Natural gas long-distance pipeline blockage positioning method

Abstract

The invention discloses a natural gas long-distance pipeline blockage positioning method, and belongs to the field of natural gas pipeline blockage detection. The method comprises the steps of 1, collecting basic data and operation data of a natural gas pipeline; 2, judging whether blockage occurs or not by adopting a pipeline simulation method; thirdly, according to the pipeline basic data and the operation data set obtained in the first step, a natural gas pipeline blockage positioning mathematical model is established through mathematical formula derivation; and 4, calculating a coefficient matrix and a result vector of the mathematical model in the step 3, and solving the mathematical model to carry out blockage positioning. According to the natural gas long-distance pipeline blockage positioning method, pipeline blockage can be quickly positioned, the problems that an existing method is short in detection distance and long in detection time are solved, and guidance is provided for safety and operation management of the natural gas conveying pipeline.

Description

Technical Field

[0001] The invention belongs to the technical field of natural gas pipeline blockage detection, and in particular relates to a method for locating a blockage in a long-distance natural gas pipeline. Background Art

[0002] During the operation of a natural gas pipeline, saturated natural gas may precipitate free water, heavy hydrocarbons, etc. due to failure of the upstream dehydration separator equipment, incomplete pipeline drying, temperature and pressure changes, etc., and easily deposit in low-lying areas of the pipeline or even form hydrates, blocking the pipeline. Existing natural gas pipeline blockage detection methods mainly include hardware-based methods and software-based methods. Hardware-based methods are difficult to detect over long distances, and software-based methods are complex to model and take a long time to detect. Based on this, the present invention proposes a blockage location method suitable for long-distance natural gas pipelines, which has simple modeling and fast calculation speed.

[0003] First, there are many reasons for blockage, and the distribution of blockage in the pipeline is also variable. In order to reduce the complexity of blockage calculation and take into account the rationality of blockage parameters, the boundary of the blockage material can be regularized, and the pipeline containing blockage can be regarded as a pipeline with a smaller inner diameter. Figure 1 It can be found that this processing method simplifies the complex blockage situation while retaining important blockage parameters, which is convenient for theoretical analysis and mathematical modeling. In this regard, when judging whether a blockage occurs along the pipeline, it can be characterized by the inner diameter of the pipeline. The smaller the inner diameter of the pipeline, the greater the degree of blockage. Secondly, the blockage may occur anywhere in the pipeline, the degree of blockage may be large or small, and the reasons for the blockage may also be varied, but the manifestation after the pipeline is blocked is that the inner diameter of the pipeline is reduced, the local resistance loss is increased, and the operating pressure of the pipeline is changed. Therefore, the present invention transforms the problem of detecting the position of the natural gas pipeline blockage into the problem of determining the inner diameter of the pipeline by establishing a relationship between the pressure boundary conditions at both ends of the pipeline and the inner diameter of the pipeline, and proposes a mathematical model for locating the blockage of a natural gas long-distance pipeline and a method and application for establishing it, which solves the problems of short detection distance and high cost of existing hardware and complex mathematical model establishment process and long detection time of software methods, and avoids blindness and unnecessary cost and time consumption in the detection process. Summary of the invention

[0004] The present invention provides a method for locating a blockage in a long-distance natural gas pipeline, which realizes pipeline blockage position detection by combining pipeline operation parameters, thereby overcoming the problems of long detection time and short distance in existing natural gas pipeline blockage detection methods.

[0005] The above-mentioned method for locating a blockage in a long-distance natural gas pipeline comprises the following steps:

[0006] Step 1: Collect basic data and operation data of natural gas pipelines, including transmission medium components, pipeline length, pipeline elevation, pipeline inner diameter, and pipeline mass flow, starting pressure, starting temperature, end pressure and end temperature data sets under various steady-state operating conditions;

[0007] Step 2: Determine whether the natural gas pipeline is blocked. There are many methods for determining blockage. The present invention uses pipeline simulation technology to calculate whether the difference between the simulated value and the measured value of the pressure difference between the starting point and the end point when the pipeline is not blocked reaches a set threshold, so as to determine whether blockage occurs. Specifically, the following steps are included:

[0008] Step 2.1, use OLGA software to model the pipeline to be inspected and set basic data, including pipeline length, elevation along the pipeline, pipeline inner diameter and natural gas quality components;

[0009] Step 2.2, setting boundary conditions, including setting the end pressure, starting temperature and mass flow rate. The starting pressure and end temperature of the pipeline under the boundary conditions can be obtained through simulation calculation;

[0010] Step 2.3, according to the obtained pipeline operation data set, repeat step 2.2 to obtain the simulation value of the pipeline starting pressure under various working conditions, calculate the average pressure difference ΔP1 and the actual average pressure difference ΔP2 between the starting pressure simulation value and the end pressure, and judge ΔP1-ΔP2>εL, ε is the given detection threshold, and L is the pipeline length. If the difference is greater than ε, it is considered that the pipeline is blocked; in the long-distance natural gas pipeline, the pressure drop is usually 15-25kPa / km, so the detection threshold ε is set to 25kPa / km.

