A CFD study of low pressure wet gas metering using slotted orifice meters

Wet gas metering is becoming an increasingly important problem to many industries, in particular the oil and gas industry. Extensive studies have been done in the past on Venturi and standard orifice differential pressure (DP) flow meters to tackle wet gas flow problems. However in recent years, the slotted orifice flow meter has been developed in the attempt to improve the performance of the standard orifice meter. The novel flow meter is shown to be insensitive to the upstream flow profile with lower head loss and faster pressure recovery. This paper describes the numerical studies to establish the effect of different geometrical perforations on the performance of the slotted orifice. Three sets of slotted orifices with varying aspect ratios (1.5≤l/w≤3.0), of rectangular perforations and one slotted orifice with a circular perforation and a β ratio of 0.40 are simulated in a 1.6 m horizontal pipe using the k-ε turbulence model over a range of parameters, i.e. gas volume fraction (GVF) and gas mass flow rate. The commercial CFD code, FLUENT 6.3 was used to model the wet gas flow. Simulation results revealed that the shape of the perforation has no effect on the differential pressure, However, a marginally better pressure recovery was observed with rectangular perforations of l/w=3.0. The relatively higher over-reading values obtained in this work are consistent with the results of Geng et al. (2006) [1] that for a slotted orifice, a low β ratio is more sensitive to the liquid presence in the stream and hence is preferable for wet gas metering. Mass flow prediction by wet gas correlations showed that the homogeneous model, Steven’s and De Leeuw’s correlations had the best performance, with a calculated mean error of 4%-5%.     

Comparison of throttle devices to measure two-phase flowrates of wet gas with extremely-low liquid loading

The online measurement of wet gas with extremely-low liquid loading (Lockhart-Martinelli parameter lower than 0.02) remains a challenge. In this study, three types of throttle devices, Venturi, orifice plate and cone, are compared experimentally with air-water two-phase flow in a horizontal pipe of inner diameter of 50 mm. High-precision correlations are established to measure the gas and liquid flowrates via a single throttle device. Results show that the two-phase mass flow coefficient (K) of the three throttle devices all increase linearly with the liquid densiometric Froude number and the K correlations are established respectively to correct the gas mass flowrate deviation. The pressure loss ratio (δ) for Venturi is sensitive and monotonous to the liquid loading, which contributes to the high accuracy of liquid flowrate measurement. By incorporating the K correlations, both the gas and liquid mass flowrates can be predicted precisely. The relative error of the gas mass flowrate predicted by the Venturi is within ±2.0% at 95% confidence level, and that of the liquid mass flowrate is within ±15% at 90% confidence level.     

Wet gas pressure drop across orifice plate in horizontal pipes in the region of flow pattern transition

As a basis for measuring the mass flow rate of wet gas using differential pressure meters, predicting the pressure drop of a wet gas flowing through orifice plates is important; however, this has not yet been solved satisfactorily, although many studies have reported on that subject. In this study, the pressure drop of wet gas across sharp-edged orifice plates was experimentally investigated in the region of flow pattern transition using air and water as the two phases, and the prediction performance of the available pressure drop models was compared based on the experimental data. The results show that the homogenous flow models overestimate the pressure drop, whereas those models based on the separated flow model often present underestimations. The models reported for wet gas are also incapable of predicting the pressure drop in this region with acceptable accuracy. Through an analysis of the prediction deviations, it is found that the Froude number of the liquid phase has a significant influence on the pressure drop of the wet gas, besides the Froude number of the gas phase. Then, three new correlations that are based on the homogeneous flow, Chisholm model, and Murdock model, respectively, were proposed based on the experimental result.     

Experimental research of single-hole and multi-hole orifice gas flow meters

As energy efficiency is becoming more important today due to limited energy resources as well as their rising prices and environment issues, it is crucial to have reliable measurement data of different fluids in production processes. Because of its simplicity, affordability and reliability, orifice flow meters are again becoming subject of numerous researches. Conventional single-hole orifice (SHO) flow meter has many advantages but also some disadvantages like higher pressure drop, slower pressure recovery, lower discharge coefficient etc. Some of these disadvantages can be overcame by multi-hole orifice (MHO) flow meter while still maintaining advantages of conventional SHO meter. Both SHO and MHO flow meters with same β ratios were experimentally tested and compared. Results showed better (lower) singular pressure loss coefficient and lower pressure drop in favour of the MHO flow meter. Experimental data indicates that MHO flow meter is superior to the conventional orifice flow meter, but further research is necessary to make the MHO a drop-in replacement for a SHO flow meter.     

