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SUGGESTIONS FOR BESTPRACTICES IN RUN-OUT
DEVICES
By:
G.Sridhar
Sr.Manager/TG
BHEL Site Office, TATA Trombay
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INTRODUCTION
During the course of erection and commissioning ofHigher Rating Steam Turbines, the need to improveupon certain existing methods of erection was felt.
Among the many that need improvement are thestandard, time consuming, laborious and not soperfect methods of:
Setting the pedestal seal bore by piano wire before
grouting the pedestals Measurement of face run-out of the IP & LP Rotors by
rotating the rotors on saddle supports
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1. MEASUREMENT OF FACE RUN-
OUT OF IP & LP ROTORS BEST
PRACTICE
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Normal Practice
The normal practice of measurement of face run-
outs of IP & LP Turbine Rotors is to place them on
saddle supports and rotate them. Lifting action of
the crane hook causes rotation of the rotor. To aid inrotation of the rotor, copper sheets are placed in
between the saddle and the rotor and, grease is
smeared to reduce friction during rotation. It has
been noticed that this method has a few problems
associated with it:-
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It is not possible to restrict the axial movement of therotor in order to avoid overshooting the range of thedial gauge.
The area of contact being very small, it requiresextreme care and control while operating the EOT, lest itshould cause an irreparable accident and lead to loss ofboth men and material.
It has often been noticed that there is quite an errorbetween the measurement made at shop and thosemade at site leading to avoidable loss of time incorrespondence, summoning experts from MUs, etc.
It gives the feeling of using a very primitive and un-
reliable method of measurement. Overall, it is never like measuring the run-outs by
anchoring the rotor on a lathe to take the measurementsand thats exactly the cause of the error.
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Suggested New Method
To tide over the above problems, a very simple device was made atsite. To make the device, the following material was made use of:
Shaft of 40mm x 1000mm long 1 no.
Ball Bearings with Plummer block for above shaft 2 nos. Supports to anchor the Plummer blocks 2 nos.
Cross arm 1 no.
Dial Gauges for mounting to X-arm 2 nos.
The sketch of the fixture is as in the adjoining slide.
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Brg with
Plummer Block
Cross Arm
Shaft Support provided by Unit
External Support
for anchoring the
Plummer Block
Dial
Pointers
Dia 40
Shaft
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Advantages:
This method is very user friendly and costs only Rs.5000/- to make.
Does not require too much of a precision in machining the shaft.
Does away with repeated rotation of the rotor for every reading.
Reduces the risk of the rotor toppling during rotation by EOT crane.
Compares well with the values attained at works on a lathe.
Overall, an improvement in the positive direction.
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2. CENTERING OF TURBINE
BEARING PEDESTALS
BEST PRACTICES
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Normal Practice
The usual practice is to draw a piano wire
coinciding with the centre-line of the TG Axis and
centre the pedestals at the seal bore by measuring
the distance between the seal bore and the pianowire by an Inside Micrometer. Though the method
has been used successfully in the past, there have
been elements of doubts cast over its correctness.
These are enumerated in adjoining slide:
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There is no scientific method to determine the amount oftension to be given to the piano wire
During windy seasons, the piano wire oscillates so muchthat no two readings are consistent, there by introducingthe element of human error.
The effect of ambient temperature on thesemeasurements has always been noticed and over theyears it has been assumed that the best period torecord the measurements is the early hours of the day
i.e.; between 0600 & 0700 hrs.
This method can be used to align the pedestal in oneaxis only. It cannot be used to set the elevation of thepedestal.
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Recommended Method
LASER ALIGNMENT is a method being used by manyan OEM to good and accurate effect. This device as isunderstood from reliable sources takes into account thevariations of temperature and also is not affected by
wind. It can be used to center the pedestal as well as setthe elevation simultaneously. This vastly reduces the timetaken for each reading and the overall time taken foralignment of the pedestals as a whole. Also, it eliminatesthe human error that is likely to occur.
The method can be extended to alignment of CondenserTube Support Plates as well.
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3. COUPLING OF TURBINE
ROTORS
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Normal Practice
The normal practice of coupling of Turbine Rotors is
to Ream & Hone the coupling, machine the bolts to
the required diameter, balance the coupling bolts,
insert and stretch them to the required values. Theentire job of Reaming, Honing, Machining of
Coupling Bolts and Balancing of the rotors is a time
consuming and costly one.
There are certain pitfalls associated with this
arrangement:
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The process is long and arduous.
It depends too much on the skill of the person carrying out
the job.
The requirements are very stringent and many a times inremote areas, facilities for high-accuracy machining is notavailable.
The process is irreversible; meaning, one mistake in themachining of the bolts and the bolt cannot be used.
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Recommendation
Since the basic idea is to develop contact pressure
and thus transmit torque, the use of coupling bolts
and nuts similar to the ones being used in BFPs can
be thought of. Once alignment is completed,coupling is a very easy and quick job in a BFP. This
concept can be extended to the Turbine as well.
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Advantages:
No time lost between alignments and coupling.
Reaming, Honing, Bolt Machining and Balancing can
be done away with. A significant reduction in cycletime considering that it takes at least a week foreach coupling.
Due to the above, the element of human error is
totally done away with. Easy on the contractor as well; the cost of Reaming,
Honing and Bolt Machining is saved.
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