Nitrogen replacement is a key process for natural gas pipeline before it is put into operation. A computational fluid dynamic model coupled to a species-transportation model has been used to investigate the gas mixture length of nitrogen replacement in large-diameter pipeline without isolator. A series of numerical simulations are performed over a range of conditions, including pipe length and diameter, inlet rate, and inclination angle of pipe. These affecting factors are analyzed in detail in terms of volume fraction of nitrogen, the maximum gas mixture length, and gas mixture length varied with time. Gas mixture length increases over time, and the maximum gas mixture length is present at outlet of pipe. Long and large-diameter pipe and fast speed of nitrogen lead to long length of mixed gas, while large inclination angle of pipe brings about short length. Several fitting formulas have been obtained, which can predict the maximum gas mixture length in gas pipelines. The used method of fitting formula is shown in the paper by examples. The results provide effective guidance for practical operation of nitrogen replacement.
Facing the increasing gas consumption, natural gas pipeline is progressing towards large diameter and long distance, resulting in high investment. It is very important to ensure the safety of gas pipeline operation. However, explosion is easily caused when the content of natural gas in air reaches 5~15% [
To ensure safety, isolator is commonly employed to separate the nitrogen and air. As shown in Figure
Schematic diagram of nitrogen replacement: (a) with isolator; (b) without isolator.
Several works in the literature focus on nitrogen injection to develop gas and gas condensate fields [
Since numerical simulation can provide detailed information of flow field which is not easily obtained by physical experiments and has advantages of low cost and short research time, in present work, CFD model coupling with a species-transportation model has been employed to investigate the gas mixture length of nitrogen replacement in large-diameter pipeline without isolator. The gas mixture length has been examined in terms of volume fraction of nitrogen. By conducting a series of numerical simulations, effects of pipe length, pipe diameter, inlet rate, and inclination angle of pipe are examined. Then, the maximum gas mixture length is analyzed and the fitting formulas are obtained. Using these formulas, we can predict the maximum gas mixture length in gas pipelines. The used method of fitting formula is shown in the paper by examples. The results provide useful guidance for practical operation of nitrogen replacement.
The remaining part of this paper is organized as follows. In Section
Straight pipe and undulating pipe with an inclined upward section are adopted in this study. Figure
Sketch of the geometry and numerical grid for computational domain: (a) straight pipe; (b) undulating pipe with an inclined upward section.
The undulating pipe consists of three sections: straight pipe section, bend section, and inclined upward section. The bend curvature (
Gas flow in pipeline is a symmetric problem, so two-dimensional flow simulation is accurate enough to capture the gas mixture length. In addition, three-dimensional simulation needs a higher CPU cost. Due to time limitations, 2D simulation is applied in this work. All geometry generation and meshing are performed using GAMBIT 2.3 mesh-generator. As shown in Figure
The velocity of gas flow is slow in pipe (2~5 m/s), so both nitrogen and air can be seen as incompressible fluids. In simulations, density and viscosity of nitrogen are defined as 1.138 kg/m3 and
The gas flows of nitrogen and air are governed by the Reynolds-Averaged-Navier-Stokes (RANS) equations, including continuity and momentum equations written as follows [
Reynolds number ranges from
There is no reaction between nitrogen and air in the replacement process, so a simplified species-transportation model is employed to capture the mixture of nitrogen and air written as [
Finite volume method (FVM) is employed to discretize above equations. All the calculations are performed using a commercial software package FLUENT 14.5. Patankar’s well-known SIMPLE algorithm [
Velocity inlet boundary condition is used for the inlet, and the inlet rates of nitrogen are taken as 2 m/s, 3 m/s, 4 m/s, and 5 m/s in comparing cases. In the outlet of computational domain, pressure outlet boundary condition is employed, and the value is defined as 0 Pa in order to facilitate comparative analysis. No slip boundary condition is imposed on the pipe wall.
Since it is an unsteady problem, the whole computational domain of pipe is defined as being filled with air at the initial time. And the time step in simulations is set to 0.001 s. The information of simulation cases is listed in Table
Simulation cases.
