BOILER INTERVIEW QUESTION & ANSWER
Today we discussed about some multiple choice questions and answer related to power plant these questions are very helpful for which do preparation for B.O.E ( BOILER OPERATION ENGINEER ) EXAM. and also for ENERGY MANGER EXAM , and also for those people which prepared for interview .
Q.(A).What is the affect of sulfur in coal when used in boiler?
Sulfur will get oxidized to SO2 and fraction of SO3 and will react with water to form sulfuric acid and this occurs at a temperature called the acid dew point which normally is about 120 oC. The sulfuric acid so formed corrodes the steel when it comes in contact with it.
Q.(B).What do you mean by IBR steam boiler.
IBR Steam Boilers means any closed vessel exceeding 22.75 liters in capacity and which is used expressively for generating steam under pressure and includes any mounting or other fitting attached to such vessel,which is wholly or partly under pressure when the steam is shut off.
Q (B.1)What do you understand by ‘water tube boilers’ and ‘fire tube boilers’?
In water tube boilers the water passes through the tubes and the hot gases passes out side the tubes where as in case of fire tube boiler the hot gases passes through the tubes and the water passes over the tubes
Q(C).What are the parameters required to estimate the boiler efficiency by ‘direct method’?
- a) Steam flow rate
- b) GCV of fuel
- c) Fuel flow rate
- d) Steam conditions ( pressure and temperature)
- e) Feed water temperature
Q (D).Why boiler blow-down is required?
As the feed water evaporate into steam, dissolved solids concentrate in the boiler. Above certain level of concentration, these solids encourage carryover of water into steam. This leads to scale formation inside the boiler, resulting in localized over heating and ending finally in tube failure. Hence blow-down is very much required for boilers.
Q (E).What is the principle of mechanical deaeration (pressure type) of boiler feed water?
The pressure-type de-aerators operates by allowing steam into the feed water through a pressure control valve to maintain the desired operating pressure, and hence temperature at a minimum of 105 °C. The steam raises the water temperature causing the release of O2 and CO2 gases that are then vented from the system. This type can reduce the oxygen content to 0.005 mg/litre.
Q (F).What is the effect of boiler loading on boiler efficiency?
- The maximum efficiency of the boiler does not occur at full load, but at about two-thirds of the full load. If the load on the boiler decreases further, efficiency also tends to decrease.
- As the load falls, so does the value of the mass flow rate of the flue gases through the tubes. This reduction in flow rate for the same heat transfer area, reduced the exit flue gas temperatures by a small extent, reducing the sensible heat loss.
- Below half load, most combustion appliances need more excess air to burn the fuel completely. This increases the sensible heat loss.
Q (G).What are the principle heat losses that occur in a boiler?
The principle heat losses that occur in a boiler are:
• Loss of heat due to dry flue gas
• Loss of heat due to moisture in fuel and combustion air
• Loss of heat due to combustion of hydrogen
• Loss of heat due to radiation
• Loss of heat due to unburnt fuel
Q (H).What do you meant by tangential firing with respect to pulverized coal fired boiler?
The method of firing used for coal firing in pulverized fuel fired boiler is the tangential firing. In this type of firing four burners are used at the corner to corner to create a fire ball at the center of the furnace.
Q (I).What are the disadvantages of ‘direct method’ of boiler efficiency evaluation over ‘indirect method’?
a) Do not give clues to the operator as to why efficiency of system is lower
- b) Do not calculate various losses accountable for various efficiency levels
J.List out the data required for calculation of boiler efficiency using ‘indirect method’.
The data required for calculation of boiler efficiency using indirect method are:
- Ultimate analysis of fuel (H2, O2, S, C, moisture content, ash content)
- Percentage of Oxygen or CO2 in the flue gas
- Flue gas temperature in 0C (Tf)
- Ambient temperature in 0C (Ta) & humidity of air in kg/kg of dry air.
- GCV of fuel in kcal/kg
- Percentage combustible in ash (in case of solid fuels)
- GCV of ash in kcal/kg (in case of solid fuels)
Q (K).Explain the different external water treatment methods.
External treatment is used to remove suspended solids, dissolved solids (particularly the Calcium and Magnesium ions which is a major cause of scale formation) and dissolved gases (oxygen and carbon dioxide). The techniques include Precipitation processes, in which chemicals are added to precipitate calcium and magnesium as compounds of low solubility.
The lime-soda process is typical of this class, but other precipitating agents such as caustic soda and sodium phosphate can be used when the composition of the raw water permits.
Ion-exchange progresses, in which the hardness is removed as the water passes through bed of natural zeolite or synthetic resin and without the formation of any precipitate. Ion exchange processes can be used for almost total demineralization if required, as is the case in large electric power plant boilers.
De-aeration, in which gases are expelled by preheating the water before entering the boiler system. Water normally contains approximately 10 mg/1 of dissolved oxygen at ambient temperature Filtration, to remove suspended solids
Q( L).What are the salient features of a ‘packaged boiler’?