[0011] If step 2 determines that the pipeline is blocked, then step 3 is entered to establish a mathematical model for blocking location.

[0012] Step 3: Based on the pipeline basic data and operation data set obtained in step 1, a mathematical model for locating natural gas long-distance pipeline blockage is established by deducing mathematical formulas:

[0013] Step 3.1, based on the discretization idea of ​​the finite difference method, divide the pipeline to be solved into n pipe segment grids and n+1 cross sections. Figure 2 Shown is a schematic diagram of pipeline discretization.

[0014] For the first pipeline grid, the following relationship can be established based on the natural gas pipeline pressure drop formula:

[0015]

[0016] Where: P1 - the pressure of the second section of the pipeline, the subscript indicates the section number, Pa;

[0017] P2——pressure of the second section of the pipeline, Pa;

[0018] a1——the first grid related calculation coefficient, the subscript indicates the grid number;

[0019] Z1——the first grid compression factor, the subscript indicates the grid number;

[0020] R——gas constant, 8314.3, m 2 / (s 2 K);

[0021] T1——the temperature of the first section of the pipeline, the subscript indicates the section number, K;

[0022] T2——the temperature of the second section of the pipeline, K;

[0023] λ1——friction coefficient of the first grid, the subscript indicates the grid number;

[0024] L——pipeline length, m;

[0025] n——grid number;

[0026] M——pipeline flow rate, kg / s;

[0027] D1——Inner diameter of the first grid pipe, the subscript indicates the grid number, m;

[0028] △S1——the elevation difference between the starting and ending points of the first grid, where the subscript indicates the grid number, m;

[0029] S1——the elevation of the first section of the pipeline, the subscript indicates the section number, m;

[0030] S2 - elevation of the second section of the pipeline, m.

[0031] in:

[0032]

[0033]

[0034]

[0035]

[0036]

[0037]

[0038] Where: g——gravitational constant, m 2 / s.

[0039] Re1——Reynolds number of the first grid, the subscript indicates the grid number;

[0040] μ1——dynamic viscosity of natural gas in the first grid, the subscript indicates the grid number, Pa.s;

[0041] M g ——Relative molecular mass of natural gas, kg / kmol;

[0042] △——Relative density of natural gas.

[0043] make:

[0044]

[0045] Substituting formula (7) into formula (1) yields:

[0046]

[0047] In summary, the corresponding relationship between the pressure, temperature, flow rate, starting and ending point elevations, pipeline length and inner diameter of the first grid pipe segment at both ends was obtained.

[0048] Step 3.2: For the second section of the pipeline grid, the mathematical relationship (8) is also established:

[0049]

[0050] Multiplying equation (9) by (1+a1△S1) and adding it to equation (8), eliminating P2, we get

[0051]

[0052] For all grids, the relationship shown in formula (8) can be established. Repeat the above elimination operation to eliminate the unknown node pressure in the middle of the pipeline, and obtain the relationship between the starting and ending pressures of the pipeline, the mass flow rate and the inner diameter of each grid pipeline:

[0053]

[0054] make:

[0055]

[0056]

[0057] Formula (11) can be written as:

[0058]

[0059] Step 3.3, for m operating conditions, the superscript represents the operating condition number, repeating steps 3.1 and 3.2, we can get the relationship between the starting and ending pressures of the pipeline, the mass flow rate and the inner diameter of each grid pipeline under different operating conditions, and the combined equations can be obtained:

[0060]

[0061] Step 3.4, the left side of each row of equation (15) can be calculated from the boundary conditions of the starting and ending pressures, flow rates and pipeline elevations under each operating condition; the right side of the equation a and b are approximately calculated according to the linear distribution of temperature and pressure along the pipeline, and c, O, and L can also be calculated based on the elevation and length of each section, which are also known numbers. Therefore, we can set:

[0062]

[0063]

[0064]

[0065] Formula (15) can be written as follows, which is the established mathematical model for locating the blockage of a long-distance natural gas pipeline:

[0066]

[0067] Write the above system of equations in matrix form:

[0068] KX=F (20)

[0069] Among them, X is defined as the solution vector, K is the coefficient matrix, and F is the result vector:

