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Preventing Leaks in Large Aboveground Storage Tanks

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Posted / Last update: 01-05-1999
Publication: Petroleum Equipment & Technology Archive
Issued: May 1999
Author: Myers Philip E.

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.

Chemical markers
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.

1. (c)
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.

2. (a)
Coatings are not used on the underside of tank bottoms because the welding process would destroy them.

3 (d)
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.

4. (d)
API 650 Appendix E is used for this purpose.

5. (b)
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.

6. (b)
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.

7. (d)
All of the above are sources of potential false alarms.

8. (a)
No downtime is required. The double bottom tank operates reliably and passively over time.

9. (d)
The double bottom is an effective corrosion prevention method that significantly increases the tank life in a variety of ways.

10. (b)
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.

 
 
 

Glossary

1. Ambient air temperature –
The average, everyday temperature and the temperature that a tank tends to become if liquid is stored in it for a long time.

2. Ambient noise level –
The background interference present in all measuring systems that tends to obscure the measurement and lead to inaccuracies of measurement.

3. Blinding –
Closing off of the end of a pipe through the use of bolted plates.

4. Bottom plates –
The steel plates that form the bottom of a tank. They are usually about six to 10 feet wide and up to 40 feet long. They are typically G inch thick.

5. Descaled plate –
Bottom plates that have had the factory mill scale removed to improve the corrosion resistance of the plates to soil-side corrosion attack.

6. Diurnal volumetric changes –
Even with no liquid entering or leaving the tank, the fluctuation of the liquid level will move in response to its temperature change on a daily basis that tracks the outside ambient temperature. This results from the well known expansion and contraction of liquids when heated or cooled.

7. Edge-cutting settlement –
One of several forms of settlement addressed in Appendix B of API 653 that covers tank settlement. Edge-cutting settlement results when the load at the tank shell is so great that it causes the bottom plates under the shell to be pushed into the soil.

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 –
Welds that join plates, which are lapped on one another (as opposed to butted up against one another). The lap welds are triangular in shape and, for bottom plates in tanks, are on the topside only. These welds are difficult to make in tank bottoms without defects. Therefore, testing is important.

9. Finished fuel –
Fuel for motor, aviation and other applications including gasolines, diesels and jet fuels.

10. Horizontal thermal gradient –
In a tank, the temperature variations that can occur on a horizontal plane across the diameter of the tank.

11. Mass measurement –
This measurement makes use of pressure measured at the bottom of a tank to determine if a leak is occurring in a tank.

12. Missed detection rate –
The false result of a leak-detection test stating that there is no leak when, in fact, there is a leak.

13. One- and two-pass welds –
If only one weld is made, this is a single-pass weld. If the welder comes back and welds on top of his first weld a second time, this is a two-pass weld.

14. Passive system –
These are systems known for reliability because they do not require any foreign power supply, air supply or any other source that must be relied upon. The only thing that passive systems require are the forces of nature such as gravity.

15. Pressure head –
The pressure exerted by a column of liquid due to gravity.

16. Probability of false alarm –
The chance of a leak-detection system indicating a leak when there is none.

17. Release prevention barrier –
A barrier to a leak under a tank. The RPB can be a plastic sheet, a double bottom tank, a reinforced concrete mat or other constructions that are passive and divert the leak to the perimeter of the tank, where it can be visually observed.

18. Soil bearing pressure –
The ability of the foundation soil to withstand the forces of the tank resting on it without undue settlement or deformation.

19. Stilling period –
When leak tests are conducted, it is necessary to stop flowing liquid into or out of the tank. It is also necessary to ensure that no energy input, such as by mixing, is allowed to occur in the tank so that the liquid is quiescent.

20. Temperature-compensated volume —
The volume that would be in the tank if the temperature were changed to a standard value, such as 60 degrees F.

21. Vacuum box –
A steel box with a glass window that allows for evacuating the air on a portion of a tank bottom weld. When the vacuum is applied, the soap solution that is painted over the weld starts to emit bubbles if there is a leak. This is the primary method of testing bottom plate weld seams.

22. Vertical thermal gradients –
If you were to measure the liquid temperature at one foot intervals vertically from the bottom to the top of the tank, the temperature would form a curve called a vertical thermal gradient. It can vary by as much as five to 10 degrees F.

23. Volumetric measurement –
A method to detect leaks by attempting to see if the temperature-compensated volume is constant after the tank has been stilled.

Author
Myers Philip E.
Biography
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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