Dear Readers, Welcome to Thermal Power Plant Interview Questions and Answers have been designed specially to get you acquainted with the nature of questions you may encounter during your Job interview for the subject of Thermal Power Plant. These Thermal Power Plant Questions are very important for campus placement test and job interviews. As per my experience good interviewers hardly plan to ask any particular questions during your Job interview and these model questions are asked in the online technical test and interview of many IT & Non IT Industries.
Thermal Power plant consists of four main circuits, they are:
Steam power plant works on the principle of Rankine Cycle.
Thermal efficiency of steam power plant is defined as the ratio of heat equivalent of mechanical energy transmitted to the turbine shaft to the heat of combustion. Generally Thermal efficiency of the steam power plant will be in the range of 30-35%.
Overall efficiency of the system is defined as the ratio of heat equivalent of electrical output to the heat of combustion. Generally Overall efficiency of the steam plant will always be less than the thermal efficiency of the steam plant, it will be of the order of 29-33%.
Overall efficiency of steam plant is determined by multiplying the thermal efficiency of the plant with efficiency of the generator (electrical efficiency).
In Steam power station, more than 50% of the total heat of combustion is lost as heat rejected to the condenser and the loss is unavoidable as the heat energy cannot be converted in to mechanical energy with out a drop in temperature. Steam in the condenser is at lowest temperature. This is the reason that the thermal efficiency of the power plant is quite low.
Efficiency of the thermal plant depends on three factors, they are:
pressure of steam entering the turbine
temperature of the steam entering the turbine
pressure in the condenser
Thermal efficiency increases with increase in temperature and pressure of the steam entering the turbine. For this reason high temperature and pressure are used. Thermal efficiency is effectively increased by decreasing the pressure in the condenser, so pressure in the condenser is kept as low as possible.
Thermal efficiency also increases by reheating the steam between turbine stages.
Almost 50% of the heat generated is lost at the condenser as heat rejection. It is unavoidable as with out heat rejection it is not possible to convert heat energy into mechanical energy and drive the turbine without drop in temperature. Therefore majority of the loss takes place in the condenser. Thus efficiency of the thermal power plant is between 30-35%.
Thermal Power plant works on the principle of Rankine cycle.
The current carrying conductor cross section depends upon the magnitude of the current it is carrying and insulation strength of the conductor depends on the maximum voltage it can withstand. Therefore while designing the generator an optimum value is chosen between the amount of the current and voltage conductor can withstand.
Some of the methods by which the efficiency of the thermal plant can be improved are:
By increasing the temperature and pressure of the steam entering the turbine.
By reducing the pressure in the condenser.
By reheating the steam between different stages between the turbine.
The formation of air bubbles and water vapour on the water surface due to the reduction in the pressure is called "Cavitation". When the pressure on the water reduces below the saturation pressure corresponding to the temperature of the water, the rapid formation of the water vapour and air bubbles starts. The bubbles suddenly collapse with the violent action and this collapsing pressure will be very high. The rapid formation and collapsing of bubbles causes the pitting of the metal surface. Cavitation also reduces the efficiency of the hydraulic prime mover causing the honey-combing of runner and blade contours which reduces the power output.
In thermal power plant Boiler Feed Pump (BFP) is the largest pump, it delivers the water to the boiler.
Different type of boiler used in thermal power plants are:
Different types of Pulverizers are:
Pulverizers serve two purposes, to dry the coal and to grind the coal. They crush the coal to size of 74microns.
Coal handling and storage, coal pulverizes water treatment plant, steam boiler, Drought systems, Ash Handling systems, Steam turbine, Circulation water system, Electrical Systems, Control and Instrumentation, Pollution Control equipment, and Fire Protection System.
Hydrogen gas cooling is employed for large size generators because of better heat carrying ability of the hydrogen. Hydrogen cooling is provided for rotors and core of the generator. Water cooling is provided for the stator of the alternator.
For low voltage operation 415/220V vacuum circuit breakers or air break circuit breakers are employed. For voltage ratings about 6.6kV and beyond SF6 circuit breakers are employed.
MVA power rating of the Generator Transformer will be equal to the alternator. Hence lot of heat will be generated while generator transformer is under operation. Oil Forced Air Forced (OFAF) type of cooling is employed for generator transformer.
Faults in the windings, Over load protection, Over heating of windings or bearings, Over speed protection, Loss of Excitation protection, Motoring operation protection, Inadvertent energisation, single phase or unbalanced current protection, out of step operation protection, sub-synchronous oscillations protection and earth fault protection.
Major electrical equipment in thermal power plant are:
Difference between safety valve and relief valve:
Relief Valve: The relief valve is used on a filled vessel. For such a valve the opening is proportional to increase in the vessel pressure. Hence the opening of valve is not sudden, but gradual if the pressure is increased gradually. A relief valve is meant to relieve pressure to prevent an over pressure condition. A relief valve may have an operator on it to assist in opening the valve in response to a control signal. The capacity of PRV are generally small.
Safety Valve: This device is used to relief on a compressible filled vessel. For such a valve the opening is sudden. When the set pressure of the valve is reached, the valve opens almost fully. A safety valve is meant to relieve pressure without operator assistance and a safety valve, or combination of safety valves, must be have a capacity to relieve more than the energy input to the volume being protected.
