SETTING TURBINE THRUST BEARINGS
by Kim A. Lovejoy, Lovejoy Controls Corporation
Copyright 2010, All rights reserved.
No duplication without written consent.
Frequently problems with spurious thrust wear alarms are attributed to the thrust monitoring device when in fact it is the setting of the thrust bearing itself which has introduced the problem. This treatise has been written to cover the critical points in properly setting a turbine thrust bearing.
A drawing exists for all turbines titled "Rotor Clearance Drawing" or "spindle Clearance Drawing". This drawing gives radial and axial clearances along with allowable tolerances between rotating and stationary elements of the turbine. The drawing is generated by the manufacturer's engineering department, and should always be referenced when any question arises as to turbine clearances.
Figure 1 shows the key axial clearances and reference dimensions found on a typical rotor clearance drawing. It is worthwhile to review these references individually and to understand their significance when setting rotor axial position.
ITEM 1, INTERNAL "K" DIMENSION
The "K" or internal reference dimension represents the normal running axial position of the rotor. It is usually the closest running clearance between the rotor and stationary elements, which in turn is usually the gap between the steam admission nozzle block and the first blade row shroud. The "K" dimension can only be measured with the rotor resting in its journal bearings in normal position on the lower cylinder half with the cylinder cover (upper half) off. The spacer for the forward thrust shoes is ground to place the rotor at "K" WHEN IN OPERATION UNDER THUST LOADING, or under equivalent pressure by means of an hydraulic or mechanical jack during measurement. On most turbines this can be approximated well enough by applying a load of 25 psi to the forward thrust shoes through jacking.
ITEM 2, EXTERNAL "L" DIMENSION
The "L" or external reference dimension defines exactly the same rotor position as the "K" dimension, but is measured outside of the turbine cylinder, usually between a gland case cover and raised collar. The "L" dimension provides an external check that the rotor is at "K". The "L" reference dimension is usually stamped on an exposed surface of the turbine.
"L" dimensions should be checked and verified prior to any thrust calibrations.
ITEM 3, FORWARD TRAVEL TO INTERFERENCE
This dimension is usually found on the exhaust side of one of the blade row shrouds to stationary, and represents how far the rotor must travel in the forward direction from "K" before interference is encountered. Travel checks should be made both prior and after setting the turbine cylinder cover/s to verify that each assembled piece does not restrict travel less than the given tolerance. The thrust bearing is not assembled for this measurement and only mild forces (NOT a hydraulic ram) are used to position the rotor.
ITEM 4, THRUST CLEARANCE
Provided Items (1) thru (4) are satisfactory, the thrust clearance may be checked and adjusted if necessary. This is accomplished by surface grinding the reverse spacer halves to obtain a value centered in the rotor clearance tolerance range for thrust clearance.
Thrust travel should be measured in each direction with 25 psi shoe loading applied by a hydraulic ram of known area and a calibrated pressure gauge. Note that most thrust bearings have fewer shoes in the reverse direction than in the forward (or normal) direction, and that therefore the reverse force jacking pressure must be proportionately decreased.
The thrust bearing must be fully assembled, including casing cover, when travel limits are measured. Without the casing cover, many thrust bearings have the freedom to rock or pivot and a false reading will be obtained.
An accurate dial indicator should be mounted directly in-line with the axial travel direction, and readings should be recorded only under jack pressures.
Although it is the ONLY proper way to measure travel within the thrust bearing (do not compromise with pry-bars or sledges), NEVER use a hydraulic ram to measure travels without the thrust bearing assembled, as jacking forces can easily damage internal components.
If it becomes necessary to replace either the forward or reverse spacers, NEVER compile the needed thickness with shims. Such action use can cause all clearance to be eliminated under capillary jacking of individual shims, or create a "spring" effect due to localized shim rollover and deflection. Always carefully surface grind thrust spacers to obtain exact thicknesses necessary in ONE set of halves per end. Spacers should be true to at least .0005 inch.
ITEM 5, TOTAL TRAVEL WITHOUT THRUST BEARING
This will be the sum of Items (1) and (3), and should also be checked with and without the cylinder cover in place.
If the thrust measurements and clearance adjustment is performed as given, spurious motions and wear alarms may be eliminated.
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