Study on the Influencing Factors of Gas Mixing Length in Nitrogen Displacement of Gas Pipeline Kun Huang 1,a Yan Xian 2,b Kunrong Shen 3,c

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1 Applied Mechanics and Materials Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Study on the Influencing Factors of Gas Mixing Length in Nitrogen Displacement of Gas Pipeline Kun Huang 1,a Yan Xian 2,b Kunrong Shen 3,c 1 Southwest Petroleum University, Institute of Petroleum Engineering, Chengdu, Sichuan, China. 2 Southwest Petroleum University, Institute of Petroleum Engineering, Chengdu, Sichuan, China 3 Southwest Petroleum University, Institute of Petroleum Engineering, Chengdu, Sichuan, China a swpuhk_yjs@126.com, b xianyan04@163.com, c Keywords: Gas pipeline; Displacement; Gas mixing length; Simulation; Influence law Abstract. The gas pipeline commissioning replacement is a vital section before the pipes being put into operation. It is related to the gas mixture length of segment regardless the replacement method, the gas mixing length theory formula is derived based on diffusion theory and convection diffusion equation. Taking the impact factors of gas movement into account, with software FLUENT, this paper simulated different flow regimes, pipe length, diameter, velocity, back pressure and temperature to influence the gas mixing length, the general influence law of various factors is identified. That is the gas mixing length increases with the increasing of pipe length, diameter, velocity, back pressure and temperature, the corresponding mixing length and displacement time are longer at laminar flow. The research results provide theoretical basis for the natural gas pipeline commissioning replacement to determine reasonable parameters. Introduction The domestic gas pipeline has made rapid progress in resent years. Many natural gas pipelines, such as Northeast and Zhong Wei-Gui Yang natural gas pipeline network, are in the production stage. Furthermore, China-Myanmar pipeline, the Sino-Russian pipeline, West-east gas transmission line three and four will also be put into operation in the years of The safe and economic operation of the domestic natural gas pipeline is the main task of the pipeline industry in the last few years. The nitrogen displacement process before the production of the natural gas pipeline stays in empirical construction stage in China. There is no theory support and it is generally through the construction to accumulate experience, where a certain blindness of the displacement exists. In order to improve the safety and efficiency in the production and replacement process of the gas pipeline, it is necessary to conduct the scientific and systemic research on the influence of gas mixing length in production replacement of natural gas pipeline. Due to the diffusion and mass transfer of natural gas pipeline, it becomes particularly complex of nitrogen injection in the replacement process, which is also influenced by the pipe length, diameter, flow rate, back pressure and temperature. This article provides a theoretical basis for the realization of the scientific, economic, efficient, safe nitrogen replacement in the natural gas pipeline by simulating six different factors on the impact of the gas mixing length. Models development There is almost no change of pipe flow and concentration fields in axis direction in the nitrogen displacement process, therefore it can be seen as a two-dimensional problem. The tube flow goes through the inlet section and then becomes full development of the flow. The geometric model and All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-06/03/16,03:35:32)