Analysis of high pressure tests on wet gas flow metering with a Venturi meter

This work deals with the flow metering of wet gas issued from high pressure natural reservoirs. Some recent results obtained from tests performed on the CEESI facilities are presented. They are performed at 75 bar with 0.6 beta ratio Venturi meter installed in horizontal pipe configuration. Correction factors obtained are compared to predictions deduced from the flow modelling inside of the meter. These results are analysed in order to explain the agreements or disagreements obtained between the experiments and the flow modelling.     

Horizontally installed cone differential pressure meter wet gas flow performance

Differential pressure (DP) meters which utilise a cone as the system’s primary element are increasingly being used to measure wet natural gas flows (i.e. mixtures of natural gas, light hydrocarbon liquids and water). It is therefore important to understand this meter’s response to wet natural gas flows. Research into the wet gas response of the horizontally installed cone DP meter is discussed in this paper. Consideration is given to the significant influence of the liquid properties on wet gas flow patterns and the corresponding influence of the flow pattern on the cone DP meter’s liquid phase induced gas flow rate prediction error. A wet natural gas flow correlation for 4 in. 0.75 beta ratio cone DP meters with natural gas, hydrocarbon liquid and water flow has been developed from multiple data sets from three different wet gas flow test facilities. This corrects the liquid induced gas flow rate prediction error of a wet gas flow up to a Lockhart–Martinelli parameter of 0.3, for a known liquid flow rate of any hydrocarbon liquid/water ratio, to ±4% at a 95% confidence level.     

Study on wet gas online flow rate measurement based on dual slotted orifice plate

According to the current technical problems existing in gas and liquid flow measurement for wet gas production, the slotted orifice-couple flow meter was developed and the basic measurement principles for gas and liquid flow was presented. A new wet gas flow meter was developed based on the dual slotted orifice transducer. The flow characteristics of liquid flow through dual slotted orifice plate, the relationship of differential pressure between the dual slotted orifice plate, pressure, temperature, and flow rate of gas/liquid of different aperture ratio were studied. A mathematical measurement model was established to be applied in the flow meter measurement system with dual slotted orifice plate. The model was tested and calibrated by on-site field experiments in the China National Center of Metrology at Daqing Oil field. The results showed that the maximum measurement error of the gas and liquid flow was less than 10% and 15% respectively, when the Gas Volume Fraction (GVF) was greater than 90 vol%. The measurement accuracy of this industrial prototype can meet the requirements of well fluids.     

Study on the similarity of wet gas pressure drop in long-throat Venturi

This study investigates the effect of pipe diameter on pressure drop with the same diameter ratio, similar pressure-sampling position and throat length of long-throat Venturi. Considering the factors including the void fraction, the friction between the two phases and the entrainment in the gas core, the one-dimensional momentum equation for gas has been solved in the axial direction of long-throat Venturi. A novel void fraction model is established, by considering the effects of dryness and gas-liquid density ratio, then predicting the distribution of wet gas static pressure between the two pressure tapings of the long-throat Venturi. The comparison between the values predicted by the model and those measured experimentally reveals that all the relative deviations of the predicted points by the modified model were within ±15%. In the same entrance conditions, the effect of pipe diameter on pressure drop in long-throat Venturi is similar.     

Wet surface heat transfer and pressure drop of aluminum parallel flow heat exchangers at different inclination angles

The effect of inclination angle on the heat transfer and pressure drop characteristics of brazed aluminum heat exchangers was experimentally investigated under wet conditions. Three samples having different fin pitches (1.25, 1.5 and 2.0 mm) were tested. Results show that heat transfer coefficients are not affected by the inclination angle. However, friction factors increase as the inclination angle increases with negligible difference between the forward and backward inclination. The effect of fin pitch on the heat transfer coefficient and on the pressure drop is also discussed. Comparison of the dry and wet surface heat transfer coefficients reveals that dry surface heat transfer coefficients are significantly larger than wet surface heat transfer coefficients. Possible explanation is provided by considering the condensate drainage pattern. The data are also compared with the existing correlation. This paper was recommended for publication in revised form by Associate Editor Man-Yeong Ha Nae-Hyun Kim is a Professor of Mechanical Engineering, University of Incheon. His area of interest spans boiling and condensation, heat transfer enhancement and heat exchanger design. He has been active in heat transfer community, and was a Chairman of Thermal Engineering Division of KSME. He holds several editorial position including Journal of Enhanced Heat Transfer. He is a recipient of Asian Academic Award awarded by SAREK and JSRAE.     

