Thursday, 24 May 2012

Turbine Balancing


 For static balancing the rotor may be simply rotated on knife edges, the position it stops in indicating the position of the excess mass, material may then be either removed or added. However, unless the material is added or removed from exactly opposite the are of excess, then an axial turning moment indicated on the diagram as Fx exists when the imbalance is acted upon by centrifugal force when rotating, a wobble will occur.


 This machine balances the rotor whilst it is still out of its casing. For best balancing the rotor is placed in its casing and run at high speed


Another example is shown below, the rotor would pass a static balance test and a low speed dynamic balance test; but the tendency for the rotor to sag would mean that at speeds near to or at critical a very heavy vibration would occur.
Hence, a high speed dynamic balance is required, and as the bearing rigidity has quite a large effect on the critical speed ( if the bearings are flexible the point of location will change increasing the distance between supports ), then the test is done by placing the rotor in its bearings in its casing.
Any out of balance will cause vibration at the critical speed
Balance is achieved by placing a weight under the shroud at one end and half weights under the shroud at the two opposite ends, directly on the opposite side of the rotor thereby maintaining dynamic balance. By trial and error the correct weights are found, material is removed on the opposite sides to the weights,and the weights removed

Critical speeds of rotors.

Even perfectly machined rotors once placed between bearings will tend to sag and hence do not run concentric. For turbines the centre of mass is by necessity very close to the centre of rotation (It is this deflection which leads to out of balance and subsequently this deflection is used in Critical speed calculations) and hence the natural frequency of transverse vibration ties in very close to the Critical speed. For its calculation the rotor is considered to be a simple beam supporting several point loads ( these can be calculated by splitting the rotor into sections and summing the mass within), these are typically due to wheel, blading. shrouding etc.
A formulae may be used;
Nc = 16.8/dc-1
Where dc is the static vibration.
For turbines whose normal max speed is higher than critical, balancing is carried out at full speed.
Turbines maybe built stiffly so that the critical speed occurs above norm max, speed. This means that to make the rotor stiff the diameter must be increased which increases the gland area and bearing loads.If the rotor is made less stiff so that critical now occurs within the normal operating revs, then care must be taken to pass quickly through the critical.

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