2 300 Mechatronics and Industrial Informatics boundary conditions of the pipeline are shown in Figure 1, the initial conditions are set as that the mass fraction of nitrogen is 100% for the pipe inlet; the rest of the pipe section, the nitrogen mass fraction is 0. Pressure inlet Wall Pressure outlet D L Fig. 1 2D pipeline geometric model and boundary conditions Influencing law of gas mixing length According to the theory of diffusion and mass transfer theory, the theoretical calculation of the gas mixing length ( L mix ) can be derived as the following equation. L mix 4LZP 0.5 ed 1 Z Where: P ed =vl/d T, L mix - gas mixing length in pipe (m);l - length of the pipe (m);v - flow rate of the medium (m.s -1 ); D T - diameter of the pipe (m);τ - displacement time (s); According to the equation, the gas mixing length is mainly affected by the gas flow rate, inlet and outlet pressure, pipe diameter, pipe length and other factors in the displacement. And because the state of the fluid flow has a great influence in the gas mixing rule, according to equation of gas state, the flow rate and pressure affect the temperature of the gas, so the gas mixing length L mix = F (Re, V, L, DT, Pout, T). The purpose of this paper is to study the general rules of gas mixing length in the nitrogen replacement process, the influence of six factors on nitrogen replacement: flow regime, inlet velocity, pipe length, pipe diameter, pressures and temperature, are investigated. 3.1 Flow regime We carry out the displacement simulation of two flow regimes using the conditions of different nitrogen inlet velocity which are shown in Table 1, so as to compare the distribution of the nitrogen molar concentration in the replacement of the same pipe length, the result are shown in Figure 2. L=1Km, DT=508mm Table 1 Gas mixing length under different flow regime Reynolds number Gas mixing length (m) P ed Displacement time (s) Laminar flow Turbulent flow Table 1 shows that the gas mixing length under laminar flow is 1.4 times than that under the turbulent state, but the displacement time of the former is about 53 times than the latter, which is far more exceeded. This is because when nitrogen displacement takes place in laminar flow, velocity distribution is uneven on the cross-sectional inside the tube. The flow rate at the center of the tube is twice than that of the average flow in the tube, so that the radial velocity gradient is large. The convective mass transfer along the path is the main reason for the aerated, which results in that the fluid in the center brakes into the front row in the form of the "wedge", increasing the gas mixing length. On the other hand, flow rate in the cross-section of the pipe is close to the average velocity

3 Applied Mechanics and Materials Vols under the turbulent flow regime. There is no parabolic velocity section, the fluid was "bullet" shaped into forward fluid, the diffusion transfer process becomes a major factor in the formation of gas mixing segment. Therefore, it is necessary to avoid the gas under laminar flow state in nitrogen displacement. N 2: a. Laminar flow b. Turbulent flow Fig. 2 Nitrogen concentration distribution under different flow regime 3.2 Velocity It can be seen from Figure 3 that as the flow rate increases, the gas mixing length increases, there is a linear correlation between them. Comparing the effect rules of the gas mixing length caused by the flow rate under different length of the tube, we can see that the longer the length of the tube, the greater the effect of the gas mixing length made by the flow rate. The growth rate of the gas mixing length is shown in Figure 4, after fitting the curve it shows that the growth rate is also increased when the velocity is relatively small, when the speed is larger, the growth rate slows down. Different displacement length has different gas mixing length, but changes in patterns and trends are the same. This is because in the displacement process, there exists molecular diffusion and convection diffusion, thus, nitrogen and air are mixed continuously and gradually the gas mixing length increases. On the other hand, at the beginning of the displacement, the velocity gradient and the concentration gradient in the tube are the largest, and slowly get smaller as the displacement goes on, as a result, the growth rate of gas mixing length gradually slows down. Fig. 3 Influence of velocity on the gas mixing length Therefore, the displacement rate cannot be too low, otherwise it will not only extend the displacement time, but also may cause the laminar flow, in which a gas wedge pours into another gas, easy to form a large number of gas mixing, increasing the length of the gas mixing. However, the

4 302 Mechatronics and Industrial Informatics speed of displacement cannot be too fast, although it may reduce the displacement time and contact time of the two gases, it increases the gas mixing length. Fig. 4 Increasing rate of gas mixing length with velocity 3.3 Back pressure It can be seen in Figure 5 that under the same flow rate, the back pressure increases, the gas mixing length will increase when displace the same length of tube. This is because the back pressure makes the gas diffusion coefficient increase to accelerate the diffusion of the gas, and as the flow rate increases, the gas mixing length increases, which is the same as the rules of the velocity. Therefore, the existence of the back pressure is unfavorable to the operation displacement of natural gas pipeline, and back pressure should be minimized. Fig. 5 Influence of back pressure on gas mixing length 3.4 Pipe length The effect of pipe length to the gas mixing length is shown in Figure 6, gas mixing length increases with the increasing of pipe length. This is because under the same conditions except for different tube length, the diffusion coefficient of the gas is almost the same. When the displacement speed is the same, the longer the pipeline is, the longer time the displacement takes, so it is the same with the gas mixing length. For the same pipe length, the gas mixing length is longer as the velocity is greater. 3.5 Pipe diameter From Figure 7 we can see that under certain conditions, the gas mixing length is different with the different pipe diameters, the gas mixing length and pipe diameter have an approximate relationship with linearly increasing. This is because when the other conditions are certain, the diffusion coefficient of the gas convection increases with the increasing of pipe diameter, so the gas mixing length also increases. And the gas mixing length is longer as the pipe length increases for the same pipe diameter.

