Preventing Leaks in Large Aboveground Storage Tanks
Posted / Last update: 01-05-1999
False signals — Some potentially false signals can come from the following sources:
• Impulsive signals seemingly from the floor can come from such other sources as roof drains, pivoted float arms and roof supports.
• Impulsive signals can be generated by condensation dripping onto the product surface.
• Impulsive signals can be generated by floating roof movement.
• Impulsive signals can come from high winds.
• Impulsive signals can come from thermal excitation of the tank shell.
The potential for false signals is also affected by other factors. For example, data collection and analysis have a significant impact on the potential for false acoustic leak signals. Also, it is critical to identify the propagation mode for signals received to locate the source of the signal.
One company recently examined 345 tanks using the acoustic emission leak-detection method. Twenty-one of the tanks were indicated to be leaking, and 19 of the 21 were internally inspected. Of those 19 suspects, 16 actually had leaks. This gives a probability of detection of 0.84 and a probability of false alarm of 0.16.
In this method a highly unique chemical can be injected into the tank that is otherwise not present in normal petroleum liquids. These markers, or tracers, then spread throughout the liquid. By sampling vapors from the underside of the tank (if a leak exists), the detection of the chemical marker in the sampled vapors indicates the existence of a leak.
This method is highly accurate and perhaps has the best probability of detection. But since the vapors under the tank must monitored, it is necessary to install sampling tubes. While this technique is feasible for small tanks, it becomes prohibitively costly at diameters exceeding 60 to 100 feet (on existing installations). Tracer Research Corporation, located in Tucson, AZ, has perfected this technique and can be consulted for further details [(520) 888-9400].
Release prevention barriers
The release prevention barrier (RPB) is the simplest but most effective of all leak-detection systems for large ASTs. It may simply be a plastic liner underneath the tank bottom.
The double bottom is a subset of the RPB. The double bottom works extremely well for retrofits, whereas a simple sheet liner buried beneath a newly constructed tank is entirely adequate. API Standard 650, Appendix I addresses the basic requirements for constructing double bottom systems.
RPBs are very simple to understand; they block the downward flow of leaks and divert them to the perimeter, where the leak really does come out on your shoes. In all cases that I am aware of, leak detection that has been accomplished by viewing the leak at the perimeter has effectively prevented environmental damage.
The RPB detects extremely small leaks. In one case, leaks occured in a new double bottom. Six years later, the very small leaks appeared as a staining at the leak-detection bottoms. The double bottom has some important leak detection characteristics:
• It is passive (i.e., it has no moving parts and does not depend on power supplies or any other maintenance or support).
• It has essentially a zero threshold leak rate—it will detect smaller leaks than any other type of leak-detection system.
• It has essentially a 100 percent probability of detection. Unlike other leak-detection systems it will not miss any leaks.
It must be understood that there are many ways to construct a double bottom tank. The foregoing discussions of double bottoms assume the use of a concrete spacer and 80 mil high density polyethylene (HDPE) liner. Concrete construction, in addition to the other advantages already cited, reduces the probability of creating serious problems when using granular filler material. Remember, however, that the concept of a liner could include a reinforced concrete mat, a plastic liner, a double bottom tank, or other similar ideas. It is the concept of using an RPB that catches and diverts the leak that is important—much more so than the details of how the system is constructed.
Answers to questions of PE&T’s Pop Quiz on large ASTs and leak detection.
Bottom construction uses fillet welds because they are the only practical way of building tank bottoms for the vast majority of applications. However, these are the most difficult types of welds to examine for tightness to leaks.
Coatings are not used on the underside of tank bottoms because the welding process would destroy them.
All of the above. While a (internal coatings) and b (cathodic protection) are universally recognized corrosion prevention measures, the double bottom, if constructed according to Chevron specifications, does reduce corrosion (1) by reducing water and contamination, changing the alkalinity of any residual water under the tank bottom and (2) by elevating the tank bottom, improving the ability of the water to drain away from the tank.
API 650 Appendix E is used for this purpose.
Volumetric testing measures the change in volume of a tank by compensating for thermal expansion of the liquid. An alternative form of volumetric testing is the mass method where the pressure head of the liquid is measured.
The impulsive leak signal is the characteristic sound emitted by a leaking tank. It is this sound that allows us to perform acoustic leak detection.
All of the above are sources of potential false alarms.
No downtime is required. The double bottom tank operates reliably and passively over time.
The double bottom is an effective corrosion prevention method that significantly increases the tank life in a variety of ways.
Cathodic protection has not been proven universally effective for protecting finished fuel tanks from internal corrosion. Coatings do that job adequately.
1 A complete discussion of this topic may be found in the book Aboveground Storage Tanks by Philip Myer and Robert Ferry.
1. Ambient air temperature –
2. Ambient noise level –
3. Blinding –
4. Bottom plates –
5. Descaled plate –
6. Diurnal volumetric changes –
7. Edge-cutting settlement –
This is more likely to occur in the Spring in locations where the heavy snow load on the tank roof adds to the pressure at the tank bottom near the shell and when the soil is soft as in the springtime during snow melt. API 653 addresses edge-cutting settlement, but the formulas are currently too stringent and are overly conservative. A task group has been formed that is revising the Appendix to provide a more reasonable method of determining what acceptable edge-cutting settlement is.
8. Fillet welds –
9. Finished fuel –
10. Horizontal thermal gradient –
11. Mass measurement –
12. Missed detection rate –
13. One- and two-pass welds –
14. Passive system –
15. Pressure head –
16. Probability of false alarm –
17. Release prevention barrier –
18. Soil bearing pressure –
19. Stilling period –
20. Temperature-compensated volume —
21. Vacuum box –
22. Vertical thermal gradients –
23. Volumetric measurement –
Myers Philip E.
Philip E. Myers, retired from Chevron Products Co., where he specialized in tank and pressure-vessel technology. He is currently consulting.
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