Improve Aboveground Storage Tank Leak Detection Inspections
Mass Measurement leak detection programs offer financial incentives when returning large aboveground storage tanks back to service after repair.
The most important time to test for leaks in a tank bottom is after it has been repaired. Basic post-repair inspection is mandated by API 653, but prudent tank operators look to the latest technology when assessing tank tightness after repair. Several methods are currently available; they include: vacuum box testing, magnetic floor scanning, gas detection, hydrotesting, and hydrotesting with mass measurement. These procedures are compared for their benefits and disadvantages when applied on large aboveground storage tanks (ASTs)
Compliance with EPA and other regulations is good reason to implement a leak detection program for ASTs. However, economic justification for leak testing above 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 a leak 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.
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 during Hydrotesting
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.
Mass Measurement is a technology-driven testing technique that minimizes the possibility of human error or omission. The accuracy of test results is built into the technology and is not likely to be degraded by improper application. Human influence on the accuracy of results is virtually eliminated.
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 tank tightness test, but hydro is often principally an attempt to assure tank 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.