5 Applied Mechanics and Materials Vols Temperature In the formula of the gas mixing length, there is no explicitly expressing of the temperature effect, but according to the gas equation of state, the temperature affects the gas remarkably, especially when diffusion and change of pressure exist. The influence of temperature on the gas mixing length and the displacement time is shown in Figure 8. From Figure 8 it is clear that the gas mixing length increases along with the increase of temperature, this is because the rise in temperature, on the one hand, the viscosity of the gas increases, the increase in the kinetic energy and convective diffusion coefficient of the gas, so the gas mixing length increases. On the other hand, as known from the gas state equation, gas volume is getting bigger as the temperature increases, so the gas mixing length increases. When the pipe length and diameter are certain, the displacement time is related to the friction coefficient of gas and the gas flow rate in pipe. Although the high flow velocity in the tube reduce the displacement time, the temperature rising result in the increasing of viscous force when gas flows inside the pipe, so the displacement time increases. The reduction of displacement caused by the gas itself is bigger than that caused by increasing of the flow rate, so the displacement time increases as the temperature rises. Fig. 6 Influence of pipe length on gas mixing length Fig. 7 Influence of pipe diameter on gas mixing length Fig. 8 Influence of temperature on gas mixing length

6 304 Mechatronics and Industrial Informatics Conclusions Flow regime, inlet velocity, pipe length, pipe diameter, pressures and temperature can affect the gas mixing length, mainly by affecting the convection diffusion coefficient and replacement time of the gas, five main results can be drawn as follow. (1) The gas mixing length increases with the increasing of replacement time, and its maximum comes out when the gas at the exit of the pipe reaches the required export concentration. This is because along the entire pipeline, the lowest pressure is at the exit of the pipe, the expansion coefficient of the gas here is maximum, resulting in an increase of diffusion coefficient, so that the gas mixing length increases. (2) In the process of Nitrogen displacement, the gas mixing length and displacement time under the laminar flow is bigger and longer than that under the turbulent flow, so it is necessary to avoid the gas in a laminar flow state. (3) The gas mixing length increases as the flow rate rises, and increases fast when gas flow rate is smaller, as the rate increases, the growth rate of gas mixing length gradually slowed down; (4) Because of the increase in displacement time, the gas mixing length increases with the increasing of tube length. Due to the increase of the diffusion coefficient of the gas convection, the gas mixing length increases with the increasing in temperature. Meanwhile, as the increase of displacement time and diffusion coefficient of the gas convection, the gas mixing length increases with the increasing in pipe diameter. (5) The existence of back pressure leads to the increase of gas mixing length, which is unfavorable for the production displacement of the natural gas pipeline, so venting in the displacement is very necessary to reduce back pressure. Reference [1] Fu ChunLi.( 2011) Influencing factors of gas-mixing length during nitrogen purging & packing course of gas pipeline, Journal of Oil and Gas Storage and Transportation, Vol. 30, pp [2] Hu Qihui. (2006) Natural gas pipelines nitrogen replacement the mixed gas segment law research. China University of Petroleum. [3] Duan wei etc. (2012) Study on Length of Gas Mixture Segment for Commissioning Replacement of Long-distance Gas Pipeline, Journal of Natural Gas and Oil, Vol. 30, pp [4] Gao Wei. (2009) Purged with nitrogen gas mixing length of natural gas pipelines influencing factors. Journal of Surface Engineering of Oil and Gas Fields, Vol. 28, pp [5] Xue Jijun etc. (2006) Study on gas mixing regulation of gas displacement operation for gas transmission pipeline commissioning. Journal of Natural Gas Industry, Vol. 26, pp [6] Guo Yimin etc. (2006) Numerical simulation on gas mixing rule in course of nitrogen isolation gas displacement technology for commissioning of gas transmission pipeline. Journal of Oil Field Equipment, Vol. 35, pp

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