Understanding Turbulence in Vibration Analysis

Definition: What is Turbulence?

In the context of vibration analysis, turbulence refers to the chaotic, random, and unstable flow of a fluid (liquid or gas) through a machine, such as a pump, fan, or turbine. This erratic flow creates pressure fluctuations that act as a forcing function, inducing a low-frequency, random vibration in the machine’s structure.

Unlike the discrete, periodic forces caused by unbalance or misalignment, the vibration from turbulence does not occur at a single, sharp frequency. Instead, it appears as a “hump” of broadband, non-synchronous energy in the FFT spectrum.

Characteristics of Turbulence Vibration

• Frequency: It is a low-frequency phenomenon, typically occurring below 10-20 Hz and well below the running speed of the machine.
• Broadband Nature: It does not produce a sharp, distinct peak. Instead, it raises the noise floor in the low-frequency region of the spectrum, often described as a “random hump” or “haystack.”
• Random and Non-Periodic: The vibration is not steady. The amplitude and phase are constantly and randomly fluctuating. When viewed in the time waveform, it appears as a chaotic, non-repeating signal.
• Direction: The vibration is typically radial and can be present in both horizontal and vertical directions.

Common Causes of Turbulence

Turbulence is a hydraulic or aerodynamic issue caused by disruptions to the smooth, designed flow of the fluid. Common causes include:

• Operating Away from the Best Efficiency Point (BEP): Pumps and fans are designed to operate most efficiently and smoothly at a specific point on their performance curve. Operating them at a significantly higher or lower flow rate than the BEP will cause the fluid to flow inefficiently, creating turbulence.
• Obstructions in the Flow Path: Anything that obstructs or disrupts the fluid’s path can cause turbulence. This includes poorly designed piping (e.g., sharp bends right before a pump’s suction inlet), partially closed valves, clogged strainers, or foreign objects.
• Air Entrainment or Cavitation: The presence of air bubbles in a liquid (entrainment) or the formation and collapse of vapor bubbles (cavitation) creates highly turbulent and impulsive conditions, which generate significant random vibration.
• Poor Sump or Inlet Design: In pumps, a poorly designed sump can create vortices that introduce air and turbulence into the pump suction.

Diagnosis and Differentiation

The key to diagnosing turbulence is its random, broadband, and low-frequency nature. An experienced analyst can often identify it by observing the “unsteady” and “beating” nature of the vibration on the machine itself.

It is important to differentiate turbulence from other low-frequency issues:

• Mechanical Looseness: Looseness also creates broadband noise, but it is often characterized by a raised noise floor across the entire spectrum and distinct harmonics of running speed, which are absent in pure turbulence.

– Oil Whirl: This is a distinct sub-synchronous peak at ~0.4-0.48X, not a broad hump of random energy.

– Rubbing: A rub can generate a wide range of frequencies, but it often includes many high-frequency harmonics and sub-harmonics, and the time waveform may show truncated or clipped peaks.

Since turbulence is a process-related issue, not a mechanical fault, the solution typically involves correcting the operational or system-design problem. This may include adjusting the operating point of the pump or fan, opening valves, cleaning strainers, or modifying the piping design.

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