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Secondary Systems > Main Turbine > Flashcards

Main Turbine Flashcards

0
Q

Where do the main turbine control valves get their signal from?

A

Electro-hydraulic control system

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1
Q

What is the purpose of the MT system?

A

1) convert thermal energy of steam into mechanical energy to drive the main generator
2) provide extraction steam for feed water heating to increase plant efficiency
3) provide hot reheat steam for operation of main feed water pump turbines

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2
Q

What are the major parts of the high pressure turbine?

A

Rotor
Nozzles
Turbine shell

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3
Q

Classify the HP turbine

A

7 stage dual axial flow turbine

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4
Q

What does MON BLOCK mean in reference to the HP and LP turbine rotors?

A

They have integral wheels which means that they were cut into the rotor after initial forging of the shaft.

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5
Q

What happens to the size of the turbine wheels and buckets progressing to the low pressure of the exhaust end of the rotor?

A

The wheels and buckets increase in size and diameter

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6
Q

Why is the nozzle box construction used?

A

To reduce distortion and thermal stresses in the first stage region of the turbine shell. It allows for thermal expansion and contraction and is not an integral part of the turbine casing

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7
Q

What forces steam through nozzle blades and into buckets on the diaphragms?

A

The inner circumference of the diaphragm has a packing ring that prevents steam from leaking into the next stage and the outer circumference forms a metal to metal steam joint with a groove in the turbine casing.

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8
Q

What type of packing is used at the turbine casing shaft penetration to prevent steam leakage?

A

Spring backed metallic labyrinth type packing.

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9
Q

What are the major differences between the HP and LP turbines?

A

Wheels in the latter stages of the LP unit are larger for max energy transfer.
Stellite erosion shields are installed on the vanes to minimize the effects of water droplet impingement
Grooves are cut in the back of the vanes to aid in the removal of water accumulation.

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10
Q

What are the primary functions of the exhaust hood (outer casing) on the LP turbine?

A

1) transition structure between the last stage exhaust and condenser
2) support structure for the rotor bearings
3) sustain various external forces imposed on it.

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11
Q

Where are the extraction pockets on the LP turbine located?

A

The inner casing.

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12
Q

What is used to ensure that the exhaust hoods don’t exceed 300F?

A

Water spray manifolds are installed downstream of the last stage buckets out of the normal steam path. It is initiated either automatically on high temperature or manually via the bypass valve at a temp of 200F. Turbine trips at 225F.

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13
Q

What prevents positive pressure from developing in the exhaust hood?

A

Rupture discs installed in the hood. Each LP turbine has 3 that will rupture at approx. 5 psig.

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14
Q

Describe the operation of the rupture disc.

A

The rubber diaphragm is dished inward normally. If excessive pressure develops, the diaphragm bows outward and is cut by a knife allowing pressure to pass. A guide bar prevents the disc from moving very far.

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15
Q

Where is the thrust bearing mounted?

A

It is mounted on the shaft between HP turbine and first LP turbine.

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16
Q

What type of bearing is the thrust bearing?

A

Tapered land type bearing. Consists of 2 stationary thrust plates and two rotating thrust collars mounted to the turbine shaft.

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17
Q

How many journal bearings are on the main turbine?

A

10 spherical seat journal bearings

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18
Q

What is the purpose of the turning gear?

A

Primary function is to rotate the shaft slowly and continuously during shutdown periods when rotor temp changes occur.
Also used to jack the rotor small amounts for inspections.
Used at startup to break away the TG rotor from standstill without steam.

19
Q

What is the problem with using the turning gear too much?

A

It can cause increased wear and rubbing of the generator rotor windings.

20
Q

What interlocks must be met for the piggyback motor to start when the switch is taken to start?

A

TG ckt breakers or motor operated disconnect open
A bearing lift pump is running
Bearing lube oil pressure adequate
TG is disengaged.

21
Q

What speed does the rotor need to be for the turning gear to engage?

A

<2 RPM and low speed permissive signal is generated by the EHC

22
Q

What happens to the piggyback motor when the turning gear is engaged?

A

The piggyback motor stops and the slow speed motor of the turning gear starts.

23
Q

What valves have control PACs?

A

Stop valves, control valves, and combined intermediate valves.

24
Q

What components make up control PACs?

