Investigation On Liquid Loading Distribution And Monitoring Technology In Hilly-terrain Wet Gas Pipelines

Wet gas gathering and transportation is common technology in gas produce,but liquid loading and slug will occur in hilly-terrain pipeline.The understandings of the generation and development of slug flow in low liquid hold-up,hilly-terrain pipelines is insufficient.In high-sulfer gas gathering and transportation system,non-intrusive method for detecting the liquid level of wet gas pipeline should be developed.In order to know the rule of liquid accumulation and distribution in gas pipeline,the flow mechanism of slug flow in hilly-terrain pipelines is studied.Experimental study on the two-phase flow with low liquid holdup in diameter of 40 mm,length of 30 m hilly-terrain pipeline is conducted and a liquid level detecting technique in wet gas pipeline based on ultrasonic echo characteristics is presented.The results of two phase flow in hilly-terrain pipeline with different velocity show that the flow regime is different in different pipe sections and there is regime transition between sections.The research on the two-phase flow with low liquid holdup in upward inclined pipe indicates that a new slug will be generated when the liquid level in the elbow of pipeline reaches a certain height and then the new slug will dissipate and back flow quickly,resulting in the increase of the slug length.When the slug length reaches a certain value,it will leave the pipe elbow.The experiment results also show that there are three kinds of slug initiation mechanisms in the low liquid holdup and hilly-terrain pipelines: wave growth initiation mechanism,co-current flow and extrusion initiation mechanism,and wave coalescence initiation mechanism.The formation of liquid slug under wave growth initiation mechanism,accords with the dividing criteria of stratified flow and intermittent flow by Taitel&Dukler.And there is disparity between the dividing criteria by Taitel&Dukler with that of slug flow and annular flow in upward inclined pipeline,due to the slug is generated under wave coalescence initiation mechanism.The dissipation of slug in the hilly-terrain wet gas pipelines is mainly influenced by minimum stabilizing slug length and the stability of slug.Fist of all,the distribution of liquid and pressure of slug flow and stratified flow are analyzed in theory,combined the two-phase flow experiments of low liquid holdup and hilly-terrain pipelines.The experiments of low holdup gas liquid two-phase flow under the terrain consisting of two sections of upward inclination reveal that the downstream flow influences the pressure of the upstream,while the upward inclined section of the upstream influences the slug formation in the downstream through liquid flow rate.The experiments with different velocity show that the average pressure at the given point of the hilly-terrain pipeline increases with the increase of the gas superficial velocity,but the slope of the pressure increasing is different because of the change of flow regime.The differential pressure monitor shows that the time that the head of slug enters the test section and the tail leaves the test section is distinctive and accords with the theory analysis.The research on change rules of slug flow characteristic parameters in upward inclined pipe indicates that the translational velocity of slug increases significantly at first as the increase of gas velocity.When the superficial gas velocity increases to a certain value,the translational velocity of slug don't increase and the flow regime becomes pseudo-slug flow.With a constant superficial liquid velocity,the slug length ranges between 7-12 D when the superficial gas velocity is small.As the increase of superficial gas velocity,slug length is mainly 7-9D.Under the condition of liquid at a constant velocity,the slug holdup decreases with the increase of the gas velocity.Under the condition of gas at a constant velocity,the liquid holdup is not influenced by the liquid velocity.When the gas superficial velocity is small,the probability density distribution curve of slug frequency is logarithmic normal distribution.While the distribution curve basically corresponds to standard normal distribution with a lager gas superficial velocity.To study the liquid distribution in hilly-terrain wet gas pipeline accurately,the liquid level detecting technology is developed based on detailed analysis on the traditional liquid level detecting technologies.Ultrasonic echo characteristic of different interfaces is studied by theoretical calculations and numerical simulation.The results show that the difference of acoustic impedance between liquid and solid is smaller than that between gas and solid.Thus,ultrasonic penetration is higher at liquid-solid interface leading to faster attenuation of the residual ultrasonic waves inside a pipeline wall.Therefore,the difference of ultrasonic echo characteristic between the cases of gas-solid interface and liquid-solid interface is obvious.Moreover,as the echoing time grows the difference becomes more obvious.Experimental investigation was done and this method was used to detect the liquid level of two-phase flow pipeline.The results show that flow regimes could be identified and gas-liquid interface could be detected accurately and the detection accuracy could be ±3mm,which agreed fairly well with the actual liquid level.The reliability of this method is further confirmed by the results detected on wet gas gathering pipeline of Puguang gas field.