It is well documented that fired heaters are a critical component of any oil refinery. Mechanical failure of process tubing contained within fired heaters can have catastrophic safety, environmental and financial effects. Historically, failure risk has been minimized by implementing various nondestructive testing management systems; however, in more recent years, small-bore smart pigging techniques have transformed the way in which this risk is managed.
Smart pigging inspections during scheduled turnaround periods can allow any general or localized tube anomalies to be accurately located and quantified, allowing further fitness-for-service assessments to be carried out. Furthermore, by combining a "baseline" smart pigging inspection prior to the heater entering commercial operation and regular smart pigging inspections at the end of each operating period, an accurate corrosion rate can be calculated. Determination of accurate corrosion rates will allow for more precise remaining life assessments of the process tubes.
Cokebusters USA was recently contracted by a client to carry out a Merlin⢠in-line inspection on the radiant section of an atmospheric crude heater as part of a scheduled maintenance turnaround in 2018. The coils that were to be inspected were newly commissioned and had not yet entered commercial operation. Upon manufacture, seamless tubes are susceptible to various manufacturing tolerances, which can result in deviations from the specified nominal tube dimensions. To obtain a complete evaluation of tube geometry, a complete inspection of all process tubes using the Merlin smart pig was requested by the client.
The Merlin Mark IV smart pig is a carbon-fiber, single-bodied, untethered device that employs a series of ultrasonic transducers to measure tube wall thickness and internal radius at equally spaced circumferential locations along the full length of the heater coil, effectively scanning the process tubes for geometric abnormalities or defects. The smart pig records and stores the received data to its on-board memory, which is later uploaded via USB to a computer. The data is then automatically interpreted and analyzed by the Merlin software, which can then output various graphical images and C-scans of the process tubes. The scanner unit employs a series of custom-built piezo-composite transducers, each constantly firing and receiving a complexity of rapid-fire ultrasonic pulses.
Multiple inspection runs were carried out using the Merlin smart pig in order to confirm and validate data. More than 230,000 individual wall thickness measurements were recorded in each tube. From the as-obtained data, it was clear the average wall thickness for each tube was significantly higher than the specified nominal values for the replacement tubes. The specification sheet for these tubes gave dimensions of 4.5 inches outside diameter x 0.25 inches average wall thickness. Inspection data showed the average wall thickness across all of the tubes was 0.305 inches, representing an increase of 18 percent.
In the next issue, Cokebusters will assess the effect smart pigging has on the determination of accurate corrosion rates and how this can alter the decision making of refinery engineers.
For more information, visit www. cokebusters.com or call (281) 537-7475.
