FEEDBACK NO. 38

BARRING GEAR A NEW DESIGN

1.0        Introduction:

With the up gradation of technology in Power field and growing competition, more & more thrust is being to reduce the house load consumption. The new design barring gear mechanism supplied in 2 x 250 MW Dahanu TPP satisfies this requirement. The power consumption in operating this barring gear mechanism is much less than the power required for functioning of conventional electrically operated mechanism (e.g. LMW design) or hydraulically operated mechanism (e.g. KWU design).

2.0        Arrangement of the barring gear motor:

The hydraulic turning gear is situated at the front end of the HP turbine front bearing pedestal.

3.0        Mode of Operation: (Please see the enclosed fig.)

The hydraulic gear motor (1) is connected to the HP turbine front bearing pedestal via the cover (4) and the casing (8). The motor of the barring gear is operated hydraulically using oil tapped from jacking oil pump discharge. The rotation of this motor is transmitted via a specially profiled shaft (2) and the shaft flange (5) to the outer ring of the over-running clutch (10). The outer ring is supported in the casing (8) by the casing ring (9) and the ball bearing (6). The inner ring of the overrunning (10) is mounted directly on the end of the main oil pump shaft (13).

The engaged elements of the over-running clutch (10) are supported by a case and are installed such that during turning-gear operation, they swing into the engaged position in which they form a non-positive lock between the outer and inner rings. Thus, the turbine rotor is profiled shaft (2), shaft flange (5) and over-running clutch (10).

4.0        What happens to sprags when turbine is run-up?

When the turbine is run-up, the engaging elements of the clutch swing out, breaking the connection. At higher speeds, centrifugal force the engaging elements, which rotate with the inner ring, so that they cause no wear during turbine operation.

5.0        Detection of alignment faults / rubbing

Turning gear speed is governed by an adjustable throttle in the feed line to the motor of barring gear. A throttle control is used so the speed of the hydraulic turning gear increases as the torque required for the turning the turbine shaft drops or rises. This permits detection of any alignment faults or rubbing.

6.0        Isolation of Mechanism:

A manual isolating valve makes it possible to separate (isolate) the turning gear motor from JOP circuit.

7.0        Lubrication Circuit:

The leakage oil from the barring gear motor flows via the leakage oil pipe (3) into the collar of shaft flange (5) to lubricate the over-running clutch (10) ball bearings (6) is lubricated by the return oil.

8.0        Protection of the Antifriction Bearings:

During turbine operation, the antifriction bearings of the barring gear motor and of the clutch must be protected against standstill corrosion damage.

After shutdown of the Barring gear, the motor of Barring gear (1) is lubricated and turned by low pressure oil (approximated 2 bar), tapped from lube oil line to bearing 1 of Turbine.

9.0        Advantages of New design barring gear:

      Low power consumption

      Highly sensitive, helps in detecting faults in alignments, rubbing and other similar abnormalities.

      Operates with jacking oil system. So emergency maintenance work on bearings and inside pedestal can be carried out in hot conditions of machine keeping set on barring mode.

      Faults / defects in barring gear mechanism can be attended by resorting to hand barring operation in hot conditions of the machine.

      Higher AOP discharge pressure also higher trip oil pressure is available especially at the time of resetting the machine & opening stop valves, as no flow from AOP discharge is needed for turning gear operation.

Attention!! When turbine is in operation & barring gear is off ensure that barring gear motor rotates at 3-5 rpm. Peeping through the hole provided at top of the motor, after removing the plug, can ensure this.


10.0    Operation of solenoid valve:

The SV cuts off motive oil supply to barring gear motor. The operation of SV is needed only after cooling down the casing to less than 120C. TG set is to be taken off barring operation.

However, by operating the manually operated valve the requirement can be fulfilled.

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