151.What are the types of fan? What is difference and give examples for their application?

Axial and Radial fan. Axial fan can provide large flow at low head. Ex. FD fan, Ignitor fan, ID  Fan, etc Radial Fan  can provide less quantity at high pressure. PA fan, Flame scanner cooling air fan, etc

150.How to control flow of fan?

By modulating the inlet vane

149.How to control process flow for centrifugal pump and positive displacement pump?

Centrifugal pump - Modulating the discharge side valve.

Positive displacement pump - Modulating  recirculation valve.

148.What are the type of pumps and give their typical example?

Positive displacement, reciprocating, centrifugal type, etc. Positive displacement type - Fuel oil pumps, Reciprocating Type - Chemical dosing pump, Centrifugal type - Feed water pump and other general service pumps.

147.What are the types of Instrument air compressors? What is loading and unloading?

Reciprocating and centrifugal type. Loading - Compressor running with suction valve open. Unloading - Compressor running with suction valve closed, ideal running

146.What are the fail-safe conditions of control valves, give typical example?

Fail safe conditions of Control valves are Fail-Open and Fail-Close (Depending on the end position reached by control valve on loss of control signal/air supply).Normally Fail open valves are used in Emergency applications like recirculation line or Bypass lines (HP bypass, Heater bypass etc). Valves used for fuel flow are fail close type.

145.What are the characters of Control valve and give example of their application?

The typical flow characteristics of a control valve are Linear, Equal percentage, Quick opening type.          The characteristic of valve depends of loop sense, differential pressure across the control valve etc.                  Fuel oil flow control - Controller is Linear characteristic. Error at various valve opening, from 0 to 100%, demands same flow change.The DP across the control valve is high at loads and decrease with respect to load. Hence the CV at low loads shall be small compared to at high loads. A Equal percentage valve will provide such CV. Fuel Oil Temperature Control - Controller is non linear characteristic. Error is temperature. Same error at small valve opening require less flow. At higher valve open demands large flow. The mass of steam required for 50% fuel can only change the temperature by 50% at 100% fuel flow. DP across the valve is constant. Hence an Equal % valve can perform better. However the valves characteristics can be matched with the process using adapting gain in DCS using valve position demand.

144.What is Constant Pressure mode and Sliding Pressure mode?

Large units will be operated at reduced steam pressure at loads less than 60% from 25%. The pressure will be maintained constant at rated value from 60 to 100% load. Typically for a 550 MW Unit, pressure will be 120 bar at 25% load and increased proportionally to 160 bar up to 60% load. This mode of unit operation is known as Sliding/Variable pressure mode. The governer valve will remain at a fixed value except for any emergency. From 60% to 100% the pressure will be maintained constant at 160 bar by modulating the governer valve, it is called constant pressure mode.In sliding pressure mode an appreciable amount energy is saved in the BFP.

143.What is Runback?

In a large units, the auxiliaries will be split to two of 2 x 60% capacity. This will help to produce atleast 50% load during any such auxiliary. Loss FDF/IDF will reduce air supply. Loss of BFP will reduce feed water supply. Loss of Mill will reduce fuel supply. When a loss of auxiliary is sensed the load shall be reduced to below 50% as quickly as possible. Loss of Fan needs 100%/min rate. This control is initiated logically.This is called Runback. Runback for FDF can be achieved successfully for almost all the boilers. However the unit can survive for loss of BFP only in the case of assisted circulation boilers. Loss of coal Mill will transfer the coal flow control to Manual.The unit will go to Turbine follow Mode for Runback.

142.What is Rundown?

During load ramping, the auxiliaries may not be able to meet the required load due to change in boiler conditions, aging of auxiliaries, etc. It is practiced to reduce the load and CMC demand slowly to a safe limit. This is known as Rundown.

141.What is Load Block?

During CMC mode the demand is applied parallel to BM and TM. The response of boiler will be slow when compared to turbine. This will create deviation in boiler loops, such as Fuel flow controller, Air flow controller, Steam temperature controller, Drum level controller, etc. It is not advisable to continue load change with a high deviation. The change in load demand will be blocked until the deviation become less.Load block will be applied for Load Increase and Load Decrease. Alarm will be initiated to alert the operator.

140.What is Co-ordinate control mode?

In order to get the Optimum response of both Boiler Follow and Turbine Follow mode, as higher level control function called Co-ordinated Master Control is designed.In this configuration, CMC will be the main Load demand controller and will parallely sending demand for fuel from MWD as a feed forward signal to Boiler Master and the MWD to the Turbine Master in a Co-ordinate manner. This helps Boiler controls to take corrective measures, even before the PV is changed and at the same time Turbine control to quickly respond to load variations utilising the stored energy in the boiler. This mode is called as MW Control or Co-ordinated Boiler Follow Mode by Vendors. Sometimes it is necessary to operate plant in Co-ordinated Turbine Follow Mode ( BM - control MW and TM - control steam pressure). This mode helps to over come the large time lag in boiler.

139.What is Turbine Follow mode?

The Power generation (Load), in a power plant can be changed by changing the fuel/air demand manually. In this condition the Turbine Master will control the steam pressure.This Process where in the TG  Master "Follows" Boiler  is known as the "Turbine Follow" mode of operation. During Turbine Follow Mode, The Boiler Master will be on Manual and Turbine control will be on Auto (controlling throttle steam pressure)

138.What is Boiler Follow mode?

The Power generation (Load), in a power plant can be changed by modulating governer valve. This will cause a change in steam flow in turn a change in steam pressure.The Boiler Master will change the fuel demand from the SF as FF and by deviation in pressure as feed back.This Process where in the Boiler Pressure Control "Follows" Turbine Load controller is known as the "Boiler Follow" mode of operation. During Boiler Follow mode, Boiler Master will be on Auto (controlling pressure) and Turbine Master will be on Manual or local Auto (controlling MW).

137.What is the meaning of MOCS (Multiple Output Control System) and where it shall be applied?