[0070] X=(d1,d2,d3...d n ) T (twenty one)

[0071]

[0072] F=(F 1 ,F 2 ,F 3 ...F m ) T (twenty three)

[0073] Step 4: Solve the linear equations of formula (20) to obtain the inner diameter of each grid pipe, which can describe the actual flow area of ​​the current pipe, determine whether blockage occurs, and locate the blockage by numbering the pipe segments. The specific steps are as follows:

[0074] Step 4.1, first, for the first working condition, according to the starting pressure, end pressure, starting temperature, end temperature and number of grids, considering that the decreasing speed of pressure and temperature changes linearly, calculate the pressure and temperature values ​​on each section;

[0075]

[0076]

[0077] Step 4.2: For each cross section, if the temperature and pressure are known, the compression factor Z of each grid can be calculated according to formula (3): i , i = 1, 2, ..., n, and then calculate a for each grid according to formula (2) i ,i=1,2,...,n; According to formula (6), the gas viscosity of each cross section is obtained, where the relative molecular mass and relative density of the gas are known values. Then, the b of each cross section is calculated by formula (7): i , i=1,2,...,n; the elevation of each section is known, the length of the grid is known, and the O of each grid can be calculated according to formulas (12) and (13): i , i = 1, 2, ..., n and c of each section i ,i=1,2,...,n+1, and then use formula (17) to get the k of each grid i , i = 1, 2, ..., n, the starting pressure, end pressure, mass flow rate and c are known n+1 The result value F of this working condition can be calculated by formula (16).

[0078] Step 4.3, repeat steps 4.1-4.2 for m working conditions to obtain the coefficient matrix K and result vector F of the mathematical model. Step 4.4, use the least squares method to solve the mathematical model, and the matrix form of the objective function can be expressed as:

[0079] f(X)=||KX-F|| 2 =(KX-F) T (KX-F) (26)

[0080] The minimum value of the objective function is where the derivative is 0, that is:

[0081]

[0082] We can get:

[0083]

[0084] Right now:

[0085] K T KX=K T F (29)

[0086] The least squares solution is:

[0087] X=(K T K) -1 K T F (30)

[0088] The coefficient matrix K and the result vector F are known, and X can be obtained by formula (30). Then the real inner diameter of each grid pipe is calculated by (18). Then the calculated inner diameter is divided by the designed inner diameter to obtain the percentage of the calculated inner diameter of the pipe to the designed value. When the percentage is less than 80%, it can be determined that the pipeline is blocked, and the location of the pipeline blockage can be determined by the grid number.

[0089] The present invention adopts the above technical solution, and can achieve the following beneficial effects:

[0090] Based on the natural gas pipeline pressure drop formula, a mathematical model for locating blockages in long-distance natural gas pipelines under various working conditions was established through formula derivation. The model was solved to obtain the true inner diameter value of each grid in the pipeline, which was used to characterize the severity of blockages at different positions in the pipeline, and to achieve rapid detection of pipeline blockage positions. The blockage location model of the method of the present invention is simple to model, and the solution method does not involve numerical iterative calculations. It has the advantages of clear calculation process, convenient operation, and fast calculation speed. In addition, the positioning accuracy of the method of the present invention is related to the accuracy of grid division. The more grid divisions, the higher the solution accuracy. After using this method to perform preliminary positioning of pipeline blockages, more accurate hardware methods can be used to accurately detect blockages. BRIEF DESCRIPTION OF THE DRAWINGS

[0091] Figure 1 This is an equivalent schematic diagram of the present invention for processing pipeline blockage.

[0092] Figure 2 It is a schematic diagram of pipeline discrete processing of the present invention.

[0093] Figure 3 This is a step diagram of the method for calculating the blockage position of a long-distance natural gas pipeline according to the present invention. DETAILED DESCRIPTION

[0094] In order to make the purpose, technical solution and advantages of the present invention more clear, the present invention is now described in detail with examples. Example: The natural gas composition of the XQDS pipeline is 96% ethane, 2% methane, 2% propane, with a relative molecular mass of 16.88 and a relative density of 0.58. The total length of the pipeline is 2000m, the starting height is 1000m, the end height is 3000m, and the inner diameter of the pipeline is 500mm. There are hydrate plugs of 5m and 3m in length at 734m and 1330m in the pipeline respectively. The blockage position is detected by the method provided by the present invention.

[0095] Step 1: Collect basic data of XQDS pipeline: The composition of natural gas transported is 2% CH4, 96% C2H4, 2% C3H8, relative molecular mass 16.88, relative density 0.58. The total length of the pipeline is 2000m, the starting height is 1000m, the end height is 3000m, the inner diameter of the pipeline is 500mm, and the operating parameters at both ends of the pipeline are shown in Table 1 below.

