1-Why Boiler is considered as explosive equipment / system?
Boiler involves the generation of steam at higher pressure & temperature upon release of this into atmosphere could cause disasters.
2-How do you specify the Boilers?
Boilers are specified by their operating pressure, temperature, steam generating capacity & even by heating surface.
3-What are the various circuits involved in Boilers?
Water circuit, Steam circuit, air circuit & flue gas circuit
4-Write down the various energy used / generated in power plants?
- Chemical energy (fuel)
- Thermal energy (Heat present in flue gases & steam)
- Kinetic energy (rotation of Turbine rotor),
- Mechanical energy (shaft power on Turbine)
- Electrical energy (In generators).
5-Why steam, is generated at high pressure & temperature?
High pressure & temperatures steam is having more heat content & also has more potential to do work.
6-At what pressure do you carryout Hydraulic test on Boilers?
For new boilers it is 1.5 times the design pressure & for old boilers it is 1.25 times the design pressure (also depends on Boiler life & its physical conditions)
7-What is the standard method or procedure followed for boiler pressure rising during hydraulic tests?
Pressure rise is 3.5 kg/cm2 per minute
8-What will happen if drum safety valves have been set at higher blow down rates?
Super heater coils may over heat due to insufficient flow of steam
8a-What are the reasons for failure of super heater coils?
- Overheating due to insufficient fuel
- Erosion due to high flue gas
- Internal scaling
9-What is the MOC of steam drum?
Carbon steel: SA 516 Gr.70
9a-What is the MOC of water wall tubes & super heater coils?
Carbon steels 210 Gr.A
Super heater coils: SA 213 Gr.T11, T22, T 91 & T92
10-Why do you carryout pre heating & post heating before welding works?
Pre heating: For expelling out the moisture present in the materials
Post heating: It is done after the welding is done for relieving residual stress due to welding
11-Why the pressure gauges installed at boilers firing floor show more reading as compared that of installed at lines?
Firing floor gauges show actual line pressure & pressure head due to height from line to firing floor (around 30 meters)
12-Why it is been recommended to operate boiler safety valves at least once in a shift or day or week?
To avoid seat stuck up due to rusting
13-What is the allowable reduction of tubes thickness?
It is around 20% of original thickness. For example a SH coil of original thickness is 5mm, then it should be replaced if its thickness at straight portion reduces up to 5-5X 20% = 4 mm
14-What are the reasons for overheating Boilers pressure parts?
Overheating is due to
- Wrongly set burners/spreaders
- High velocity of flue gases
- More firing rate
- Internal scaling
15-On what factors Boilers thermal expansion depends?
It depends on
- Boiler operating temperature
- Boiler tube materials composition
- Length of tubes
16-What are the reasons for fish mouth failure of boiler tubes/coils?
Erosion & corrosion
16a-What do you mean by priming in Boilers?
It is the carryover of water particles into steam
17-What could be the probable causes of priming?
- Drum level fluctuation
- Boiler load fluctuation
- Boiler parameters fluctuation
18-What is the reasons for monitoring of Oxygen level in flue gases?
To control combustion & to achieve maximum efficiency of combustion by optimising excess air
19-What are the functions of steam drum?
- Steam drum acts as water storage device
- Separates steam & water
- Provides space for internal chemical treatment
- Removes sludge from boilers through blow down
- Accommodates safety valves to relieve excess pressure during abnormal operating conditions
20-What is the function of start-up vents in boilers?
- To control steam pressure during abnormal operating conditions
- To provide minimum flow through super heater coils
- To increase main steam temperature during low loads
21-Why steam drum dish end thickness is lesser than that of cylindrical portion
Because: Stress produced in dish ends are circumferential & are less as compared to stress developed in longitudinal portion.
22-What do you mean by an equivalent evaporation?
Quantity of water evaporated at 100 deg C to dry saturated steam at 100 deg C
23-What are the reasons for more main steam temperature?
- More excess air
- High moisture fuel
- More convective heat transfer at super heater zone due to loss of turbulence
- Failure of attemperator control valve
- Operating the Boiler at lower feed water temperature at economiser inlet.
