HRSG BOILERS

Introduction of BOILERBy Nadeem khanWhat is Boiler?A closed vessel in which water or other liquid is heated, steam or vapor is generated, steam is superheated, or any combination thereof, under pressure, for use external to itself, by the direct application of energy from the combustion of fuels, from electricity or nuclear energy."Types Of BoilersGenerally there are two types of boilers:

Fire Tube boilers

Water tube boilers

Fire tube BoilerA fire-tube boiler is a type of boiler in which hot gases from a fire pass through one or more tubes running through a sealed container of water. The heat of the gases is transferred through the walls of the tubes by thermal conduction heating the water and ultimately creating steam. The operation upper limit for fire tube boiler is around 14bar.

Water tube BoilerA water tube boiler is a type of boiler in which water circulates in tubes heated externally by the fire. Fuel is burned inside the furnace, creating hot gas which heats water in the steam-generating tubes. In smaller boilers, additional generating tubes are separate in the furnace, while larger utility boilers rely on the water-filled tubes that make up the walls of the furnace to generate steam, the heated water then rises into the steam drum. Here, saturated steam is drawn off the top of the drum and steam will reenter the boiler through a super heater to become superheated. Such water tube boilers in thermal power station are also called heat recovery steam generating units. Water tube boilers are capable of operating even in supercritical pressures.

Heat Recovery Steam Generator BoilerA heat recovery steam generator or HRSG is an energy recovery heat exchanger that recovers heat from a hot gas stream. It produces steam that can be used in a process or used to drive a steam turbine or a boiler that captures the exhaust from a natural gas or diesel engine, a combustion turbine, or other process and uses it to generate steam or it is used to recover the energy produced by hot exhaust gases in power generation or other thermal process and then uses this energy to produce steam that can then be used in other processes like that of driving a steam turbine. There are a few variations on the standard design. Some include supplemental or duct firing, which provide more energy, which in turn produces more steam. Other variations include diverter valves to control the inlet flow, or emissions controls.

Boiler ThermodynamicsHeat TransferThe heat energy from fuel is delivered to water / steam inside the boiler by three methods:Radiation: Radiation does not require a transmission medium. It travels like light waves through vacuum and air. The most common example of the radiation is the heat we feel from the sun.(2)Conduction: Conduction is the transfer of heat from a warm molecule to a cooler one. Some materials conduct heat better then others. For example gases and vapors are poor conductors, liquids are better and metals are best. Materials that are poor heat conductors are called insulators, heat travels through insulator but at slower rate.Convection: Convection hest transfer takes place by movements of the heated materials itself. In a heated room warm air rises and cold air falls. In a boiler the hot water rises and the cold water falls to the bottom.

Three forms of heat transfer work together in the boiler. Evaporator and Economizer receive their heat by Radiation from hot flue gases and burner flame (IF ANY), by convection from the hot flue gas travelling over the surface and water / steam travelling inside the tubes and by conduction through the tube metal and onto the water. Steam & Types of Steam SteamSteam is favorable for power generation because of:a) Its higher thermal capacityb) Its wide availabilityc) Its non toxic natureWet steamMixture of liquid and steam of the same substance in which both are at saturation temperature. Saturated SteamIf additional heat is added to the wet steam at constant pressure, the temperature remains constant until all liquid is evaporated (saturated steam);Superheated Steamit is only at this point that the temperature increases above the saturation temperature (superheated steam).Flash SteamWhen hot water at its saturated temperature is released to a lower pressure Flash steam is produced.

Steam Generation Step in BoilerEconomizerPreheats water close to saturation temperature (Sensible Heat input).EvaporatorChange phase from water to steam (Latent heat input). Super HeaterAdditionally heat saturated steam (Sensible heat input)

Steamwater separation There are three Types of Separation:Natural gravity: Can be applied only low pressure and very low steam generation rates. Its difficult to have an effective separation with this type.Baffle assisted gravity: Can be applied for low pressure and low steam generation rates with better separation because of addition of baffles and perforated plates which act as changing mixture direction, maximize mixture travel path.(Cyclones) Mechanical separation: Is the steam / water separation by means of centrifugal force, the force that tends to move objects outward from the centre of rotation. The steam/water mixture isdirected to a series of cyclones. Water is thrown to the side by centrifugal force and return back to the drum while steam travel on the boiler outlet. In high capacity boilers mechanical separation in the most effective way to separate steam from water.Besides the above mentioned separation methods which are considered to be the primary separators, drum also contain secondary separators or scrubbers. Steam leaving the primary separator passes through scrubbers or perforated plates in order to reduce flow of small water droplets if still exists in steam flow.Boiler Main PartsBoiler DrumDe-aeratorEconomizerBoiler Blow downFlash & Blow down tanksDrainsVentsPressure Safety ValveAttemperatorFeed water pumpBoiler Internal water treatment

Boiler Drum functions:

(a) The primary function of drum is water / steam Separation by natural gravity, Baffles plats and mechanical mean Cyclones.(b) Mix new coming feed water from economizer with the saturated water remaining inside drum.(c) Mix the corrosion control and water treatment chemicals with the saturated water inside the drum.(d) Remove part of water which contains solids to control boiler water chemistry (Blowdown).(e) Provide limited water storage (Retention), to accommodate for rapid changes in boiler load.

