阅读说明：本技术 防渗膜条件下罐底外壁强制电流阴极保护方法 (Forced current cathodic protection method for tank bottom outer wall under impermeable membrane condition ) 是由 牟华 黄春蓉 马含悦 唐强 屠海波 张国虎 刘海禄 施岱艳 张平 于 2019-10-15 设计创作，主要内容包括：本发明公开了一种防渗膜条件下罐底外壁强制电流阴极保护方法,包括如下步骤：第一步、收集阴极保护现场数据；第二步、确定阴极保护电流；第三步、确定阴极保护电源设备；第四步、确定阴极保护电源设备的保护电位调节范围；第五步、设置辅助阳极；第六步、确定参比电极。与现有技术相比,本发明的积极效果是：通过采用恒电位仪作为阴极保护供电电源,有效地将罐底外壁保护电位恒定维持在需要的区间范围；通过分析、计算、测试,在防渗膜工况下阴极保护电源设备保护电位调节范围确定为0V～-30V；采用网状阳极布置方式电位分布更均匀,效果更优。(The invention discloses a forced current cathodic protection method for the outer wall of a tank bottom under the condition of an impermeable membrane, which comprises the following steps: firstly, collecting cathodic protection field data; secondly, determining the cathodic protection current; thirdly, determining cathode protection power supply equipment; fourthly, determining the protection potential adjusting range of the cathode protection power supply equipment; fifthly, arranging an auxiliary anode; and sixthly, determining a reference electrode. Compared with the prior art, the invention has the following positive effects: by adopting the constant potential rectifier as a cathode protection power supply, the protection potential of the outer wall of the tank bottom is effectively kept constant within a required interval range; through analysis, calculation and test, the adjustment range of the protection potential of the cathode protection power supply equipment is determined to be 0V to-30V under the working condition of the impermeable membrane; the potential distribution is more uniform and the effect is better by adopting a mesh anode arrangement mode.)

1. A forced current cathodic protection method for the outer wall of a tank bottom under the condition of an impermeable membrane is characterized by comprising the following steps: the method comprises the following steps:

firstly, collecting cathodic protection field data;

secondly, determining the cathodic protection current;

thirdly, determining cathode protection power supply equipment;

fourthly, determining the protection potential adjusting range of the cathode protection power supply equipment;

fifthly, arranging an auxiliary anode;

and sixthly, determining a reference electrode.

2. The method for the cathodic protection of the forced current on the outer wall of the can bottom under the condition of an impermeable membrane according to claim 1, characterized in that: the cathodic protection field data of the first step includes:

(1) design data for tank farms and tanks;

(2) the anticorrosion layer data on the outer wall of the tank bottom is as follows: the coating comprises the types, insulating property and construction mode of anticorrosive coatings;

(3) power supply data: including the location of peripheral available power sources, possible sources of interference, and the presence or absence of stray currents;

(4) geological data: the method comprises the steps of (1) natural potential of a site where a storage tank is located, soil resistivity, underground water level, soil corrosion condition, frozen soil layer depth, bedrock depth and site conditions;

(5) peripheral cathodic protection operating data: including cathodic protection potential, output current, failure rate;

(6) peripheral metal structure data: includes the electric insulation and electric continuity between the storage tank and the peripheral metal structures and the connection mode with the ground electrode of the field.

3. The method for the cathodic protection of the forced current on the outer wall of the can bottom under the condition of an impermeable membrane according to claim 2, characterized in that: the tank farm and storage tank design data also includes the adjacent above-ground and below-ground metal structure distribution and cable conduit paths, as well as the type of impermeable membrane planned to be laid on the bottom of the storage tank, insulation, and location in the tank bottom foundation.

4. The method for the cathodic protection of the forced current on the outer wall of the can bottom under the condition of an impermeable membrane according to claim 2, characterized in that: the second step of the method for determining the cathodic protection current is:

(1) determining the value range of the protection current density according to the integrity and the insulativity of the anticorrosive coating on the outer wall of the tank bottom;

(2) on the basis of determining the value range of the protection current density, the cathodic protection current demand calculation of the outer wall of the tank bottom is carried out by utilizing a forced current cathodic protection calculation formula of the outer wall of the tank bottom.

