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HYDRAULIC TURNING GEAR 1.0 Introduction:
The function of hydraulic turning gear is to rotate the
Turbo-Generator shaft
system at a sufficient speed during the period before start-up
and the period after
shut down. During cold or hot rolling of turbine, it is
necessary to measure and
monitor the eccentricity of the rotor even before the admission
of steam into the
turbine in order to ensure that no vibration is caused and no
rubbing between the
moving and the stationary parts takes place. The eccentricity of
the rotor is caused
when it becomes unstraight due to bending. Hydraulic Turning
gear is useful to
rotate the Turbo generator shaft system at a sufficient speed
during the period
before start up and the period after shut down. Mechanical
Barring gear is also
available as a backup.
2.0 Necessity Of Turning Gear:
Rotor is said to be rotating with eccentricity when its axis of
rotation does not
coincide with its true centerline of mass. Eccentricity of the
rotor is caused during
cold or hot rolling.
Before cold rolling of turbine, a natural deflection of rotor is
there due to the
non-uniform distribution of its own weight since the weight of
the different stage
differ. This is an initial condition of eccentricity. More over
uniform heating up of
rotor is necessitated while admitting the steam for sealing at
glands on raising
vacuum. Due to the above reasons, slow rotation of turbine rotor
is done with the
help of turning gear for a sufficient duration of time prior to
cold rolling. As the
turbine speed is gradually increased, the rotor starts to
straighten itself and the
eccentricity gets greatly reduced beyond the critical speed.
The conditions in hot rolling are different. When the turbine is
tripped, the
rotor comes to rest from its rated speed. The turbine starts
cooling very slowly since
it is well insulated. Due to the difference in the area exposed
in different sections of
the cylinder, viz. the cylinder top and bottom, the rate of
cooling varies leading to
uneven cooling. In such conditions if the rotor is allowed to be
stationary, after it
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comes to rest, it would be exposed to uneven temperatures,
resulting in thermal
stresses, bending and eventually eccentricity in the rotor. This
problem is also
overcome by having a slow rotation of rotor by means of turning
gear. The blade
ventilation during turning operation provides good heat transfer
at the inner wall of
the casing, which is conducive for temperature equalization
between the top and
the bottom cylinder.
3.0 Hydraulic Turning Gear:
Hydraulic turning gear is located in the bearing pedestal
between IP turbine
and the HP turbine. Mechanical barring gear is available as a
back up to this.
During turning gear operation, the turbo-generator shaft system
is rotated by
a double row blade wheel, which is driven by oil. The oil is
supplied by the auxiliary
oil pump and it flows through a gate valve gearing and a nozzle
box. Gate valve
gearing is an electrically operated valve through which the high
pressure oil is
supplied to turning gear thro Nozzle box. Nozzles guide the jet
of oil towards the
moving blades. Nozzles increase the velocity of oil and guide
the jet of oil towards
the moving blades. This flow of high velocity jet of oil through
the two rows of
moving blades result in slow rotation of the Turbo generator
rotor system. Speed of
rotation of TG rotor during the turning gear operation is 120
rpm without condenser
vacuum and 160 rpm with condenser vacuum.
In order to reduce the gap losses at the moving blades, sealing
strips are
caulked into the nozzle boxes. After passing through the moving
blades, the oil
drains into the bearing pedestal and flows along with the
bearing drain oil into the
return flow piping.
To overcome the initial breakaway torque on startup and to
prevent dry
friction, the bearings are relieved for a short time by jacking
oil supplied below the
shaft. The shafts are thus slightly.
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4.0 Mechanical Barring Gear:
The turbo generator is also equipped with a mechanical barring
gear, which
enables the combined shaft system to be rotated manually in the
event of failure
of the normal hydraulic turning gear.
4.1 Construction: The mechanical barring gear consists of a gear
machined on the rim of the
turning gear wheel and a pawl. The pawl engages with the ring
gear and turns the
shaft system when operated by means of a bar attached to a
lever. The pawl can
be engaged or disengaged by using a lever. The lever is held in
position by means
of a latch.
