WO2011121415A1

WO2011121415A1 - High-precision leak detection in underground storage tanks

Info

Publication number: WO2011121415A1
Application number: PCT/IB2011/000657
Authority: WO
Inventors: Máté KAJTAR
Original assignee: Kajtar Mate
Priority date: 2010-03-29
Filing date: 2011-03-29
Publication date: 2011-10-06
Also published as: HU1000167D0; HUP1000167A2

Abstract

The leaking tanks may engender financial losses and environmental damages up to a sum of billions in "controlled" circumstances. In order to avoid these, the invention proposes such triple-walled tanks, where the soil side interstitial space is divided in a quadratic lattice of close spaces cells (3). In addition, the invention proposes special designed pre-produced ribs with a special hollow shape (4) in order to be interstitial spaces too, so their leakage can be monitored separately. Any leakage can be indicated immediately and with higher precision than by means of every earlier method using the cheapest detection systems (1) (2). In the case of an external leakage the soil side exploration of the tank is not required. The space bordered by quadratic lattice belonging to the location of the leak must be discovered only, and the tank can be operated again with safety following the quick reparation. This is an outstanding economization for the operator and inappreciable environmental security for all of us.

Description

High-precision leak detection

in underground storage tanks

Up to the recent decades underground fuel storage was implemented with single-walled steel tanks all over the world. However, the application of plastic tanks looks backs already on several decades in the USA, but in Europe - perhaps owing to the interest of the strong steel industry -even today the structures made of steel are the most widespread.

The possibilities for leak testing of the single-walled tanks are limited. The sensitivity of leak tests conducted in compliance with the testing standards of the horizontal and vertical single-walled tanks (filling with water, perhaps pressure test) according to the research of the Environmental Protection Agency (EPA) are maximum 0.1 gal.h-1 , that is 0.38 l/h. This means that a single-walled fuel tank can lose unnoticed 9 litres of hazardous liquid a day, amounting to 3300 litres a year. The tank operators of the countries newly acceded to the European Union shall comply with the environmental regulations of the EU, but this - calculating with a minimum number of 100 000 underground tanks - is obviously an extremely time-consuming and costly task. In theory, the afore-mentioned number of single-walled underground tanks may have lost up to 330 million litres of hazardous liquid a year into the ground in "controlled" circumstances.

At market price, the annual loss may exceed 330 million euros. It is easy to imagine - even if it is difficult to assess in terms of numbers - the environmental damage that may have arisen so far in addition to the direct financial loss.

Though plastic tanks - safer from the point of view of corrosion - have appeared in several European countries, but their order of magnitude is not comparable with that of the steel tanks.

The safe storage of the flammable and explosive liquids - posing an ever increasing problem all over the world and damaging the environment - has been implemented all over the world with double-walled tanks recently. From the point of view of environmental safety this method meant a huge advance in comparison with the application of the single-walled tanks. The unquestionable advantage of this solution is that in the case of an external or internal leak the immediate contamination of the environment does not have to be anticipated. Namely, it is true that in the case of the leakage of the internal wall the stored dangerous liquid leaks not directly into the soil, but into the common space bordered with the external wall, where the fact of the leakage can be identified by means of different detection methods. Furthermore, it is also true that if there is a leakage in the external wall, even then the direct contamination has not to be anticipated because the liquid "is taking a rest" safely in the internal tank. In this case the situation is more complicated, as not all methods are suitable to detect the external leak, in addition the location of the leak in the external wall cannot be determined because it is invisible without excavating around the tank or lifting it up from the ground. So the question remains: Where is the leak?

For the sake of understanding it, let us look at several leakage monitoring methods widely used in practice:

- Warning system with floating switch: perhaps it is the most widely used method in practice, by means of which the leakage of the internal wall can be indicated safely. This is a cheap solution, it is based on the idea that in the case of a leakage the stored liquid penetrating into the space between the two walls lifts up the buoy, and a micro-switch actuated gives a signal (sound/light) on the display panel. Though it is a cheap solution, it is not able to provide complete safety because - in the case of damage to the external wall - it will give a signal only if high ground-water surrounds the tank. Otherwise the instrument "remains silent" because there is no liquid to lift the buoy and to actuate the micro-switch. So, in the case of damage to the external wall we cannot decide with absolute certainty if there is a leak, and if yes, then where.

