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