Advances in Nde Techniques for Delayed Coke Drums

Delayed coke drums are operated under severe conditions of cyclic heating and forced cooling that apply repetitive thermal stresses to the drum walls. It has long been recognized that the ultimate failure mechanism for coke drums is weld cracking due to low cycle fatigue caused by these thermal stresses. It is also known that coke drums distort and bulge in service and that these bulges can be used as pointers to potential weld failure areas. CIA Inspection (CIAI) operates a laser profiling service that locates and measures distortions in coke drums. Consistent and accurate measurement of surface deformities has allowed operators to focus on areas of concern, but it has not been able positively identify and size cracks. TSC Inspection Systems (TSC) is a developer of electromagnetic inspection technology, Alternating Current Field Measurement (ACFM). CIA Engineering (CIAE) is an engineering/design company with experience in developing automated systems for a variety of industrial applications. To further assist in the evaluation of the vessels condition, CIAI in conjunction with TSC and CIAE is developing a robotic technique, which allows further evaluations of the indicated areas. This capability will provide operators with a remotely operated Non Destructive Examination technique which can be selectively applied to local, internal surface areas. This paper outlines the evolution of Remote Coke Drum Inspection discussing a suite of technologies which have been developed to investigate and monitor the condition of delayed coke drums. A key focus of the paper is centered around the introduction of new sensor technology and the specific development of a robotic device which places the sensor remotely, on the inside surface of the vessel. This innovative technology is designed to identify and size flaws without the need to scaffold, remove insulation or blind the vessel. www.ndt.net 3rd MENDT Middle East Nondestructive Testing Conference & Exhibition 27-30 Nov 2005 Bahrain, Manama 1 BACKGROUND OF CIA INSPECTION SERVICE CIA Inspection (herein referred to as CIAI) developed and operates a specialized laser surface profiling system designed to internally inspect coke drums during the short time period between coke cutting and refilling. The engineering company, (now known as CIA Engineering or CIAE), started development of the remote inspection equipment in 1990 and created the commercial arm, CIAI, to perform contract service inspections in 1994. To date, CIAI has performed over 500 site inspections throughout the world. CIAI’s inspection system uses a remote sensor package deployed from the coke drum’s drill stem as shown in Figure 1: Figure 1: Coke Drum Inspection System with Laser Profiler A color video camera with zoom lens permits a detailed remote visual inspection of the inside of the drum capable of identifying surface flaws such as cladding defects or potential weld cracks. A scanning laser range finder produces an accurate and dense surface profile of the entire inside surface of the vertical walls of the drum. The capabilities of the laser scanner have elevated surface profiling from an occasionallyused qualitative method to a much more useful tool which: • profiles the entire inside surface of the vertical drum walls on a 1” x 1” 25mm x 25mm) grid. This ensures that no deformations are “missed” • measures depths of deformations to 1/8” (3.2mm) accuracy • measures drum ovality as well as local bulges or wrinkles • measures repeatably over time and therefore provides a means of accurately comparing initial scans with subsequent inspections. The quality of the surface profile data produced by CIAI’s inspection system has allowed coke drum operators to: Laser scanner Drill Stem Operator control room on ground www.ndt.net 3rd MENDT Middle East Nondestructive Testing Conference & Exhibition 27-30 Nov 2005 Bahrain, Manama • compare the degree of deformity among their different drums, thereby identifying which drums are likely to require more inspection effort • focus further inspection efforts on welds near deformed areas • compare the change in drum deformities over time through the repeated use of CIAI’s inspection system • use actual drum profiles as input to Finite Element Analysis or to other third-party analyses (such as Stress Engineering’s Bulge Intensity Factor – “BIF” analysis) This use of CIAI’s automated inspection service has allowed operators to accurately trend the shape of the vessels as they age. This capability has been useful, as many sites have been increasing throughput by reducing coker cycle times which make the drums age faster. By “benchmarking” a coke drum’s surface profile before changing cycle times and then rechecking the profile after several hundred cycles at the new cycling rate, operators can determine whether the cycle change has led to an increase in drum bulging. This information can then be used to formulate pro-active inspection plans to maintain vessel reliability while operating at increased throughput. 