PECM Issue 21 2016 | Page 52

Separating The Good From The Bad: Prioritise Your Storage Tank Maintenance Risk-based inspection systems and advanced non-destructive testing methods are But active cracking is not the only source helping storage tank owners comply with ever-increasing stringent regulations, as well as of AE. Chemical processes such as reducing maintenance costs, says Tim Bradshaw, General Manager, MISTRAS GROUP, UK corrosion spalling, fracture and debonding Operations. are very emissive. or owners of above ground It also considers the consequences of such The AE arising from the corrosion process process storage tanks, a failure. As a result, out-of-service tanks on the floor of a storage tank, for example, including both atmospheric that exhibit potential problems can be will travel through the product in the tank, and pressurised storage inspected, while continuing to operate through the tank wall and into the sensors assets that are in ‘good’ condition. attached to the outside. based around EEMUA 157 and/or API 653. A combination of inspection methods A ring of sensors can be used to pinpoint According to both of these standards, the can be used to monitor and grade the location of the AE from within the purpose of an inspection is to determine the condition of tanks. One of these is tank and so, in the space of an hour or so, if tanks are safe for continued service techniques is acoustic emission (AE). The provide a very detailed picture of where in terms of their mechanical integrity. application of mechanical or thermal stress the corrosion is, and how bad it is. Originally, inspection plans were strictly to a material results in elastic energy being regulatory compliant, utilising traditional stored in that material. F systems such as bullets and spheres, tank inspection programmes are normally manual, non-destructive testing (NDT) Tank floors remain largely unseen and are traditionally uninspectable during The stress field sustained by the material operation. Normally, this involves tends to concentrate at localised costly shutdown and decontamination mechanical instabilities, which exist in followed by detailed local inspection. almost all practical mechanical structures. Often, this is executed from an inspection If the applied stress is high enough, schedule according to a predetermined To help tank owners comply with ever- the material will fail at such local stress period of time in service. To know when increasing stringent regulations, risk-based concentrations and crack, until the the condition of a tank justifies being inspection (RBI) systems and advanced propagation of the crack is such that the taken offline and cleaned out, in order non-destructive testing (ANDT) methods material has become stress free and the to prioritise and target inspection and can be used to assess the condition and stored elastic energy has been dissipated. maintenance activity accordingly, is of techniques such as visual, ultrasonic and settlement surveys, which were used for both in-service and out-of-service inspections. mechanical integrity of an individual tank or a complete tank f arm. This tank-specific RBI approach enables owners to focus their inspection on those assets that have the highest probability of potential problems. The continued operation of healthy tanks, along with large reductions in budget spent on preparing tanks for traditional internal inspections (including cleaning, degassing and waste disposal) provides tank owners with better profitability and improved operational effectiveness. The method by which this energy is released is a step-like process, where the crack grows in a chaotic cascade of distinct, discrete snaps. Each snap provides a discrete pulse of energy that propagates throughout the surrounding material in the form of a transient elastic wave. The frequency content of these pulse-like transient waves is broadband, ranging from a few KHz to a few MHz. Much of this is in the ultrasonic region, detectable by using specialised AE sensors. The identification, location and evaluation of structural defects and active cracks in pressure vessels, pipelines and storage tanks is now routinely applied to process plants. huge economic (and environmental) benefit to tank owners. AE process for tank floors AE sensors are mounted on the wall around the tank’s circumference and connected to a data acquisition system. To assess the condition of the tank floor, a window of one hour is required to gather enough data for a valid statistical assessment of the floor. To achieve a ‘quiet hour’ all activity likely to cause product movement must have been stopped and the tank allowed to settle for a period of 6 to 12 hours. Agitators, heater coils, level measurement systems, etc. must all be turned off. Disruption to operations is thus no more than one working day per tank.