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Fig.No.1a HYDRAULIC TURNING GEAR & MANUAL BARRING GEAR
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4.2 Operation of STG:
The following steps of operation are to be made
Remove the cover and unlatch, Attach a bar to the lever, Barring
of lever will rotate the combined turbo-generator shaft system.
After the barring has been completed return the lever and the pawl
to
disengaged position.
Secure the lever by means of latch and replace the cover. The
barring gear shall be operated only after the turbo-generator
shaft
system has been lifted with jacking oil. If it is hard to start
turning by means of the
mechanical barring gear, this may be due to incorrect adjustment
of the jacking oil
system or due to rubbing of the shaft. Corrective action must be
taken before the
steam is admitted into turbine. After shut down of turbine,
turning gear should be in
operation till the maximum metal temperature reaches 120 C.
5.0 Hydraulic Lifting Device:
When the turbine is started up or shut down, the hydraulic
lifting device is
used to maintain the oil film between rotor and bearings. The
necessary torque for
rotation is reduced in this way when the hydraulic device or
manual turning device
is in service.
The turbo-generator bearings are supplied with high-pressure oil
delivered by
a jacking oil pump. The high pressure oil lifts the rotor when
it is forced under the
journal of the bearings. To avoid damage to the bearings, the
jacking oil pump
must be switched ON at turbine speed below 510 RPM.
(approximately) during shut
down and it should be switched OFF at turbine speed above 540
RPM.
(approximately) during start-up.
5.1 Need Of The Jacking Oil For Lifting: The way in which
liquids lubricate can be explained by considering the
example of a plain journal bearing as shown in fig no.4.
As the shaft (journal) rotates in the bearing, lubricating oil
is dragged into the
loaded zone. Since the loaded zone will be the point at which
the shaft and the
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bearing surfaces are closet together, the entry into this zone
is tapered, like a
curved wedge. As the oil is forced to move into the narrower
part of the wedge, its
pressure increases, and it is this hydrodynamic pressure which
supports the shaft
load.
Increasing load reduces the oil film thickness while increasing
hydrodynamic
pressure increases the oil film thickness. The hydro dynamic
pressure, in turn, is
determined by the viscosity of the oil and the speed at which it
is squeezed into the
wedge shaped entry zone. Thus the rise in hydrodynamic pressure
and therefore
the thickness of the film will depend on the shaft speed and the
lubricant viscosity.
The relationship between speed, viscosity, load, film thickness
and friction
can be understood by considering a graph shown in fig no.4. In
this graph, the co-
efficient of friction is plotted against expression V/P where,
V/P = (Oil viscosity *
Shaft speed)/ Bearing pressure.
There are three different zones in the graph, separated by the
points A & B.
At B, the co-efficient friction is at its minimum, and this is
the point at which
the oil film is just thick enough to ensure that there is no
contact between the shaft
and the bearing surfaces. The zone 3, to the right of B, the oil
film thickness is
increasing and the co-efficient of friction also increases (as
the film thickness
increases). This increase in the oil film thickness is because
of increasing viscosity or
increasing shaft speed or reducing the bearing load. Zone 3 is
the zone of
hydrodynamic lubrication or Full film lubrication.
As the conditions change from B towards A, the oil film
thickness reduces
and hence the shaft and the bearing rub against each other, the
amount of
rubbing, and the friction increases as the oil film thickness
decreases, zone 2,
between A & B, is known as the zone of mixed lubrication or
partial lubrication. The
shaft load was supported by a mixture of oil pressure and
surface contacts,
At A, the oil film thickness has been reduced to nil and the
load between
shaft and bearing is carried entirely on surface contact. In
zone1, the co-efficient of
friction is almost independent of load, viscosity and shaft
speed. Zone1 is the zone
of boundary lubrication.
