JP2009097887A - Leakage risk evaluation method for underground storage tank facility - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To estimate the risk of leakage for underground storage tank facility, using an evaluation method having high degree of certainty. <P>SOLUTION: This leakage risk evaluation method for underground storage tank facility, equipped with a steel underground storage tank for storing combustible fluid, or the like, is such that the data of corrosion amount on the outer surface of the underground storage tank, or the generation of thinning caused by corrosion are generated, relative to many underground storage tank facilities formed of the same coating material; data for each ground potential of the underground storage tank facilities and each number of elapsed years, after burying the underground storage tank facilities are collected; relation data between the corrosion amount on the outer surface of the underground storage tank or the generation of thinning caused by corrosion, and the ground potential and the number of elapsed years, after burying the underground storage tank facility are determined; and a corrosion-generating probability or leakage risk due to the corrosion of other underground storage tanks that constitute the same coating material is predicted, based on the relation data. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  TECHNICAL FIELD The present invention relates to a leakage risk evaluation method for underground storage tank facilities, and in particular, corrosion occurs on the outer surface of underground storage tank facilities such as underground storage tanks for storing flammable fluids such as gasoline and underground piping for flowing flammable fluids. The present invention relates to a leakage risk evaluation method for an underground storage tank facility that predicts the probability of occurrence of perforation through a wall surface or the leakage risk due to corrosion of the outer surface of the underground storage tank facility.

  For example, underground steel structures consisting of tanks that store fuel oil such as gasoline, kerosene, light oil, and heavy oil are installed in service stations, fuel bases, factories, offices, and buildings with heavy oil / light oil boilers. Yes. The outside of the underground storage tank, which is an underground steel structure, is covered with asphalt, tar epoxy, FRP (Fiber Reinforced Plastics) or the like so as not to be corroded by soil. Furthermore, in the case of fuel oil or the like, the inside of the underground storage tank does not corrode the steel plate, which is a normal tank material, so that the steel plate is in a bare state without taking special anticorrosion measures.

Conventionally, a method for diagnosing corrosion deterioration of underground storage tanks related to the risk assessment of the above-described corrosion leakage accident has been developed in the United States and the like (for example, see Non-Patent Document 1). The leakage risk assessment method for underground storage tank facilities according to Non-Patent Document 1 is based on underground storage tank data (capacity, structure, material, elapsed years after embedment, etc.), chemical properties data of the surrounding soil (soil ratio) Resistance, chloride concentration, sulfide concentration, pH, etc.) and the amount of corrosion progress of the underground storage tank are measured, and the relationship between the soil data and the amount of corrosion progress of the underground storage tank is statistically analyzed, and the underground storage tank It is a technique to develop a program for predicting the amount of corrosion degradation. With the prediction program developed by this method, if the underground storage tank data of the diagnosis target underground storage tank and the chemical property data of the soil are collected and applied to the prediction program, the corrosion deterioration amount of the underground storage tank can be predicted. is there.
ASTM standard G158-98, method A

  According to the method of Non-Patent Document 1 described above, the data on the site to be collected is soil chemistry data, which indicates whether the environment in which the underground storage tank is embedded is corrosive. .

  However, even if the environment (soil) in which the underground storage tank is buried is corrosive, if the coating of the underground storage tank is healthy, the underground storage tank will not be corroded. That is, it cannot be determined whether or not corrosion occurs in the underground storage tank, which is the object of diagnosis, only from the soil chemistry data.

  Therefore, the diagnostic method based only on the soil chemistry data is considered to have low accuracy in leak prediction of underground storage tank facilities and low risk assessment for leakage accidents.

  This invention is made | formed in view of said problem, Comprising: It aims at providing the leakage risk evaluation method of underground storage tank equipment with high accuracy.

  In order to solve the above problems, the present invention has the following means.

The present invention is a leakage risk assessment method for an underground storage tank facility comprising a steel underground storage tank for storing a flammable fluid,
For a number of underground storage tank facilities formed of the same coating material, a procedure for creating data on the amount of corrosion on the outer surface of the underground storage tank or occurrence of thinning due to corrosion,
A procedure for collecting data on the ground potential of the steel underground storage tank facility and the number of years elapsed since the steel underground storage tank facility was buried;
A procedure for determining the relationship between the amount of corrosion of the outer surface of the underground storage tank or the occurrence of thinning due to corrosion and the ground potential and the number of years elapsed since the underground storage tank facility made of steel,
A procedure for predicting the corrosion occurrence probability or the risk of leakage due to corrosion of another underground storage tank made of the same coating material based on the relational data;
By including the above, the above-mentioned problems are solved.

