Thursday, 24 May 2012

Turbine blade fixing

Turbine blade fixing
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Blade stresses

The predominant stress in turbine blades is centrifugal and concentrated at the root Vibration is set up in blades due to fluctuations in steam flow.Particularly in impulse turbines where partial admission is used
Further stress is caused by expansion and contraction as well as bending stresses due to the action of the steam
In addition to these stresses occur during manoeuvring due to speed changes.

Fixed Blades

Although not subjected to centrifugal force, the fixed blades of curtis velocity compounded turbines are subjected to vibration in a similar way to the rotating blades. The root fixture must, by necessity, be secure to prevent fretting

Reaction Blades

Blades are rolled to correct shape then cut to length. Up to 50 blades are then assembled in a jig of correct radiurwith a distance piece to give the correct spacing.
The root is drilled and the upper part machined so as to accept shrouding fro end-tightening, or thinned for tip tightening.
After assembly on the jig a hole is drilled though the base and a wire passed through. The whole assembly may then be removed and brazed or spot welded to form a solid curved section.
The arc is then machined to the desired root form. Shown below is a single blade section of the arc showing typical root form.
The segment is dropped into position pushed axialy and a caulking piece fixed
A gate is formed in the final blade which receives a further thin section piece made of copper which is caulked in.
The fixed blades in reaction turbines are made in a similar fashion except that the end blades as held in by a screw and locking strip as the horizontal joint. ALso the root may be of a simpler design due to the lack of centrifugal stress.
For higher speed, higher rated turbines the built up method may not be acceptable due to the stresses.
These blades may then be made of soild individual sections. The blades enter through a gate with the final blade being caulked into position.
The gates for each groove are staggered to assist balancing. The lacing wire/shrouding is then fitted.

Impulse Blades

The most common form is the dove tail.
The groove is cut away to form a gate to allow the fitting of the blades. The final blade is riveted in position.
Blades subjected to higher centrifugal stresses, for example the longer tapered blades found in the final stages of the LP turbine, may have the fir tree root method which allows increased contact area without weakening root or wheel rim.
To reduce centrifugal stress on the wheel straddle root form of blade fixing may be used thinning the wheel rim. The straddle may be a simple fork design or of fir tree root. Rivets are added for strength.
Inverted fir tree root
Fir tree root attachment is very strong but requires accurate machining and manual blade fixing is not possible. The gate is filled with a machined block with no blade and then riveted to secure.
Multiple forks
For very large blades, say at the end if the LP turbine, the root, and thus wheel rim, would be required to be very large. Multiple forks may be used which are comparitively easier to machine.
Straddle 'T'
Straddle 'T' used rather than inverted 'T' so that the holding faces on the rim can be easily inspected for defects.
Stal Laval bulb root
The main advantage of this system is that the blades are introduced into the rim axially. Therefore the individual fitting of the blades required with circumferential root arrangements is unnecessary
Where the distances between the bulb becomes so small as to risk failure of the rim, staggered bulb root depths are used with alternating short and long shank lengths.
For these types of blades the shrouds are part of the blade. On this shroud are two tabs. A shrouding wire is passed around the circumference over the shroud and the tanbs are bent over. This has the advantage that in the event of root failure some support is given to the blade. Multiple shroud wires are filled rather than a singe one for ease of manufacture allowing smaller tabs, and also to reduce mechanical stress. On more modern designs the groove is moved to the end of the shroud and a welded shroud wire fitted.

Sizing the rim

When the rim is first cut and the entrance gate formed, a test blade with slightly too large root ( or feet) is carefully filed and then tapped around the rim. This blade is then discarded. The real blades are then carefully filed and fitted taking into account the wear on the rim. The nating face of the blades are filed to ensure even blade pitch. A tight fit is essential with a steam turbine, if not then severe fretting and failure will occur.

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