Flow induced vibration of two-phase flow passing through orifices under slug pattern conditions
Introduction
Two-phase flows are found in various industries such as oil, gas, power generation, food, chemical, and water treatment. Two-phase flow patterns can include bubbly, slug, churn, and annular depending on the piping geometry, size, orientation and flow conditions. The slug flow found in these applications represents common conditions in which a higher superficial gas flow exists. In such applications, slug flow can induce vibrations when passing through piping components, such as valves, tees, elbows and orifices; for example, orifices are commonly found in the downstream side of a blow-down valve in order to control the flow rates. In these situations, the system may experience high levels of vibrations. The vibrations can be attributed to the momentum change of the phases in addition to the unsteadiness in local flow properties, such as void fraction and phase velocity. As a result, the components experience fluctuations in the local pressure, which leads to dynamic stresses that may be large enough to cause failure of the structure (NTSB, 2004).
Vibration due to slug flow is a challenge because of the large fluctuations in the local mass flux (Yih and Griffith, 1968). Although these systems are designed to operate in more homogeneous conditions, slug flow might still exist during the start-up and the shutdown of the system. Furthermore, slugging may also occur due to changes in the slope of the pipe and the presence of piping components, which may cause a pressure drop (De Henau and Raithby, 1995, Bendiksen and Espedal, 1992).
Slug flow tends to induce more lateral vibrations in piping components when compared to other flow patterns as explained by Miwa et al. (2015). They identified two main sources of excitation in slug flow-induced vibrations: the collision of the slugs with the piping components, and fluctuations in pressure or each phase momentum. The former occurs at a lower order of frequencies, depending on the length of the Taylor bubble, the volumetric flux, and the void fraction. The latter occurs at higher frequencies due to flow redistribution. Moreover, vibration in the structure may become a major concern if either of the aforementioned sources of excitation are in the proximity of the natural frequency of the structure. These structures are often well supported; therefore, existing research suggests that slug flow-induced vibration is expected to occur in the neighborhood of the slug frequencies (Miwa et al., 2015).
In the experiments performed by Liu et al. (2012) for piping systems with an elbow in a vertical section, the slug flow was found to induce distinct excitation frequencies when compared to other flow patterns. The RMS of the fluctuating forces increased with an increase in the volumetric quality and the time average momentum fluctuations (Liu et al., 2012). Experimental work that focused on the internal two-phase flow damping reported that two-phase flow damping was strongly dependent on the flow pattern, and that the dissipation of damping was affected by the bubble drag (Béguin et al., 2009, Charreton et al., 2015).
While some research has been done on two-phase flow-induced vibrations in piping components, an understanding of the effect of the slug flow characteristics on the structure is still lacking. In previous works (Païdoussis, 1982, Goyder, 2002, Ahmed et al., 2012), several researchers have reported that a large amplitude of vibration may be expected when the slug collides with a piping component; however, a detailed correlation between the flow and the vibrations does not exist. The current research aims at examining these physical correlations.
Section snippets
Experimental set-up
The experimental set-up used in this work allowed for testing various test sections to study two-phase flow-induced vibrations in piping components. The test rig was comprised of a closed loop of air–water piping as shown in Fig. 1. Deionized water at room temperature was pumped by a centrifugal pump from a 310 liter steel tank through a two-phase mixer. Air from a compressed air line was injected into the mixer, after which the two-phase flow mixture proceeded into the test section. The speed
Results and discussion
It is important to carefully study the detailed flow structure with an emphasis on the two-phase parameters that affect vibration response. Therefore, the study of vibrations in terms of the local two-phase flow properties of the mixture makes the slug flow pattern difficult to predict but equally interesting to study.
Conclusions
Experiments were conducted to investigate the slug flow-induced excitation in a piping structure with a flow-restricting orifice. The effect of two-phase flow properties, such as slip ratio, slug length, and the void fraction, on the amplitude and frequency of vibration may vary depending on the superficial velocity of the liquid phase (). The pipe response was found to be a superposition of quasi-periodic excitation due to the slug momentum transfer and broad-band excitation of turbulence
CRediT authorship contribution statement
Olufemi E. Bamidele: Experimental setup, Data collection, Writing - original draft. Marwan Hassan: Conceptualization, Funding acquisition, Methodology, Writing - original draft, Writing - review & editing. Wael H. Ahmed: Conceptualization, Funding acquisition, Methodology, Writing - original draft, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors are thankful for the financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC RGPIN-2016-03800).
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