The features of package boilers are:
• Small combustion space and high heat release rate resulting in faster evaporation.
• Large number of small diameter tubes leading to good convective heat transfer.
• Forced or induced draft systems resulting in good combustion efficiency.
• Number of passes resulting in better overall heat transfer.
• Higher thermal efficiency at lower capacity (say below 1 ton) levels compared with other boilers.
Q (M).What are the two main classification of a stoker fired boiler?
1. Chain grate or travelling grate stoker
2. Spreader stoker
Q (N).Briefly explain the principle involved in ‘reverse osmosis’?
When solutions of differing concentrations are separated by a semi-permeable membrane, water from less concentrated solution passes through the membrane to dilute the liquid of high concentration.
If the solution of high concentration is pressurized, the process is reversed and the water from the solution of high concentration flows to the weaker solution.
Q (O).What are the various methods available to control the ‘excess air’ in a boiler?
Various methods are available to control the excess air:
• Portable oxygen analysers and draft gauges can be used to make periodic readings to guide the operator to manually adjust the flow of air for optimum operation. Excess air reduction up to 20% is feasible.
• The most common method is the continuous oxygen analyzer with a local readout mounted draft gauge, by which the operator can adjust air flow. A further reduction of 10-15% can be achieved over the previous system.
• The same continuous oxygen analyzer can have a remote controlled pneumatic damper positioner, by which the readouts are available in a control room. This enables an operator to remotely control a number of firing systems simultaneously.
Q (P).Describe ‘chain grate’ and ‘spreader stoker’ type boiler.
Chain-Grate or Travelling-Grate Stoker Boiler
Coal is fed onto one end of a moving steel grate. As grate moves along the length of the furnace, the coal burns before dropping off at the end as ash. Some degree of skill is required,
particularly when setting up the grate, air dampers and baffles, to ensure clean combustion leaving the minimum of unburnt carbon in the ash. The coal-feed hopper runs along the entire coal-feed end of the furnace. A coal grate is used to control the rate at which coal is fed into the furnace by controlling the thickness of the fuel bed. Coal must be uniform in size as large lumps will not burn out completely by the time they reach the end of the grate
Spreader Stoker Boiler
Spreader stokers utilize a combination of suspension burning and grate burning. The coal is continually fed into the furnace above a burning bed of coal. The coal fines are burned in suspension; the larger particles fall to the grate, where they are burned in a thin, fast-burning coal bed. This method of firing provides good flexibility to meet load fluctuations, since ignition is almost instantaneous when firing rate is increased. Hence, the spreader stoker is favoured over other types of stokers in many industrial applications.
Q (Q).Write short notes on ‘intermittent blow down’ and ‘continuous blow down’ with respect to boilers.
The ‘intermittent blown down’ is given by manually operating a valve fitted to discharge pipe at the lowest point of boiler shell to reduce parameters (TDS or conductivity, pH, Silica and Phosphates concentration) within prescribed limits so that steam quality is not likely to be affected. In intermittent blowdown, a large diameter line is opened for a short period of time, the time being based on a thumb rule such as “once a shift for 2 minutes”. ‘Intermittent blow down’ requires large short-term increases in the amount of feed water put into the boiler, and hence may necessitate larger feed water pumps than if continuous blow down is used. Also, TDS level will be varying, thereby causing fluctuations of the water level in the boiler due to changes in steam bubble size and distribution which accompany changes in concentration of solids. Also substantial amount of heat energy is lost with intermittent blow down.
There is a steady and constant dispatch of small stream of concentrated boiler water, and replacement by steady and constant inflow of feed water. This ensures constant TDS and steam purity at given steam load. Once blow down valve is set for a given conditions, there is no need for regular operator intervention. Even though large quantities of heat are wasted, opportunity exits for recovering this heat by blowing into a flash tank and generating flash steam. This flash steam can be used for pre-heating boiler feed water or for any other purpose. This type of blow down is common in high-pressure boilers.
Q (R).Explain the reasons for carrying out “blow down” in a boiler?
Water contains certain percentage of dissolved solids. The percentage of impurities found in boiler water depends on the untreated feed water quality, the treatment process used and the boiler operating procedures. As a general rule, the higher the boiler operating pressure, the greater will be the sensitivity to impurities. As the feed water materials evaporate into steam, dissolved solids concentrate in the boiler either in a dissolved or suspended state. Above a certain level of concentration, these solids encourage foaming and cause carryover of water into the steam. This leads to scale formation inside the boiler, resulting in localized overheating and ending finally in tube failure.It is therefore necessary to control the level of concentration of the solids and this is achieved by the process of ‘blowing down’, where a certain volume of water is blown off and is automatically replaced by feed water – thus maintaining the optimum level of total dissolved solids (TDS) in the water. Blow down is necessary to protect the surfaces of the heat exchanger in the boiler.
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