Thursday, 24 May 2012

Turbine blade vibration

    Damping wires, Lacing wires and shrouding are fitted to
    • reduce stress due to vibrations in the blade excited by such as steam flow fluctuations as the blades pass the nozzles. This is referred to as the 'passing frequency'. This particularly occurs with partial admission
    • To prevent spreading of the long thin blades found in the final stages of the LP turbine. Shrouding is not fitted to these blades to allow adequate drainage. Due to the high specific volume losses due to spillage is relatively small
    • Steam changing direction as it passes over the blade tends to build up in the concave face. There is a tendency to flow to the tip where if unchecked it can spill over leading to considerable loss of efficiency. This is particularly important in parsons turbines expecially as the initial stages of the HP turbine where the steam has a low specific volume.
The vibration associated with turbine blades is referred to as the 'clamp-pin' type and is determined by vieing the blades in their packets i.e. blade groups attached by their shroud.

Frequency types

The lowest frquency is of the whole packet vibrating.
Higher frequency is where as equal number of blades bow in oposite directions
Higher still frequencies occur where each blade vibrates

Lacing, Damping and Binding wires

    There are four sources of vibration damping under normal operating conditions
    • Internal damping of the blade material
    • Inherent dry friction damping of the blade assembly at the root and tip
    • Fluid damping or viscous damping due to the steam environment
    • Mechanical damping through fitting of damping aids such as damping or lacing wires etc
Lacing wires fitted at an anitnode provide a very effective form of dampening. However, the antinode may exist at different positions for the different types of vibration so a compromise on the position has to be reached.
A Damping wire which is 'free fitting' is free to move within the holes. Centrifugal force throws the wire to the outside of the hile where frictional effects help dampen the vibration. The disadvantage of damping wires is that heavy fretting can eventually cause the holes to widen to an extent that the rotor has to be rebladed.
Lacing wires are brazed in and are therefore strengthening and hence are not necessarily placed at an antinode but rather where the blade is thickest.
Binding wire is used to strengthen the trailing edge of the blade. This is a very old fashioned technique and is little used.
The use of round wire can lead to aerodynamic losses

Snubber or bumbing blocks may be cast or forged into the blade. These have a highly aerodynamic form.

The damping is then achieved by both the bumbing of the blades and the following resistance to breaking as a vacuum formed at the joined faces tries to hold them together. A certain amount of fluid damping also occurs.

Shrouding

May be fitted by brazing, welding or riveting.
The shrouding is fitted over the blade, the tenon is then either riveted with 4 or 5 blows or welded. Care must be taken either method of fixing as it can lead to crack formation. Once the shrouding is fitted the surveyor may request a pull off test. The pull is determined by calculation and governed by the expected centrifugal stress on the shroud during normal operations.

Centenary Shrouding

For blade batches where the centrifugal stress on the shroud of very large LP blades is significant, then centenary shrouding is employed.

1 comment:

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