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The different lubrication zones also have an influence on wear,
the amount
of wear which takes place depends on the severity with which two
surfaces rub
against each other. In zone 3, there is no contact between the
surfaces and
therefore the wear is minimum. As the oil film thickness becomes
thinner in zones 2
and 1, there is a greater tendency to wear.
When the turbine is on Turning Gear during start-up or shut
down, the shaft
speed is much less compared to its normal operating speed. Hence
the shaft
rotates in the region of boundary lubrication. Since the oil
film thickness is minimum
during low shaft speed, there is increasingly severe contact
between the shaft and
the bearing surface resulting in increased wear and reduction in
life of the bearing.
To avoid this, a jacking oil system also known as hydraulic
lifting device is
necessitated to supply high pressure oil called as jacking oil
under the journal of the
bearing thereby slightly lifting the journal. Slow rotation of
turbine rotor during
turning operation is thus done in slightly lifted condition so
as to avoid damage to
the bearings. Hence the shaft rotates now in the region of
Hydrodynamic
lubrication.
5.2 Jacking Oil System: To supply the high-pressure oil for the
lifting device, two jacking oil
pumps of each 100% capacity are provided on the main oil tank.
When one pump
is intended to be in service, the other one is stand by.
5.2.1 Jacking Oil Pump: The jacking oil pump is a self-priming
screw spindle pump with three spindles
and internal bearings. The screw spindle pump is connected
vertically to the cover
plate (2) of the Main oil tank via a support (5) and immerses
with the suction casing
(15) into the oil. The drive is an electric motor that is bolted
to the cover plate. The
oil flows into the suction branch of the suction casing from
underneath and is
supplied to the jacking oil system by the pump via the pressure
pipe (3).
The driving spindle (16) and the two moving spindles (20) run in
the inner
casing (13). Due to the special profile given by the sides of
the threads, the three
spindles form-sealed chambers, the contents of which are
continuously being
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moved axially from the suction side to the pressure side of the
pump as the spindles
rotate.
There is a balancing piston in the form of a shrunk on sleeve in
the main drive
spindle just above the screwed portion of the main drive spindle
and this runs inside
the throttle bushing (11). Pressure oil of a small quantity
flows in a very small gap
between the throttle bushing (11) and the driving spindle (16)
in an upward
direction. This gap is known as throttling gap since the
pressure of oil which is
coming out of this gap is very much reduced. The oil that leaves
the throttle gap
flows via the grooved ball bearing (7) and lubricates it. This
bearing serves as both
support and thrust bearing. There after the oil flows through
the support to the main
oil tank itself via an opening in the support. The driving
spindle is fixed by means of
the grooved ball bearing in the bearing carrier (9) that is
bolted to the pressure
casing (12) of the pump. The drive main spindle is a solid one.
The cumulative axial
thrust generated by the main drive spindle screw is countered by
the balancing
piston in the form of the shrunk on sleeve. The p across the
balance piston and the annular area of it are so designed to match
with the cumulative axial thrust
generated by the main drive spindle.
There are two idler screw spindles, which are hollow and are
driven by the
main drive spindle whose continuous helical screw is in mesh
with the continuous
helical screws of them. Pressure oil in a very small quantity
flows via gaps in the top
of the screwed portion of the main drive spindle though the
hollow spaces of the
two idler screw spindles in a downward direction.
The two idler screw spindles also exert a cumulative axial
thrust in a
downward direction, which are to be balanced, to perform this
task, each idler
screw spindle is having a balancing bushing (21) in its bottom.
These are fixed to the
support plate (18), which also supports the inner casing (13) by
means of distance
pipes (17) attached to it. The balancing bushing has a small
piston with a guide pin
in its bottom and can move only in a vertical direction in a
small cylinder, which is
open in its top. The piston is located just below the ending
point of the continuous
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Fig. No. 5 Jacking oil pump
helical screw of the idler spindle. Pressure oil is supplied in
the bottom of the piston
via hollow space of the idler spindle through a small opening.