24-Why it is been not recommended to give blow down during high steaming rates?
Sudden opening of IBD valves or water wall bottom header valves will affect circulation rate & may lead to failure of water wall tubes.
25-Why Deaerators are placed at higher elevation?
To give NPSH to boiler feed pumps
26-What are the functions of Deaerator?
- To remove dissolved oxygen
- To store the feed water
- To increase the feed water temperature
- To give NPSH to boiler feed pumps
- Allows space for LP dosing (Oxygen scavenger dosing)
27-More height of Chimney creates what?
Creates more natural draught & helps to reduce load on ID fans
28-What is the function of draught system?
- To force air for combustion
- To expel out products of combustion from Boiler
29-What are the different types of draughts used in Boilers?
Forced draught, induced draught & Balanced draught?
30-On what factors chimney construction depends?
- Quantity of flue gas generated per hour
- Draught to be produced
- Sulphur content in fuel
- Environment clearance
31-Why the ID fans are not situated between APH & ESP?
If placed between ESP & APH, Fan size increases due to higher specific volume of gases. As flue gas volume decreases as its temperature decreases.
32-Where do you use steel Chimneys?
For economy at lower flue gas flow steel chimneys are more preferred
33-What is the velocity of flue gas inside the chimney?
It is 10 to 14 m/sec
34-What is the velocity of flue gas inside the ESP?
It is around 0.75 to 1 m/sec
35-What factors affect the draught produced in Chimneys?
Flue gas temperature & air temperature
35a-What is the velocity of wind considered for Boiler Construction?
Around 39 m/sec
36-What do you mean by cold air inversions in chimney?
It happens when outside air pressure is more than flue gas pressure inside the chimney
37-When can cold air inversions happen?
This phenomenon happens when number of boilers operating at various loads are connected to a single Chimney
38-What is the disadvantages of concrete chimneys over metal chimneys?
- High susceptible for thermal shock
- High cost of construction
39-What does it mean that Chimney is producing good draught?
If chimney is producing good draught means, flue gas temperature is more & hence Boiler efficiency is less
40-How do you say that high pressure & temperature power plants have greater efficiency?
High pressure & temperature steam will have more heat content & high enthalpy drop will be available in turbine expansion.
41-What do you mean by circulation ration in boilers?
It is the inverse of mass steam content
Circulation ratio = Mass flow rate of steam & water mixture / Mass flow of steam through tubes
42-What do you mean by a load control range of a Boiler?
Boiler operating generation range in which boilers auto controls work.
43-In which cases do you take emergency shutdown of Boilers?
- During tubes leakage
- Overshooting of steam temperature
- Boiler furnace explosion
- Boiler brick work damage
- Furnace draught fluctuation
- Failure of Boiler feed pumps
- Failure of drum level gauges (local & remote)
44-What do you mean by MCR & ECR in Boilers?
MCR: Maximum continuous rating
ECR: Economic continuous rating
45-In what load Boiler will have higher efficiency?
46-What is the lowest load range of boiler to operate it comfortably?
It is around 30 to 40%
47-In all most all cogeneration & Thermal power plants lowest range of super heater temperature is 485 to 500 deg C, Why?
This is for protecting Turbine last stage blades from water particles erosion damage
48-What adverse effect do you observe in an overloaded Boiler?
- It affects circulation velocity
- Flue gas temperature increases
- Steam temperature increases may result into SH coil failure
49-Why do you measure O2 & CO2 Percentages in flue gas?
To know about combustion, whether it is proper or not.
50-How do you differentiate hot, warm & cold start-up of Boilers?
- Hot start up: Boiler is restarted within 6 to 10 hours shutdown (Within a shift)
- Warm start up: Boiler is restarted within 10 to 70 hours shutdown (Within 3 days)
- Cold start up: If boiler is started after 70 to 90 hours of shutdown (more than 3 days)
51-What are the various methods of Boilers preservation during shutdown?
