Plant equipment in the process, power and refining industries undergo exposure to conditions that may result in degradation, corrosion, or failure. In today's competitive climate, the operators of these plants come under increasing pressure to improve equipment reliability, while reducing overall costs.
Conventional approaches to plant maintenance take the form of 'time based', preventative models, or indeed 'reactive' ones (i.e., failure before remedial action is carried out). Currently, there is significant focus on Risk Based Inspection (RBI) and Reliability Centred Maintenance (RCM), showing high promise for improving the overall performance of plant and cost controls. Both RBI and RCM require extensive use of non-destructive examination and inspection technologies. However, the performance of the inspection technologies requires that several factors be considered. Are the speed and coverage of the examination adequate for the particular component and are the sensitivity and the reliability of the technique being used suitable for the suspect damage mechanism or degradation. Time of Flight Diffraction (TOFD) is an ultrasonic technique that offers the most efficient and cost-effective inspection tool yet developed for the non-intrusive examination of plant equipment.
TOFD differs from conventional pulse echo ultrasonics in that it depends on diffracted energy rather than reflected energy, monitored in the form of signal height or amplitude. This energy response is then compared to an 'equivalent' defect, normally in the form of a side-drilled hole or notch of a specific size for a specific material thickness. TOFD, however, relies on time-based, low energy, diffracted signals from the tips of flaws, allowing an exact or absolute position and size for the flaw to be measured and imaged. The technique is accomplished by using two probes, a transmitter and a receiver; normally placed either side of the weld joint or area to be examined. Because the technique is not amplitude dependent, as in conventional ultrasonics, it does not suffer from the constraints of probe location to flaw orientation, poor coupling, uneven or changing material surfaces.
Some Benefits of TOFD
Can be applied to new construction welds, in lieu of radiography (ASME Code Case 2235), providing an excellent record of vessel or pipe condition at birth. Can also be used for in-service defects, such as cracking, corrosion, erosion, etc. Has a very high temperature limitation (700 F), with slight loss in sensitivity.
Some Applications of TOFD
Weld examination of pressure vessels, piping and storage tanks and spheres; Hydrogen damage detection and sizing; Pre-service examination on new construction welds (cost savings by replacing RT); Weld examination during new construction (replacing RT); Detection and sizing of IGSCC; Critical steam piping examinations.
Until recently, the perception prevailed that automated ultrasonics and TOFD, is prohibitively expensive. This was true when TOFD was being used solely as a sizing tool in the 1980's. With the advent of smaller, less expensive imaging systems, combined with the expanded use of the TOFD technique, day rate charge has been reduced considerably. However, when considering the cost of TOFD, as with any comprehensive examination, one should not focus solely on the day rate fee. Other, more subtle total costs need to be considered. The production rates on TOFD are extremely high - many times that of radiography or manual ultrasonics. The inspection can be accomplished on-line with no loss of plant production, resulting in major savings. If defects are noted, they can be accurately monitored, again allowing for more effective asset management decisions. The ability to conduct the examination during a busy shut down, without the traditional clearing of an area to perform radiography, provides massive savings in man-hour costs. Consequently, in applying TOFD, the total costs versus the hourly costs need to be considered before assessing the technique as expensive.