The control system which control single PV using a process PID controller as Master and an auxiliary Integral controller as slave is called MOCS.When the PV is controlled by more than 1 final element, using MOCS will provide automatic gain modification for number of A/M stations on Auto. Also provide facility to use Feed Forward loop for such loop. One example is the Furnace Pressure control using 2 x 60% ID Fans. The FF will be from the FD Fan inlet vane demand.

136.How to compensate the gain for multiple drives for a single process value?

When as single controller is designed to control multiple control elements in parallel, depending on the number of the control elements in operation, the controller demand (common) has to be varied for the same process deviation. This  is achieved by dynamically varying the Controller Gain depending on the number of ouputs active at any given point of time (Dynamic Gain changer).

135.How to over come the non-linear characteristic of a control’s final element?

If the characteristics of process variations are linear w.r.t valve opening, but the control valve characteristics are non-linear, a suitable characterisation curve can be designed at the ouput of controller to overcome the non-linearity

134.When to use derivative control? When to use dynamic compensation?

Derivative control action is designed when the process variable has a tendency to change rapidly. Hence an enhanced and immediate control action is needed with respect to rate of change. It needs careful application as improper tuning may result in process swinging out of control. Dynamic compensation action is designed to aid the control action, by the influence of other parameters (not controller PV), which may recognise the anticipated variation of controlled PV.The controller response needed may required to be varied under different conditions and hence typically Feedforward control concepts are used for dynamic compensation.

133.What is Over-firing and Under-firing? What parameter of the boiler will be affected much? How to overcome?

When load is increased in a boiler, as a dynamic compensation, fuel fired will be more than that required for steady state. This is called over firing. Similarly when load is decreased in a boiler fuel fired will be less than that of steady state. This condition is under firing.

The temperature pick-up in the final super heater will vary considerably. The value of change is proportional to the % of mismatch. The feed forward loop input shall be compensated suitably to over come the effect of mismatch.

132.What is three element control in drum level control? Why it is necessary?

Drum level is controlled using three measurement parameters-- Steam Flow, Water Flow and Drum level signals. During rapid load changes, the water level in boiler drum swells or shrinks due to change in Drum pressure. This would result in erroneous reading in actual Drum level measurements. Hence level measurement alone will not be adequate for control purposes. In order to control level effectively, the control loop is designed to vary the water into drum by balancing the Stem flow (boiler OUT) and Feed water Flow(boiler IN).The controller signal is corrected thru the actual Drum Level deviations to continuously correct the steady state level variations.

131.What is generator purging?

During Power generation, Generators radiate enormous heat. Hence the Generator casing needs to be cooled. Hydrogen gas is used for fillling the Generator casing as it is most effective. However, H2 will easily react with air (O2) unless it is >98% pure. In order to ensure that there are no entrapped air in the Gen casing,the casing is first purged with Nitrogen gas as N2 is an inert gas.Subsequently, the N2 gas is displaced with H2 and purity maintained > 98%.

130.What is furnace purge? Why 30% air flow? Why not 100% air flow? Why 5 Min?

Furnace Purge would involve allowing some minimum quantity of air to flow through the furnace (without fuel) for a specified period of time to remove any unburnt fuel. This activity is done every time boiler is started by running Air fans and Draft fans as required. It is expected that with 30% airflow maintained for a period of 5 min, the volume of air is adequate for the furnace to be purged completely of all the unburnt fuel that may be residual in the furnace during the previous firing operations thereby removing possible uncontrolled ignition/explosion during next firing operation.The Purge cycle is performed with furnace maintained under the design pressure(draft) conditions.

129.How to confirm the performance of control system?

As a part of Performance Guarantee, C&I contractor is expected to limit the deviations in critical parameters within limits specified (+/- % age) both for steady state and dynamic load variations (Step and Ramp). By proper design of the control system including feed forward/feedback signals and by correct tuning of control loops, the performance of control system is established in limiting the deviations within limits.

128.What is the meaning of load test and what is the use of it in the Control system?

Operation of a plant at various loads is known as Load tests. The parameters used in Function generators, which are load dependent are collected after 30 min at each load. Ex. MW, Steam Flow, Fuel flow, Air flow, O2 in flue gas, SH out temperature, At outlet temperature, Burner tilt/Gr fan vane position, etc are the minimum to be collected. Main purpose of the test is to correct all FX as per the present plant conditions

127.When the gain of a control loop will need to change?

When the response of the Process is different at different conditions (eg: Oil or Coal or Gas firing, Load increase or decrease, No of Mills in operation, no of Fans/Pumps in operation etc) the control loop gain needs dynamic variation design.

126.How to protect the super heater from saturation steam?

By providing Temperature limiting controls to the Setpoint of the Secondary loop in a Steam Temperature control.

125.When air limit fuel alarm will appear and how to handle it?

During Load demand variations, if the airflow cannot be maintained as per the fuel demand (refer lead-lag control), the fuel flow will be limited by actual airflow--air limit fuel alarm. In such a case, the load demand (fuel flow demand) has to be reduced OR additional air fans(FD) have to be started to increase air flow. We should also check healthiness of Oxygen analyzer as the error in its reading can give wrong airflow values for control.

124.Under what circumstances, the boiler master output will hunt and what is the solution?

Boiler Master demand (PV:Bolier outlet pressure) will hunt when the Load demand controlled by turbine is varying (eg: excessivegrid  frequency variation). In such circumstances, it is better to switch to "Turbine Follow Mode". Boiler controls will look at Load Variation and Turbine controls will maintain Boiler outlet pressure.

123.What is balance-less transfer and how to achieve it?

This is experienced in a controller which is controlling more than one secondary output simultaneously. If one of the secondary o/ps is switched to manual or fails, the remaining o/ps have to be adjusted(balanced) automatically to attain the desired parameters. This action of automatic balancing of the active o/ps (in a multi o/p controller) without experiencing a bump is known as Balance-less transfer (achieved thru slow ramping). The combination of Auto Gain change and Tracking techniques are used to achieve the same.