  According to the present invention, data on the actual amount of thinning due to corrosion of a large number of underground storage tanks of the same coating material that has already been buried and used underground, the ground potential of the tank, and the data of years of burial are collected. Makes it possible to predict the probability of corrosion of other underground storage tanks made of the same coating material or the risk of leakage due to corrosion based on the relational data, which is more accurate than the prediction method based on the corrosiveness of the soil. Predictive results can be obtained, and it becomes possible to increase the reliability of the risk assessment of leakage due to corrosion of underground storage tanks.

  The best mode for carrying out the present invention will be described below with reference to the drawings.

  FIG. 1 is a longitudinal sectional view showing an embodiment of an underground storage tank facility. As shown in FIG. 1, the underground storage tank 12 is constructed so that an iron plate is processed into a cylindrical shape and both ends of the cylinder are closed by welding or the like, and an asphalt or epoxy having an electrically insulating property on the surface thereof. Coating is applied using resin.

  On the ground of the filling station, there are provided a meter 14 for refueling, a manhole 16 communicating with the underground storage tank 12, an oil filling port 18 of the underground storage tank 12, and a vent 20 communicating with the upper space of the underground storage tank 12. ing. In the underground storage tank 12 buried underground in the fueling station, an oil supply pipe 22 communicated with the oil supply system of the measuring machine 14 and an oil supply pipe 24 communicated with the oil supply port 18 are inserted. A ventilation pipe 26 communicated with the vent 20 is attached to the upper part of the underground storage tank 12.

  FIG. 2 is a longitudinal sectional view showing a construction state of the underground storage tank 12. As shown in FIG. 2, the underground storage tank 12 is installed so as to be supported by an installation table 34 provided on a concrete base 32. In addition, concrete pillars 36 are erected on both sides of the installation table 34, and a concrete upper wall 38 covering the upper part of the underground storage tank 12 and a concrete pavement 40 surrounding the manhole 16 are supported by the pillars 36. Has been. Thus, the underground storage tank 12 is reinforced so that a load does not act even when a tank truck (not shown) passes through the concrete pavement 40.

  In addition, soil 42 such as mountain sand is backfilled around the underground storage tank 12 to absorb vibrations and to fix the underground storage tank 12.

  Here, the form which the outer surface of the underground storage tank 12 embed | buried underground as mentioned above corrodes is demonstrated. As one of the corrosion forms, for example, there is electrolytic corrosion that occurs when a leakage current from a track such as an electric railway becomes a stray current and flows into an underground steel structure or the like. Another form of corrosion is concrete / soil macrocell corrosion that occurs due to electrical contact between the underground storage tank 12, the piping 22, 24, 26 system and the reinforcing bars in the reinforced concrete structure (such as the foundation of the facility or between soils). is there. Further, as another form of corrosion, there is dissimilar metal macrocell corrosion caused by contact between an underground tank / piping system and a dissimilar metal such as a copper alloy of a ground rod.

  The natural potential of carbon steel embedded in the soil is -500 to -600 mV (saturated copper sulfate electrode standard), whereas the potential of the reinforcing steel in the concrete is as high as -200 to -300 mV. Therefore, the contact between the underground storage tank 12 or the pipes 22, 24, 26 and the reinforcing bars creates a current loop of the reinforcing bars → the underground storage tank 12, the pipes 22, 24, 26 → the soil → the reinforcing bars, and the underground storage tank 12, the pipes 22, 24, Corrosion that progresses quickly occurs in the electrically insulating deteriorated portion of the coating 26. This is concrete / soil macrocell corrosion. Corrosion caused by contact between a high-potential copper alloy and an underground tank / piping system, similar to rebar, is dissimilar metal macrocell corrosion.

  These corrosion forms are 10 to 20 years when macro-cell corrosion such as electric corrosion or concrete / soil occurs even in underground steel structures made of thick steel plates due to the rapid progress of corrosion, or If the corrosion is moderate, a part of the underground steel structure will be perforated in about 30 years, and a liquid accident such as storage of dangerous materials will occur. Therefore, determining whether or not this corrosion has occurred is a point for performing corrosion diagnosis of underground steel structures with high accuracy.