Prediction of pressure drop in Venturi based on drift-flux model and boundary layer theory

Venturi, as the primary flow measurement sensor, is widely used in various industrial fields of oil and natural gas. Pressure drop of the Venturi is a crucial factor in the process design of exploitation and transportation of natural gas. Based on the drift-flux model and boundary layer theory, a pressure drop prediction model is established. Except for divergent section, a uniform void fraction model is established basing on drift-flux model. The thickness of boundary layer grows rapidly due to the existence of adverse pressure gradient in the divergent section, which results in an increase of the irrecoverable pressure drop. Considering the influence of slip between gas and liquid, weight coefficient is used to adjust the proportion of displacement thickness in the cross section of the Venturi. Compared to experiment, the theoretical model is applied to stratified wavy flow and annular mist flow. For different diameter, the relative deviations of experiment points are within ±15%.     

A CFD study of wet gas metering over-reading model under high pressure

The venturi flow meter is increasingly being preferred in multiphase flow measurement because of its shorter upstream and downstream straight sections, less influenced by the flow pattern and relatively small pressure loss. However, when the venturi is used for wet gas measurement, the over-reading phenomenon occurs due to the presence of a small amount of liquid. Many scholars have established over-reading models to correct the measured values of wet gas. Regrettably, the applicability of these over-reading models under actual high pressure operating conditions has not been verified. Therefore, this review focuses on numerical simulation of the flow of wet gas in the venturi tube under high pressure conditions (11MPa/13MPa/15 MPa). The discrete phase model (DPM) and the standard k-ε model was employed in this review. The simulations results reveals the flow characteristics of wet gas in venturi tube, which includes the flow field distributions, droplet concentration distributions and wall pressure profile distributions, and indicates that the over-reading values increases with the increase of Lockhart-Martinelli parameters and gas volume flow rate, but decreases with the increase of pressure. Moreover, the ISO model has the best performance under high pressure conditions.     

基于三轴加速度探头的涡街湿气分相流量测量

针对涡街湿气测量过读问题,提出了基于加速度检测的涡街过读校正和分相流量测量方法。设计了高频响三轴加速度探头,分别对敏感元件、探头尺寸和封装进行了优化设计。干气标定结果表明,在4.43×10⁴～1.81×10⁵雷诺数范围内,测量精度为±1.0%,线性度为1.06%。然后,在不同湿气工况(载气压力和流量、液相流量)下测试了输出频率和加速度幅值特性,以气、液相韦伯数为参数,分别建立了涡街过读和两相加速度幅值模型。最后,联立两方程建立了湿气测量模型,并利用牛顿迭代算法进行求解。预测结果表明,气相测量误差在±1.0%以内,不确定度0.46%,液相全量程误差在±15%以内,不确定度10.04%。与未过读校正时最大8%的测量误差相比,气相测量精度大大提升,同时实现了湿气中分相流的在线测量。     

A modified model for wet gas flowrate measurement considering entrainment downstream of a cone meter

To develop a reliable wet gas flowrate measurement model, the relationships between pressure drop characteristics and entrainment downstream of the cone are investigated experimentally. The equivalent diameter ratio of the cone is 0.45. The experimental fluids are air and tap water with X_LM in the range of 0–0.3. The two-phase mass flow coefficient and pressure loss ratio are employed to establish the measurement model. The piecewise characteristics of pressure loss ratio are disclosed innovatively, which is explained by the different intensity of entrainment downstream of cone caused by gas-liquid jetting. A simplified method for evaluating the degree of entrainment is proposed to facilitate the establishment of the modified measurement model. Under the present experimental conditions, the relative error of liquid fluctuates within ±20% when X_LM is larger than 0.02, and the relative error of gas flowrate is within ±5%. Compared with the model without piecewise consideration, the relative error of the liquid flowrate of the modified model reduces obviously under low wetness conditions (0.02<X_LM<0.1). The modified measurement model provides a reliable and cost-effective technology for wet gas flowrate measurement.     