Image of pressure relief valve:
The relief valve (PRV) is a type of valve used to control or limit the pressure in a vessel or system during an overpressure of the system. The primary purpose of a pressure Relief Valve is protection of life and property by venting fluid from an over pressurized vessel. Many electronic, pneumatic and hydraulic systems exist today to control fluid system variables, such as pressure, temperature and flow. Each of these systems requires a power source of some type, such as electricity or compressed air in order to operate.
A pressure Relief Valve must be capable of operating at all times, especially during a period of power failure when system controls are nonfunctional. The PRV is provided on the on the main steam line to control the line pressure. The PRV is generally operated through pneumatic pressure.The PRV operated according to the set pressure of rhe controller. The valve set down when the pressure reach to normal value.
This is also known as one way valve which allow the fluid flow in one direction only. As its name implies non return means there is only one direction flow.All the assembly are provided with a valve bonnet.This valves are generally used in pump discharges and steam water pipe lines.This device ensures there is no back flow of fluid from the source. It has two lines one is inlet line and other is outlet line.There is a direction symbol provided on the valve which show the direction of flow fluid.
Types of NRV:-
Tilting disc check valve -This is a disc type check valve.The disc provided fixed with a hinge which swing up and down. As the fluid flow in the line it lift by the fluid force and allow the fluid to flow in the pipe. When there is no fluid flow in the circuit it closes due to gravitational force.
Ball check valve - Here a spherical ball is used to fervent the fluid flow. The ball is spring loaded which close the valve when there is no fluid flow in the line. The pressure of fluid lift the ball and let the fluid to flow.
Diaphragm type NRV - This is consists of a rubber diaphragm clapper ,Which works on the differential pressure When the pressure in the upstream is more it open the diaphragm and allow the fluid to flow and when the pressure is equalize is closes diaphragm and prevent to fluid.
Stop check valve - It’s construction is same as the swing check valve except that here is a external control mechanism of handle or lever provided.
Lift check valve - Here a lift or disc is provided which operates on the working pressure of the fluid.
function of steam trap- It is a device is used in steam pipe line to discharge condensate and gasses, prevent to escape of steam through the line. It ensures that steam is not wasted. This is a self contained valve which automatically drains the condensate from a steam containing device. This is generally provided in steam drains or before steam drain valve.
Types of steam trap:
Mechanical trap - Works in the principle of difference in density between steam and condensate. This type of trap operate according the condensate level in the port if the condensate level increase the float operates and allow to condensate pass through the pipe ,when condensate level decrease is close the valve and doesn’t allow the condensate. Types of mechanical trap i) Float type, ii)Float with lever type, iii)Inverted bucket type, iv)Open bucket type.
Thermodynamic type steam trap - Works in the principle of difference in between thermodynamic property of steam and condensate.This types of traps operate due to velocity change in flow of compressible and non compressible fluids.As the steam starts condensate there is a temperature difference which allow the trap to operate. Types of Thermodynamic type steam trap i) Disc type ii) orifice type.
Thermostatic type steam trap - This trap works in the principle of difference of temperature in between steam and condensate. Types of Thermostatic type steam trap i) Bimetallic type, ii) Metallic expansion type.
We can control steam temperature by :
Using gas recirculation method-Hot flue gas is circulated for maintain steam temperature.
By providing excess air- By providing excess air it reduce the furnace temperature for some timing hence control temperature.
Burner tilting method- Here the tilting of burner are provided by a pneumatic cylinder. The burner can be tilted 30° up and down for control temperature.
Attemporation control-This is the best method for temperature control in boiler. Here the attemporator are provided to spray water in steam in steam pipe line. Ceramic thermal sleeves are provided to avoid thermal shock due to temperature difference of steam and spray water.
Elevation of fuel firing – The temperature is controlled by choosing upper or lower elevation of fuel burner.
The deposited soot act as a poor conductor of heat which reduce the heat transfer rate and increase flue gas temperature.
The deposits block the flue gas path which increase the drought loss.
The deposit may lead to corrosion.
Due to falling of large size of soot it damage the tube in dry ash conveying system.
It Is the carrying amount of droplet water in the steam, witch leads to carryover of Salt crystals on the super heater and turbines. priming may be caused due to sudden change of load and maintaining of high level of water.
Prevention of priming and foaming:
The best remedy for foaming and priming carryover is the proper blow down of TDS. The continuous blow down should be regulated to maintain the TDS at 3,000 to 4,000 ppm. More blow down means less TDS but more blow down is the more loss of the boiler so the blow down should be within control level.
It is the generation of foam In boiler due to high concentration of solids, organic matter, bubbles are build up on the surface of boiler water and passed out with the steam. This is generally caused due to high concentration of solids in boiler.
Thermal Power Plants can be operated near the load centers unlike hydro and nuclear plants.
Requires less space compared to hydro plants and cost of construction is less.
Running or operating costs are less compared to diesel or gas plants.
Can able to handle over load for certain period of time.
Emits green house gases and causes pollution.
Coal and Ash handling requires large area.
Efficiency is low.