8.4 CONCLUSIONS

 The most important causes of either asynchronous or synchronous vibrations can be very well simulated during calculation so that a good forecast of the rotor dynamic behaviour is available.

Moreover, the parallel growth of instrumentation technology provides the possibility of thorough verification not only ofthenmchanical running conditions of the machine but also, and consequently, of the theoretical assumptions taken as design basis, therefore confirming the statements made in the first paragraph.

Note: The rotors are balanced through the following procedure:

1. The impeller is mounted on the balancing equipment The whole unit is then mounted on the balancing machine and must be turned by hand to check correct mounting; to do this, measure eccentricity on external diameter of impeller seal (max. permissible value: 0.02 mm). Next balance impeller at a higher speed, compatible with the machine's limits in accordance with its weight, by removing material on hub and shroud until final unbalance is within permissible range gi ven by API 617. The impeller must then be subject- ed to overspeed test.
2. Subsequently the impeller, mounted on the special shaft with two adaptor disks used for balancing, is fitted onto the vertical overspeed unit. Overspeed test is then carried out maintaining the same level for about 10 minutes, in accordance with the values given in the specification after the trip device of the unit has been set at a speed 2% greater than per specification. Check that vacuum remains at an absolute pressure of less than 1 Torr and that the vibrations measured on the drivinq turbine are less than 6 mm/sec. Test values must be recorded; the impeller must then be rechecked by penetrant dye liquids and then forwarded to be assembled onto the shaft.
3. Balancing of the rotor Mount shaft on balancing machine resting it on its journal bearings; fit false half-keys and begin balancing process, temporarily adding filler on surfaces for end locking rings, using adhesive tape for this purpose. Balancing speed is selected with reference to the characteristics of the machine, in compliance with the degree accuracy required by API 617 (Oct. '73). Next, mount one impeller at a time and after each mounting, balance by removing material from hub and shroud. The end seal rings must then be fitted on and the tem ora ' " p ry weights, added previously, removed. Next, proceed to balance by removing material,by driliing, from the rings themselves. Mount thrust bearing block and corre t ' c its unbalance by machining the outer diameter. Mount specified joints and check balance. For final checking, turn connection joint on balancing machine by 180° and check balance again. After the operation is completed, the test certificate shall be drawn up.

N.B The above is based on the basic criterion for a flexible rotor: to prevent internal moments arising during assembly of rotor, the rotor is balanced at different intervals, i.e. after mounting each individual part (impeller, spacer etc.) the rotor undergoes balancing.

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