Invariant system for measuring the flow rate of wet gas on Coriolis flowmeters

One of the important tasks of modern flow measurement is to measure the flow rate and the amount of wet gas. This task becomes especially important when there is a need to obtain information about the amount of dry gas at the production facilities. The paper presents the principle of operation and structure of the invariant wet gas measurement system. The work of the invariant flow measurement system is based on the application of the principle of multichanneling and the method of partial flow measurement. Coriolis flowmeters are used as the main elements of the system. The proposed measurement system does not imply the use for gases with abundant droplet moisture. The authors propose a method of verification of the measurement system, which is currently limited by the measurement error of moisture in the gas equal to 5%. The article also provides information on the results of the test of the system.     

基于EEMD-Hilbert谱的气液两相流钝体绕流流型识别

为了有效识别气液两相流的流型,以水和空气为实验介质,以涡街流量计为元件诱发钝体绕流,通过管壁差压法获取气液两相流钝体绕流的尾迹波动信号,采用集总经验模态分解对信号进行分解,通过Hilbert变换得到Hilbert边际谱,利用最大互相关系数法对固有模态函数进行筛选,选取特征固有模态函数能量比分别与体积含气率、两相雷诺数构建流型图。结果表明,构建的两类新流型图对单相水、泡状流、塞状流、弹状流等典型流型的识别率分别可达91.67%和88.89%,能较好地满足工程实际应用的需求。     

Identification of gas/liquid two-phase flow regime through ERT-based measurement and feature extraction

Gas/liquid two-phase flow is of great importance in various industrial processes. As the most important characteristic of a two-phase flow, the flow regime not only characterizes the flow condition in an explicit way, but also determines the measurement model in each measuring method. Based on the application of Electrical Resistance Tomography (ERT) to a gas/liquid two-phase flow on a vertical pipe, features reflecting the characteristics of gas/liquid two-phase flow are extracted directly from the measured data without reconstruction of the cross-sectional images. The statistical features are derived through time series statistical analysis. Meanwhile features in the wavelet-scale domain are derived through both one-dimensional and two-dimensional wavelet transform. All extracted features are considered as the input of a Support Vector Machine (SVM) algorithm to recognize the flow regime. The preliminary results show that the feature extraction methods of multi-feature fusion and high-dimensional wavelet transform are suitable for the identification of gas/liquid two-phase flow regimes.     

差压式流量传感器测量一般气体流量时的温度压力补偿方法

简要介绍使用差压式流量传感器进行一般气体流量测量时的温压补偿方法;指出了差压方式流量传感器测量一般气体的通用流量温压补偿公式,并写出了公式的推导过程;与线性流量传感器温压补偿方法进行对比,强调指出了采用差压式流量传感器时进行温压补偿的注意要点.对公用工程中的一般气体的流量计量工作有一定的指导作用.     

自力式油气水三相流气液分离装置的设计与实验

针对油井油气水三相流量测量难、测准更难这一实际问题,对溢气型集流伞的结构作了优化和改进,设计出一种自力式油气水三相流气液分离装置,装置上安装了涡轮流量计和电导持水率计,完成油气水产出剖面测井仪的研制。利用多相流测试系统对产出剖面测井仪进行了流量测量实验,实验结果表明,产出剖面测井仪能有效降低油井气相对油井液相流量和持水率测量的影响,可提高油田测量精度及采收率。     

A new correlation of wet gas flow for low pressure with a vertically mounted Venturi meter

Wet gas metering has become an increasingly important technique for many industries. However, the over-reading phenomenon reduces the accuracy of Differential Pressure meters. This research fills the vacancy of correlations and presents a new correlation for low pressure between 0.82 and 1.52 MPa with a vertically mounted Venturi meter to calculate the over-reading coefficient accurately. Based on the correlational analysis, the over-reading coefficient is a function of the Lockhart-Martinelli parameter, density ratio, and gas Froude number. The constant coefficients in this correlation are obtained by nonlinear regression. Effect of low gas velocity with gas Froude number under 1.5 is taken into consideration as well. The average relative error is 1.9% and the root mean square error is 3.0%. Furthermore, a new method to calculate the over-reading coefficient for industrial applications is put forward due to the difficulties of online measurements of the Lockhart-Martinelli parameter which is substituted with the void fraction. The void fraction is calculated by an empirical correlation using quality and an approximate algorithm is utilized to obtain gas Froude number. For this new method, the average relative error is 2.3% and the root mean square error is 3.7%. This quality-based method will be helpful to resolve the limited applicability of gamma-ray attenuation for wet gas flow metering in industry regarding vertical low pressure conditions.