  In addition, in the corrosion form, there is microcell corrosion between the carbon steel surface and the soil of the electrical resistance deteriorated portion of the coating such as a tank, but the underground storage tank 12, the pipes 22, 24, and 26 are mountain sand and the like. It is backfilled with low corrosive soil, and the rate of progress of the corrosion is slow, about 0.02 to 0.05 mm / year, and it does not become a big problem on leakage risk in underground storage tanks made of thick steel plates. . From the above, in the risk assessment of leakage accidents due to corrosion of the underground storage tank 12 and the pipes 22, 24, 26, it is necessary to diagnose the presence of the above-described macro corrosion such as electric corrosion and concrete / soil.

  Here, considering the mechanism of the occurrence of macro-cell corrosion such as electric corrosion and concrete / soil, in the case of electric corrosion, whether there is inflow of stray current around the underground storage tank 12 and the pipes 22, 24 and 26. It becomes the first element.

  In the case of macrocell corrosion such as concrete / soil, the second factor is whether there is contact between the underground storage tank 12 and the pipes 22, 24, and 26 and different metals such as reinforcing steel or copper alloy in the concrete. However, even if these two elements are present, the coating state of the coating of the underground storage tank 12 is healthy (the electrical insulation between the underground storage tank 12 and the soil is maintained well). Does not cause electric corrosion, macrocell corrosion such as concrete / soil, or even microcell corrosion.

  The fire fighting law stipulates that the outer surface of steel underground storage tank 12 for storing flammable fluid in Japan should be coated with asphalt roofing or epoxy resin of a certain thickness or more to prevent corrosion. A steel underground storage tank 12 having a structure conforming to this law is buried and used. That is, in the underground storage tank 12 buried nationwide, it is considered that a steel tank with a certain quality coating is buried.

While these coated underground storage tanks 12 are buried underground, they are subjected to the following loads (1) to (4). Become.
(1) Repeated load capacity by entering and exiting the combustible fluid to be stored. For example, in the case of a 10 KL gasoline storage tank 12, it receives a repeated load caused by 5 to 7 tons of gasoline coming and going.
(2) Expansion and contraction due to temperature difference between stored oil and underground temperature. For example, in summer, the temperature of the oil loaded in the tank truck from the oil tank station may be higher than about 35 ° C, while the underground temperature in which the underground storage tank 12 is buried is about 14-18 ° C. Therefore, the temperature difference between the two is large. When unloading hot oil in the summer to the underground storage tank 12, the entire underground storage tank 12 expands and is cooled by the soil and shrinks over time. Since the difference in expansion coefficient between resin and oil and fat materials and carbon steel is more than an order of magnitude, by repeatedly unloading high-temperature flammable fluid to the underground underground storage tank 12, stress is applied to the coating adhesion part, It becomes a factor that paint coat peels off.
(3) Change in earth pressure (4) Vibration load due to tank lorry etc. traveling on underground storage tank 12 By receiving the above loads (1) to (4) for many years, the coating years will advance It is considered that local peeling and cracking occur as the time elapses. The groundwater penetrates into the part where these peelings occur, and the electrical insulation between the carbon steel of the tank and the soil begins to deteriorate. Moreover, since coating such as asphalt is hygroscopic, it causes a slow deterioration of electrical insulation.

  In the case of the coating material having a certain quality defined by laws and regulations as described above, it is considered that the above-mentioned deterioration occurs almost uniformly after a certain number of years such as 15 to 20 years.

  In this way, the coating begins to deteriorate, and when the electrical insulation between the soil and the tank carbon steel surface begins to deteriorate, a corrosion current flows between the soil and the carbon steel, causing iron ionization and corrosion. Will occur. In particular, the underground storage tank 12 and the pipes 22, 24, and 26 are subjected to electric corrosion due to stray current, or in an environment where stray current that is a condition of macro cell corrosion occurrence such as concrete / soil occurs, or underground with reinforcing steel and dissimilar metals in concrete. When the steel in the storage tank 12 and the pipes 22, 24, and 26 is in an environment where they are in contact with each other, electrolytic corrosion or concrete / soil or dissimilar metal macrocell corrosion that occurs quickly occurs.

  Considering the occurrence and progression of galvanic corrosion, the larger the value of stray current, the faster the progression of galvanic corrosion.