- Detection with vacuum: it is an exceptionally sensitive method for monitoring the leakage of the double-walled tanks. In the case of damage to either the external or the internal wall the vacuum decreases, and under a certain value the alarm goes off, and the failure diagnosis can be commenced. What is the problem here? It can be identified immediately whether an external or internal leakage occurred. If fuel appears in the monitoring space, then the internal wall must be damaged, if nothing or ground-water appears, then the leak is in the external wall. Safe operation means the greatest concern. The vacuum between the two walls must be maintained by means of a pump, and this is an extremely expensive solu- tion, considering that this may only be an explosion-proof construction because of the possibly penetrating flammable liquid. The other problem in this system is that in the case of damage to the internal wall, contamination appears in the space between the walls owing to the vacuum, fills up with the dangerous liquid more quickly, the removal of which is expensive, and no success can be guaranteed. When damage is done to the external wall, we still do not know.

- Leakage indication with overpressure: this is one of the safest solutions, because it indicates the leakage in both the internal and external walls. Its additional advantage is that it does not contaminate the interstitial space between the walls - as the vacuum method does - because here the gas is bubbling through the stored liquid owing to the overpressure. Its disadvantage is that it cannot be identified immediately whether the external or the internal wall is damaged, but only after discharging the tank. A further problem is that this method can be applied exclusively in the case of such tanks whose internal wall has been dimensioned because indentations may appear on it owing to the overpressure prevailing between the two walls. In turn, this makes the tank obviously much more expensive and we still do not know where is the leak.

- A signaling system making use of liquid fill-up. The interstitial space between the two walls is filled up with liquid, and either the internal or the external wall is damaged, the level of the liquid goes down, thus indicating the leak. There are several problems with this method. The first is that the liquid used shall be non-freezing. In the majority of cases a liquid with low freezing point is used for this purpose. If the external wall is damaged, it leaks into the ground, thus posing a danger at least as great as the stored liquid. If it leaks into the stored liquid, it "contaminates" that. We still do not know where the leak is.

By way of summary, it can be noted in connection with all the enumerated methods that there is no way at all to determine the location of the damage if the leakage occurs in the external wall. For this reason, there is no other solution, than either lifting out and replacing the complete tank or entirely exploring it by excavating the tank around, locating the leak by means of a measuring instrument, then repairing it. This is also a very expensive solution, let alone the loss sustained by the operator of the fuel station because of the business interruption. Most recently a patent description has appeared in the USA about the application of triple- walled tanks (WO 99/38785). But not even this method makes it possible to identify the location of the leak if it is in the external wall. Where is the leak then? The US patent description no. 6.551. 024 attempts the identification of the leak on the external wall, but as it uses the liquid fill-up method to detect the location of the leak, this solution brings us back to the problem mentioned. The patent description referred to can only identify the location of the leak with great inaccuracy. If the level of the ground water is high, then the level of the detecting liquid in the interstitial space can only go down as low as the level of the ground water. What can be asserted with great accuracy is that the leak is under this level. The location can only be identified - and only with low probability - with the application of environment polluting methods (introducing compressed air into the interstitial space, pressing the detecting liquid into the ground through the leak). This is the most promising method so far, but as it can monitor maximum one ringed space - an area between two reinforcing ribs - (a significant proportion of the tank's surface), we still do not know exactly where the leak is.

The idea that the present invention propose solves all those problems in a cost saving manner and with great safety, which are raised by the application of the underground tanks. It is based on triple-walled tanks which should be introduced into practice. The presented method uses the cheapest leakage indication procedures, and it is able to identify the leak with greater precision than ever before in the case of an external leak. A further advantage of the above is that whereas it provides a high-level of safety, it is a cheap solution.

During the production of the double-walled tanks on the external cylindrical wall several ribs are put in place, which are required to resist the pressure of the soil. The essential point of present solution is that these ribs are also used to partly divide the interstitial space between the second and the third wall into parts. The interstitial space adjacent to the soil is divided into any number of squared spaces cells (3). The pre-produced ribs have a special hollow shape (4) in order to be interstitial spaces too, so their leakage can be monitored separately with the same pressure sensor (1). The entire wall thickness of the proposed triple-walled tank will be not greater than that of the double-walled one, therefore hardly any use of excess material is required. The application of the tanks manufactured in this way provides numerous advantages in comparison with the tanks used so far.

- The cheapest procedures can be applied conveniently for monitoring the leakage. For example, the interstitial space adjacent to the stored material (5) can already be monitored safely with the cheapest floating switch (2), while the overpressure ^'type indication (1) can be applied on the soil side, since the indentation strain does not require an excess use of material in developing the double interstitial space.

- On the soil side even the vacuum type monitoring can be applied, which does not entail excess expenses either, and - since the external double-walling is not physically connected to the internal wall - it is not required to install the vacuum pump in an explosion-proof design at an extremely high cost.

