The answer to this month’s trouble shooter is provided by MISTRAS Group Limited.

Published:  08 September, 2017

Q:

How can I assess the structural integrity of process storage tanks without busting the maintenance budget or causing costly plant downtime?

A:

Risk-based inspection (RBI) systems and advanced non-destructive testing methods can be used to assess the condition/mechanical integrity of process storage tanks, including both atmospheric and pressurised storage vessels. This tank-specific RBI approach enables owners to focus their 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 tanks that exhibit potential problems can be inspected, while continuing to operate assets that are in ‘good’ condition, thus minimising maintenance costs and preventing costly downtime.

A combination of inspection methods can be used to monitor and grade the condition of tanks. One of these is techniques is acoustic emission (AE) monitoring. 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 AE sensors attached to the outside. A ring of sensors can pinpoint the location of the AE from within the tank and so, in the space of an hour or so, provide a very detailed picture of where the corrosion is, and how bad it is. Disruption to operations should be no more than one working day per tank.

The result of condition monitoring is a tank floor grading, from A (no damage) to E (major repair required). Plots can also be generated 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 by plant management to prioritise tank inspection and maintenance programmes. If the vessel is in good condition, leading to the deferral of internal inspection, significant savings can be made.

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