[0096] Table 1 XQDS pipeline operation parameter set

[0097]

[0098]

[0099] Step 2: Determine whether a blockage occurs.

[0100] Step 2.1, use OLGA software to model the pipeline to be inspected, the total length of the pipeline is 2000m, the starting height is 1000m, the end height is 3000m, and the inner diameter of the pipeline is 500mm;

[0101] Step 2.2, set the boundary conditions. Taking the first working condition as an example, set the end pressure to 4000 kPa, the starting temperature to 270 °C and the mass flow rate to 20 kg / s. Through simulation calculation, the starting pressure under the boundary conditions is 4531 kPa.

[0102] Step 2.3, repeatedly calculate the starting pressure of the unblocked pipeline under multiple sets of operating conditions, as shown in Table 2 below.

[0103] Table 2 Pipeline simulation results

[0104]

[0105] The average pressure difference between the start and end points before the blockage is calculated to be 584 kPa, and the current operating pressure difference is 653 kPa, a difference of 69 kPa, which exceeds the set threshold of 25 kPa / km×2km=50 kPa. Therefore, it can be determined that the pipeline is blocked at this time.

[0106] Step 3: divide the pipeline into grids n=20, the number of operating conditions 20, and establish the pipeline blockage detection mathematical model shown in formulas (20) to (23).

[0107] Step 4: Calculate the relevant parameters of the mathematical model in step 3, solve the blockage location mathematical model to obtain the inner diameter of each grid pipe, and locate the blockage by numbering the pipeline segments.

[0108] Step 4.1, for the first working condition, the starting pressure is 4.59MPa, the end pressure is 4.00MPa, the starting temperature is 270K, the end temperature is 260K and the number of grids is 20. Considering that the decreasing speed of pressure and temperature changes linearly, calculate the pressure and temperature values ​​on each grid interface, see Table 3 below.

[0109] Table 3 Calculation results of temperature and pressure along the pipeline

[0110]

[0111] Step 4.2: For each grid, if the temperature and pressure are known, the compression factor Z of each section can be calculated according to formula (3): i , i=1,2,...,n+1, and then calculate a of each section according to formula (2) i ,i=1,2,...,n+1. The gas viscosity of each cross section is obtained according to formula (6), where the relative molecular mass and relative density of the gas are known values. Then the b of each cross section is calculated by formula (7): i ,i=1,2,...,n+1. The elevation of each section is known, and the grid length is known. The O of each section can be calculated according to formulas (12) and (13). i ,i=1,2,...,n+1 and c i , i=1,2,...,n+1, and then the k of each section is obtained by formula (17) i , i=1,2,...,n+1, the starting pressure, end pressure, mass flow rate and c are known n+1 It can be calculated by formula (16) to get F = 1.184 × 10 10 The calculated values ​​of the intermediate coefficients are shown in Table 4.

[0112] Step 4.3, repeat steps 4.1-4.2 for each working condition to obtain the coefficient matrix K and result vector F of the linear equations, as shown in Table 5.

[0113] Step 4.4, given the coefficient matrix K and the result vector F, the solution vector X is calculated by formula (30), and the inner diameter of each grid is shown in Table 6. Table 6 shows that there is pipeline blockage at 700-800m and 1300-1400m. If there is more operating data, more grids can be divided to improve positioning accuracy. In addition, hardware detection methods such as ray detection can be used to accurately detect the blockage of these two pipelines.

[0114] Table 6 Calculation results of pipeline mesh inner diameter

[0115] Grid Number 1 2 3 4 5 6 7 8 9 10 Pipe inner diameter (m) 0.5 0.5 0.5 0.5 0.5 0.5 0.22 0.5 0.5 0.5 Grid Number 11 12 13 14 15 16 17 18 19 20 Pipe inner diameter (m) 0.5 0.5 0.25 0.5 0.5 0.5 0.5 0.5 0.5 0.5

[0116] The improvements of this method are as follows: 1. The proposed mathematical model for locating natural gas pipeline blockage is simple to model. It only requires the preparation of a program in advance, the input of boundary condition parameters, natural gas relative molecular mass and relative density, pipeline structure parameters and the number of grids to quickly solve the model correlation coefficient; 2. The proposed model can be applied to pipelines of any transmission distance; 3. The model is solved directly by matrix, without the need for iterative calculation, which greatly reduces the calculation time.