  On the other hand, in the case of concrete / soil, dissimilar metal macrocell corrosion, the greater the amount of rebar or dissimilar metal in the concrete that comes into contact with the steel in the underground storage tank 12, piping 22, 24, 26, the greater the source of corrosion current, The progress of corrosion is considered to be fast.

  Here, underground storage tank is used as a method for determining the presence or absence of contact between the underground storage tank 12 and the pipes 22, 24 and 26, which are the first conditions for the corrosion of concrete / soil and different metal macrocells, and the reinforcing bars or different metals in the concrete. There is a measurement of the ground potential of the tank 12 and the pipes 22, 24 and 26.

  Next, the ground potential measurement method will be described. Usually, the contact between the underground storage tank piping system, which is a requirement for the occurrence of corrosion of concrete / soil, dissimilar metal macrocells, and the reinforcing bars and dissimilar metals in the concrete, the ground potential of the underground storage tank 12 and the piping 22, 24, 26 The measured value is an index. The measurement diagram is shown in FIG. In FIG. 3, the ground potential of any of the pipes 22, 24, 26 connected to the underground storage tank 12 in the vicinity (which is also electrically connected) is measured using the reference electrode 54 of the terminal box 50. It shows the state.

Since the pipes 22, 24 and 26 and the underground storage tank 12 are connected in the vicinity, this measured value can be regarded as the ground potential of the underground storage tank piping system. The ground potential measuring device 60 measures a detection signal (potential) from the terminal box 50 buried underground, and stores the detection signal in a memory. The terminal box 50 includes an electric wire 52 that is electrically connected to the pipes 22, 24, and 26, and a verification electrode 54 that is in contact with the soil 62 above the pipes 22, 24, and 26.
Here, generally, the ground potential (natural potential) of iron embedded in the soil 62 is a value of about −500 mV to −600 mV. On the other hand, since the reinforcing bars in the concrete are in an alkaline atmosphere, the ground potential is about -200 mV (both are cases where a saturated copper sulfate electrode is used as the reference electrode 54). Accordingly, when the underground storage tank facility is in a state of being in electrical contact with the reinforcing bar at any place, the ground potential of the underground storage tank 12 exhibits a value of about −450 mV to −250 mV.

  Here, since the ground potential of copper is also about −200 mV, considering the value of the ground potential of the underground storage tank 12 and the pipes 22, 24, 26, the reinforcing bars that are in contact with the underground storage tank 12, the pipes 22, 24, 26 When the amount of copper alloy is large, that is, when the supply amount of corrosion current is large, the ground potential of the underground storage tank 12 and the pipes 22, 24, 26 becomes a value close to the potential on the reinforcing bar side or the copper alloy side. When the amount of reinforcing steel and copper to be contacted is small, the value is close to the natural potential of steel.

  That is, when the ground potential of the underground storage tank 12 and the pipes 22, 24, 26 is as high as about −300 to −250 mV, the strength of the macrocell corrosion of the generated concrete / soil is considered to be high and the progress is rapid. When the ground potential is as low as about −400 to −500 mV, the strength of the generated macrocell corrosion such as concrete / soil is low and the progress is considered to be slow.

  The applicant (inventor) measured the amount of corrosion of a large number of underground storage tanks 12 of the same coating material that has been used for a long period of time with an ultrasonic wall thickness measurement device, and simultaneously measured the ground potential. The data of ground potential, the amount of corrosion of the underground storage tank 12, the structure of the underground storage tank 12, the age of burial, etc. are collected.

  In this data, it is a result that no corrosion has occurred in an underground tank having an elapsed age of 20 years or less after the burying of the underground storage tank regardless of the value of ground potential of the underground storage tank facility. In addition, when data with an elapsed age of about 25 years is picked up when the ground potential is higher than −300 mV, the corrosion rate becomes about 100%, and the corrosion rate decreases as the ground potential becomes lower. ing.

  This means that in the case of the same coating, the electrical insulation between the soil and the soil begins to deteriorate in almost a fixed number of years. When the amount of metal is large, that is, when the ground potential is high, it is estimated that the tank 12 is rapidly reduced in thickness due to corrosion.

  Even if there is contact with reinforcing bars and dissimilar metals, when the amount of contact is small and the strength of corrosion is low, that is, when the ground potential is low, it is presumed that thinning due to corrosion occurs late. Naturally, since the underground storage tank 12 that has been rapidly reduced in thickness is rapidly corroded, there is a high risk that a leak will occur due to drilling.

