SHAFT TURNING GEAR

1

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

2 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.

3

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.

4

5

Fig.No.1a

HYDRAULIC TURNING GEAR & MANUAL BARRING GEAR

6

7 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

8 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 coefficient 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. Zone’1’ is the zone of boundary lubrication.

9

10 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

11 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 Δp 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

12

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 pump’s 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 pressurelimiting 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

15 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  HYDRAULIC TURNING GEAR & JACKING OIL SYSTEM – CONTROL  DESK 

JACK OIL  PUMP 2

JACK OIL  PUMP 1 

GATE VALVE  GEARING

ON/OFF 

 

SLC   J.O.P 2

SLC   J.O.P 1 

    SUB‐GROUP    SHUT DOWN   

SLC TURNING GEAR

OFF                            ON  FAULT 

SUB‐GROUP  START UP 

SGC                     OIL     SUPPLY

  SHUT DOWN       

     ‐‐ PUSH BUTTON    ‐‐ INDICATION 

 ON/OFF 

START‐UP 

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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. (i)

Bringing SLC of JOP ‘A’ to ‘ON’ 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. (i)

Bringing SLC of JOP ‘A’ to ‘OFF’ 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.

3.

When SGC oil supply is ‘ON’ and the shut down programme is at STEP

(or)

‘54’. (or) 4.

6.2.2

Auto start command for JOP ‘B’ exists.

SLC OF JOP ‘B’: a. (i)

Bringing SLC of JOP ‘B’ to ‘ON’: 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. (i)

Bringing SLC of JOP ‘B’ to ‘OFF’ 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)

19 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 JOP’B’ is off (irrespective of SLC of JOP’A’ 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.

b.

i.

Fire protection 2 Channel 1 has operated

ii.

Fire protection 2 Channel 2 has operated 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 turbogenerator 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

22 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.