50. What are the typical trouble spots for air ingress in a condenser?

1. Turbine to Condenser joints
2. Turbine expansion diaphragms
3. Turbine atmospheric valves
4. LP glands and gland housings
5. LP bled-steam lines and feed heaters
6. Condenser fittings ( Gauge glass, etc.)
7. Any drain line passing back to the steam spaces of condenser, erosion at pipe bends can be a particular problem

49. What are the methods used to detect the tube leak with vacuum and with out vacuum?

Detection methods requiring vacuum include :

1. Foam
2. Halogen gas
3. Bubbler leak
4. Indiplugs
5. Acoustic meters
6. Conductivity measurements

Those that do not require vacuum includes :

1. Fluorescein
2. Bubbler leak
3. Furmanite guns
4. Corrodograph survey

48. What are the trip interlocks for a steam turbine?

1. Vacuum very low
2. Hot-well level very high
3. Vibration very high
4. Exhaust steam temperature very high
5. Speed very high (>1.1 times rated speed)
6. Boiler Trip
7. Generator trip
8. Steam temperature very low

47. What are the trip interlocks for Generator?

a.        Turbine trip

b.       Loss of excitation

c.        Stator earth fault

d.       Stator phase to phase fault

e.        Stator interturn fault

f.         Negative phase sequence current high

g.       Rotor temperature very high

h.       Loss of synchronism

46. What is the limit for pressure raising with respect to drum metal Differential temperature? What is the normal pressure raising and reducing rates in a boiler?

The differential temperature between top and bottom shall not be greater than 50 Deg.C.

Pressure change rate depends on saturation temperature of drum water. At low pressure the rate of raise is very small. The minimum time required to raise from 0 to 1 bar shall be 1 hour assuming the initial drum water is at atmospheric temperature.

Rate of increase of saturation temperature is 83 Deg.C

Rate of decrease of saturation temperature is 56 Deg.C

45.What is mean by start-up characteristic? What are the types?

The requirement of steam pressure and temperature during starting of a turbine depends on the metal temperature. The characteristic showing speed/load vs steam pressure//Temperature is start-up characteristic.

The three type of start-up in any plant are :

1. Cold star
2. Warm start
3. Hot start

44. What is performance diagram of a Generator? What are the limiting factors?

The curve between MW (Generator) vs MVAR is the basic performance cuve.

  

The limitations are :

1. Minimum excitation
2. Practical stability limit
3. Rated current
4. Rated MW output
5. 90 deg. Rotor angle
6. Maximum excitation
7. Minimum MW output
8. Transformer Tap limit
9. Generator transformer overfluxing lmit

43.What is Potier voltage and Potier reactance? Explain the relation between Potier voltage and zero pf curve?

Refer to Modern Power Station Practice, Vol.G, Station Operation and Maintenance, Chapter 3, para 5.3 (CEGB)

42.Define Stability? What is the two kind of stability?

Stability of a generator is defined as its ability to remain in synchronism with the power system to which it is connected.

The two regimes of stability are :

1. Steady state stability
2. Transient stability

The Steady stability of a generator defines its ability to remain in synchronism under the conditions of steady state operation. In practice, this also includes load drift, voltage drift and small fairly slow load changes.

Transient stability defines the ability of the generator to return to its former state of stable operation after having been disturbed from this state due to sudden changes of loading or voltage conditions. The type of changes included here are those due to power system faults, load swing, sudden tripping of sections of power system network, etc.

  

41.What is power angle equation and power angle characteristics of a generator?

P = VxE/Xd Sin D

P                      - Power generated

V                     - Terminal voltage

E                      - Generated Emf

Xd                   - Generator reactance

D                     - Delta, Power angle (The direct angle between rotor load axis and No load axis

The characteristic between Power vs D is Power angle characteristic of a generator.

40.What is function of AVR in the control of steady state stability?

In case of a generator connected to an isolated system with no other synchronous plant present, then the function of the ACR is solely to maintain a constant busbar voltage, as in the static system.

When applied to a multi-generator system, then voltage regulation forms one of its functions. Its main effect is to maintain machine rotor angle and, therefore, to assist in maintaining steady state stability.

39.What is Synchronizing power?

When a generator suffers a transient disturbance such as load surge, the rotor oscillates about its mean power position. The accelerating and retarding action being assisted by flow power from or to the system. This is synchronizing power.

This is transient in action and responsible for the machine locked to the system.

38.What are types of electrical motors used in power plant?

Induction motor and DC motor are mainly used in a power plant.

37.What is asynchronous operation of a generator?

Asynchronous operation is its operation out-of-synchronism with the rotor unexcited.

36.What are the abnormal operating conditions of a generator?

