Flow induced vibration of two-phase flow passing through orifices under slug pattern conditions

Abstract

Flow restricting orifices are important components for flow and pressure control in the oil, gas, and power generation industries. In many situations, the piping systems are operating under two-phase flow conditions. These orifices affect the redistribution of the two phases and induce vibrations in the piping structures. These vibrations often occur during a slug flow pattern and can severely affect the integrity of a structure; therefore, in the present work, both the vibrations of the piping structure and the characteristics of the two-phase flow dynamics are experimentally investigated in the presence of an orifice. In this study, an air–water​ two-phase flow mixture was utilized at different superficial velocities ranging from 0.12 m/s to 1.7 m/s for air and 0.46 m/s to 1.3 m/s for water. Synchronized flow visualization images, along with local void fraction and vibration measurements, were used to evaluate the response of the structure as the slug flow passed through the orifices. Detailed instantaneous local void fraction measurements were carried out to understand the underlying physics of the phenomenon and their effect on the vibration response of the structure. The results showed that not all slug flow cases excited the piping at the slug frequency. The amplitude of vibration around the slug frequency was found to increase as the superficial velocity of the gas phase increased.

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.