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.