122.What is Bump-less transfer and how to achieve it?

Controllers can be on Cascade/Auto/Manual Modes. Whenever the controller status is changed, the process should not experience a variation(BUMP). In order to achieve this, the output should be held in its last value before the change in status and the output has then to be varied as per the conrol requirements. Different "Tracking" computations can be configured to achieve the same(PV-SV track etc.), so that the controller does not see a deviation during Mode change.

121.How to make sure the failure of transmitter?

Failure of transmitter is determined thru signal out of range (above 106% (20.96 mA) or below -6%(3.04 mA)) detection of DCS .

120.What will be logic when the transmitter fails?

When both transmitters are healthy, operator can select any transmitter or average value for control (on Auto).When one transmitter fails, the control shall changeover to the other healthy transmitter and the alarm (failure) status to be notified to the operator. When both transmitters are faulty OR there is a deviation between the two transmitter readings (beyond specified margin), the control loop shall be forced to Manual and Failure to be alarmed to operator including forced manual status.Force the controller to Manual mode, if there is no redundancy

119.What will be operation logic for selection of transmitter when redundant transmitters are used for control?

When both transmitters are healthy, operator can select any transmitter or average value for control (on Auto).When one transmitter fails, the control shall changeover to the other healthy transmitter and the alarm (failure) status to be notified to the operator. When both transmitters are faulty OR there is a deviation between the two transmitter readings (beyond specified margin), the control loop shall be forced to Manual and Failure to be alarmed to operator including forced manual status.

118.How to ensure air rich in the furnace?

The Boiler Master Controller controls Boiler outlet Steam pressure by regulating Fuel (Coal/Oil/Gas) and Air required for combustion. For any combustion to be effective , the volume of air required has to be equal to atleast the Schiometric ratio. However, it is necessary to prevent Fuel rich conditions in the bolier as it can lead to unburnt carbon and un-controlled combustion resulting in possible explosion. Hence, the control system design shall ensure an air rich condition always. to accomplish the same, whenever there is a demand to increase fuel, the actual airflow shall be increased first and then fuel flow (Air leads Fuel). When the Boiler Master demands decrease in fuel flow, first fuel has to be reduced and then air flow (Air lags Fuel). The Cross limiting circuit ensures the above to happen--actual Airflow and Fuel flow measurement shall be used to cross limit increase/decrease (thru Max/Min selectors) to ensure air rich condition always.

117.What is the meaning of lead/lag control with respect to Boiler Master? What is cross limit and cross bias?

The Boiler Master Controller controls Boiler outlet Steam pressure by regulating Fuel (Coal/Oil/Gas) and Air required for combustion. For any combustion to be effective , the volume of air required has to be equal to atleast the Schiometric ratio. However, it is necessary to prevent Fuel rich conditions in the bolier as it can lead to unburnt carbon and un-controlled combustion resulting in possible explosion. Hence, the control system design shall ensure an air rich condition always. to accomplish the same, whenever there is a demand to increase fuel, the actual airflow shall be increased first and then fuel flow (Air leads Fuel). When the Boiler Master demands decrease in fuel flow, first fuel has to be reduced and then air flow (Air lags Fuel). The Cross limiting circuit ensures the above to happen--actual Airflow and Fuel flow measurement shall be used to cross limit increase/decrease (thru Max/Min selectors) to ensure air rich condition always.

116.Why to maintain 6 – 7 Kg/cm2 dp across the feed water control valve

Normally in a boiler the spray water will be tapped of at the inlet of Feed Water Control valve. At full load the pressure drop between the tapping point and the attemperator nozzle will be 10 bar without any pressure drop across the FWCV. On minimum auxiliary power consumption point, this is ideal operating condition. The boiler feed pump speed will be modulated to provide the quantity of feed water flow to drum.

The 10 bar dp across the spray control valve is enough to provide the required spray water flow.

At 50% the dp across the spray water control valve, with same 0 dp across FWCV, will be 2.5 bar. This dp will not be enough to provide the spray flow. More over the quantity of spray at 50% will be maximum and that at full load will be minimum.

The required dp across TCV can be provided by maintaining a minimum dp across the FWCV. Hence in Drum Level control loop, the feed water flow is controlled by LC and the dp across FWCV is controlled by the BFP’s speed control.

The steam temperature control is a case cade loop. The secondary will control after attemperaor temperature. Assume that spray water required to reduce 1 Deg.C at full load is 1 T/H. The spray water demand at 50%  load to 1 Deg.C. will be 0.5 T/H. If the dp across the TCV is maintained constant, a Equal % valve will provide satisfactory results in this application. 

If the dp across the FWCV is controlled constant at 7 bar from 50% to 100%, the dp across TCV will be  9.5 bar at 50% load and at 100% load it will be 17 bar. In this case an adaptive gain shall be applied .

The dp across TCV can be maintained constant at 17 bar by setting the FWCV dp at 14.5 bar at 50% and 7 bar at 100%.

115.What are the type of control algorithms used?

Proportional, Integral, Derivative are the three algorithm of feed-back control

114.What are the reasons for high steam temperature at the primary super heater outlet? How to handle it?

The heat transfer in this zone is purely by convection. In this the steam flow and mass flow of flue gas shall be matched. If the flue gas mass is more than the steam flow, the steam temperature will increase. No attemperator is possible for this section. By properly adjusting the air flow for this low load will maintain the primary super heater steam temperature within limits. When the Automatic combustion is tuned, the fuel/air ratio shall take this into consideration in fixing the excess air.

113.What will happen if a soot blower is tripped on over current during forward motion?

Normally, if the SB motor trips in the forward position of blower, the blower can get damaged due to exposure to furnace heat. Hence, either the blower has to be withdrawn using manual operation from local.

112.What are the parameters to be monitored when they are operated manually?

SB steam pressure and steam temperature, Time cycle and sequence of blowers.

111.Whether the blowing pressure will be same for all the blowers? How to set the blowing pressure?