  In the underground storage tank 12 where the ground potential is lower than −500 mV and the underground storage tank 12 and the pipes 22, 24, and 26 are considered not to come into contact with reinforcing bars or dissimilar metals, the burial years are long for about 30 years. However, no thinning due to corrosion was observed. Since we are comparing underground storage tanks with the same coating material, it is considered that the coating is almost deteriorated, so the condition of microcell corrosion is lower than that of macrocell corrosion. Therefore, it is presumed that no metal loss due to corrosion occurred. This is because when underground tanks are buried, the underground storage tank is backfilled with sand, etc., so that there is no component with an abnormally high concentration of chloride or sulfide that accelerates the progress of microcell corrosion. It is thought that this is also caused by being buried in

  Here, the procedure of the leakage risk evaluation method for the underground storage tank facility including the steel underground storage tank 12 for storing the flammable fluid and the like will be described.

  In the procedure 1, for a large number of underground storage tank facilities formed of the same coating material, the amount of corrosion on the outer surface of the underground storage tank or the occurrence of thinning due to corrosion is measured and data is created.

  In the procedure 2, the measured value of the ground potential of the steel underground storage tank facility and the data of the number of years elapsed after burying the steel underground storage tank facility are collected.

  In step 3, the relationship between the amount of corrosion of the outer surface of the underground storage tank or the occurrence of thinning due to corrosion, the ground potential, and the years elapsed since the steel underground storage tank facility was buried is obtained.

  In the procedure 4, the corrosion occurrence prediction rate data table 70 as shown in FIG. 4 of the corrosion occurrence probability or leakage risk due to corrosion of other underground storage tanks made of the same coating material is created based on the relational data.

  In FIG. 4, ◎ indicates that the rate of occurrence of thinning due to corrosion is extremely low, ○ indicates that the rate of occurrence of thinning due to corrosion is low, and Δ indicates the rate of occurrence of thinning due to corrosion. Indicates an intermediate value, x indicates that the rate of occurrence of thinning due to corrosion is high, and xx indicates that the rate of occurrence of thinning due to corrosion is extremely high.

  Here, regarding steel underground storage tank facilities, there is a risk of leakage whether the tank should be replaced, whether the tank should be subjected to anticorrosion (external power supply method), or can be used as it is. When the evaluation is performed, the data on the locations corresponding to the buried years of the underground tank and the value of ground potential (O, Δ, ×, etc.) are obtained from the corrosion incidence prediction data table 70 shown in FIG. What is necessary is just to determine with reference.

  In the above embodiment, the case where the leakage inspection of the underground storage tank facility for storing fuel oil such as gasoline is performed is described as an example. However, the present invention is not limited thereto, and the present invention is not limited to the flammable fluid other than fuel oil. Of course, it can also be applied to underground storage tank facilities for storage.

It is a longitudinal cross-sectional view which shows one Example of an underground storage tank installation. It is a longitudinal cross-sectional view which shows the construction state of the underground storage tank 12. FIG. It is a figure which shows the measuring method of a measurement of ground potential. It is a figure which shows typically the corrosion generation | occurrence | production prediction rate data table 70. FIG.

Explanation of symbols

12 Underground storage tank 14 Weighing machine 18 Lubricating port 20 Venting port 22 Lubrication piping 24 Lubrication piping 26 Venting piping 32 Foundation 34 Installation base 50 Terminal box 52, 54 Reference electrode 60 Ground potential measuring device 70 Corrosion occurrence rate prediction data table

Claims (1)

A leakage risk evaluation method for an underground storage tank facility comprising a steel underground storage tank for storing a flammable fluid, etc.
For a number of underground storage tank facilities formed of the same coating material, a procedure for creating data on the amount of corrosion on the outer surface of the underground storage tank or occurrence of thinning due to corrosion,
A procedure for collecting data on the ground potential of the steel underground storage tank facility and the number of years since the underground storage of the steel underground storage tank facility;
The procedure for obtaining the relationship data of the amount of corrosion of the outer surface of the underground storage tank or the occurrence of thinning due to corrosion and the ground potential and the years elapsed since the underground storage tank equipment made of steel,
A procedure for predicting the corrosion occurrence probability or the risk of leakage due to corrosion of another underground storage tank made of the same coating material based on the relational data;
A leakage risk evaluation method for underground storage tank equipment, characterized by including:

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