Research Paper
Wet gas measurements of long-throat Venturi Tube based on forced annular flow

https://doi.org/10.1016/j.flowmeasinst.2021.102037Get rights and content

Highlights

  • A new type of wet-gas measuring device composed of a cyclone and a long-throated venturi is proposed.

  • Numerical simulation and experimental study on the performance of the new type of wet-gas measuring device were carried out.

  • The accuracy of the new wet-gas measuring device is better than that of the traditional long-throat venturi nozzle.

Abstract

A wet gas dual-parameter measuring device composed of a cyclone and a long-throated Venturi tube is proposed to overcome the difficulty of measuring the liquid content of wet gases and reduce the error caused by the wet gas flow pattern. The flow pattern is transformed into an annular flow by a cyclone. In this study, the proposed device was compared with a traditional non-cyclone long-throat Venturi tube; furthermore, the pressure difference ratio W between the contraction and expansion sections of the long-throat Venturi tube was introduced as a parameter. Through numerical simulations, the relationship between W, the gas Froude number, over-reading, and liquid-gas mass flow ratio was analyzed, and a new wet gas flow measurement model was established. The reliability of the measurement model was verified through indoor experiments. The experimental results showed that the traditional wet gas measurement device had gas phase and liquid phase errors of ±4.5% and ±10%, respectively; on the other hand, the cyclone-based wet gas measurement device had gas phase and liquid phase errors of ±3% and ±8%, respectively. Thus, the performance of the wet gas measurement device with the cyclone was higher than that of the traditional wet gas measurement device.

Introduction

Gas–liquid two-phase flows widely exist in the fields of oil exploitation, transportation, and storage. When natural gas is produced underground and pumped to the surface, it usually carries with itself a part of the liquid phase. Generally, the mixture of natural gas and liquid phase produced in this type of gas well is called “wet gas” There is no unified standard for defining the wet gases. According to ISO/TR 11583 [1], if the volumetric gas content of a gas–liquid two-phase flow is greater than 95%, it is considered as a wet gas. Thus, a wet gas flow is a subset of gas–liquid two-phase flows [2]. Compared with a single-phase flow, wet gas has a velocity slip between the gas and liquid phases, and its flow pattern is significantly affected by time, operating pressure, and temperature. These factors significantly increase the difficulty of wet gas measurements. Currently, separation measurement methods are widely used in various oilfields; however, traditional techniques utilize equipment with large dimensions, complex designs, and high cost of development [3]. With the development of gas-liquid metering devices for miniaturization, high precision, and low cost, the accurate measurement of wet gases has become increasingly difficult. Therefore, a new economic, reliable, accurate, and practical measurement method is urgently needed to replace the traditional measurement methods to realize the accurate measurement of wet gases and control the investment costs.

Owing to the simple structure of a differential pressure flowmeter, good measurement repeatability, and low cost [4], its output is stable in gas–liquid two-phase flow measurement, and it is widely used in wet gas flow measurements. Differential pressure flowmeters produce over-reading when measuring wet gas. Several experiments have been conducted on various throttling devices through differential pressure flowmeters at home and abroad, and a series of over-reading correction models have been established. The most widely used models are the Murdock [5], Chisholm [6], Lin [7], De Leeuw [8], and Steven models [9]. However, these models are based on the known liquid holdup established under these circumstances; however, the liquid phase holdup is unknown in the actual production process; therefore, the practicability of the above-mentioned over-reading correction models is greatly restricted. In 2020, Yufei [10] adopted a wet gas measurement device composed of a cyclone and a Venturi tube, proposed a dual-parameter measurement based on forced annular flow, and established a new wet gas measurement model.

