FIV and AIV (Flow-Induced Vibration and Acoustic-Induced Vibration) are critical considerations in the design and maintenance of piping systems. These phenomena, caused by fluid dynamics and acoustic energy, can lead to significant structural issues if not properly addressed.
Understanding FIV and AIV, along with their respective causes and mitigation strategies, is essential for ensuring the safety, reliability, and longevity of systems in industries such as oil and gas, petrochemicals, and power generation.
This blog delves into the fundamentals of FIV and AIV, highlighting their differences, potential impacts, and practical solutions to mitigate their effects.
Flow-Induced Vibration (FIV):
Flow-Induced Vibration occurs when the flow of fluid (liquid or gas) through a piping system causes vibrations. These vibrations can lead to fatigue failures in the piping system, which can be costly to repair and pose safety risks. FIV is typically associated with low-frequency vibrations and is more common in systems with liquid flow1.
Acoustic-Induced Vibration (AIV):
Acoustic-Induced Vibration is caused by high-frequency acoustic energy generated by pressure changes in the system, such as those caused by relief valves, control valves, or orifice plates. AIV can lead to fatigue failures, especially in small bore connections, and is generally associated with gas flow2.
Key Differences between FIV and AIV:
Frequency: FIV is low-frequency, while AIV is high-frequency.
Flow Type: FIV is associated with liquid flow, whereas AIV is associated with gas flow.
Sound: AIV produces sound waves within the range of human hearing, while FIV produces less audible sound.
Mitigation Strategies
To prevent failures caused by FIV and AIV, proper design and engineering methods are essential. This includes:
Proper Spacing: Ensuring adequate spacing between pipes to reduce vibration.
Supports and Dampers Using pipe supports and dampers to absorb vibrations.
Material Selection: Choosing materials that can withstand vibration-induced stresses.
To perform Flow-Induced Vibration (FIV) screening, you’ll need to follow a series of steps to evaluate the potential for vibrations in your piping system. Here’s a general outline of the calculation process:
Steps for FIV Screening Calculation:
Determine Momentum Flux:
Calculate the momentum flux using the formula: Momentum Flux = 𝜌 x 𝑣2 where:
𝜌 is the fluid density (kg/m³)
𝑣 is the fluid velocity (m/s)
Typically, if the Momentum Flux value is below 10,000 kg/ms², the risk of failure is considered negligible.
Values ranging from 10,000 to 20,000 kg/ms² indicate a medium susceptibility to failure, while values exceeding 20,000 kg/ms² are categorized as high risk.
To effectively address these susceptibility concerns, it is essential to follow the guidelines set forth by the Energy Institute (EI) for appropriate mitigation strategies.
Assess Span Length Between Supports:
Determine the span length between pipe supports. This helps in understanding the flexibility of the piping system.
Calculate Natural Frequency:
Use computational tools or empirical formulas to determine the natural frequency of the piping system. This frequency should be compared with the excitation frequency caused by the fluid flow.
Evaluate Likelihood of Failure (LOF):
Based on the momentum flux and natural frequency, calculate the Likelihood of Failure (LOF) using guidelines such as those provided by the Energy Institute (EI). The LOF score helps in assessing the risk of vibration-induced fatigue.
Mitigation Measures:
If the LOF score indicates a high risk, consider implementing mitigation measures such as adding supports, dampers, or changing the pipe layout to reduce vibration.
Example Calculation:
Let’s say you have a fluid with a density of 1000 kg/m³ flowing at a velocity of 5 m/s through a pipe with a span length of 10 meters between supports.
Momentum Flux Calculation: Momentum Flux = 1000 (kg/m³) x 5 m/s = 25000 kg/ms²
Natural Frequency:
Assume the natural frequency of the system is 2 Hz (this would be determined through analysis or empirical data).
Likelihood of Failure (LOF):
Using the EI guidelines, you would input the momentum flux and natural frequency into a screening tool to calculate the LOF score.
Acoustic-Induced Vibration (AIV) Calculations are essential for ensuring the safety and integrity of piping systems, especially those carrying gases or vapors. Here’s a more detailed look at the calculations involved:
Key Components of AIV Calculations:
Sound Power Level (PWL):
The sound power level is calculated based on the pressure drop across a pressure-reducing device (e.g., relief valve, control valve, orifice plate) and the flow rate of the gas or vapor.
The formula for calculating PWL is:
Lw = 10 log[M2 x (P1−P2/P1)3.6x (T/W)1.2] + 126.1 + K
where:
Lw = sound power level in dB
M = mass flow rate in kg/s
P1 = upstream pressure in kPa absolute
P2 = downstream pressure in kPa absolute
T = temperature in Kelvin
W = molecular weight
K = Correction factor to account for sonic flow. Its value is 0 for nonsonic flow and 6 for sonic flow conditions
Likelihood of Failure (LOF):
The LOF is calculated to assess the risk of vibration-induced fatigue failure at discontinuities in the piping system.
The LOF calculation considers factors such as the outside diameter and wall thickness of the mainline pipe, the outside diameter of the branch connection, and the sound power level at the point of assessment.
The LOF is determined using guidelines from the Energy Institute (EI) and other industry standards.
Mitigation Measures:
Based on the calculated LOF, appropriate mitigation measures are recommended to reduce the risk of AIV-induced failures.
Common mitigation strategies include increasing pipe wall thickness, using sweepolet branch fittings instead of weldolet fittings, and adding full encirclement pads for supports at pressure safety valve (PSV) outlets.
Example Calculation:
Let’s consider a scenario where a relief valve causes a pressure drop from 10 bara to 1 bara in a gas line with a flow rate of 50 kg/s and an upstream temperature of 300 K.
Sound Power Level Calculation:
Lw = 10 log[M2 x (P1−P2/P1)3.6∗(T/W)1.2] + 126.1 + K
Lw = 10 log[502 x (10−1/10)3.6∗(300/1)1.2] + 126.1 + 0
Lw = 10 log[2500 x (0.9)3.6x(300)1.2] + 126.1 + 0
Lw = 10 log[2500 x 0.6843 x 938.7404] + 126.1 + 0
Lw = 10 log[3439.4247] + 126.1 + 0
Lw = 3.5364 + 126.1 + 0
Lw = 3.5364 + 126.1 + 0
Lw= 129.6364 Db
Assuming
𝑆𝐹𝐹=0, the PWL can be calculated accordingly.
Likelihood of Failure (LOF):
Using the calculated PWL and the dimensions of the piping system, the LOF can be determined using the EI guidelines.
Tools and Resources for FIV and AIV:
Computational Fluid Dynamics (CFD): Simulates fluid flow and predicts potential vibration issues.
Finite Element Analysis (FEA): Models the structural response of the piping system to acoustic excitation.
Experimental Testing: Physical tests on scaled models or full-scale prototypes to validate predictions and design modifications.
All these calculations and screening details are included in the stress analysis report or prepared as a separate report.
By performing detailed FIV and AIV calculations and implementing appropriate mitigation measures, engineers can ensure the safety and reliability of piping systems in Liquid, gas and vapor services.