The blowing steam pressure for soot blowers will be different between them. The blowers for final super heater/re-heater will be maximum. The pressure depends on the temperature in  that zone. Pressure at super heater zone will be 22 bar, convection super heater zone is 12 bar, economizer zone is 10 bar and it is 8 bar for air heaters.

The soot blower steam will be taken after primary super heater before attemparator . It will be reduced to 25 bar. Each soot blower will be provided with a valve. The lift of the valve can be adjusted by a nut. Hence the pressure after the valve,

the blowing pressure, can be adjusted as requires. Throttling of the valves shall be avoided if possible.

First the soot blower far away from the reducing station in super heater zone shall be selected. A pressure gauge shall be fixed after the valve. The valve shall be left full open. Confirm the blowing pressure. Adjust the soot blower steam header pressure to get the blowing pressure of the soot blower.

Move upward towards reducing station and set the blowing pressure by throttling the valve. The temporary pressure gauge shall be used for confirmation.

Re-adjust the station if it is to be increased to get the highest blowing pressure and repeat downward and set the pressure bu adjusting the lift of the blower valve.

110.What is soot blower? What are the types?

Soot blowers are devices used to blow steam on water walls to remove carbon deposits/soot. Two types Rotary (Wall blowers) and Long Re-tractable (furnace walls).

109.The distance between top and bottom tapping in a drum for level measurement using DP transmitter is 1200mm. There are 600 mm apart from center of drum. The normal water level will be 250mm below the center line. The span of the level indicator is plus or minus 250 mm. What will be the calibration range of the dp transmitter?

The minus pressure from Drum minimum level to the constant column vessel middle is 4 mA.

 Ie., minus 1100 mm of WCL – 4 mA                                                                                  

The minus pressure from the drum maximum llevel to the contant column vessel middle is 20 mA.

Ie. Minus 600 mm WCL – 20 mA.                                                                                                                 

108.Whether the drum level between the transmitter, direct level gauge and electronic level monitors will be same on load? If no, what is the reason and how the level will be indicating?

The DP transmitter measures the difference of force ( due to pressure + mass of the liquid/steam) applied at the two ports. When used to measure level, the same shall be corrected for any change of Density of the liquid in the legs. When drum level is measured with DP transmitter, the density of water and steam in one port and the reference coloumn water are applied deriving from drum pressure. This is a real time correction applied from 0 to rated pressure of the boiler. Hence the level indicated by the transmitter will be nearer to the actual in the drum for all load.                                   

However, direct level guages and electronic level monitors (like Hydarstep) do not provide this correction. The water in the measuring vessel will be less thatn that of drum. Typically the temperature of the drum will be 340 Deg.C and the level gauge will be 150 Deg.C. The density of drum water will be 0.6 and that in the level coloumn will be arround 0.8. This is known sub- cooling. This will cause a considerable difference in the level between the DP transmitter and the direct level gauge. It can be 120 mm for a boiler of 180 bar drum pressure. Mounting/ insulation of measurement legs on which these devices are mounted will have considerable bearing on measurements of Drum level vis-a-vis actual level in the drum. The direct mounted devices will read always lower level compared to compensated DP Transmitter measurement.

107.Whether the drum level between left and right will be same on load? If different what is the reason?

There will some differences between Left and Right side Drum level measurements primarily due to unbalanced evaporation between side walls. This is caused by shift in fire ball in a corner fired boiler or unbalance firing of burners with respect to drum center. It can be to the extend of 50 mm.

106.How to know reverse flow in the case of GR Fan and how to handle it?

A reverse flow of flue gas in the GR system will produce high temperature at the outlet of GR duct after the fan.

A shut of damper as close as possible at the inlet to furnace will stop the reverse flow. However during operation of GR fan a positive feedback from the shaft of fan end will ensure no reverse flow in the GR system. This is a must when hydraulic coupling or equivalent methods are used to control the speed of the fan.

105.What is Re-heater? How to control re-heat outlet steam temperature?

In large boilers, to increase the efficiency, the steam from the HP cylinderr outlet  will be taken to the boiler again and passed through the set of heating coils and used at the MP cylinder. This heating coil is known as re-heater.

In a wall fired boiler the flue gas from Eco-inlet will be pumped through a fan to the furnace. This is known as gas re-circulation. This way the flue gas mass will be more than the steam flow. This will help to increase the temperature pickup in the re-heater. In this case the re-heater pickup may be mainly by convection.

Some times flue gas will be bypassed over the re-heater.

In a corner fired boiler, burners will be tilted up or down to control re-heat temperature. In this case the Effective Projected Radiant Surface (EPRS) of the re-heater is changed. In this type of arrangement, re-heater coils are in such a way that the effective heat pickup is by radiation.
Two methods are in practice: Reheat Spray control, Burner tilt/Bypass damper/Gas re-circulation control systems.

104.Assume that there is a load reduction in the Unit. The control system has changed to manual. How you will control, Boiler Master, Drum Level and Steam Temperature control?

Boiler Master
     Check/adjust Boiler Master demand proportional to the steam flow.
Drum Level
      Check/adjust Feed water flow nearest to the steam flow.
Steam Temperature
      Check/adjust super heater inlet temp nearest to that corresponds to steam flow.

103.What are the control final elements used?

Control Valves (pnuematic, Hydraulic and electric actuator), Power cylinders, Variable speed controllers are the main final control elements used.

102.What are the feedwater heaters in the system and what is their function?

Feedwater heaters are nothing but heat exchangers between BFP and Boiler drum. These heaters will impart heat to feed water to prevent large temp differential between water in boiler drum and the in-cpminf feedwater. Large temp difference would result in water hammering. these heatersoving efficiecy of overall thermal cycle.

101.What is the function of De-aerator? What is the operating pressure?

De-aerator helps in removing dissolved O2 in the condensate thereby preventing possible corrosion inside the pipelines. Normally it is maintained at Positive pressure sufficient to maintain NPSH for BFP at all loads. Normally, for 210 Mw plant,the Dearator will be maintained at 1.5--3.5 bars.