The top of the piston
is exposed to pressure less oil in the tank. The upward counter
thrust provided by the
balance piston in the balancing bushing encounters the
cumulative axial thrust
exerted by the continuous helical screw of each one of the idler
spindle screws in
the downward direction. There is provision for leakage oil to
escape to the main oil
tank from the balancing bushing on the pressure side.
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5.2.2 Jacking Oil Supply: Ref fig no.6. The discharge pressure
oil piping of both the jacking oil pumps is connected
in parallel to supply high-pressure oil to the common jacking
header. In each
discharge line, a check valve is provided to prevent the jacking
oil returning from
the header to the oil tank, if the pump concerned is not in
operation. A spring
loaded safety relief valve is provided between the jacking oil
pump and the check
valve. This is to prevent any damage to the jacking oil pumps
discharge piping in
case that the concerned jacking oil pump is in operation and the
check valve
continues to remain in closed position.
The pressure in the common jacking oil header is maintained at a
constant
value (approximately 120 bar) by means of a pressure-limiting
valve. The pressure-
limiting valve can be relieved by a bypass valve. The
superfluous flow from the
pump is conducted into the main oil tank.
The jacking oil required for each bearing is supplied from the
common
header as detailed below.
Bearing No of lines
Hp front One
Hp Rear journal cum thrust One
IP Rear Two
LP Rear Two
Generator Front Two
Generator Rear One
In each supply line, a fine control valve and a check valve are
provided. The
necessary jacking oil pressure sufficient to lift the shaft
varies with respect to bearing
load. The lift will be of 0.03 to 0.05 mm. The required jacking
oil pressure is set for
each bearing by means of a finer control valve. The pressure
gauges mounted in
the downstream pipes of these finer control valves indicate the
jacking oil pressure
required for lifting. A check valve provided in the jacking oil
supply pipings prevent
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Fig.No.6 Jacking oil system
the lub oil from flowing out of the bearings into the header
during the normal
operation of turbine since the jacking oil pumps are out of
service.
The finer control valve, the check valve and the pressure gauge
for each line
are arranged in boxes, which are connected laterally to the
bearings. At the
generator free end bearing alone, they are arranged in the
hacking oil piping
outside the bearing housing.
The lift in mm of the shaft at the bearings is about 0.04 to
0.08 mm.
Bearing Jack oil pressure (in ksc) where the shaft lifts.
(When JO header pressure is 120ksc)
Hp front 40
Hp Rear journal cum thrust 60
IP Rear a) 62 b) 82
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LP Rear a) 50 b) 36
Generator Front a) 80 b) 70
Generator Rear 40
The values are given above for the purpose of indication
only.
6.0 Logics:
6.1 Hydraulic Turning Gear:
6.1.1 Sub Loop Control Of Turning Gear: a) Bringing SLC of
Turning gear to ON.
i. SLC of Turning gear can be made ON by giving manual
command
from the control desk.
ii. When Sub Group Control (SGC) Oil supply is ON and the start
up
programme is at STEP 4, SLC of Turning gear is made ON
automatically.
b) Bringing SLC of Turning gear to OFF.
i. SLC of Turning gear can be made OFF by giving manual
command
from the control desk.
ii. SLC of Turning gear gets switched OFF automatically when any
one
of the following conditions appears.
1. Fire Protection 2 Channel 1 has operated. (OR),
2. Fire Protection 2 Channel 2 has operated. (OR),
3. When SGC Oil supply is ON and the shut down programme at
STEP 51, is executed.
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FIG.NO.7
HYDRAULICTURNINGGEAR&JACKINGOILSYSTEMCONTROLDESK
ON/OFF
OFFON FAULT
SHUTDOWN ON/OFF STARTUP
PUSHBUTTON INDICATION
JACKOILPUMP1
JACKOILPUMP2
GATEVALVEGEARING
SLCJ.O.P1 SLCJ.O.P2 SLCTURNINGGEAR
SUBGROUPSHUTDOWN
SUBGROUPSTARTUP
SGC OILSUPPLY
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6.1.2 Gate Valve Gearing: 1. Protection close:
When the lub oil pressure before the thrust bearing becomes less
than 1.2
ksc (MAV 43 CP 012), the gate valve gearing will protection
close.
