Fired heaters or furnaces remain the key fundamental process units within any oil refinery. The elevated temperatures and pressures to which the process tubes are exposed mean they are extremely vulnerable to a plethora of corrosion and metallurgic damage mechanisms. Traditionally, metallic flaws and defects have been identified and classified using various NDT techniques such as handheld ultrasonic testing (UT), radiography and other labor-intensive, time-consuming processes. More recently, the introduction of small-bore, ultrasonic smartpigs, which are able to navigate short-radius return bends and plug headers down to a 2.5-inch internal diameter, has led to a significantly more time-efficient and accurate method of identifying defects within the process tubing compared to conventional, manual UT techniques. Furthermore, smartpigging techniques have been proven to acquire considerably more tube wall thickness data, which are typically in the region of 30,000 individual measurements per meter of tubing. A comprehensive and thorough physical assessment of the process tubing can be carried out by exploiting and utilizing these large quantities of data correctly.
A fitness-for-service assessment is a multidisciplinary approach used to determine if a piece of equipment contains metallic flaws and is fit for continued operation based on quantitative information obtained from the inspection process. The assessment is also able to provide recommendations for future operating parameters and maintenance scheduling. Moreover, the remaining life of the equipment or component can be calculated. In fired heaters and boilers, these calculations are usually based upon the extent of the damage and the allowable stress limits within the tube materials when exposed to different operating temperatures and pressures.
The API 579-1/ASME FFS-1: Fitness for Service standard sets out the procedures and calculations necessary to define and assess a range of metallic flaws common to fired heaters and boilers. These flaws include general metal loss, local metal loss, dents, pitting, laminations, gouges, hydrogen blistering and creep damage. These damage mechanisms are commonplace in fired heater and boiler process coils, and repairs or tube replacements in the process coils can be costly and labor intensive. It is therefore advisable any metallic flaws detected from the inspection process are individually assessed in accordance with the procedures described in the API 579 standard and the remaining structural integrity of the process tubes is comprehensively evaluated.
The mass quantity of wall thickness data obtainable using modern smartpigging techniques allows for precise contour mapping of any particular flaw. Creating this complete geometric profile of the flaw using the input data obtained enables a much more accurate fitness-for-service and remaining life assessment to ensue. This will ultimately lead to continued, safe operation of the equipment and maximum profitability.
For more information, contact Dr. David Thewsey at dthewsey@coke busters.com.
