Integrity created a higher level of verification called Advanced Decoking & Cleanliness Verification (ADCV) in response to these concerns. ADCV empirically detects and measures internal fouling eliminating any uncertainties in coil cleanliness. This is accomplished using specialized ultrasonic based technology, software and proprietary cleaning methods.
This service also includes additional benefits for fired heater owner/operators. Performing a fouling verification tool run early on in the cleaning process creates a baseline of the fouling that can be used to identify where concentrations of fouling exist in a coil. With this information, cleaning efforts can be focused on specific locations of fouling. For example, if no fouling is present in convection tubes, the verification tool runs are limited to just the radiant tubes. This front-end step eliminates unnecessary travel through the portions of the coil free of fouling resulting in reduced mechanical cleaning run times and wear and tear on the coil. When coupled with periodic verification runs to gauge the progress of the cleaning efforts throughout the decoking process, ADCV can significantly reduce cleaning times, which can be of great importance for heaters in a critical path.
Another added benefit of the service is the ability to identify operational concerns with the heater by evaluating the location and thickness of fouling. The presence of fouling in the upper part of a convection section or excessively thick areas of fouling in radiant tubes could reveal underlying operational issues, such as improper burner operation or inadequate process flow, which may lead to uneven heat distribution and elevated tube metal and process temperatures that will cause internal fouling.
This level of information can lead to more effective temperature monitoring programs and allows operators to make informed decisions on future operating and shutdown procedures.
* Quest Integrity obtained this information from over 2,400 heater inspections
The Benefits on Display
Quest Integrity was called out to perform an Advanced Decoking and Cleanliness Verification on two coils located in a fired heater. The heater was configured with a horizontal convection section and two separate vertical radiant cells (see Figure 2). Shortly into the mechanical cleaning portion of the project, a verification tool was deployed in both coils to establish fouling levels and determine where cleaning efforts should be focused. The verification runs detected a higher than the expected buildup of fouling in one of the radiant cells (see Figure 3). The other radiant cell showed much less fouling. This discovery provided excellent operational information showing the temperature in the first radiant cell was too high. As a result, the tube metal temperature (TMT) limit for the tubes in the cell with heavy fouling was reached much sooner. Had the temperature been more evenly spread across both radiant cells, the TMT limit would have been reached much later, resulting in a longer run time and improved profitability.
The first fouling verification runs also revealed horizontal grooving patterns on the inside tube walls (see Figure 4). Further investigation concluded that the previous cleaning company that had decoked the coils was overly aggressive in its cleaning efforts resulting in damage (wall loss) to the tube walls. This was most likely from the combined effects of using oversized cleaning pigs, extra hard cleaning appendages and excessive cleaning pig runs.
To the last point, without a way to conclusively determine that a coil is completely free of fouling, companies may resort to more cleaning runs than are required, causing unnecessary wear and tear on bare tube walls. ADCV eliminates this problem by providing 100% assurance on the cleanliness of coils during the decoking process, effectively minimizing the amount of cleaning runs to what is required and nothing more. This precise and economical approach to decoking can reduce cleaning times thereby allowing asset owners to restore operations sooner.
The benefits of mechanical decoking are numerous and necessary. Refinery and Petrochemical owners decoke heaters to preserve the integrity of their tubes, improving throughput and operating efficiencies. When performing this type of standard maintenance activity, it is reasonable to expect solutions that do not diminish tube integrity and overall heater performance. By applying Advanced Decoking and Cleanliness Verification, owner/operators are able to use data-driven solutions that can reduce decoking times, reveal unexpected operational issues and validate cleaning efforts, allowing assets to return to service faster and with better operational assurances.