vibrations overview

8/17/2019 Vibrations Overview

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Alignment

Centrifugal CompressorAlignment


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The measure of the mechanical behavior of a compressor is given by the amplitude and frequency of the rotor vibrations.

The rotor vibration amplitude must not cause:

• Contact between rotating and stator parts;

• Dry gas seals overloading;

• Fatigue in the bearings.

The typical vibrations of the centrifugal compressors can be classified with reference to the frequency and the nature of the forcecausing vibrations.

According to the first classification (frequency) the vibration may be:

• Synchronous

The vibration frequency corresponds to the machine rotation speed ( ω R)• Asynchronous

The vibration frequency is different from the machine rotation speed ( ω R)


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Alignment

=

The most common causes are:

• Unbalance in the rotor system;• Rotor and stator parts contact;• Fouling deposit.

It is necessary to do another distinction between frequency of the vibration and machine rotational speed

• Frequency of the vibration multiple of ω R

• Frequency of the vibration lower or higher than ω R

=


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Alignment

=


• Coupling misalignment;

• Loose of interference in some rotor components.

= 2


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• High clearance in journal bearings

• Oil whip;

• Aerodynamic effects (rotating stall in diffusers)

• Cross - coupling

= 0.5

= ( 0.4 − 0.5 )

= ( 0.1 − 0.2 )

= 0.2


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Alignment

Frequency

V i b r a t i o n a m p

l i t u d e

Subsybchrounous Vibration

Frequency of rotation Supersynchrounous Vibration


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Alignment

To achieve this objective has been defined the Amplification Factor (AF).According to API 617, it is an index of maximum vibration amplitude.

=−

Amplitude

Speedn criticaln lower n upper


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Amplification Factor Determine:

•

System Stability:• Amount of System dampening needed .

< .

> .

≫ .

Amplitude

Speed


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Alignment

NC1 = Rotor first critical, center frequency, cycles per minuteNcn = Critical speed, n thNmc = Maximum continuous speed, 105 percentN1 = Initial (lower) speed at 0,707 x peak amplitude (critical)N2 = Final (higher) speed at 0,707 x peak amplitude (critical)SM = Separation margin

0.707

=−


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•

When the rotor amplification factor is greater than or equal to 2.5, the corresponding frequency is called a critical frequency,and the corresponding shaft rotational frequency is called a critical speed;

• If the AF is less than 2.5, the response is considered critically dumped and no SM is required;

• If the AF is 2.5 to 3.55, a SM of 15% above the maximum continuous speed and 5% below the minimum operating speed isrequired.


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Alignment

Vibrations damping is performed by means of journal bearings, the damping effect depends on rotational speed and bearingclearances.To ensure that vibrations are damped correctly a test, lateral analysis, is performed.Lateral analysis reports only the rotordynamic behaviour of the compressor, torsional analysis reports the rotordynamicbehaviour of the whole train, including drivers, gearboxes etc.


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Alignment

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