5. The method for the cathodic protection of the forced current on the outer wall of the can bottom under the condition of an impermeable membrane according to claim 4, characterized in that: the third step is that the method for determining the cathodic protection power supply equipment comprises the following steps:

(1) selecting a potentiostat with the functions of constant potential, constant current, manual regulation, power on and power off test, overcurrent protection, lightning protection, alternating current interference resistance, automatic switching during fault and the like as cathode protection power supply equipment;

(2) determining the rated output current and the rated output voltage of the cathodic protection power supply equipment:

and determining the model of the cathodic protection power supply equipment according to the calculation result of the cathodic protection current, and ensuring that the rated output current, the rated output voltage and the power of the cathodic protection power supply equipment have margins of 10-50%.

6. The method for the cathodic protection of the forced current on the outer wall of the can bottom under the condition of an impermeable membrane according to claim 1, characterized in that: and fourthly, determining the protection potential adjusting range of the cathodic protection power supply equipment to be not less than 0V to-30V.

7. The method for the cathodic protection of the forced current on the outer wall of the can bottom under the condition of an impermeable membrane according to claim 1, characterized in that: fifthly, the auxiliary anode is a net-shaped anode formed by vertically and crosswise welding a titanium plated noble metal oxide anode strip and a titanium conducting strip together, and the anode net is laid in a sand cushion layer above the impermeable membrane and below the tank bottom plate.

8. The method for the cathodic protection of the forced current on the outer wall of the can bottom under the condition of an impermeable membrane according to claim 7, characterized in that: the connection point of the mesh anode and the anode cable is not less than 2.

9. The method for the cathodic protection of the forced current on the outer wall of the can bottom under the condition of an impermeable membrane according to claim 1, characterized in that: and sixthly, the reference electrode is in a double-electrode form of a long-acting copper sulfate electrode and a zinc reference electrode.

Technical Field

The invention relates to a forced current cathodic protection method for the outer wall of a tank bottom under the condition of an impermeable membrane.

Background

The external wall of the bottom of a domestic large vertical storage tank is generally applied with cathodic protection, but if an impermeable membrane is added at the bottom of the tank, the change of the corrosion environment at the bottom of the tank is caused due to the shielding and isolating action of the impermeable membrane on a cathodic protection electromagnetic field, the difference is generated on the parameters of the existing cathodic protection system such as equipment type selection, anode selection, resistance voltage drop and the like, and the deviation and the failure of the cathodic protection can be caused by the difference. The conventional large-scale vertical storage tank bottom outer wall cathodic protection is mainly provided with the following design key points:

1. forced current cathodic protection system for outer wall of tank bottom

The cathode protection method for the outer wall of the tank bottom of the domestic large vertical storage tank is divided into a sacrificial anode mode and a forced current mode, and the forced current cathode protection mode is mainly used. The forced current cathodic protection system is generally formed by connecting a cathodic protection power supply device, an auxiliary anode, a power-on point, a reference electrode, a testing device and the like through cables.

2. Cathodic protection power supply device type

The power supply device used by the forced current cathodic protection system mainly comprises two types of a rectifier and a constant potential rectifier.

The rectifier is a device for converting alternating current into direct current, has the advantages of simple structure, continuously adjustable output voltage and output current and simple and convenient operation and maintenance, but does not have the functions of automatically adjusting the output current and automatically controlling the protection potential when the load changes, and needs manual adjustment. In the design of a conventional storage tank, after a cathode protection system is put into operation, the interference degree of the outside to the tank bottom is low, the protection potential of the electrifying point on the outer wall of the tank bottom is relatively constant, and power supply equipment adopts a rectifier to meet the requirement.

3. Potentiostat control potential range

The potentiostat is a device which takes the measured potential of a reference electrode as a sampling signal and compares the sampling signal with a control potential to realize control and regulation of polarization current output and keep a current-carrying point on a set control potential, and has the functions of automatically regulating output current when load changes, automatically controlling protection potential and ensuring the constant potential of the current-carrying point. The potentiostat typically controls the potential range from 0V to-3V (relative to a copper/copper sulfate reference electrode), and the potential adjustment range is determined by the IR drop. IR drop refers to the resistance drop caused by the cathodic protection current flowing in the medium. The cathodic protection potential at the bottom of the tank is measured as the potential between the reference electrode and the protected body.

