Thermal power station: Difference between revisions

Revision as of 03:02, 8 September 2005

Revision as of 03:12, 8 September 2005

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===Barring gear===

Barring gear is the term normally used for the complete mechanism provided for the rotation of the turbine generator shaft at a very low RPMspeed (revolutionsof the order of one revolution per minute) after the unit stoppage duefor to any reason. The requirement for this is very important for all capacities of turbine generator units. Once the unit is "tripped" (i.e. the turbine stop valve is closed cutting off steam to the turbine) the

unit starts slowing down (also known as "coasting down"). When it reaches a dead stop, if it is allowed to be in one position for some time there is a tendency for the turbine shaft to deflect or bend.

This deflection is because of the heat inside the turbine casing tends to get concentrated onin the top half of the casing as compared to bottom half casing, thus making the top half portion of the shaft hotter than the bottom half. The shaft therefore warps or bends by millionths of inches, only detectable by monitoring meters such as the eccentricity meter on the operators’ console.

But this small amount of shaft deflection would be enough to cause vibrations and wreck the whole turbine generator unit when the huge mass started spinning. Even the whole concrete building vibrating has been observed in some cases where barring was not used. Therefore the shaft is not allowed to come to a dead stop, but automatically taken over by a mechanism known as turning gear or barring gear automatically takes over to rotate the set at a particular set low speed.

If the unit has to be taken down for major maintenance requiring inspection of turbine bearings, etc., then barring gear has to be kept in service till the temperatures of the casings and bearings, as seen on the operators’ console, isare sufficiently low as prescribed by the manufacturers.

The drivingbarring motor supply is therefore madetaken from a reliable source. In very small units of capacities of the order of about 500 KW and below, the mechanism may be hand operated to change the shaft position by 180 degrees frequently. This is done using a bar temporarily attached to the shaft - hence the term "barring".

All the necessary monitoring instruments such as turbine casing temperatures, shaft eccentricity meters, vibration indicators, etc. with alarms for various operating limits, are provided at operators’ console, in addition to starting and stopping controls.

All the necessary monitoring instruments such as turbine casing temperatures, shaft eccentricity meters, vibration indicators etc. with alarms for various operating limits, are provided at operators’ console, in addition to starting and stopping controls.

===Auxiliary oil system===