1.1 The Traditional Response to Flaw Indications When areas of concern have been identified by CIAI’s inspections, options are considered whether to immediately inspect or wait for the next turnaround with the hope that potential flaws will not progress to a thru-wall condition during the run cycle. Because insufficient information is available to determine the nature of the indication, reliability people are unable to verify whether the visual or distortion indication goes beyond the cladding and into the pressure boundary. Production pressures often force the decision to ignore the indications and continue operating, potentially with disastrous results. The gamble to run to the next turnaround date may not pay off and an unplanned outage could be forced upon operations because of a thru-wall failure. Once a drum has failed, repair and inspection personnel are placed under tremendous pressure to affect a repair and put the unit back in service. Due to the metallurgy of modern coke drums, repairs require sophisticated weld procedures and care must be taken to follow the procedures or the crack condition can reoccur. Figure 2 outlines the typical features of flaws found in the clad restoration weld of coke drums Figure 2. Low cycle fatigue crack caused in the clad restoration weld of coke drum Traditionally, inspection and reliability technicians employ conventional ultrasonic NDE methods such as SHEAR WAVE or TOFD to these areas of concern to determine both the location and magnitude of the flaw indications. These conventional technologies are applied to the external surface of the vessel. Access to these areas is very difficult and requires the installation of scaffolding and the removal of insulation. Once the surface has been exposed, careful and thorough surface preparation must be performed to remove mill scale, rust and dirt. After all this preparatory work has been completed, inspection contractors can then perform a variety of NDE inspection technologies to verify the presence and size of flaws in the circ welds. www.ndt.net 3rd MENDT Middle East Nondestructive Testing Conference & Exhibition 27-30 Nov 2005 Bahrain, Manama Interpretation problems are often encountered when the “shadow effect”, caused by the boundary between the internal stainless steel cladding and parent material, makes it difficult to interpret the results. Because of this boundary condition, cracks initiating from the inside of the drum in the restoration weld of the cladding and penetrating into the parent metal are often overlooked. The only way to thoroughly inspect this area from the outside of the vessel is to utilize a new development referred to as PHASED ARRAY in combination with DUAL TOFD. This recently perfected technology is supplied by SUMITOMO Heavy Industries Ltd. and is offered as a contract service through CIAI. 1.2 Phased Array and TOFD Simultaneous Inspection Phased Array (hereinafter called PA) technology generates an ultrasonic beam by setting beam parameters such as angle, focusing depth, and number of the active elements of PA probes using a computer to control the system. The PA probe typically consists of a one-dimensional array of small transducer elements. In order to control the beam characteristics, the excitation pulse is applied at different times to the elements of the PA probe. PA Linear Scan is a technique which applies ultrasonic beams to the clad restoration weld and its heat-affected zone. The PA probe is mounted on the outside weld surface and traverses around the weld via a magnetic crawler. The sliding direction of ultrasonic beams is perpendicular to the scanning direction. The scanning data is normally obtained at every 1 mm in the scanning direction. Since a PA probe is mounted on the outside weld surfaces, the outside weld protrusion must be removed to make a flush condition. PA Linear Scan (straight beam technique) is well suited to detect weld defects caused during the manufacturing stage including porosity and lack of fusion between the clad restoration weld and base metal weld. PA Sector Scan inspects the weld joint by using a pair of PA probes which are mounted adjacent to the weld joint and spaced equally around the centerline of weld. This configuration allows the ultrasonic beams to be focused on the clad restoration weld and its heat affected zone. The swing angle range of the focused shear wave is between 30 and 35 degrees. The scanning data is normally obtained at every degree, and at every 1 mm (0.04”) in the scanning direction. Since PA Sector Scan is the angle beam technique, it is well suited to detect low cycle fatigue cracking in the clad restoration weld and its heat affected zone. Because of the dual probe configuration, PA Sector Scan does not require removal of the outside weld protrusion. While these techniques are very thorough and gaining in popularity, they still require access to the exterior of the drum and that requires scaffolding, insulation removal and some preparation of the surface prior to implementing the technique. All of this preparation work means that this sort of inspection can only occur during a planned turnaround. 2 SEARCH FOR NEW TECHNOLOGY As part of ongoing research to improve inspection capabilities for our clients, CIAI has been searching for a means to quickly verify the presence of cracks in the failure prone areas. From CIA’s point of view, a key requirement for the system would be the ability to perform remote NDE inspection from inside of the vessel without having to scaffold, remove insulation or prepare the surface of the area to be inspected. A variety of technologies were considered for the task and, after careful consideration of many aspects, ACFM was selected as the most appropriate technology for this particular application.