Wet preservation method
Dry preservation method
52-Where do you use wet preservation method?
Used for standby boilers & they should be available for immediate use
53-Where do you use Dry preservation method?
Used for Boilers which are kept shutdown for long periods & they are not required for immediate use
54-What is the concentration of Hydrazine for wet preserved Boilers?
It is >200 ppm
55-What is the alternative chemical used for wet preservation
It is Sodium sulphite & concentration maintained is 350-400 ppm
56-What is the significance of Slag screens?
These are the staggered tubes installed in flue gas path to avoid clogging of tubes as a result of cooling & adhering of molten slags.
57-What are the drum internals?
These are the internal parts of steam drums utilized for separation of steam water mixture & for chemical dosing
Drum internals are
1. Cyclone separators
4. HP dosing connection
5. Feed water connection
6. CBD line
58-What do you mean by steam separation?
It is the process of separating bulk water particles from steam
59-What are the various methods used for separation of water particles from steam?
- Centrifugal separation method
- Use of baffles
- Abrupt change of steam water mixture direction
- By gravity method
- Direct hitting on plates
60-What is dry pipe in Boilers?
Dry pipe is the perforated pipe fitted at the most height level of boiler to provide dry steam (around 98%)
61-What are the Boiler auxiliaries?
These are the system or equipments used in Boilers to boost of the Boiler efficiency & performance
- Super heaters
- Air preheaters
- Boiler fans & pumps
- Soot blowers
- Water storage tank
- Chemical dosing system
62-What are boiler mountings?
These are the devices used for safe operation of Boilers
- Safety valve
- Boiler level gauge
- Steam stop valve & NRV
- Start-up vent valve
63-Why do ID fans & FD fans are not installed before ESP & after APH respectively?
This is for reducing fan size & load on it .As flue gas & air volume is more at higher temperatures, in order to drag & push these high temperature flue gas & air need to install bigger size fans
64-What type of Impellers used in ID & FD fans & why?
Backward curved fans are used, as these offer very less resistance to rotate
65-Why do you use super-heated steam in power plants?
More heat is with SH steam & more enthalpy drop available & hence more efficiency. And also SH steam is having no water particles due to this equipments life increases
66-What is the necessity of forced circulation in high pressure boilers?
As the pressure increases beyond 150 kg/cm2 up to 220 kg/cm2 density of steam & water becomes same & hence there will not be any chance for natural circulation due to density difference. Hence there need arises for forced circulation
67-Do you prefer soot blowing after shutdown of Boilers?
Generally NO, because soot blowing may result into explosion due to leftover unburnt fuel & hot ash
68-What do you mean by wire drawing?
It is the process of steam expansion & no any work done or loss. It happens in orifice, flow meters & throttled valves.
In PRV & PCVs stations does not contribute in work done or loss & hence upon reducing pressure of normal (saturated steam) steam, it becomes super-heated.
69-What will happen if economiser inlet feed water temperature drops?
Lower feed water temperature at economiser inlet lead to more absorption of heat from flue gases. This causes drop in flue gas temperature, may lead to acid corrosion.
70-What will happen if economiser outlet feed water temperature increases more than design?
More feed water temperature at economiser outlet may lead to steaming of water, which could ultimately cause overheating & failure of economiser tubes. This happens in low load operation.
71-What is the recommended flue gas temperature at APH & ESPs?
It is more than acid due point temperature to avoid corrosion of materials. For coal & biomass fired boilers it is always >140 Deg C
72-What will happen to the enthalpy of steam if its pressure increases & temperature remains constant?
Its enthalpy decreases
73-What will happen to the enthalpy of steam if its temperature increases & pressure remains constant?
Its enthalpy increases
74-What will happen to the enthalpy of steam if its temperature & pressure increase?
75-What will happen to the enthalpy of evaporation of saturated fluid if its pressure increases?
Enthalpy of evaporation decreases if pressure is increased
76-What will happen to the enthalpy of evaporation of saturated fluid if its temperature increases?
Enthalpy of evaporation decreases if temperature is increased
77-What will happen to the density of water if its temperature increases?
