Economic
sense. Compliance with regulations is a good reason to implement
a leak detection program for ASTs. However, economic justification
for leak testing ASTs after inspection or repair is equally important.
The value of ensuring that tanks are not put back in service with
small, undetectable leaks or with leak rates high enough to require
tank reentry can be quantified. The return on investment for leak
detection results in benefits that more than justify the cost
of incorporating this system in an inspection program.
After
repairs are made to a tank bottom, it is imperative that the entire
tank floor be inspected for leaks. When a tank has been cleaned,
heavy equipment that has been active inside the tank can cause
an old tank floor to leak. There is always a possibility that
the repair was implemented unsatisfactorily. The most popular
procedures for inspecting the tank bottom for leaks include:
Post
repair inspection in empty tanks. The principal method used
today to test tanks for leaks after repair is vacuum box testing.
It is a simple and effective procedure when used with care. This,
coupled with the recommendation of API 653, explains its widespread
use. Vacuum box testing is an inspection method intended for weld
joints and is not usually applied to the entire tank bottom. For
this reason and due to occasional human error in its application,
the vacuum box testing can miss leaks.
Other
popular, post repair, tank bottom inspection tools are magnetic
flux floor scanning and ultrasonic thickness detection. These
methods attempt to locate possible leaks by finding areas of reduced
thickness in the tank bottom. Ultrasonic testing is a spot testing
procedure and gives an excellent evaluation of the spots tested.
A very small percentage of the total tank bottom is actually measured.
Magnetic
flux scanners can cover most of the tank floor. The scanners
only miss the areas close to the wall and places where physical
obstructions prevent the machine from performing. The magnetic
flux scan is not as accurate an indicator of bottom thickness
as the ultrasonic method. Neither of these methods completely
inspect the tank bottom and can miss leaks.
Gas
detection. Helium or other gas can be injected under the tank
floor and detected inside the tank if a leak is present in the
tank bottom. This method has proven effective in locating leaks
in tank bottoms. This method requires that a hole be drilled in
the tank floor where the gas can be injected. The most important
aspect of the method is that the gas must migrate to all areas
of the tank floor. The gas will migrate to most sections of the
bottom, but can be obstructed. Two problems with the gas migration
are: 1) the weight of the tank wall makes it difficult for the
gas to migrate near the edge of the tank and 2) when a viscous
product has been leaking under the tank, the product may restrict
gas movement. These gas migration problems can prevent helium
from detecting all leaks.
Leak
detection during hydrotesting. Tank-bottom repair will not
usually require a true hydrotest of the tank. The hydrotest is
a structural test that is only required if significant repairs
are made in the area near the tank walls. In spite of this, many
operators fill their tanks with water after a bottom repair as
a leak test. The hydrotest can be supplemented with dye to assist
in using this test. However, even with dye in the water, it is
not considered a leak test. Most tank bottom leaks will not migrate
past the tank wall. The leak will go into the ground and will
not be evident outside the tank. Even when the tank is considered
to be sitting on clay or other soil with limited permeability,
the weight of the tank wall will usually prevent migration of
leaks outside the tank perimeter.
Mass
measurement of the tank contents during hydrotesting can turn
this test into an effective leak test. With this method, extremely
sensitive measurement to the tank contents over a two or three
day period will determine if the tank is losing fluid. Effective
measurement of tank contents depends on several things. The most
important is the sensitivity of the measurement method. Fortunately,
since this leak detection technique measures a leak quantitatively,
the leak detection equipment can be quantitatively evaluated.
The mass measurement technique has been used in underground tank
leak detection because the federal government mandated tank testing
for underground tanks. The government also provides procedures
for evaluating the leak detectors.
The
problem with using the typical underground tank testing equipment
to evaluate leaks in large above ground tanks is that testing
equipment will only work well with small tanks. Very few companies
have devised mass measurement techniques sensitive enough to accurately
detect leaks in large tanks. But the EPA technique used to evaluate
the test systems needs to be modified only slightly to evaluate
leak detection systems for large tanks.
The
best feature of the mass measurement method during hydro is that
it measures the actual leak with the tank floor load. Since this
method measures actual fluid loss from the tank, no leak within
the detection threshold of the test system can be missed. The
important phrase here is "within the detection threshold." If
you use mass measurement to detect leaks, the detection threshold
for the system must be certified by a third party evaluator. There
are extreme differences in the ability of mass measurement systems
to detect leaks. The only way to have any confidence in a leak
detection system is for it to have been evaluated.
Leak detection pays for itself. Saving the value of lost products,
eliminating tank re-entries, reduced internal inspection time/cost
combined with reduced water usage during leak test hydro makes
tightness testing a cost benefit rather than an expense.
Use less water during hydro. Mass-measurement leak detection
can further reduce the cost of putting the tank back in service
by providing leak detection with only 6 to 10 feet of water in
the tank during hydrotesting. This will not be a complete structural
test, but hydro is often principally an attempt to assure tightness.
The values of the water-cost savings vary widely with each operator,
but for some operations, is a significant part of placing a tank
back in service.
Mass
measurement method provides the most accurate precision leak detection
available to meet the new requirement of API 653. The good news
is that including leak detection in tank inspection programs will
typically save money. The money spent on leak detection will be
returned due to: 1) saving the value of lost products 2) eliminating
tank re-entries 3) reduced internal inspection time and 4) reduced
water usage during leak test hydro.