100.What is the condensate pump? What is the critical point of NPSH for the pump and how to handle it?

Condensate pumps help circulate water (condensate) from Condenser thru LP heaters to De-aerator. Net Positive Suction Head (NPSH) is the minimum Head (pressure) required at the suction of the Pump to function properly. The NPSH can be maintained by a combination of the Pressure and the Level at the vessel from which the pump is taking its suction from.

99.What is the function of cooling water? How many types of cooling systems are there?

A circulating water system at atmospheric temperature is used to remove exhaust medium temperature is Cooling water system.

Condenser cooling water system and Auxiliary Cooling water system are the 2 main cooling water systems in use in a power plant.

Condenser cooling water system is a once through system when sea water is used for cooling. This is the most common practice in the world. However river water with cooling towers are also used in inland plants.  Natural cooling tower and forced cooling tower are two types.

Bearing lub oil, Turbine operating oil, lub oil, etc are cooled in a tubular heat exchangers using a closed circuit cooling water system. This cooling water is mostly raw water used for water treatment plant.

98.What is the function of condenser?

Condensers are used to remove Heat from Steam and thereby convert steam into water. Normally this is at the steam exit of Turbine.

97.What is the function of Air-ejector? How many types are there?

Normally Ejectors are used for creation of vacuum in a vessel. There are two types--Air ejectors and Steam ejectors.

96.How to control NOx emission?

Keeping the flame temperature less than 950 Deg.C will reduce the NOX emmission. The achieve the above condition the O2 in the air can be reduced to 18% by adding flue gas from economizer outlet in the wind-box. Alternately air can be admitted at the top of the flame. 

95.How to reduce SOx emission?

By using fuel with lower sulphur content and also by installing de-sulphurisation plants to remove sulphur from flue gas at chimney.

94.What are the chemical parameters shall be monitored at the chimney exit?

CO, CO2, SOx, NOx, dust burden & smoke density.

93.What is the function of Chimney and its principle?

The main function of chimney is to dispose the flue gas at a high level so as spread it to a wide area and thereby reducing the pollution.

92.What is the Smoke Density and what is its regulation limits and operation limits?

Flue gas consists of large quantity of CO is smoke. When light can pass through the flue gas fully, the smoke density is 0%. When no light pass through the flue gas, smoke density is 100%.

Smoke emission of 20% in the chimney is allowed. However a good practice is to operate a boiler within 0 to 5% smoke density in the flue gas.

Operation of a boiler at more than 40% for more than 5 min in a day is an offense and subject to penalty by Government. 

91.Explain the principle of EP and their components? What may the reason for low current and High voltage and High current and Low voltage? How to set the operating condition manually?

Two Electrodes at high potential difference of DC supply will be build in the flue gas path after air heater before ID fan. The emitting electrode is a thin wire. The potential strength around wire will ionize the gas. This is known as Corona. The ionized gas will be with Electrons and positive ions. The collecting electrode are plates and provide large area.

The electrons are traveling from discharge electrode towards the collecting electrode at a high velocity. ( Normally the gap is 250 mm).The flue gas consists of dust is passing through the electrode at a low velocity of 1 m/sec or less. The dust particles are bombarded by  the electrons and attached to the dust. The dust will be now charge –vely. They are attracted by collecting electrode plates. The velocity at which the dust is moving towards collectors is known as migration velocity.

Migrations velocity will be more than the gas velocity.  This bring the dust to the collectors. There they are attached to the plates. The efficiency of a zone of ESP will be around 90%.

Discharge electrodes, collecting electrodes, Insulators, high voltage transformer/rectifiers, hammering systems for the electrodes and  controllers are some of the main components of an ESP.

Low current and High voltage is caused by the deposit of fine dust on discharge electrode.

High current and low voltage is caused by deposit of coarse dust on the collecting plates. High resistive dust will make the condition worst in this case. There will be corona near collecting plated when high potential strength is established by the dust deposit. This phenomenon is called Back Carona. The electrons from this corona will combine with the positive ion from the main corona and similarly the positive ion from back corona will combine easily with electrons from main corona. The dust will escape from ESP to chimney.

When control is on Manual, increase the current until there is spark. Leave at a level where 2 to 5 sparks/min is occurring.

90.What is the function of dust collectors and what are the types of dust collectors are there? What are the limits of the precipitators? What is the environment limits?

Ash and unburned carbon will leave the furnace along with flue gas. These dusts shall not be left to chimney. To control pollution in the atmosphere, the dust shall be collected by some means before going to chimney. The equipment serves this function is dust collectors.

Mechanical separators, Electro static precipitator and bag filters are some of the dust collectors used commercially in boilers.

The finesse is the limit in the case of mechanical separators .

Electrical resistance is the limit for ESP

40 mg/M3 of dust burden is accepted in many countries. However dust burden specified is 20 mg/M3. 

89.What are the main parameters shall be controlled during start-up of turbine?

Pressure and Temperature are the steam parameters to be matched with turbine metal temperature.

88.What is the function of Circuit Breakers and what are the types and their applications?

A device used to close/Open a electrical equipment to the power supply is called

Circuit Breaker.

They are named by the cooling or arc quenching media used in the device. When an equipment is disconnected from a power supply, the potential strength will be more. This will produce heat in between the surfaces and cause the media to ionize. This is spark. If the spark continues  and it will be called as Arc. The heat energy will damage the device.

To reduce the effect of heat generation, the area provided to the breaker  will be increased. The time to separate the equipment from power supply will be minimized by using spring. However the arc can be controlled by cooling or using media which can withstand high potential strength.

       Air, Oil and SF6 gas are some of the quenching media used in a circuit breaker. The breaker is named based on them.            They are Air Circuit Breaker (ACB), Oil Circuit Breaker (OCB) and SF6 Circuit Breaker

87.How to test and maintain Battery?

By periodically measuring End cell voltage and  the density of Acid to be within limits. Acid Topping up is needed if density goes low.Nowadays, Maintenance free batteries are available, whose life is 10--15 years. 

