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Transcript of Barring Gear
What is a Barring Gear Barring gear (or "turning gear") is the mechanism provided to rotate the turbine generator shaft at a very low speed after unit stoppages. Once the unit is "tripped" (i.e., the steam inlet valve is closed), the turbine coasts down towards standstill. When it stops completely, there is a tendency for the turbine shaft to deflect or bend if allowed to remain in one position too long. This is because the heat inside the turbine casing tends to concentrate in the top half of the casing, making the top half portion of the shaft hotter than the bottom half. The shaft therefore could warp or bend by millionths of inches. The turning gear turns the turbine rotor slowly, about 3-7 rpm, during shutdown, prior to starting the turbine, or when the turbine is hot. Turning the rotor slowly ensures that it is heated or cooled evenly. If the rotor is allowed to come to a rest when hot, temporary bowing and excessive vibration can result. Distortion
of the turbine casing also results because the hotter steam rises to the top of the casing. Basic components of a Typical Barring Gear? A typical turning gear consists of an electric motor driving a speed reducing gear train. The gear train drives a pinion gear or "clash pinion" as it is often called, that can swing in towards and out away from the "bull gear" mounted on the turbine rotor. The "bull gear" is normally mounted between the turbine and generator couplings Turning gear systems are usually automated. Turbines with automated turning gears are equipped with a device to sense when the turbine shaft has come to rest. This device, commonly called a "zero speed indicator" may be mechanical- hydraulic or electronic.
lf a hot turbine is shut down without turning the turbine
rotor, this leads to rotor distortion on turbine
cooldown. If the turbine is started up again in this condition,
the resulting eccentricity could lead to rubbing
or destruction of the blade tip sealing or even of the
blades themselves. Furthermore, the ensuing unbalance
of the rotor could give rise to excessive vibration.
To rule out this hazard during turbine startup, the turbine
rotor should be turned either continuously or at
regular intervals. The hydraulic barring gear meets this
requirement by continually turning the turbine rotor
through a set angle.
To ensure that this position is maintained even on interruption of hydraulic oil supply p1, hydraulic oil p2 is drawn from the control oil system and used to continue loading the space above the hydraulic ram via path p5.
A check valve 8 cuts off oil supply from the control oil
system when the barring gear pump is in operation. Mode of Operation
After starting the barring gear and provided that all start criteria are met, the solenoid valve 13 opens and hydraulic oil p1 can flow from the pump to under the ram.
The force resulting from p4 and p5 moves the ram upward together with the drawframe.
Each upward movement of the ram turns the rotor by 15 degrees.
When top limit position for the ram (100% lift) is acquired by a proximity switch and is signalled to the control system.
The control system then operates the solenoid valve to isolate hydraulic oil supply so as to drain the space under the ram to the tank. This causes the ram to move downward due to the hydraulic oil acting above the ram.
When the bottom limit position (0% lift) is reached the control system allows hydraulic oil p1 to flow to under the ram. During normal operation this cycle is repeated until the turning gear is stopped.
The direction of movement of the ram is indicated by a mark on the ram rod. When the hydraulic ram operated turning gear is stopped, the ram moves beyond the "0% ram travel" position until the drawframe reaches the mechanical stop on the rest. Ram and drawframe are held in this position by the oil pressure acting above the ram. Any faults during turning gear operation are detected and annunciated as appropriate for the control system fitted. The shaft is therefore automatically turned at low speed (about one revolution per minute) by the barring gear until it has cooled sufficiently to permit a complete stop. Other reasons why its a barring gear is useful What is a Barring Gear? Hydraulic Barring Gear The hydraulic barring gear is supplied with hydraulic oil p1 by a dedicated pump 18. The oil flows via a throttle 12 and a solenoid valve 13
integrated into the control block 7 and via port p4 to
the barring gear hydraulic cylinder, where it acts on
the underside of the ram. Upstream of hydraulic oil path p1 a branch leads off to
pressure reducing valve 11 . Here hydraulic oil pressure
is reduced to approx. 8 bar and this oil supplied to
above the ram via path p5. As a result, the ram moves
to the 0% ram travel position as soon as hydraulic
oil supply to below the ram is cut off by the solenoid
valve 13 in the control block. The limit positions for the ram are acquired by
proximity switches 3 and 5. The output signals are
transmitted to the control system to reverse the direction
of movement of the ram. Design
lts major components are a cylinder 6, a ram 4, a ram rod and a drawframe 2; the drawframe is pivoted to the ram rod by a pin.
On each upward movement the drawframe engages in a gearwheel / mounted on the turbine shaft and turns the rotor by 15 degress Any Questions ?? by Saleel Maraj