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VibraMac

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Reply with quote  #1 
40 MW condensing turbine (HP/LP) with a sliding front pedestal into which the thrust bearing is fitted.  The rear pedestal is fixed to the foundation.  Turbine casing is attached to both pedestals.  Proximity probes are mounted on the rear fixed pedestal 'looking' at the turbine coupling.  Are these probes measuring Differential Expansion (DE) or Rotor Expansion (RE)? 
John from PA

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Reply with quote  #2 
In the instance of a differential expansion measurement with a single probe, the probe is mounted to the casing and observes the rotor, thus establishing the differential between the two.  If the probe is mounted to the foundation, then it is observing the rotor expansion relative to the foundation.  In your instance, it sounds as if you may have a setup for rotor expansion.  What I'm confused about is your use of the plural "probes".

In the attached, see 6.6.1, 6.6.2 and 6.63 (page 17) for the differential expansion, and 6.6.5 (page 19) for the rotor expansion.

 
Attached Files
pdf GEA31795-TSI-Application-Note_R8.pdf (1.21 MB, 20 views)

VibraMac

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Reply with quote  #3 
Thanks John.  I have the same attachment which also makes me believe that RE is being measured.

The other theory (client) is since the thrust bearing is attached to the sliding pedestal (i.e. casing) the rotor will  move with the casing and therefore the rotor is positioned relative to the casing hence a DE measurement. 

After reading the attachment again I'm more confused.  My thinking, since the thrust bearing is in the sliding pedestal, the rotor expansion probes need to be fitted to a fixed casing (i.e. page 10, figure 7 right) hence the OEM has used the fixed pedestal and therefore the application is correct for RE readings.

The problem is the DCS description and the client's interpretation is that these probes are measuring DE.

I hope the OEM see these probes as RE and have used the correct clearances values.

Currently the turbine is experiencing sporadic rubs during transient speed events.

Two probes are being used (similar to that seen in fig 7).

Would appreciate your comments

 
John from PA

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Reply with quote  #4 
Quote:
Originally Posted by VibraMac
Proximity probes are mounted on the rear fixed pedestal 'looking' at the turbine coupling.


In my opinion, if this quote is true, then the probe is measuring what the shaft is doing relative to a fixed object, so that is rotor expansion but may also include axial movement allowed by thrust bearing clearance.  The probes as described aren't even aware of what is going on at the sliding front pedestal.  Usually to measure axial movement you want the axial probes to be with about 12 inches of the thrust bearing so that thermal expansion is a small portion of the indicated movement.  In this case, since the thrust bearing is at a significant distance from the probes (I assume 12 to 15 feet or 4 to 5 meters) rotor expansion is the major portion of the indicated gap change and thus the arrangement measures rotor expansion. 

You may also find the attached of use, see section 8.4 on page 13.




 
Attached Files
pdf GEA31971A-Centrifugal-Compress-App-Note_R4.pdf (5.28 MB, 12 views)

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