86.What is UPS and what is its function and what are their components?

UPS is Un-interruptable Power Supply System. The main componenets are Charger, Inverter and Battery bank. Normally,these are used to provide power supply for limited period to Control and Instrumentation systems during Main AC power failure.

85.What is Jacking oil pump?

A separate high pressure oil pumping system is designed to facilitate Baring operation and the Pump is known as Jacking oil pump. This pump is switched on both at the the beginning of Baring operation.

84.What is baring gear?

Normally, the TG rotor is a heavy mass and rests on the bearings, when no steam is admitted to Turbine. In order to prevent metal to metal contact/erosion during initial steam admission, the rotor mass is lifted (Jacked up) slightly away from bearing metal thru thin film of high pressure Oil.Baring Gear is the motive force to Rotate the TG rotor in slow speed. This is needed to faciliate easy rotation of TG on steam admission. The TG rotor is also put on barring on Turbine stop to assist in uniform cooling of rotor mass until the turbine metal reaches the room temperature.

83.What is the function of Exciter and how many types of exciters are there?

Exciter is the device which is providing the supply to provide field current of a generator.

Mechanical with a normal DC generator

Static Exciter – Provide necessary supply from the AC supply by rectifying by using Thyristors.

82.What is the function of AVR and AQR? How to maintain the export power factor?

AVR – To maintain the terminal voltage of a Generator

AQR – To maintain the Power Factor

Using Unit Transformer tap change in conjunction with AQR, the power factor of the export power factor can be maintained.

81.What is synchronizing? How to synchronize?

Connecting an incoming power supply to the grid or existing system is Synchronizing.

Frequency, phase angle and magnitude of voltage shall be same for both incoming and running power supply. Normally an indicating instrument will be provided to compare them. When they are satisfied the meter will be at 12 O’clock position. The breaker shall be closed at this condition.

80.How many types of over speed trips are available for turbine and what are the setting?

Electrical Over speed protection

Mechanical Over speed protection

Trip setting for Electrical is 1.05 times the rated speed and that of Mechanical is 1.1 times the rated speed.

A PID speed control will be provided as over-ride to the load control. A Troop control also will be provided with 0.25 Hz dead band. The normal troop setting will be 4 to 8% . The range will be upto 20%.

Troop control is a proportional control. Example 4% troop will produce a 100% output with a deviation 2 Hz. Assume a Unit is running at full load with GOV demand as 95%. The GOV demand will be 0 at 51.9 Hz.

In addition to avoid a turbine tripping due to over speed, Anticipatory Load Schedule logic will be provided. In such case during a load reduction more than 25%, the electrical and thermal load will be compared and if thermal load high after 2 sec, the GOV will be demanded to close fully for a few second and release to the position required by the electrical load before closing.

79.How to check the Generator and Unit Transformers?

Generator is tested by the above test.

Unit Transformer

1. Oil centrifuging
2. Open Circuit Test
3. Short Circuit Test

Unit transformer is checked by back charging. However some companies will nto agree for back charging. They are tested using Generator by slowly increasing voltage.

78.What are the pre-commissioning activities of Generator?

1. Excitation
2. Open Circuit Test
3. Short Circuit Test
4. Synchronizing

77.What are the pre-commissioning activities of a steam turbine

1. Steam blowing of pipes
2. Oil flushing
3. Vacuum test
4. Initial rolling
5. Trial rolling

76.What is SCAH? And how many types of them in the market?

As explained above the air will be pre-heated before admitting to rotary air heater using steam. This heat exchanger is known as Steam Coil Air Heater (SCAH).

Coil type and vertical tube are the two types used in the market. Coil type is using simple instruments and control. But efficiency is low. The Trap used at the outlet of steam can only extract the latent heat from the steam. The steam pressure will be 10 to 14 bar. The exit temperature will be around 200 Deg.C.  In a vertical tubular type the water is maintained in the tubes and the heat can be extracted as low as 70 to 100 Deg.C.

75.What is the need for an Airheater? How many types are there? What is cold end corrosion? How to protect from cold end corrosion?

If the heat input from fuel is extracted as much as possible, the efficiency of the boiler will increase. The flue gas temperature at the outlet of economizer is around 300 Deg.C. The process media, which is less than 300 Deg.C., is air used for combustion. The heat ex changer used to extract the heat from flue gas after economizer to air is called Air heater.

Two types are in the market. One is Tubular and the other one is Rotary air heater. Tubular air heater is used in small boilers. Rotary air heaters are used in large size boilers above 110 MW.

The average temperature between the exist flue gas and entering air is the temperature at the exit of air heater. This end is called cold end.

If the cold end temperature goes below sulphur dew point, suphuric acid will form on the metal. This will cause corrosion of the metal. The metal shall be protected from this possible corrosion.

One method is using non metallic material at the cold end. Rotten iron and Ceramic are used in rotary air heaters. Another method is to keep the cold end temperature higher than sulphur dew point. For this purpose, the air will be pre-heated by steam before admitting to rotary air heater. Both of them are practiced in majority cases.

74.What is economizer? How to protect the economizer from evaporation?

To increase the boiler efficiency, the feed water is heated further using flue gas. This section of heat recovery coils are known as Economizers.

Main criteria are that the steaming shall be avoided for smooth flow of feed water in to drum. Otherwise hammering will take place in the feeder tubes.

The flue gas temperature at the exit of economizers shall be maintained well above Sulphur dew point for the fuel in use.

A feeder pipe with non return valve and an isolating valve (for maintenance) will be connected at the bottom header of the economizer. This keep the water level in the economizer will be same as that of drum when there is no feeding from boiler feed pump. This will avoid evaporation in the economizer.

73.What is super heater and how many types are there?

Additional heat is added by increasing the steam temperature through bundle of tubes.

These are Super heaters.

There are two kind of super heaters used in a boiler. Convection types are placed in the second pass and heat transfer will be from flue gas by convection.

