Avoid Bad Turns with Rotating Equipment

Identify damaged and worn components before they cause problems.

By Amin Almasi, WorleyParsons Services Pty. Ltd.

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Vibration-based condition monitoring can provide crucial insights about a process plant's rotating machines. Indeed, analysis of vibration data can pinpoint a number of common malfunctions and problems.

Rotors usually are the principal source of vibrations. These vibrations eventually are transmitted to the machine's pedestals, casing and foundation. So, measuring vibrations at their actual source is vital for correct evaluation of equipment health. That's why the generally accepted practice is to utilize two non-contacting displacement transducers installed in an orthogonal XY configuration (usually +45° and -45° from vertical) on or near each radial bearing. Vibration measurements on a machine's casing from velocity pickups or accelerometers just provide indirect information about the vibration source. In addition, it's extremely useful to install axially oriented non-contacting proximity transducers for the rotor assemblies.


Let's now look at how vibration data can identify specific issues, including unbalance, misalignment, loose stationary and rotating parts, and worn bearings.

UNBALANCE
This is the most common malfunction in chemical processing rotating machines. It stems from unequal mass distribution at each section of the rotating assembly. In an unbalanced condition, the rotor's mass centerline doesn't coincide with the axis of rotation. The unbalance represents the first fundamental mechanism to transfer rotational energy into vibration. The unbalance vibration speed equals the rotor assembly rotating speed. The unbalance vibration is a typical sinusoidal waveform. The rotor response depends on both the rotor combined dynamic stiffness (i.e., the effects of stiffness and damping) and the unbalance excitation. Therefore, any changes in the rotor-dynamic response may result from either a change in unbalance or a change in the restraining rotor-dynamic stiffness components (e.g., a shaft crack can reduce rotor stiffness).

MISALIGNMENT
The second most common malfunction is a rotor getting out of alignment. This causes a constant radial force that pushes the rotor assembly to a side. The same result can occur from a strong radial component of the fluid flow in the rotating machine.

Because of the radial force, the rotor gets displaced from its original position, increasing rotational eccentricity; this is particularly problematic at bearings and seals. The rotor assembly also may become bowed and rotate in a bow configuration. These conditions lead to nonlinear effects, adding higher-order harmonics of vibration components (particularly second-order (2×) harmonics) to the rotor's synchronous component (1×) response. The appearance of 2× harmonics with comparable vibration amplitude to the first harmonic is a good indication of an alignment issue.

Axial measurements additionally may indicate an impending excessive radial load or misalignment problem on a rotating machine.

Continuously acting radial load causes reversal stresses in the rotor. For a high-speed continuously operating machine, the number of the stress reversal cycles may swiftly reach the fatigue limit — and thus prompt rotor cracking and premature failure.

ROTOR RUBBING
Contact between the rotor and the stationary part of a rotating machine is a serious malfunction that could lead to a catastrophic failure. Rubbing usually occurs as a secondary effect of a primary malfunction, such as unbalance, misalignment, fluid-induced dynamic effects or self-excited vibrations. Rub-related failures occur quite often in chemical processing equipment.

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