For the cathodic protection system of the tank bottom, as the reference electrode is arranged in the sand cushion layer below the bottom plate of the tank, the potential measured under the electrified condition between the reference electrode and the bottom of the tank consists of ① natural potential of the bottom plate of the tank, ② cathodic polarization potential of the bottom plate of the tank, ③ voltage drop caused by transition resistance between the bottom plate of the tank and the sand cushion layer, ④ voltage drop caused by loop resistance, ⑤ potential offset between the reference electrode and the anode and the like, wherein the ①② th item is a parameter which truly reflects the polarized degree of the bottom of the tank and is a capacitance property, and the other items are parameters of resistance property, and the voltage drop caused by the items can be collectively called IR drop.

In order to effectively evaluate the cathodic protection condition, the real cathodic polarization potential of the tank bottom plate needs to be obtained, and the actual polarization potential of the tank bottom plate can be known only by eliminating IR drop through the potential measurement value under the electrified condition. Since the IR drop is a constant of current and resistance, in order to eliminate the IR drop effect, a power-off potential test method recommended by the buried steel pipeline cathodic protection parameter measurement method (GB/T21246) is generally adopted, and the measured power-off potential is regarded as a polarization potential after the IR drop is eliminated. The value of the IR drop can be considered as the difference between the power-on potential and the power-off potential.

4. Auxiliary anode

The auxiliary anode matched with the tank bottom outer wall forced current cathodic protection system can select several modes of installing a deep well anode and a shallow high-silicon cast iron anode outside a tank foundation and installing a net anode, a linear anode and a shallow high-silicon cast iron anode in the tank foundation (usually in a sand cushion layer).

Because the ring beam (mainly steel bars in the ring beam) plays a certain role of insulating shielding layer on the anode outside the tank, the method of installing the net anode, the linear anode or the shallow high-silicon cast iron anode in the sand cushion layer of the tank bottom foundation is more suitable for the anode outside the tank (deep well anode outside the tank foundation and shallow high-silicon cast iron anode).

In the installation form of the tank bottom close to the anode, the linear anode is called as a linear anode, a flexible anode or a cable-shaped anode because the linear anode can be flexibly bent and is shaped like a thick cable, and the arrangement mode of the linear anode is generally divided into 2 types of spiral type and clip type. The linear anode has the characteristics of reliable conductivity, low anode consumption rate and long service life, and has the advantage of simple and convenient construction compared with a mesh anode. The installation position of the net anode is similar to that of the linear anode and is embedded in the sand cushion layer below the tank bottom plate, so that uniform protection current can be provided for the tank bottom plate, and the service life is longer; the disadvantages are that the construction is more complicated than that of a linear anode, and the number of welding points between the anode strip and the conducting strip is more. The price of the high-silicon cast iron anode is the lowest, but the high-silicon cast iron anode has the defects of large anode size, possible interference between anodes, insufficient uniformity of current distribution and the like.

5. Corrosive environment of tank bottom outer wall

For a vertical storage tank, after coating of paint on the outer wall of a tank bottom plate is finished, the tank bottom plate can be welded, a certain welding heat affected zone is formed near a welding seam, and the coating or repair of the paint on the welding seam and the steel surface of the heat affected zone cannot be carried out.

Tank bottom basis usually refers to the geotechnical layer that is surrounded by the girt under the tank bottom plate, and from the tank bottom plate down conventionally be: bituminous sand bedding-crushed stone bedding-original soil layer (or rammed soil layer). The asphalt sand cushion layer is in direct contact with the tank bottom plate, although the corrosion environment is lighter compared with the actual soil environment containing water, oxygen and various chemical media, the asphalt sand cushion layer can be influenced by the soil capillary phenomenon, the leakage of the edge of the tank bottom plate and the leakage of the concrete foundation part, and the moisture outside the foundation can permeate into the space between the outer wall of the tank bottom plate and the asphalt sand cushion layer through the soil capillary phenomenon, the cracks of the asphalt sand cushion layer, the gaps between the edge of the tank bottom plate and the concrete foundation and the like, so that accumulated water is generated, and the corrosion is caused to the weak part of the coating of the outer wall of the tank bottom plate.