[0117] Therefore, the natural gas long-distance pipeline blockage locating method proposed in the present invention makes up for the shortcomings of the existing methods of short detection distance and slow speed. It has the characteristics of clear calculation process, convenient operation and fast calculation speed, and provides guidance for the safe, economical and efficient design and operation management of natural gas transmission pipelines.

[0118] Table 4 Calculation results of intermediate parameters

[0119] Grid Number 1 2 3 4 5 6 7 8 9 10 Z 0.909 0.91 0.91 0.911 0.912 0.912 0.913 0.913 0.914 0.915 <![CDATA[a×10 -6 ]]> 9.6 9.62 9.63 9.64 9.65 9.66 9.67 9.68 9.7 9.71 O 1.01 1.011 1.012 1.013 1.014 1.015 1.016 1.017 1.018 1.019 S 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 c 1 1.001 1.002 1.003 1.004 1.005 1.006 1.007 1.008 1.009 <![CDATA[b×10 3 ]]> 32.91 32.86 32.81 32.76 32.71 32.66 32.61 32.56 32.51 32.46 <![CDATA[k×10 3 ]]> 3324 3325 3327 3328 3330 3331 3332 3334 3335 3336 Grid Number 11 12 13 14 15 16 17 18 19 20 Z 0.915 0.916 0.916 0.917 0.918 0.918 0.919 0.919 0.92 0.921 <![CDATA[a×10 -6 ]]> 9.72 9.73 9.74 9.76 9.77 9.78 9.79 9.8 9.82 9.83 O 1.02 1.021 1.022 1.023 1.024 1.025 1.026 1.027 1.028 1.029 S 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900 c 1.01 1.011 1.012 1.013 1.014 1.015 1.016 1.017 1.018 1.019 <![CDATA[b×10 3 ]]> 32.41 32.36 32.31 32.26 32.21 32.16 32.11 32.06 32.01 31.95 <![CDATA[k×10 3 ]]> 3338 3339 3340 3342 3343 3344 3345 3347 3348 3349

[0120] Table 5 Coefficient matrix and result vector calculation values

[0121]

[0122]

Claims

1. A method for locating a blockage in a long-distance natural gas pipeline, characterized in that: The following steps are involved: Step 1: Collect basic data and operation data of natural gas pipelines, including transmission medium components, pipeline length, pipeline elevation, pipeline inner diameter, and pipeline mass flow, starting pressure, starting temperature, end pressure and end temperature data sets under various steady-state operating conditions; Step 2: Use pipeline simulation to calculate whether the difference between the simulated value and the measured value of the pressure difference between the starting point and the end point when the pipeline is not blocked reaches a set threshold, so as to determine whether blockage occurs; Step 3: If there is a pipeline blockage, a mathematical model for locating a natural gas long-distance pipeline blockage is established by deducing the basic pipeline data and operation data set obtained in step 1 through mathematical formulas; Step 4: Calculate the coefficient matrix and result vector of the mathematical model in step 3, solve the blockage location mathematical model to obtain the inner diameter of each grid pipe, and locate the blockage by numbering the pipeline segments.

2. A method for quickly locating a blockage in a long-distance natural gas pipeline as claimed in claim 1, characterized in that: The mathematical model for locating the blockage of a long-distance natural gas pipeline established in step 3 is as follows: The pipeline is discretized and m operating condition data are combined to establish a mathematical model for locating blockage in long-distance natural gas pipelines: KX=F (1) Among them, X is defined as the solution vector, K is the coefficient matrix, and F is the result vector: X=(d1,d2,d3...d n ) T (2) F=(F 1 ,F 2 ,F 3 ...F m ) T (4) Where: m——number of pipeline operating conditions; j——pipeline operation condition number; n——Number of pipeline discrete grids; i——pipe section / grid number; Subscript 1 indicates the starting point, n+1 indicates the end point, and the pressure and temperature values ​​on each grid interface are calculated as follows; Where: T——pipeline section temperature, K; P——pipeline section pressure, Pa; F is calculated as follows: Where: M——pipeline flow rate, kg / s; The coefficient c of each section is calculated as follows: In the formula: a——correlation calculation coefficient; △S——elevation difference between the starting and ending points of the grid, m; g——gravitational constant, m 2 / s; Z——grid compression factor; R——gas constant, 8314.3, m 2 / (s 2 ·K); the coefficient k of each grid is calculated as follows: in: Where: L——pipeline length, m; μ——dynamic viscosity of natural gas, Pa.s; Mg——relative molecular mass of natural gas, kg / kmol; S——elevation of pipeline section, m; △——Relative density of natural gas; The coefficient d for each grid is calculated as follows: Where: D is the inner diameter of the grid pipe, m.

Images