Prioritising storage tank maintenance
19 July 2016
Tim Bradshaw, general manager at Mistras Group, UK operations, explains how risk-based inspection systems and advanced non-destructive testing methods are helping storage tank owners comply with increasingly stringent regulations, as well as reducing maintenance costs.
For owners of above ground process storage tanks, including both atmospheric and pressurised storage systems such as bullets and spheres, tank inspection programmes are normally based around EEMUA 157 and/or API 653. According to both of these standards, the purpose of an inspection is to determine whether tanks are safe for continued service in terms of their mechanical integrity. Originally, inspection plans were strictly regulatory compliant, utilising traditional manual, non-destructive testing (NDT) techniques such as visual, ultrasonic and settlement surveys, which were used for both in-service and out-of-service inspections.
To help tank owners comply with increasingly stringent regulations, risk-based inspection (RBI) systems and advanced non-destructive testing (ANDT) methods can be used to assess the condition and mechanical integrity of an individual tank or a complete tank farm. A tank-specific RBI approach will enable owners to focus their inspection on assets which have the highest probability of potential problems. It also considers the consequences of such a failure. As a result, out-of-service tanks that exhibit potential problems can be inspected, while continuing to operate assets that are in ‘good’ condition.
A combination of inspection methods can be used to monitor and grade the condition of tanks. One of these techniques is acoustic emission (AE). The application of mechanical or thermal stress to a material results in elastic energy being stored in that material. The stress field sustained by the material tends to concentrate at localised mechanical instabilities, which exist in almost all practical mechanical structures. If the applied stress is high enough, the material will fail at such local stress concentrations and crack, until the propagation of the crack is such that the material has become stress free and the stored elastic energy has been dissipated.
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. But active cracking is not the only source of AE. Chemical processes such as corrosion spalling, fracture and debonding are very emissive.
The AE arising from the corrosion process on the floor of a storage tank, for example, will travel through the product in the tank, through the tank wall and into the sensors attached to the outside. A ring of sensors can be used to pinpoint the location of the AE from within the tank and so, in the space of an hour or so, can provide a very detailed picture of where the corrosion is, and how bad it is.
Tank floors remain largely unseen and are traditionally uninspectable during operation. Normally, this involves costly shutdown and decontamination followed by detailed local inspection. Often, this is executed from an inspection schedule according to a predetermined period of time in service. To know when the condition of a tank justifies being taken offline and cleaned out, in order to prioritise and target inspection and maintenance activity accordingly, can be an economic (and environmental) benefit to tank owners.
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 six to 12 hours. Agitators, heater coils and level measurement systems must all be turned off. This means that disruption to operations amounts to no more than one working day per tank.
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.
The overall tank floor and potential leak grades can be used to prioritise tank inspection and maintenance programmes. Cost savings from the use of this technology are obvious. 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 that have been 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 monitarily.
It is important to treat the AE solution 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. On the other hand, it will detect and locate regions of overstress or areas where 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 be 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. It is a management tool to be employed by senior maintenance personnel with a direct interest in managing maintenance budgets and directing inspection personnel to where they are really 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 can introduce new stresses to the vessel which were otherwise not present.
The most effective approach to tank 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, Out-of-Service inspection of tanks can be recommended that exhibit potential problems, while continuing to operate assets that are in good condition.
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.
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