Along with the increase of the service time of the storage tank, the risk of degradation and viscosity loss exists in the anticorrosive coating of the tank bottom plate, and the corrosion risk also exists in the later stage of the tank bottom plate. Extensive investigations have shown that the corrosion of the outer bottom of the tank is generally severe without cathodic protection or with incomplete cathodic protection.

6. Influence of liquid level change of storage tank on cathodic protection of tank bottom

The taper of 0.8% -1.5% is designed for the bottom plate of a 10-ten-thousand-square storage tank generally, when the tank is full, the bottom plate of the tank is tightly contacted with an asphalt sand cushion layer, the cathodic protection current can fully protect the position where the bottom plate of the tank is deficient, but when the oil storage amount is reduced, the contact degree of the bottom plate of the tank and the asphalt sand cushion layer is changed, the obvious change is that a central hollow drum is generated, the distance of nearly 7mm can be generated at the maximum position of the center, the cathodic protection current cannot reach a protected body due to the gap between the protected body and electrolyte, and the output current of cathodic protection power supply equipment can be changed along with the change of the liquid level.

In recent years, along with the attention on environmental protection, the situation that an impermeable membrane is additionally arranged in the foundation of a domestic large-scale storage tank is more and more. The impermeable membrane is an insulating geomembrane which is laid in a tank foundation and is usually positioned below a bituminous sand layer and a sand cushion layer, and the insulating geomembrane takes high-density polyethylene and geotextile as main materials. The main function of the anti-seepage film is to prevent the diffusion and pollution to the surrounding environment when oil and water media are leaked. Because the anti-seepage film has an insulation effect, when the anti-seepage film and the ring beam form an insulation shielding layer together, the electromagnetic field environment of the cathode protection system on the outer wall of the tank bottom is different from that of a conventional storage tank without the anti-seepage film. Therefore, under the condition of impermeable membrane, the design method for cathodic protection of the outer wall of the bottom of the vertical storage tank needs to be improved.

Disclosure of Invention

In order to overcome the defects of the prior art, the invention provides a method for cathodic protection of forced current on the outer wall of the tank bottom under the condition of an impermeable membrane.

The technical scheme adopted by the invention is as follows: a forced current cathodic protection method for the outer wall of a tank bottom under the condition of an impermeable membrane comprises the following steps:

firstly, collecting cathodic protection field data;

secondly, determining the cathodic protection current;

thirdly, determining cathode protection power supply equipment;

fourthly, determining the protection potential adjusting range of the cathode protection power supply equipment;

fifthly, arranging an auxiliary anode;

and sixthly, determining a reference electrode.

Compared with the prior art, the invention has the following positive effects:

1. power supply device type selection

The invention determines the relation between the contact area between the outer wall of the tank bottom and the soil cushion layer of the tank bottom and the required cathodic protection current through numerical simulation calculation and field verification test of the cathodic protection potential of the storage tank during the empty and full tank periods under the impermeable membrane condition and the output current and the protection potential of the power supply equipment, and effectively maintains the protection potential of the outer wall of the tank bottom in a required interval range by adopting a constant potential rectifier as a cathodic protection power supply.

2. Power supply device control potential parameter

The core of the potentiostat control module is a comparison amplifier, as long as the amplification factor of the instrument is calculated and designed correctly, the sampling signal characteristic and the input and output characteristics of the instrument have enough linearity and are consistent, the regulation is good, and the potentiostat can make the potential of the electrified point of the protected object consistent with the set control potential within the rated working range.

The amplification is typically determined based on the cathodic protection loop resistance. Under the condition that an impermeable membrane is arranged in a storage tank foundation, the IR drop value in a cathode protection loop far exceeds the output potential regulation range of a conventional potentiostat through the test and calculation of the built storage tank, and the requirement can not be met by 0V-3V. The invention determines the protection potential regulating range of cathode protection power supply equipment to be 0V to-30V under the working condition of an impermeable membrane through analysis, calculation and test.

3. Auxiliary anode optimum mode

After the cathode protection system for the outer wall of the tank bottom is put into operation in China, the uniformity of protection potential in various anode distribution modes is not specially researched, and by simulation calculation and field test verification of the cathode protection system of the built storage tank under the condition of an impermeable membrane, the auxiliary anode for cathode protection for the outer wall of the tank bottom can adopt three modes, namely a net anode, a linear anode spiral mode and a linear anode clip mode, wherein the potential distribution of the net anode arrangement mode is more uniform, and the effect is more excellent.