A

Hydraulic cylinder that operates on 1600 psig control oil (CO system)
Disk dump valve assembly mounted to the bottom of the hydraulic cylinder
Various servos and solenoid valves and pressure controlled shutoff valves mounted to the lower end cap block “B” of the hydraulic cylinder

25
Q

How many types of control PACs are there?

A

Two.
One for controlling steam valves (including stop vlv #2 and control valves 1-4 and intercept valves 1-3.)
One for non-controlling steam valves (including stop valves 1,3,&4 and intercept valves 4-6)

26
Q

What is the difference between a PAC for a controlling steam valve and a PAC for a non controlling steam valve?

A

Controlling steam valves have a servo valve that controls oil pressure via a small electric signal.
Non controlling steam valves have a solenoid operated test valve.

Both have pressure controlled shutoff valves and fast acting solenoid valves for tripping the turbine generator and for trip testing.

27
Q

What does the fast acting solenoid valve on a PAC do?

A

Trips the controlling valve when it goes through its last inch of travel in a closing direction during a valve test.

28
Q

Why is a bypass provided for MSSV #2?

A

Because the MSSVs are unbalanced and can’t open without pressure equalization across them.
The bypass is also used to slowly warm up the stop valve below the seats and the control valve bodies.

29
Q

What components make up a main turbine stop valve?

A 
Power actuator
Pressure seal head, valve, and bushing assy
Valve seat and valve disc assy
Steam strainer
Hydraulic control
30
Q

What is the purpose of the MSSV?

A

to serve as a back up to the CVs to quickly shut off steam flow to the turbine under emergency conditions.

31
Q

What is the purpose of the main turbine control valves?

A

control main steam flow into HP turbine during startup, shutdown, and regulate flow during normal ops.
They also serve as the first line of defense against an overspeed condition.

32
Q

Describe the make up of the combined intermediate valve.

A

two valves; an intercept valve and a stop valve both contained in one body.

33
Q

Why do we have the combined intermediate valve?

A

The intercept valve shuts off steam flow to the LP turbines from the cross around section. This section of piping has a large storage capacity and could drive the unit to an overspeed on a loss of load. The stop valve is a back up to the intercept valve.

34
Q

Describe the “master/slave function of the combined intermediate valves.

A

CIV 1,2,&3 are the master valves and 4-6 are the slave valves. During a rapid load rejection, 1-3 close to control the overspeed condition. When these close to 50%, the slaves trip closed. After the overspeed condition is passed, when the master valves open to 90%, the slave valves fully open.

35
Q

What parameters are monitored by the turbine supervisory system?

A

Shaft straightness is monitored by the Eccentricity Unit when the shaft is on the turning gear.
Vibration monitors are used to take filtered and unfiltered vibration readings. Filtered are read at the TSI display on the back of the control panel and Unfiltered are displayed in the control room. The trip set point is 12 mils unfiltered.
Shell expansion is monitored to try to minimize rubbing caused by excessive rotor/shell temperature differentials.

36
Q

How does the main turbine overspeed trip device work?

A

an unbalanced ring is actuated by centrifugal force against a spring when a turbine over speeds. The ring strikes a trip finger and moves a mechanical linkage to operate the trip valve that closes the turbine steam admission valves.

37
Q

How many speed sensors are mounted on the main turbine?

A

6

38
Q

What does the thrust bearing wear detector do?

A

it detects axial position of the turbine shaft with respect to the thrust bearing casing and transmits the position signal to its pilot valve. the pilot valve is used to alarm and trip the emergency trip system.

39
Q

Where are the main turbine drain valves controlled from?

A

Control room (Jog open/Jog closed)

40
Q

When do main turbine drain valves open?

A

on a main turbine trip to depressurize the steam piping u/s and d/s of the HP & LP turbines, Hot and Cold reheat lines, MSR shell, & extraction steam lines. steam goes to the main condenser.

41
Q

Where are the drain lines located on the LP turbine?

A

number 8 and 9 extraction lines leading to #4 & #3 LP heater shells.

42
Q

Where is extraction steam taken from on the HP turbine and where is it sent to?

A

3rd stg => MSR 1st stage reheater bundles & 7th point feedwater heater
5th stg => heating steam to 6th point feedwater heater

43
Q

When is steam off the D MSR used to supply a main feed pump turbine?

A

at higher main turbine loads.

44
Q

How many stages of the LP turbine is extraction steam removed from?

A

4 stages to heat the low pressure feed heaters.

45
Q

Which LP turbine stage does not supply extraction steam?

A

10

Secondary Systems (36 decks)

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