The exit coils will be placed at the top of furnace. Heat transfer will be mainly by Radiation. This is the Final super heaters.

When the tubes are staggered horizontally, between the two headers, they can be provided with drains. This is known as drainable super heaters. This arrangement is practiced in oil/gas fired boilers. In a coal fired boiler, they are placed vertically and known as Platen super heaters. The condensate in the bottom of each coil shall be made to evaporate to allow passage for steam. This is achieved by slow firing with vent open. The starting time will be more in this case before closing vents.

72.What is steam separators and how many types will be there in a drum?

The steam from the raiser tubes will be mixture of steam and water particles. This is known as wet steam. Steam shall be separated from water particles. The devices used are called steam separators.

This functions are done in the drum. The steam from raisers ate taken back to drum and there it is made to pass through the separators.

Centrifugal type of buckets of many in number will be provided. This is Primary separators. The steam will be passed through radiator kind of separators before leaving the drum at the top. This is known as Secondary separators.

71.What is the function of Bucks- stay in a boiler?

The furnace will expand in all the 3 axis. To keep the shape it will be fixed at the middle of each side by a strong beams and moving connections will be provided at the corners. This frame or structure is called Bucks-stay

70.How many place the expansion will be checked?

Drum, in the furnace at least 3 places, burner area and at the end.

69.What is the boiler expansion? How much will be the expansion of a boiler at burner level?

Boilers will expand when subject to heating. To allow the expansion, they will be either supported from top by hanging or from bottom.

As a thumb rule, for every 100 Deg.C. per meter, the expansion from fixed support, will be 1.5 mm.

A 800 T/H capacity boiler, will expand to a maximum at the end will be 150 mm.

68.What is electrical relief valve and how to set up?

Electrical relief valve is used in M.S  line to relieve the excess pressure,

thereby reduce  the frequency of lifting the M.S line S.H safety valve.

The ERV is set for  lifting  electrically   (by solenoid ) to operate below

 the   set pressure  of  super heater safety valve.  

67.How to set a safety valve? What is Lift and Blow down? How to adjust them?

Raise boiler steam pressure. Near set pressure, lift the stem manually by using rope on the hand pop level. Increase pressure and confirm that the valve is opening at the required set value. Start from the highest set pressure valve. Jack close the low set pressure valves.

Confirm blow down. It shall be between 2 to 5%.

Lift : The amount of  raise  of  stem  while lifting is called  “lift”.

Blow down  : The difference  between set pressure & reseat pressure

                       Expressed  in Æ  percentage of set pressure.

Adjustment :

          Lower ring adj :- To set a clear popping, to close slowly. (To avoid

                                      Simmering & chattering ).

                                      To  reduce simmer,  move the lower ring  upward

                                      ie.  Anticlockwise direction.

                                      1 notch = 42.2ksc.

        Upper ring adj. :-  To adjust blow down  .

                                      To reduce blow down,  move the upper ring upward,

                                       ie., turn anticlockwise.

                                       To increase  blow down , move the ring downward

66.How many safety valves shall be provided for a boiler and where?

The following are the main safety valves typically in a boiler :                                                                             

Boiler drum (L) - 2 nos                                                                                                                           

Boiler drum (R) - 1 no                                                                                                                         

Super heater outlet (R) - 1 no                                                                                                                 

Reheater Inlet (L) - 1 no                                                                                                                               

Reheater Outlet (R) - 2 nos                                                                                                                

Reheater Outlet (L) - 1 no                                                                                                                    

Super heater Outlet (L) - 2 nos

65.What is safety valve and Relief valve?

A valve provided on any pressure vessel which will open to release the pressurizing media when the pressure exceeds the set value.

Safety valve will open at a preset value and sit again at a fixed reset pressure.

Relief valve also will open at set value and sit when the pressure is less than set value.

64.What is steam blowing? Why it is necessary? How many type of steam blowing are in the practice? What is deciding factor for the completion of steam blowing and who shall decide the completion?

A process undertaken during initial plant installation, to remove any mill scale in SH tubes and pipes by blowing steam and venting out the same is known as steam blowing. Continuous and Puff Steam blow methods are used. In continuous method the boiler shall be prepared for full load and the steam at full load flow, pressure and temperature shall be blown for some hours. In Puffing method, pressure in the boiler will be raised to 250 Deg.C, at around 40 bar, and the stop valve in the temporary pipe is opened quickly. This will create a sudden change in velocity in the pipe. It is claimed that this the best economical method. A target steel plate is put at the end of the pipeline sections and the same is observed for the indentations after the steam blow to establish the cleanliness of the steam pipes. Since the steam admitted to turbine should not contain any contaminant (damages the turbine internals), the Turbine commissioning in-charge will give clearance for the completion of satisfactory steam blowing operation.

63.What is the procedure for Alkali-boilout, Acid cleaning, passivation, etc.?

Alkali boil out - It uses Alkali such as NAOH, CACO3, etc. The purpose is to remove grease in the tubes. The boiler will be pressurized to 20 Kg/cm2 and kept for 24 to 48 hours. Blow-down will be opened for 1 min for every 1 hour. End point is no traces of grease in the effluent.

Acid Cleaning – It uses diluted Sulphuric acid. The purpose is to remove loose mill scales in the boiler tubes. The end point is the saturation of iron in the mixture or 2 hours, which one is early. Sometime acid cleaning at very mild concentration will be applied for old boiler to remove any scaling in the water wall tubes.                                                                                 

Passivation – The reaction of acid shall be cleared after acid cleaning. For this purpose, the mixture Ammonia phosphate at 200 ppm and Hydrazine at 200 ppm will be left to evaporate at 20 Kg/cm2 for 24 hours.

The effectiveness will be confirmed by the colour on the coating tube inside.
All chemical cleaning are applied for water wall tubes. SH shall be protected during Acid cleaning

62.What are the chemical cleanings for a boiler and end point for them?