2. Permissives for manual and auto operation of gate valve
gearing are
as under.
Differential pressure between the generator seal oil and
hydrogen gas should be greater than 0.9 KSc. (as sensed by PS MKW
01 CP 003)
Generator seal oil pressure (turbine side) should be greater
than 3.8 KSc (as sensed by pressure switch MKW 01 CP 001).
Generator seal oil pressure (Excitation side) should be greater
than 3.8 KSc (as sensed by the pressure switch MKW 01 CP 002).
(OR)
Turbine speed should be greater than 15 RPM. (as sensed by MYA
01 FS 001)
3. Manual open
The gate valve gearing can be opened manually from the
control
desk (after keeping SLC of Turning Gear in OFF position)
provided that the
permissives detailed in Sec.2. are available.
4. Automatic Open
When SLC of Turning Gear is ON and the turbine speed is less
than 200
rpm (MYA 01 FS 001), the gate valve gearing will open
automatically
provided that the permissives detailed in Sec.2 are
available.
5. Manual close
The gate valve gearing can be closed manually from the control
desk
after keeping in SLC of Turning Gear in off position.
6. Automatic close
When SLC of Turning Gear is on and the turbine speed is greater
than
250 rpm (MYA 01 FS 001), the gate valve gearing will close
automatically.
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6.2 Jacking Oil Pumps:
6.2.1 SLC of JOP-A: a. Bringing SLC of JOP A to ON
(i) SLC of JOP A can be made ON by giving manual command from
the
control desk (or)
(ii) SLC of JOP A is made ON automatically when SGC oil supply
is ON
and the start-up programme is at STEP 5.
b. Bringing SLC of JOP A to OFF
(i) SLC of JOP A can be made ON by giving manual command from
the
control desk (or)
(ii) SLC of JOP A gets switched OFF automatically when any one
of the
following conditions exits.
1. Fire Protection 2 Channel 1 has operated. (or)
2. Fire Protection 2 - Channel 2 has operated. (or)
3. When SGC oil supply is ON and the shut down programme is at
STEP
54.
(or)
4. Auto start command for JOP B exists.
6.2.2 SLC OF JOP B: a. Bringing SLC of JOP B to ON:
(i) SLC of JOP B can be made ON by giving manual command from
the
control desk. (or)
(ii) SLC of JOP B is made ON automatically when SGC oil supply
is ON
and the start-up programme is at STEP 5.
b. Bringing SLC of JOP B to OFF
(i) SLC of J.O.P. B can be made OFF by giving manual command
from
the control desk. (or)
(ii) SLC of J.O.P. B gets switched off automatically when any
one of the
following conditions exists.
1. Fire protection 2 Channel 1 has operated (or)
2. Fire protection 2 Channel 2 has operated (or)
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3. When SGC oil supply is ON and that shut down programme is
at
STEP 54.
6.2.3 JACKING OIL PUMP A a. Protection.
JOP A will trip on actuation of any one of the following
protections
i. Fire protection 2 Channel 1 has operated (or)
ii. Fire protection 2 Channel 2 has operated
b. Manual Starting.
JOP A can be started manually from the control desk provided
that
JOPB is off (irrespective of SLC of JOPA position).
c. Automatic starting.
JOP A gets started automatically when all the following
conditions exist.
i. SLC of JOP A is ON.
ii. Turbine speed is less than 510 rpm (MYA 01 FS 001) (OR) (MYA
01 DS
001) and
iii. JOP B is off.
d. Manual Stopping.
JOP A can be stopped manually from the control desk after
keeping
SLC of JOP A in off position.
e. Automatic stopping.