- The most important difference between the solution proposed above and the procedures applied so far is that we can not only identify whether the external or the internal wall is damaged. With the application of the reinforcing ribs the interstitial space of the tank adjacent to the soil has been divided into spaces that can be separately monitored, and the ribs are interstitial space themselves. This means that the location of the leak can be identified with great precision. This precision can be increased without any limit because the interstitial space on the soil side can be further divided into any squared spaces cells (3) depending on the required precision. Without this division the whole tank should be excavated, sheet piling should be used, and the tank could not be operated for a minimum of five days. The method I propose would make any excavation unnecessary as the external damage to the tank -once we know its location - can be repaired from the inside.

- The fact that the damaged tank does not have to be taken out of service immediately is an invaluable advantage for the operator, since regardless which interstitial space is damaged, the tank remains double-walled. The repair may be made at a time when the stoppage does not cause a major loss of revenue or production. Since the repair can be planned, it can be made more quickly, which results in additional significant cost reduction. Even a total of over 90% cost reduction can be achieved. The most important advantage is - and it is an advantage for the whole community - complete environmental safety!

The underground storage of flammable and explosive liquids dangerous from an environmental aspect meant a huge environmental problem all over the world. In order to avoid this, the safe storage of such liquids has been solved by means of double-walled tanks recently. Descriptions about the application of triple-walled tanks have also been published. Their obvious advantage is that leakage either in the external or in the internal wall does not result in the immediate contamination of the environment. Nevertheless, these storage methods have numerous shortcomings in practice, which can be eliminated with a simple method.

Figure 1 is showing the leakage monitoring system of a triple-walled tank with divided interstitial space composed by one pressure sensor which is for the monitoring of the cells of the soil side interstitial space, a floating switch which is for the monitoring of the interstitial space from the stored material side (5) and special shaped hollow ribs which can be monitored separately by the same pressure sensor (1).

The proposed method solves - in a cost saving manner and with high safety - all those problems, which are raised by the application of the widely used "modern" tanks. I would introduce - triple-walled tanks into the practice, would divide their interstitial space on the side of the soil into any number of squared spaces cells (3) with a simple procedure, making it possible to identify the external leak with the required precision. Its advantages compared to the widely used solutions are the following:

- In the case of leakage it can be decided immediately whether the external or the internal wall is damaged.

- In the case of double-walled tanks, the vacuum generation in the space between the two walls is one of the most sensitive methods to detect the leakage. One of the problems of this method is that when an internal leakage occurs, the stored liquid penetrates into the monitored space more quickly, and cleaning the interstitial space is rather difficult, almost impossible. The second concern is that the pump required to maintain the vacuum can only be of an explosion-proof design because of the stored inflammable liquid, and this is very expensive. In the case of triple-walled tanks the interstitial space adjacent to the stored material (5) and the interstitial space adjacent to the soil can be monitored separately. While on the side of the stored material the cheapest floating switch can be applied as leakage indicator, the interstitial space on the side of the soil - as no hazardous liquid can penetrate into it - can be monitored by means of several methods. - While in the case of a leak in the external wall, the soil had to be excavated around the whole tank to determine the location of the leak. The method that I propose makes it possible to precisely identify the location of the leak. It means that no external excavation of the tank is required to repair it, as we know the location of the leak and thus we can repair it from the inside, with a minimum cost!

- Otherwise the procedure can be used to improve the operational safety of single- or double-walled tanks (steel / plastic), already in service. In this case the expensive replacement of the tank is not required, it is sufficient to reconstruct the tank with this simple method, thus complying with the most stringent environmental and safety requirements.

- In the case of the leakage of any wall the tank remains double-walled, so its safe condition is maintained, and its immediate repair is not required. The operator can choose the most appropriate date and time for the repair, since environmental damage is excluded. Since the repair can be planned, it can be made at a lower cost; in addition the loss of revenue arising from the stoppage of the service is significantly smaller.

Claims

1. High-precision leak detection method in underground storage tanks using a triple-walled tank with the soil side interstitial space divided in squared spaces cells (3) and having ribs (4) with a special shape where the leakage monitoring is made with one pressure sensor (1) for all the cells (3) and a floating switch (2) for the interstitial space adjacent to the stored material (5).

2. Leak detection method as claimed in claim 1, wherein the soil side interstitial space can be divided in any number of squared spaces (3) depending on the required precision for detection of the leak.

3. Leak detection method as claimed in claim 1, wherein the cells from the soil side interstitial space (3) can all be monitored separately with the use of a single pressure sensor (1).

4. Leak detection method as claimed in claim 1, wherein the tank ribs (4) have a special hollow shape which gives the possibility to be monitored separately by the same pressure sensor (1).

5. Leak detection method as claimed in claim 1, wherein the interstitial space adjacent to the stored material (5) is monitored with a floating switch (2).