BARRING GEAR & AVOIDING STALLING

Barring gear is mechanism which rotate turbine shaft (with generator) when motive steam is not available i.e. when turbine is in trip condition.

Steam turbine is on barring means turbine shaft rotates on a speed of 60-90 rpm without vacuum & while on vacuum rotates 120-140 RPM. At this low speed the friction is very high between the shaft & bearing surfaces. To reduce friction between them high pressure jacking oil is supplied at the bottom of all bearings. For different bearing jacking oil pressure is different. the turbine is comes on stand still in case of any rotating parts come to contact with stationary parts like seal fins etc.

Barring gear stalling takes place in hydraulic barring gear system. In motorized barring gear not stalled. In steam turbines motor driven turning gear is provided which will continue to rotate the turbine shaft even if there is touching of rotating and stationery parts and hence normally the problem of Barring gear stalling does not take place, even though abnormal noise may be there however is extreme case motor will trip on overload.

This system is provided for rotation of turbine shaft at a nominal speed ( RPM as according to OEM).

(a) After trips to prevent top/bottom temperature difference induced thermal bending in hot conditions.
(b) To prevent sagging due to gravity in cold conditions.

IN MODERN LARGE SIZE STEAM TURBINE BARRING GEAR IS DRIVEN BY FOLLOWING MECHANISM

• Hydraulic Barring gear mechanism (Hydraulic turbine wheel located at HPT front shaft).
• Motorized barring mechanism.
• Hand barring mechanism (provided at LPT shaft).

MAJOR REASONS FOR BARRING GEAR STALLING

In case of Hydraulic Barring Gear, Turbine not coming on Barring Gear or stalling is not a problem but a symptom that
turbine unfit for rolling. Admission of steam is only acceptable when turbine is on barring.

This could be because of any one of the above reason or any other.

• Abnormal gland steam containing condensate.
• Abnormal Mechanical clearances, casing / glands.
• Wet steam/water entry into Casing.
• Air chilling of glands.
• Loss of jacking oil Lift pressure/Hose puncture/groove on journal.
• Loss of CW and emptying of water box in spring deck-some units.
• Speed slow down on CW pump stoppage.

TO PREVENT BARRING GEAR STALLING

The following points are to be looked into when a unit trips out or shutdown is taken:-

• Barring gear speed slowdown is observed when lubricating oil temperature reduced.
• Due to excessive cooling due to sticking of valve under extreme position and fault appears in 3 oil temperature control valve. Maintain lubricating oil temperature 45⁰C.
• Ensure that CW water box remains filled with water.

IN CASE OF BARRING GEAR STALLING

• Check jacking oil header pressure & individual lift oil pressure.
• HPT/IPT Casing top & bottom differential temperature normal.
• Hand barring with one person possible then Seal steam should be kept charged along with evacuation.
• Maintain auxiliary seal steam temperature to 300⁰C.
• If turbine is still on stall then start control fast cooling of turbine.
• Gland steam temperature as per rated parameters i.e 300⁰C and ensure no steam is condensed upstream of Seal steam supply control valve. Maintain auxiliary gland steam temperature 300⁰C.
• Keeping continuous warm up line open drain of auxiliary steam supply line. Check that no condensate is in auxiliary PRDS.
• Check source HT  aux. Steam header temperature is greater than 300⁰C.

AFTER UNIT TRIP OR SHUT DOWN

• Do not keep Gland seal steam charged for more than 1 to 2 hours.
• Do not empty condenser water box (HPT temp < 290⁰C is achieved).
• Keep At least one CW pump running till then for preventing effect on disturbance in turbine seal fin clearances.

VACUUM KILLING SEQUENCE

• Stop Vacuum pump.
• Kill Vacuum by opening Vacuum breaker.
• Isolate Gland seal steam at Back pressure of 0.7-0.8 kg/cm².
• Stop Gland steam Exhauster immediately after isolation of gland seal steam.

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