Page 143 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Compressor Best Practices Be st Practice 3.3

Fig 3.3.10 Rotor natural frequency mode shapes (Courtesy of Elliott Co.)

method is to input a value of 8 A.P.I. acceptable unbalance Rotor mode shape data is important to the designer because it
limit ð4WÞ . allows determination of modifications to change critical speed
N
This results in a rotor response input unbalance of 32W . values.
N
The location of the unbalance is placed to excite the various For the end user, this data provides an approximation of
critical speeds. Typically the unbalances are placed as noted the vibration at any point along the shaft as a ratio of the
below: measured vibration data. As an example in Figure 3.3.10,
determine the vibration at the shaft mid span if the vibration
Location To excite
measured by the probe C 2 when operating at NC 1 is 2.00
Mid span NC 1 mils. From Figure 3.3.10, the vibration at the shaft mid span
Quarter span (2 identical unbalances) NC 2 when operating at the first critical speed of 3327 RPM (50 in
At coupling NC 2 ,NC 3 location) is:

Failure to accurately determine the value and location of 1:00
residual rotor unbalance is one of the major causes of correlation :1 or 10 the bearing vibration
differences between predicted and actual critical speeds.
Ten (10) times the value at C 2 or 20.0 mils!
Rotor response (output) Mode shape data should always be referred to when vibra-
tion at operating speed starts to increase and your supervisor
The output from the rotor response study yields the following: asks

- Relative rotor mode shapes ‘When do we have to shut down the unit?’
- Rotor response for a given unbalance or
‘Can we raise the radial vibration trip setting?’
Figure 3.3.10 shows the relative rotor mode shapes for NC 1 , In this example, the bearing clearance may be 0.006 or 6 mils,
NC 2 ,NC 3 and NC 4 . Usually, the rotor will operate between and an honest request would be ‘We’ll replace the bearing at the
NC 1 and NC 2 . turnaround, please run to 7.0 mils vibration’.

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