Case | The length of straight pipe, |
Pipe diameter, |
Inlet rate, |
Inclination angle, |
The length of inclined pipe, |
---|---|---|---|---|---|
1 | 50 | 647.2 | 3 | 0 | 0 |
2 | 150 | 647.2 | 3 | 0 | 0 |
3 | 300 | 647.2 | 3 | 0 | 0 |
4 | 400 | 647.2 | 3 | 0 | 0 |
5 | 500 | 647.2 | 3 | 0 | 0 |
6 | 600 | 647.2 | 3 | 0 | 0 |
7 | 700 | 647.2 | 3 | 0 | 0 |
8 | 800 | 647.2 | 3 | 0 | 0 |
9 | 900 | 647.2 | 3 | 0 | 0 |
10 | 1000 | 647.2 | 3 | 0 | 0 |
11 | 600 | 346.0 | 3 | 0 | 0 |
12 | 600 | 851.2 | 3 | 0 | 0 |
13 | 600 | 1000.2 | 3 | 0 | 0 |
14 | 600 | 647.2 | 2 | 0 | 0 |
15 | 600 | 647.2 | 4 | 0 | 0 |
16 | 600 | 647.2 | 5 | 0 | 0 |
17 | 600 | 647.2 | 3 | 10 | 600 |
18 | 600 | 647.2 | 3 | 20 | 600 |
19 | 600 | 647.2 | 3 | 30 | 600 |
20 | 600 | 647.2 | 3 | 40 | 600 |
Case 6 is adopted as the standard case. Figure
Gas mixture length at different times.
It is seen that the head of the mixed gas shows a bullet-shaped distribution. Frictional resistance at pipe wall is the major cause for this phenomenon. It is well known that there is a viscous sublayer near pipe wall. The closer the gas is to the wall, the greater the viscosity resistance gas gets. So the gas in the axis of pipe moves ahead. It is worth noting that the gas mixture length increases over time. At the time of 50 s, the gas mixture length is 9.5 m, while it increases to 18.5 m at the time of 200 s. In the mixed gas, the volume fraction of nitrogen gradually decreases along the pipe axis direction. And the decreasing rate is gradually reduced with time. This is because nitrogen is mixed with air more fully as time passes. However, the growth rate of mixture length is reducing over time. In the period from 50 s to 100 s, the mixture length is increased by 4 m, while it is increased by 3.5 m in the same interval of time (from 100 s to 150 s). And the increment is just 1.8 m for time increased from 150 s to 200 s. It can be explained that kinetic energy of nitrogen flow is consumed continuously as time goes. Therefore, the driving force and forward speed decrease gradually.
Since gas mixture length increases over time, the maximum mixture length will appear at the outlet of pipe. Figure
The maximum gas mixture length for different pipe lengths.
The gas mixture length curves, as shown in Figure
The gas mixture length varied with time in pipes of different lengths.
The longest pipeline used in simulation is just 1000 m. However, actual piping often has hundreds or thousands of kilometers. In order to predict the maximum gas mixture length in longer pipelines, the maximum gas mixture length (
The maximum gas mixture length (
If the actual parameters are the same as that used in this paper such as
Therefore, the actual replacement time is not simply equal to pipe length divided by the flow speed. Qualified replacement is achieved until the mixed gas flows out thoroughly. So the replacement time should be more than the sum of pipe length divided by flow velocity and the maximum gas mixture length divided by flow rate.
Figure
The maximum gas mixture length for different pipe diameters.
The gas mixture length varied with time in pipes of different diameters is shown in Figure
The gas mixture length varied with time in pipes of different diameters.
The actual pipe diameter is not only the four we have studied. So we also get the fitting formula of
The maximum gas mixture length (
Figure
The maximum gas mixture length at different inlet rates.
The gas mixture length varied with time at different inlet rates is shown in Figure
The gas mixture length varied with time at different inlet rates.
As shown in Figure
The maximum gas mixture length (
Taking the terrain into account, four undulating pipes with different inclination angles are analyzed. Figure
The maximum gas mixture length at different inclination angles.
Figure
The gas mixture length varied with time at different inclination angles.
The fitting formula of
The maximum gas mixture length (
A computational fluid dynamic model coupled to a species-transportation model has been used to investigate the gas mixture length of nitrogen replacement in large-diameter pipeline without isolator. Effects of pipe length and diameter, inlet rate, and inclination angle of pipe are examined by conducting a series of simulations. Based on the numerical results, the following conclusions can be drawn. Gas mixture length increases over time, and the maximum gas mixture length is present at outlet of pipe. Long and large-diameter pipe and fast speed of nitrogen lead to long length of mixed gas, while large inclination angle of pipe brings about short length. Four fitting formulas have been obtained, which can predict the maximum gas mixture length in gas pipelines. Besides that the formula for pipe length is a quadratic polynomial, the other formulas meet linear relationship. The calculation results provide effective guidance for practical operation of nitrogen replacement.
The authors declare that there is no conflict of interests regarding the publication of this paper.
Research work was supported by Open Fund (no. PLN1210) of State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University) and Key Project of Sichuan Provincial Education Department (No. 12ZA189). Without the support, this work would not have been possible.