Increment - CAESAR II - Help

CAESAR II Users Guide

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CAESAR II
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CAESAR II Version
13

Specifies the frequency increment used to step from Starting Frequency to Ending Frequency. The harmonic forces or displacements are applied at each frequency along the specified increment. This is an optional value. If no value is entered, the software uses a default increment of 1.0 Hz.

The frequencies for harmonic excitation are taken from each defined frequency range. Individual frequencies for excitation are calculated using a "do loop" type of logic to determine the frequencies in a specified frequency range:

X = STARTING FREQUENCY

5 CONTINUE

COMPUTE SOLUTION FOR FREQUENCY "X"

X = X + INCREMENT

IF( X .LT. ENDING FREQUENCY+0.001) GO TO 5

The sign of the frequency increment may be modified by the software to properly step from the starting frequency to the ending frequency. The starting frequency, the ending frequency, or the increment may be given as a fraction.

Example

Find harmonic solutions for the following group of turbine equipment speeds:

  • Warm up speed: 100 rpm

  • Speed increments to bring turbine online: 400, 800, 1200, 1600, 2000, 2400, 2800, 3200 rpm. Speeds are passed through very slowly while coming up to operating speed.

  • Operating speed: 3600 rpm

Convert rotations per minute to cycles per second (Hertz) by dividing by 60:

  • Warm up speed: 100/60

  • Speed increments: 400/60 to 3200/60 by increments of 400/60

  • Operating speed: 3600/60

A low frequency field vibration exists in the piping system at about 3 Hertz:

  • Approximate field-observed excitation frequency: 3 Hz

The response of the piping system when the dynamic load is applied at 3 Hz is almost zero. This is true regardless of the magnitude of the dynamic load. The maximum varying pressure load was applied, and there were still no appreciable dynamic displacements when the excitation frequency was 3 Hz. Apply the dynamic load over a range of frequencies around 3 Hertz and see if any dynamic response can be observed.

  • Group of field-observed frequencies: "Guessed"

  • Excitation frequency: 3 Hz

  • Defined by the input below are:

    (2.5, 2.6, 2.7, ..., 3.3, 3.4, 3.5) Hz.
    2.5 3.5 0.1