Alkali boil out - It uses Alkali such as NAOH, CACO3, etc. The purpose is to remove grease in the tubes. The boiler will be pressurized to 20 Kg/cm2 and kept for 24 to 48 hours. Blow-down will be opened for 1 min for every 1 hour. End point is no traces of grease in the effluent.

Acid Cleaning – It uses diluted Sulphuric acid. The purpose is to remove loose mill scales in the boiler tubes. The end point is the saturation of iron in the mixture or 2 hours, which one is early. Sometime acid cleaning at very mild concentration will be applied for old boiler to remove any scaling in the water wall tubes.                                                                                 

Passivation – The reaction of acid shall be cleared after acid cleaning. For this purpose, the mixture Ammonia phosphate at 200 ppm and Hydrazine at 200 ppm will be left to evaporate at 20 Kg/cm2 for 24 hours.

The effectiveness will be confirmed by the colour on the coating tube inside.
All chemical cleaning are applied for water wall tubes. SH shall be protected during Acid cleaning.

61.Can it be possible to open the inlet vane of an IDF during trial run and why?

It may not be possible. The fan may be over loaded. A fan handle volume. When the inlet vane is fully open, the volume of gas/air will be the maximum. ID Fan handles flue gas at 120 to 140 Deg.C. The density of gas will be less compared to air

( Air density - 1.29 Kg/M3 at 15 Deg.C) . The load of motor is proportion to mass of the gas. Hence the motor may over loaded. Care shall be taken during trial run of ID Fan. The inlet vane shall be opened to the rated current. 

60.How to operate the SADs in a wall fired and corner fired boiler?

SADs will be opened fully in wall fired boiler. The air flow to each burner in service will be always equal. Hence unbalanced loading of fuel between mills shall be avoided except starting. In a corner firing, the fuel air damper will be function of mill load. The AAd will maintain the Wind-box/furnace differential pressure constant at 40 mm WCL before any mill is in service and at 180 mmWCL after any mill is taken to service

59.What are the Secondary Air Dampers and how they differ between wall fired and corner fired boilers?

The damper controlling the combustion air to fuel at the burner is known as Secondary air. The air which is used to transport fuel to the burner is Primary air. Normally secondary air will be admitted very close to the fuel. In a wall fired boiler the secondary air will be spinned around the fuel spary. If the secondary air axial velocity component is greater than flame propagation velocity, then the flame will become unstable. Hence the excess air will be supplied at the outer circle. This air is known as Tertiary air. In a corner fired boiler, the secondary air is known as Fuel air, the damper is called FAD. The air will be admitted axially at the bottom and top of the fuel nozzle and will rotate at the peripheral of the fire ball. The excess air will be admitted at he next  level of each FAD outlet and called Auxiliary air and damper is called AAD.

58.How to categorize the type of boilers?

 a) According to the use – Utility  Boiler, Processing type Boiler.

 b) According to the tube content -  Fire tube , Water tube Boilers.

c) According to the furnace  position – Externally fired & Internally fired boiler.

d) According to the circulation  - Natural

e) According to furnace pressure – Balanced draft & pressurized & Forced circulation.

57.What is the Gasket of valves? Name the type of bonnet gaskets and gland gaskets for valves?

The soft material used between the body and the bonnet of a valve is called gaskets.Asbestos reinforced Paper, rubber, metallic gaskets (Spiraket gaskets), etc.

56.What is mechanical seal?

For any pump, there will be glands between the rotor and the body at the end before bearing. This is to avoid leak along the shaft. In a high pressure pumps, instead of asbestos gland ropes, a self lubricating material made of carbon and iron will be used. This is called Mechanical seal.

55.Explain the procedure of alignment of a rotating equipment? What is reverse and soft foot alignment?

Two rotating shaft shall be aligned  such that the shear forces and bending moments at the coupling are zero.

Final alignment of rotors are obtained by adjusting the bearings ( for turbine rotors) and by adjusting the motor for Pumps, fans, etc to give parallelism and concentricity at each pair of couplings.

The alignment is carried out by measuring the face gaps and periphery errors between couplings.

Gap measurements, using suitable gauges, are taken at the top, bottom and two side positions and a repeat set taken with both shafts rotated through 180 deg. The average of the two sets indicates the true parallelism of the coupling faces, eliminating any errors due to out of truth between couplings and shaft.

Concentricity is checked by measuring between the peripheries of the two couplings, using either dial indicators or feeler gauges, to a finger attached to one of the coupling. To eliminate coupling errors, both shafts are turned together and readings taken at the top, bottom and two side positions.

Calculations using ratios and similar triangles, taking account of shaft length, distance between bearings and coupling diameter, determine the adjustment required at the bearings to obtain shaft alignment.

With solidly –coupled shafts, it is usual to work to very close tolerances of the order of plus/minus 0.025 mm.   

54.Explain the balance drum of a BFP?

Normally the differential pressure between discharge and suction sides of Boiler Feed Pumps are quite high. Due to this, the rotor section experiences very high trust in one direction. The movement of rotor (Axial shift), if it exceeds manufacturer's tolerances, will result in rubbing of mechanical seals and damage the same. A Balance Drum is created by interconnecting suction and discharge sides suitably and the wear out of seals can be inferred by measuring Balance drum pressure.

53.What are the trip commands for a pump?

Typical Trip interlocks for Pumps are: Bearing Temp OR Vibration High,Lube oil press Low, Suction valve Closed , Electrical switch gear faults.

52. What are the trip commands for a Fan?

Typical Trip interlocks for fans are: Bearing Temp OR Vibration High,Lube oil press Low,  Electrical switchgear  faults.

51.What is partial loss of flame?

In a wall fired boiler when a flame scanner sense no flame, the respective burner shut-off valves are demanded to close. If they are failed to close after 10 sec. it is announced as Partial Flame loss.

If the number burners are less than a number which can cause unsafe condition, MFT will be initiated. Sometimes a load factor less then 75% is also ANDed for MFT.

Conservatively some plants are initiating MFT for Partial Flame loss irrespective of any condition of the boiler.