FAILURE OF BARRING GEAR CLUTCH ASSLY COUPLING

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FAILURE OF BARRING GEAR CLUTCH ASSLY COUPLING

• U2,33MW TG. BORL,BINA PROJECT

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DESCRIPTION

• Unit-2 TG is provided with a motorised barring
  gear unit which drives the turbine at 150 rpm
  barring speed.
• The barring gear unit comprises mainly of one
  electric motor(37KW), worm reducer, SSS clutch
  assly ,worm gear and a soft starter.
• Drive is transmitted from motor through worm
  gear, clutch assly to Gear box pinion shaft and
  then to main turbine.
• The clutch assly is connected to the pinion shaft
  of the gear box with a coupling consisting 8nos
  studs.

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• When Barring gear is switched on, the worm shaft
  rotates the input assly. As input assly start
  rotating , the helical sliding assly moves
  axially towards drive end. This results in
  engagement of gear teeth of sliding assly and
  output assly. Thus barring gear gets engaged.
• The m/c was first put on Barring Gear operation
  on 03.07.10 and was running on BG for about an
  hour.

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• Before steam rolling while m/c was being put on
  Barring Gear on 12.08.10 suddenly all the 8nos
  coupling bolts of output assly got sheared off
  and motor got disengaged.
• Before starting the motor, JOP header pr and
  lift of rotor was checked and found in order.
  Rotor was also free in hand rotation.

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ACTION TAKEN

• The failure occurred just before steam rolling
  program of the unit. The barring gear operation
  was possible only after replacement of the
  sheared studs of output assly.
• A complete barring gear unit was available for
  U3. It was dismantled and studs were removed
  from the coupling and fitted in the affected
  clutch assly.
• Barring gear could be restored in 15 hrs which
  otherwise could have taken 5-7 days for
  procurement of coupling bolts.

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ANALYSIS

• As per opinion of M/s Triveni Engg , the
  supplier of the Barring Gear unit, the studs have
  sheared-off due to excessive loading of the
  turbine rotor probably caused due to drop in JO
  pressure at the time of switching on the motor.
• M/s Triveni also suggested the signal of min JOP
  which is a start permissive for BG motor, should
  be taken from individual brg instead of JOP
  header as per existing scheme.

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REMARKS

• With soft starter and overload relay in service,
  Barring gear motor is well protected against any
  overloading.
• Failure of coupling bolts of M/s Triveni
  supplied Barring gear unit was reported from few
  other sites.
• In view of the above BHEL site feels that bolts
  are to be designed for higher capacity.