JOP A gets stopped automatically when any one of the
following
conditions exists.
i. SGC oil supply is ON and shut down programme STEP 54 is
executed.
ii. SLC of JOP A is ON and
Turbine speed is greater that 540 rpm (MYA 01 FS 001) or (MYA 01
DS
001) (OR)
Auto start command for JOP B exists.
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6.2.4 JACKING OIL PUMP B a. Protection starting
JOP B will be protection started on actuation of any one of the
following
protections.
i. Fire protection 2 Channel 1 has operated
ii. Fire protection 2 Channel 2 has operated
b. Manual starting
JOP B can be started manually from the control desk provided
that JOP
A is off. (irrespective of SLC of JOP B position)
c. Automatic starting of JOP B along with stopping of JOP A
On occurrence of any one of the following conditions.
1. JOP B gets started automatically.
2. JOP A gets stopped automatically &
3. SLC of JOP A is made off.
Conditions:
1. (i) SLC of JOP B is ON.
(ii) JOP A is off.
(iii) JOP A Discrepancy.
(or)
2. (i) SLC of JOP B is ON.
(ii) Turbine speed is less than 510 rpm.
(MYA 01 FS 001) (OR) (MYA 01 DS 001).
(iii) Jacking oil pressure is less than 100 Ksc. (Time delay 5
Sec) (MAV 35 CP
001)
(OR)
3. (i) SLC of JOP B is ON.
(ii) Turbine speed is less than 2800 rpm (MYA 01 FS 001).
(iii) A.C. Voltage for JOP A failed.
d. Manual stopping
JOP B can be stopped manually from the control desk after
keeping SLC
of JOP B in off position.
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6.3. Alarms: 1. SLC Turning Gear System Not on alarm will appear
when SLC gate valve
gearing is off and temperature of HP casing top (50%) is greater
than
1200C (MAA 50 CT 053A)
2. Gate Valve Gearing Not closed alarm appears if the turbine
speed is
greater than 540 rpm and the valve remains open.
3. SLC Jacking System not ON alarm will appear when SLC of JOP A
is
off SLC of JOP B is off and when the turbine speed is greater
than 15
rpm. (MYA 01 FS 001)
4. Jacking oil pressure low alarm appears when the jacking oil
header
pressure drops to a value less than 100 Kg/Cm2.
7.0 Guide Lines For Turning Gear Operation:
1. After shut down of turbine, turning gear should be kept in
operation till the
maximum metal temperature comes down to 120 C.
2. The mechanical barring gear shall be operated only after the
turbo-
generator shaft system has been lifted with the jacking oil. If
it is hard to
start turning by means of the mechanical barring gear, this may
be due to
incorrect adjustment of the jacking oil system or due to rubbing
of shaft.
Corrective action must be taken before steam is admitted into
the turbine.
3. Emergency operation of Hydraulic Turning Gear.
On admission of the oil for driving the hydraulic turning gear
(after opening
of Gate Valve Gearing), if the Turbo-generator rotor fails to
rotate, Manual
barring of the rotor should be immediately started.
After manual rotation of TG rotor for a short interval, if the
rotor begins
to rotate due to hydraulic turning gear, manual barring gear can
be
stopped.
In case that the rotor does not rotate at all, due to hydraulic
turning
gear, even after manual barring for some time, manual barring
has to be
continued such that the rotor is rotated by 180 for every five
minutes. This
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has to be continued until the rotor becomes straight due to its
own cooling
and begins to rotate due to hydraulic turning gear.
8.0 Technical Data:
8.1 Jacking Oil Pump: Number of pumps per unit : 2
Type :SDF 40 R54
Manufactured by :M/S AllWeiler
Capacity : 1.26 dm cubes/sec
Discharge Pressure :120 Bar
Speed : 49.16/sec.
8.2 Motor Of Jacking Oil Pump: 1. Rated Voltage : 415 V
2. Rated Power : 30 KW
3. Rated Current : 57 A
4. Starting Current : 365 A
5. Manufactured by : M/S Siemens.
Bearing No of lines