Many vibration problems are a result of interactions among a system's pump, motor, fluid, piping, and structure. This interactive relationship requires a systems approach to vibration analysis, rather than the investigation of individual components. Neglecting important factors during pump installation can lead to vibration issues that are expensive and time-consuming to resolve. All pump installations must adhere to manufacturers' recommendations for system-vibration problems to be avoided. However, if a vibration problem occurs, all field and vibration data must be analyzed to determine the cause. Once identified, the correct, most cost-effective solution can be applied to eliminate or minimize the vibration.

WHAT IS VIBRATION?

Vibration is defined as mechanical oscillations or the repetitive motion of an object about an equilibrium point. Simply stated, any object that moves back and forth is vibrating. Examples of vibration include a clock's pendulum in motion, a guitar string that has been plucked, or a tuning fork that has been struck. These examples use oscillation — or vibration — to obtain a desired result. However, vibration in a centrifugal pump can be undesirable and lead to component failure.

Vibration is an indication of a pump's condition. All pumps vibrate at some level of intensity throughout their life cycle. A vibration analysis can help identify normal vibrations that may not be harmful as well as vibrations that exceed acceptable levels and can shorten a pump's life. Because an analysis also may determine the source or cause of a vibration, it can be used as a tool for troubleshooting and preventive maintenance.

WHAT CAUSES VIBRATION?

Able to originate internally or externally, an excitation force is the only cause of vibration. Repeating forces create the vibration problems most commonly associated with centrifugal pumps. These forces often are caused by the rotation of imbalanced, misaligned, or worn pump components.

HOW IS VIBRATION MEASURED?

Vibration measurements should be taken at the inboard and outboard bearings of a pump and motor in axial, horizontal, and vertical directions. If a more intensive analysis is required, measurements also should be taken at several places on the pump base. Electronic probes or sensors can be attached at each of these locations, sending electronic signals to a vibration analyzer. Measurements should be taken at operating speed for constant-speed motors and at varying speeds for pumps operating on variable-speed drives.

HOW IS VIBRATION DESCRIBED?

Vibration analyzers convert complex mechanical movements into equivalent electrical signals that can be filtered and measured using efficient algorithms. The resulting spectrum can describe each vibration by frequency, amplitude, and/or phase and used to identify the root cause of a pump's vibration. Confirmation of a vibration source is critical to any major corrective action based on a spectrum.

Frequency

Frequency describes the oscillation rate of vibration or the speed at which an object vibrates. It is a measure of the number of complete cycles that occur in a specified period of time, such as cycles per second (cps) — also measured in hertz (Hz) — or cycles per minute (cpm). Using cpm to describe vibration frequency allows it to correlate to the rotational speed of the pump, which typically is expressed in revolutions per minute (rpm). If pump speed is 1,800 rpm, and vibration frequency is 1,800 cpm, frequency can be expressed as:

1 × rpm

If pump speed is 1,800 rpm and vibration frequency is 3,600 cpm, frequency can be expressed as:

2 × rpm

The rotational frequency of a pump typically is referred to as the system's fundamental frequency. Many vibrations simply are a multiple of fundamental frequency.

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