78-What will happen to the density of water if its pressure increases?
79-What happens to density of steam if pressure & temperature increase?
Density also increases
80-What is the function of bypass valve arranged for main steam stop valve?
It is mainly for pressure equalizing to open the main stop valve & also for initial charging of steam line
81-When do you use single & 3-element drum level controllers?
Single element is used when the Boiler load is less than 30% of MCR
3-element controller is used when boiler load is >30% of MCR
82-A 85 TPH boiler is operating on 80% load & its blow down is 0.8%.Calculate the makeup water required
Boiler operating load = 85 X 80 / 100 =68 TPH
Blow down water quantity = 68 X 0.8 / 100 =0.544 TPH
So make up water quantity is 0.544 TPH
83-What will happen to the boiler tubes if pH is more or less than recommended?
If pH is less it leads to corrosion of tubes & if more it leads to scaling
84-What are the scale forming contents of water?
Salts of calcium & magnesium, sodium carbonates, nitrates & silica
85-A feed water sample is not showing the residual hydrazine, then what does it mean?
It means that there is no treatment for oxygen scavenging, or chemical dosed is insufficient. This could lead to corrosion of pressure parts.
86-Which chemicals are used in Boilers HP & LP dosing systems?
HP dosing: Tri-sodium phosphates
LP dosing: pH booster & Oxygen scavengers
87-When your chemist asks you for giving IBD?
IBD is given, when following parameters in Boiler water exceed?
Silica, Phosphate, pH & conductivity
88-What is the major loss in coal fired & bagasse fired boilers?
In coal fired Boilers major loss is heat loss due to dry flue gas. And in Bagasse fired boilers major loss is heat loss due to moisture present in fuel
89-What do you mean by air to fuel ratio?
It is the amount of air required to burn 1 kg of fuel
90-What do you mean by steam to fuel ratio?
It is the amount of steam generated on burning 1 kg of fuel
91-What is the velocity of flue gas at various zones of Boilers?
- Furnace: 1 to 2 m/sec
- Super heaters: 2 to 4 m/sec
- Economiser: 6 to 7 m/sec
- APH: 8 to 12 m/sec
- ESP (Inside): 0.75 to 1 m/sec
- ID fan inlet: 13 to 16 m/sec
92-What is the function of SCAPH?
SCAPH (Steam coil air pre heater) is used to heat FD air which is entering into APH or some type SCAPH is used an alternative of APH.
93-Which contents of fuels are responsible for more excess air?
Bagasse moisture, hydrogen & carbon contents cause more requirement of combustion air
94-Which content of the fuel has very least on combustion air requirement?
95-Which content of the fuel upon increase in % causes less combustion air?
96-What is the excess air required for combustion of Bagasse, Coal, Gas, Oil & Wood?
- Bagasse: 25-35%
- Coal: 20-25%
- Gas: 5-10%
- Oil: 10-15%
- Wood: 35-45%
97-Which fuels require more & least excess air for complete combustion?
Wood needs more excess air & Gas needs less excess air among all fuels
98-What can cause if Boiler line safety valve is set at higher blow down?
- Drum level fluctuation
- More load on Super heater coils
- Steam loss
99-What is the function of over fired Air (OFA)?
OFA is used to create turbulence & to increase residence time of fuel particles in furnace
100-What are the reasons for incomplete combustion?
- Less excess air
- More ash content in the fuel
- More moisture in the fuel
- Not achieving the 3Ts (Temperature, Time & Turbulence) in furnace
- Unbalanced draught in furnace
101-What happens to the velocity & pressure of steam in steam line if flow is increased more than design?
Velocity of steam increases & pressure drop occurs
102-How do you reduce excess load from ID fans?
- By maintaining optimum excess air
- By arresting all cold air ingress points
- By arresting APH tubes leakages
- By conducting regular maintenance of fans
103-How do you reduce excess load from FD fans?
- By maintaining optimum excess air
- By arresting all air & flue gas leakages
- By controlling moisture level in the fuel
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