De-aerator & its function:Oxygen in the boiler causes corrosion in the deep pits. De-aerator works on the principle that hot water holds less dissolved gas then cold water, heated water in a pot has bubbles that form under the surface long before the water boils. These are dissolved gases coming out of solution as the water warms up.Function:Removal of dissolved non-condensable gases (air) such as oxygen, nitrogen and carbon dioxide. Heating of feed waterStorage of feed waterEconomizerThe flue gases having passed through the main boiler and the super heater, it will still hot, the energy in these flue gases can be used to improve the thermal efficiency of the boiler to achieve this the flue gases are passed through an economizer. The economizer is a heat exchanger through the feed water is pumped. its a mechanical devices intended to reduce energy consumption, or to perform another useful function like preheating a fluid.Boiler Blow downBoiler blow down is the removal of water from a boiler. Its purpose is to control boiler water parameters within prescribed limits to minimize scale, corrosion, carryover, and other specific problems. Blow down is also used to remove suspended solids present in the system. These solids are caused by feed water contamination, by internal chemical treatment precipitates, or by exceeding the solubility limits of otherwise soluble salts. In effect, some of the boiler water is removed (blown down) and replaced with feed water. The percentage of boiler blow down is as follows: quantity blow down water /quantity feed water X100 = % blow down There are Two Types of Blow down

1) Manual or Intermittent blow down2) Continues blow downManual or Intermittent blow downIntermittent manual blow down is designed to remove suspended solids, including any sludge formed in the boiler water. The manual blow down take-off is usually located in the bottom of the lowest boiler drum, where any sludge formed would tend to settle. 2) Continues blow downContinuous blow down, as the term implies, is the continuous removal of water from the boiler. It offers many advantages not provided by the use of bottom blow down alone. For instance, water may be removed from the location of the highest dissolved solids in the boiler water. As a result, proper boiler water quality can be maintained at all times. Also, a maximum of dissolved solids may be removed with minimal loss of water and heat from the boiler.Flash & Blow down tanksSince the blow down is operated continuously considerable amount of heat can end up down the drain. In order to recover the energy content in blow down water the hot water is discharged into flash tank, which is at a lower pressure then the boiler. Some hot water flashes off as steam, which can be used for heating or process systems or in the De-aerator as pegging steam. The remaining dirty water then flows into the blow down tank, which reduce the temperature and pressure of the water and routes the water into the sewer. Drains and VentsDrainsAll boilers must be equipped with drains complete with valves connected to the lowest points in the water space so that the boiler can be emptied.VentsAll boilers must be equipped with vents complete with valves connected to the highest points, so that air can be removed while the boiler is being filled with water. This is to prevent air or non-condensable gases from entering to the system when the boiler is put on line. The vent valve is also opened when the boiler is being shut down or emptied in order to prevent vacuum formation. Pressure Safety valve & relief ValvePressure Safety valve (PSV)A safety valve is a valve mechanism for the automatic release of a substance from a boiler, pressure vessel, or other system when the pressure or temperature exceeds preset limits.Pressure Relief Valve (PRV)The relief valve (RV) is a type of valve used to control or limit the pressure in a system or vessel which can build up by a process upset, instrument or equipment failure, or fire.Difference Between PSV & PRVpressure safety valve is automatic relieves the static pressure on a gas, usually it opens completely while relief valve relieve the static pressure in a liquid filled ,its opens proportionally with increasing pressure.PSV is used in case of gas and PRV is used in liquid.Attemperator & Feed water pumpAttemperatorThe attemperator is an apparatus for reducing and controlling the temperature of a superheated fluid passing though it. This is accomplished by spraying high purity water into superheated steam pipe either inside the super heater interconnection or outside as externally.Feed water pumpFeed water pump coverts the velocity (kinetic energy) of the moving water into the discharge pressure (potential energy) by means of rotating impellers inside stationary vaned diffusers. The flow from the vaned diffusers is collected in a circular casing and then discharged through the outlet pipe. Several stages of impellers and diffusers can be bolted to each other in order to obtain higher discharge pressure. The discharge pressure can also be increased by increasing the Speed (RPM) or by increasing impeller diameter.

Boiler Internal water treatmentInternal water treatment is applied inside the boiler and is used to prevent the corrosion and scale formation. The most common method of internal control is the use of chemicals like phosphate to remove scale formation. This chemicals change the calcium carbonate and magnesium carbonate into a precipitate or sludge. Other compounds need to be added to keep this slug from sticking to metal surface. The sludge is then removed by drain, for the reaction to proceed additives like caustic soda may be required to maintain PH level of the water in required range.EPQL HRSG BoilerOverviewThe HRSG is the NG-BS109-R dual Pressure, with supplementary firing. Natural circulation boiler without Bypass Stack, which is design and manufactured by hangzhou Boiler Factory. Its matched PG9171E Gas turbine, and it is the main equipment of the gas-steam turbine combined cycle power plant. The total length from inlet duct expansion flange face to the out side edge of stack platform of HRSG is about 40meter, and approximate 15meter (Top platform) in width, the elevations of HP/LP drums center are 27meter and 26.4meter respectively, and the top elevation of stack is 60meter.

Heat surface ModulesModule # 1HP Superheater 2Module # 2HP Superheater 1BHP Superheater 1AModule # 3HP EvaporatorModule # 4HP Economizer 3HP economizer 2BModule # 5HP economizer 2AModule # 6LP SperheaterLP EvaporatorHP Economizer 1

Parameters of Gas turbine for BoilerGas Turbine Exhaust parametersa) Gas turbine fuelNatural gas / Light diesel oilb) Exhaust duct pressure of GT