Insights Into Fixed Equipment Vibration – How a Piping System Was Affected by Flow-Induced Vibration

Introduction

Excessive vibration is a problem frequently encountered in industrial plants and their peripheral equipment, including piping systems, pressure vessels, and steel structures [1, 3]. Long term excessive vibration can lead to fatigue crack propagation and consequently, may cause system failure. Therefore, once a vibration problem occurs it should be assessed within a short period of time. The critical nature of excessive vibration has raised significant interest in investigating and assessing underlying issues in greater detail. Fixed equipment vibration is considered a multidisciplinary field which requires knowledge in process engineering, pressure relief system design, fluid mechanics, fluid-structure interaction, rotating equipment vibration, structural vibration, and acoustic vibration. A precise vibration assessment consists of five main steps:

1. Implementing a successful vibration monitoring program to measure the vibration levels and frequencies,
2. Comparing the vibration levels against associated standards to identify regions of concern,
3. Investigating different excitation mechanisms, particularly flow-induced vibration (FIV), as well as identifying the sources of vibration,
4. Modeling the system to determine the vibration parameters including: fundamental frequencies, mode shapes, and system dynamic response, and
5. Adopting design considerations and system modifications to eliminate excessive vibration.

Excessive vibration may be initiated as a result of coinciding excitation frequencies with natural frequencies of a system. Vibration measurement plays an important role in quantifying the severity of the vibration, prioritizing repairs, and adopting a proper vibration control approach. Depending upon the nature of a system’s response and excitation mechanisms, different parameters should be measured to determine the sources of vibration, such as pressure fluctuation, acceleration, and dynamic strain.  Several factors may affect the implementation of a successful vibration monitoring program. These factors include: identifying critical locations to take the measurements, properly installing sensors, adopting mounting solutions for high temperature equipment, and defining proper signal conditioning parameters.

In addition to quantifying the level of vibration, the measurement of frequency response can provide valuable insight into the excited modes of a system and its excitation sources. The sources of vibration could be categorized as low frequency (Hz), medium frequency (30 Hz to 300 Hz), and high frequency (>300 Hz). These categories can better enable one to identify the excitation mechanisms [1, 4]. Different phenomena may cause excessive vibration in a piping system, including flow-induced excitation, pressure pulsation, and high frequency acoustic-induced vibration. Flow-induced vibration is usually initiated due to geometric constraints in the system. Vortex-shedding and turbulence excitations are the most common type of flow-induced excitations [5]. Pressure pulsation may be caused by a piece of rotating equipment and generate distinct frequencies which are integral multiples of the equipment operating speed [4].

In this study, vibration assessment steps were explored in greater detail for a piping system subjected to flow-induced vibration. The vibration data was acquired using a portable multi-channel vibration analyzer and industrial uni-axial accelerometers. Different sources of vibration, including pressure pulsation caused by rotating equipment, high frequency acoustic, turbulence, and vortex shedding excitations are discussed. In addition, a modal analysis was performed utilizing the piping stress analysis software, CAESAR II [6]. This analysis provided corrective actions to mitigate the excessive vibration by ensuring a twenty percent separation between the natural frequencies of the system and the likely excitation frequencies. The corrective modifications may increase thermal stress on the piping system, which enforce the necessity to evaluate the piping thermal stress.