PECM Issue 21 2016 | Page 53

The result of the condition monitoring exercise is a tank floor grading, from A (no damage) to E (major repair required). Also, plots are presented to show the location(s) of any particularly active areas and/or potential leak sites. MISTRAS has conducted more than 1,000 such tests on tanks in Europe alone and more than 3,000 worldwide. Huge savings to be made The overall tank floor and potential leak grades can be used by plant management to prioritise tank inspection and maintenance programmes. Cost savings from the use of this technology are very clear. If the vessel is in good condition, leading to the deferral of internal inspection, savings are enormous. The cost of preparing a large crude oil tank for internal inspection can reach £350,000 and for large cryogenic tanks up to £750,000. More than 50% of the ‘suspect’ tanks inspected by MISTRAS were proven not to require subsequent offline inspection and maintenance, saving millions of pounds for refinery and distribution terminal operators. For pressure vessels, the figure increases to 95%. This reveals how poorly targeted most shutdown maintenance is. At the other end of the scale, the skeptic might suggest that a £7,000 AE assessment which confirms that a suspected poor vessel does not need to be opened for repair is money wasted. However, costs can be measured in safety and environmental terms, as well as in pounds. Complementary methods It is important to treat the AE method as complementary to other methods. This is because, unlike ultrasonic testing, AE integrity assessment does not detect static, non-growing defects, nor does it measure their size. The system wall needs to be free of obstructions. The method involves sending long range ultrasonic signals into the tank from the outside annular (which must be in reasonable condition). The tank itself does not need to be removed from service. Indications are compared to a test plate in order to evaluate the severity of corrosion. On the other hand, it will detect and This technique can be used on steel and locate regions of overstress or areas where fiberglass tanks and vessels. micro structural problems exist, which are structurally significant and which are easily missed by conventional localised methods. The effectiveness of both methods is improved by using them together. AE should thought of as condition monitoring for static plant: a tool for determining which tanks need conventional inspection, where and when. It should not be assigned to, or used by, inspection departments. Remote UT crawlers and cameras can also be used to map the thickness of hard to reach areas. Magnetic Flux Exclusion To more accurately determine the extent of potentially flawed areas, Magnetic Flux Exclusion (MFE) or Low Frequency Electromagnetic Technique (LFET) scanning tools can be used to inspect 100% of the tank floor. Characterisation of defects is performed using traditional nondestructive testing techniques including It is a management tool to be employed ultrasonics, penetrant, magnetic particle by senior maintenance personnel with a and vacuum box inspections. direct interest in managing maintenance budgets and directing inspection Conclusions personnel to where they are really The most effective approach to tank needed. There is often a temptation to repair tanks simply because they are offline, particularly when inactive cracks not identified by AE are found by other methods. Unnecessary repair often introduces new stresses to the vessel which were otherwise not present. Annular ultrasonic testing inspection blends the regulatory compliance to current codes and standards with Risk Based Inspection and Advanced Non Destructive Testing methods. The Tank Specific RBI ap¬proach allows owners to concentrate inspection on those assets that have the highest probability of potential problems. It also considers the consequences of such a failure. As a result, Tank Annular Long Range Ultrasonic Out-of-Service inspection of tanks can Testing (TALRUT) is an automated long be recommended that exhibit potential range ultrasonic technique developed problems, while continuing to operate by MISTRAS to qualita tively map the assets that are in good condition. condition of the annular. www.mistrasgroup.co.uk Issue 21 PECM 53