To overcome the influence of liquid holdup on wet gas measurements, several experts have proposed new methods of wet gas online measurement. Among them, the Venturi meter is one of the most reliable devices used to measure multiphase mass flowrates (e.g., oil–water–gas) [11]. It has the advantages of a simple structure and small pressure loss; therefore, it has attracted increasing attention. In 2009, Reader-Harris [12] conducted multiple experiments on a standard Venturi with different throttling ratios and media. The Chisholm model was modified based on the experimental data, and the outflow coefficient of the standard Venturi tube in the wet gas measurement was also studied and modified accordingly. In 2012, Zhang Qiang [13] proposed a double differential pressure wet gas flow measurement method based on a long-throat single Venturi. In 2014, Xu Ying [14] analyzed the influence of different pressure taking positions of the long-throat Venturi on wet gas measurements and obtained the best pressure taking position for the same. In 2015, Chao Yuan [15] revealed the flow characteristics of wet gas by using the double differential pressure ratio of a Venturi. The relationships among differential pressure ratio, gas liquid density ratio, gas Froude number and Lockhart–Martinelli parameters of Venturi are analyzed. By using these parameters, a new wet gas measurement model is established. In 2016, Denghui He [16] used V-cone meter to study the influence of liquid density Froude number, gas density Froude number and gas-liquid density ratio on the two-phase mass flow coefficient. The pressure loss ratio of V cone meter is presented. Then the wet gas measurement model is obtained. In 2018, Xu Ying [17] used a computational fluid dynamics (CFD)-based simulation method to design four types of structures for a long-throat venturi with a diameter of 50 mm and a throttling ratio of 0.55. Considering the expansion angle and shape of the expansion section, he found an optimal structure suitable for the wet gas flow. In 2019, Yanzhi pan [18] studied the vertically mounted Venturi meter, and obtained the over-reading coefficient under low pressure by using the nonlinear regression method. In 2019, Xuebo Zheng [19] conducted an experimental study on the pressure drop of wet gas flow with ultra-low liquid loading by using orifice meter, V-cone meter and Venturi meter. The influence law of pressure drop and ratio of the permanent pressure loss on liquid loading of three kinds of meters is revealed. In 2020, Fang Lide [20] established a new gas–liquid two-phase flow phase holdup measurement model by combining near-infrared spectroscopy with high-speed photography; the new near-infrared system was used to locate the position of the long-throat Venturi. In 2021, Fan Zhao [21] studied the pressure drop characteristics and the entrainment downstream of the cone flow. The piecewise characteristics of pressure loss ratio are revealed. Then the wet gas measurement model is established by using the two-phase mass flow coefficient and pressure loss ratio. Although the influence of liquid holdup on wet gas measurement has been solved, the influence of wet gas flow pattern on wet gas measurement has not been considered adequately in the literature; this has led to many limitations in the application of the measurement model.

Therefore, this study proposes to set up a cyclone before the long-throat Venturi to modulate the complex and variable wet gas flow pattern into a circular flow. A wet gas measurement model of the long-throat Venturi tube under this forced annular flow was established to eliminate the influence of the flow pattern on the wet gas measurement. By comparing with a long-throat Venturi without a cyclone, the dual-parameter measurement performance of the long-throat Venturi under the forced annular flow state was studied.

Section snippets

Measuring device

A wet gas measuring device comprising four swirling blades with an inclination angle of 45°, and a long-throated Venturi tube was designed in this study. Fig. 1(a) shows a schematic of the design. The measuring device is equipped with three pressure tappings. These include (i) P1 upstream of the long-throated Venturi, (ii) P2 on the throat, and (iii) P3 on the downstream. The wet gas measurement is performed by measuring the pressure differences △P1 and △P2 between two adjacent points, where △P1

Numerical simulation method

The structures of the two measuring devices are shown in Fig. 2. The two devices were meshed using the MESH software. To ensure the accuracy and efficiency of the simulation, a structured and unstructured hybrid grid was used; grid independence studies were carried out; and it was determined that the optimal number of grid count for the wet gas measurement device with the cyclone was 1,460,080, and that of the wet gas measurement device without the cyclone was 1,300,704.

The key geometric

Experimental method and equipment

The experiments were conducted on an experimental device at the Multiphase Pipe Flow Laboratory of the China Petroleum Gas Lift Experimental Base of Yangtze University. The device comprised a gas supply system, liquid supply system, metering device, separation device, horizontal-loop pipe section, and computer control and data acquisition system. The flow chart of the experimental setup is presented in Fig. 13.

The media used in the experiments were air and water, which were mixed after being

Conclusions

  • (1)

    To negate the influence of the wet gas flow pattern on the wet gas measurement, a dual-parameter measurement method for a long-throat Venturi nozzle, based on forced annular flow is proposed. Its performance was compared with that of a traditional wet gas measurement device, and through numerical simulations and indoor experiments, the influencing factors of the over-reading model were analyzed.

  • (2)

    This study introduced a parameter W, which represents the pressure difference ratio between the

Credit author statement

Xingkai Zhang: Conceptualization, Methodology; Weibiao Zheng: Data curation, Writing- Original draft preparation, Writing- Reviewing and Editing; Ruomiao Liang: Investigation, Supervision, Ruiquan Liao: Funding acquisition; Project administration, Dong Wang: Software, Validation.

Declaration of competing interest

We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Acknowledgements

The authors gratefully expressed their thanks for the financial support for these researches from the Foundation of the Educational Commission of Hubei Province of China (Grant Number: Q20191310), National Natural Science Foundation of China (Grant Number: 61572084), and National Major Scientific and Technological Special Project (2016ZX05046004-003).

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