Faults can occur on operational plant. It may be required to operate the plant at less flexible operation with a non correct rated equipment. This condition of operation is abnormal operation of the generator.

1. Failure of generator transformer
2. Failure of exciter. Using separate DC supply
3. Modified stator with reduced conductors

35.What are the fault conditions of a generator?

1. Stator earth fault

b.    Stator phase to phase fault

c.     Stator interturn fault

d.    Negative phase sequence current high

e.    Loss of excitation

f.    Pole-slipping

g.   Rotor fault

34.What is cold Pull with respect to pipe support?

Steam pipes will be supported by spring loaded hangers to accommodate thermal expansion. The hangers will support the maximum load in the vertical position.

The expansion during normal operation will be calculated. The position from which it will obtain vertical position will be determined. The support will be fixed in the calculated position at an angle in the cold condition. The is cold Pull.

  

33.How to preserve SF6 switch-gear? Is it possible to test the breaker in the preserved condition and why?

The SF6 breakers will be filled with Nitrogen to prevent corrosion.

It is not possible to test the breaker in that condition.

The insulation (dielectric strength) of Nitrogen is much less than SF6. The gas will break down and create earth fault.

32.What is zero check in the case of pressure and differential pressure transmitters?

In certain plants, it is practiced to check the transmitter zero between certain period.

Pressure transmitter and DP transmitters will be isolated from process. The drain will be opened for PTs. The equalizing valve will be opened for DPTs. The output shall be 4 mA. Otherwise the zero will be adjusted to get 4 mA..

This is similar to one point calibration for zero. The above is known as zero check

31.What is temperature compensation of direct drum level gauge? How it is achieved and how to operate this provision?

The flow through the normal gauge glass is due to the condensation. This will cause an appreciable drop in temperature in the gauge glass. The density of water in the gauge glass will be higher than that of the water in the drum. Typically, the density of water in the drum will be 0.6 and that of gauge glass is 0.8. Since it works on manometer principle, the level in the gauge glass will be lower than that in the drum.

This suppression of level is known as cold water suppression.

To make the levels equal, it is necessary to maintain the temperature of the gauge glass same as drum water. The drain of the gauge glass will be connected to the down-comer. The continuous flow through the gauge glass created by the drain will maintain temperature. This is called temperature compensation.

More flow of drain will create a low level in the gauge glass. To avoid such condition, the flow shall be regulated. A suitable orifice will be provided in this line. The size will be fixed during commissioning of the boiler.

30.What is the procedure for pre-commissioning a soot blower after completion of erection?

Soot blower shall have a minimum slope of 5 deg. Towards the furnace, to clear the condensate after operation. The furnace will have expansion of furnace when it is on load. The other end will be supported from structure with slot for sliding.

The cold condition shall have slope to provide the above slop during hot condition.

29. What are the Turbo Supervisory Instruments?

The turbo supervisory Instruments are  provided to monitor essential moving parts during start-up and shut-down. The parameters are :

1. Turbine Speed
2. Differential Expansion
3. Eccentricity
4. Bearing vibration
5. Valve positions

28. How the Speed control will work after synchronizing in a turbine control?

Speed control will act as Droop control after synchronizing.

27. What is the allowable voltage variation for a Generator?

Plus 5% and minus 7%

26. What is HOT alignment ?

Some times the shaft will be shifted upwards by thermal expansion of the driven equipment, when running. The shaft shall be horizontal during running. To achieve this, the shaft of the motor and driven equipment will be miss-aligned in the cold condition.

25. Why minimum recirculation is necessary for a pump and how much the value will be and what are methods to predict the loss of minimum flow?

Without re-circulation, the fluid in the pump will be churned repeatedly resulting in heating/vaporization up of the fuild.This will result in sealing glands/pump internals being damaged. Typically, inferred from low flow by Flow switches In suction pipes or high differential temperature between suction and discharge by temperature swicTCH.

24. What are starting interlocks for pump?

Suction valve open is the minimum interlock for a pump before start.

23. What are the starting interlocks for Fan?

Suction damper closed is the minimum interlock for a fan before start.

22. Which Fan will suffer Stalling and why?

Axial fans.

Forced draft fan will suffer stalling. Since it has a variable resistance device

(Rotating air heater) Whose resistance to flow will increase due to choking during operation are mal function of secondary air damper .

21. What is Stalling with respect to Fan?

When a fan is loaded with discharge damper is closed or no through opening pass at the discharge, the pressure will be higher than the normal for the flow. This cause, escape of air in between the casing and the blades from discharge to suction. This will cause in reduction of pressure. The fan will behave the same way. The pressure will increase and decrease and increase. This is known as Stalling of a fan. The current also will behave in the same cycle. If allowed, the blades may break and will cause a major demage.