Detailed Description

A forced current cathodic protection method for the outer wall of a tank bottom under the condition of an impermeable membrane comprises the following steps:

the first step is as follows: cathodic protection field investigation

(1) Collecting tank field and tank design data

Investigating and knowing design information of a tank field and a storage tank, distribution of adjacent overground and underground metal structures, paths of cable conduits and the like; in particular the type of impermeable membrane planned to be laid at the bottom of the storage tank, the insulation, the position in the tank bottom foundation, etc.;

(2) designing or collecting tank bottom outer wall anticorrosive coating

The anticorrosive coating is the main means for preventing the surface of the steel from being corroded, and the cathode protection system is used for supplementing the defects of the anticorrosive coating. The outer wall of the tank bottom is provided with an anticorrosive coating with excellent insulating property, and the type, insulating property, construction mode and the like of the anticorrosive coating are collected;

(3) investigating power supply situation

Investigating available power source positions around, possible interference sources, the existence of stray current and the like;

(4) geological survey

Investigating natural potential, soil resistivity, underground water level, soil corrosion condition, frozen soil layer depth, bedrock depth, field conditions and the like of a site where the storage tank is located;

(5) investigation of peripheral cathodic protection

Existing or planned cathodic protection systems and their operating parameters around the site;

(6) investigation of surrounding metal structures

The electrical insulation and continuity between the storage tank and the surrounding metal structures, the connection mode with the ground electrode, and the like.

The second step is that: cathodic protection current calculation

(1) Determining a protection current density value range

And determining the value of the protection current density according to the completeness and the insulativity of the anticorrosive coating on the outer wall of the tank bottom, which are known through investigation. If the outer wall of the tank bottom is not coated with an anticorrosive coating, or the anticorrosive coating has poor integrity and is seriously peeled or falls off, the value range is preferably 10mA/m according to the regulation of GB 50393 technical Specification for anticorrosive engineering of steel petroleum storage tanks²～20mA/m²(ii) a If the integrity and the insulativity of the anticorrosive coating are good, the value range is preferably 5mA/m according to the stipulation of SY/T0088 technical Standard for cathodic protection of the outer wall of the bottom of the steel storage tank²～10mA/m²。

(2) Cathodic protection current calculation

Substituting the estimated protection current density value according to a calculation formula of the cathodic protection of the forced current on the outer wall of the tank bottom specified in GB 50393 technical Specification for anticorrosion engineering of steel petroleum storage tanks to calculate the cathodic protection current demand on the outer wall of the tank bottom.

The third step: cathodic protection power supply equipment model selection

(1) Cathode protection power supply equipment selection type and main function

The cathodic protection power supply equipment is of a potentiostat type, and the potentiostat has the functions of constant potential, constant current, manual regulation, power on and power off test, overcurrent protection, lightning protection, alternating current interference resistance, automatic switching in case of failure and the like.

(2) Determination of rated output current and rated output voltage of cathodic protection power supply equipment

According to the calculation result of the cathodic protection current, the model of the cathodic protection power supply equipment is determined, namely the rated output current and the rated output voltage of the equipment, and the power of the power supply equipment has a margin of 10-50%.

The fourth step: protection potential adjustment range of cathode protection power supply equipment

Because the tank bottom is provided with the anti-seepage film, the protection potential adjusting range of the cathode protection power supply equipment under the condition of the anti-seepage film is not less than 0V to-30V.

The fifth step: selection of auxiliary anodes

(1) The anode mesh is formed by welding a titanium plated noble metal oxide anode strip and a titanium conducting strip together in a vertical and crossed manner, and is laid in a sand cushion layer on the impermeable membrane and below the tank bottom plate. The spacing between the anode strips and the conducting strips is selected according to the specification of GB 50393.

(2) In order to ensure the uniform distribution of cathodic protection current in the anode network, the connection point of the anode cable and the mesh anode is not less than 2.

And a sixth step: reference electrode

The reference electrode is preferably a dual-electrode form of a long-acting Copper Sulfate (CSE) electrode and a zinc reference electrode.