20. What will be effect when a motor driven equipment is started on load?

The starting current which will be 6 to 8 times the Rated current will remain same, unless the supply voltage is higher than rated. The starting time will increase and the ppequipment may trip on thermal over load.

19. What are the possible reasons for a motor to trip by Instantaneous over current and what action to be taken?

Voltage may be higher than rated value or insulation may be failed. Confirmed by the Electrical Engineer by checking the voltage/meggaring the motor, calbe, etc.

18. What is the control solution to over come the problem of electrical valve actuator tripping on Torque limit switch while start opening?

By-pass open torque switch by close limit swich for 3% open. The limit switch shall open and bring the open torque switch from 3 to 100% open.

17. What action shall be taken if a electrical actuator trips on Torque limit switch upon start opening?

Normally in case of Globe valve where valve close will be limited by torque switch the valve will stop upon receiving open command by the activation of open torque switch. 
(operation solution) - Valve shall be open manually for one or more turns.
 ( Control solution) - The open torque switch shall be disabled for five percent.

16. How to set-up the limit switches for different type of valves?

All types of valves shall be set to stop by limit switch ( position switch) upon opeing. Torque switch shall be used as back-up. Closing shall be stoped by limit or torque sitch depending upon the type of valve. Globe Valve - by torque switch, Gate valve avd others by limit switch and torque switch shall be used as back-up.

15. What are the types of limit switches provided in an electrical valve actuator?

Limit or position switches and Torque switched both for opening and closing

14. What is allowable winding temperature?

100 to 120 Deg.C

13. What is the allowable vibration level?

100 micron

12. What is the allowable bearing temperature?

92 Deg.C to 120 Deg.C

11. What is the action to be taken if the bearing temperature increases for a greased bearing?

 Grease shall be replaced.

10. Describe the method for High Voltage Test for Motors?

May be for the first time the motor will be tested for 1.5 x V Rate + 1 KV ( DC). If AC is used it will be 1.5 x V Rated x Sqrt 2 + 1.5 KV. At site it is advisable to test for rated voltage.

9. What are the parameters shall be monitored for equipment’s trial operation?

Current, bearing temperature, Winding Temperature, Noise Vibration, speed,    Cooling water temperature & Lube oil temperature.

8. What are the pre-commissioning activities in Boiler, Turbine, Generator and others?

Boiler :
a. Trial run of Auxiliaries which includes the logic confirmation using simulated input/output signal from field/marshalling panel
b. Confirmation of logic and protection for BMS
c. Trial Light up of burners
d. Alkali boil-out
e. Acid cleaning and passivation
f. Steam blowing
g. Safety valves testing

Turbine :
a. Trial run of Auxiliaries which includes the logic confirmation using simulated input/output signal from field/marshalling panel
b. Oil flushing
c. Trial start-up
d. Speed test, confirmation of Auto Run-up System (ARS)

Generator :
a. Trial run of Auxiliaries which includes the logic confirmation using simulated input/output signal from field/marshalling panel
b. Back charging of Transformer. Some Power companies are not allowing this test. Transformers shall be test after generator commissioning
c. Confirmation of Excitation, AVR System
d. OC and SC Test
e. Synchronizing and initial loading

7. What is the Standard applied for BMS?

National Fire Protection Association - NFPA, USA. NFPA 85 covers boiler and various fuel firing

6. What is Unit Interlock Diagram?

This includes Interlocks, Logic between Main equipments such as Boiler, turbine, Generator, Generator transformer, Unit transformer. 

5. What is SQC?

Permissive, Trip interlocks and Operation and Control of fans, pumps, Vanes, Valves and Dampers, etc.,

4. What is BMS?

Burner Management system which includes, Interlocks, Trips condition and logic for leak test, total failure, loss of flame failure, Partial flame loss and master fuel trips. This is mainly a system for a protection of furnace explosion. However it includes Boiler protection interlocks such as Drum level HH/LL Reheat protection.   It also includes sequence of start and stop of fuel firing equipment. FSSS--Furnace Safegaurd Supervisory System is also used by some manufacturers.

3. How you will understand the meaning of the symbols?

From Legends of the drawing

2. What is P&ID?

P&ID is the abbreviation for Piping and Instrumentation Diagram. Also understand as Process and Instrumentation diagram. A drawing which indicates all the process components, piping details along with control and instrumentation details is P&ID.

1. What are the Engineering documents to be studied before starting an assignment in a Power Plant?

1. Process and Instruments diagram of various system (P&ID)
2. Automatic Plant Control (APC)
3.Analog flow diagram
4.Analog logic diagram
5.Burner management system
6.Sequence control diagram
7.Permissive, Trips and Alarms setting list
8.Function setting list
9.Unit Interlock Diagram
10.Operation and Maintenance manual