Steam/Gas Turbines

Bleeder Turbine

(can remove moisture through lines) -steam is removed from the turbine at some intermediate point or points between inlet and exhaust -amount of bleed steam (~20% of total steam) is not usually controlled but it simply changes as the load on turbine changes -bleed steam usually used for heating purposes (FW heating) -pressure varies with load

Disadvantages of gas turbines?

-$$ -responds to load changes slowly -less suitable for low-power applications -efficiency low at lower loads (need improvers) -need cycle improvers to increase efficiency (cost increases) -complex design

Describe low NOx emission control for NOx reduction

-7ppm, use lean air/fuel mix compared to stoichiometric ratio -unstable combustion at low loads -use primary fuel nozzle at start up and low loads -use secondary fuel nozzle at full lean load

Describe the reaction blades:

-A reaction turbine has rows of stationary blades alternating with rows of moving blades. The stationary blades, due to their shape acts as nozzles. -moving blades, with the same basic shape as the nozzles, receive an impulse from the steam, but also produce a reaction force.

What are the two types of gas turbine bearings used and what type of gas turbines are they used?

-Anti-friction bearings (roller and/or baller) >aero-derivative gas turbines (lightweight rotors) -Radial bearings (journal or tilt-pad) >heavy-duty (heavy rotors)

What is the main oil pump?

-It is a pump that is directly driven from gears on the turbine shaft. -It is the main source of lube oil when the turbine is in operation -When the pump is insufficient (and during start-up/shut-down) two other pumps, driven by electrical motors are used.

Explain Thrust Bearings

-Main purpose: >keep the rotor in exact position in the casing >absorb any axial thrust on the rotor -the thrust bearing must be strategically placed on the shaft of the turbine to ensure that shaft is free to expand in one direction (normally located at inlet of turbine where blade clearance is most critical) -simple thrust bearings used on impulse turbines -Kingsbury thrust bearing uses a number of tilting pads attached to a stationary plate, mounted on the casing. Pads force against a second, rotating thrust collar, machined into or mounted on the shaft.

What are NOx emissions and how are they produced?

-NOx are NO and NO2 formed by the oxidation of nitrogen at about 2100 C (combustion) -formed during combustion from oxygen and nitrogen -rate of NOx formation decreases exponentially as temperature decrease -main way to decrease NOx is to burn lean >decrease in combustion T also promotes formation of CO (incomplete combustion)

What is power oil used for?

-Power oil, acting in servo motors, provide hydraulic pressure to open the emergency stop valves and governing valves -governor relay oil act as a regulating medium >both must be at high pressure

Describe some instrumentation for Gas Turbines

-RPM for each shaft -Air in Temp -delta P across intake filters -compressor discharge pressure -vibrations (bearings) -bearing T -fuel gas flow, P, and T -oil P and T -generator output

Summary of blade sealing in turbines

-Radial fins minimize leakage between blade tips and casing (rotating blades) -single knife used between rotating and stationary blades -double knife used between stationary blade tips and rotor in an reaction turbine

What is the impulse turbine operating principle?

-Steam at high pressure pressure expands through a stationary nozzle, the pressure of steam drops and the velocity of the steam increases -steam leaves the nozzle as a high-speed jet -some of the steam's momentum will be transferred to the the blade which produces an impulse force on the blade, causing movement. -impulse turbines are good for velocity, but lots of centrifugal force.

Describe the flow of steam through an reaction turbine:

-Steam expands first in the stationary blades, gaining velocity as pressure decreases. -it then enters the moving blades, where the direction of flow is changed, producing a force on the blades. -at the same time, the blade shapes reduces the pressure of the steam and the expansion creates a further force (reaction) on the blades

Explain stationary blade attachment on impulse turbines and the sealing used

-The stationary blades (nozzles) in an impulse turbine are mounted in diaphragms (consists of two halves) -diaphragms secured in casing by keys -nozzles fit in machined slots, in the circumference of the diaphragms.-labyrinth seals used to seal diaphragm and shaft

Describe throttling Governing

-a single valve at the inlet to the turbine throttles the steam flow into the turbine casing -directed to each nozzles equally -always used in reaction turbines because you need a constant steam admission to the full circumference of the blades

Oil Purification

-a small stream of oil is continuously removed from the bottom of the oil tank, which is pumped through a purifier filter (which removes dirt/water) then returned to the tank. -it also uses a centrifuge (rotation removes water out of oil)

Advantages of gas turbines?

-ability to produce large amounts of power -high power to weight ratio (can be used off shore) -ability to use wide variety of fuels -ability to start rapidly -high availability/reliability -minimum outage time (quick replacement) -remote operation (minimal manpower requirements)

Mixed Pressure Condensing Turbine

-addition steam is added to the turbine part way between inlet and exhaust -this design allows some flexibility in turbine load capacity, with extra steam being added if the load demands more power from the turbine -commonly used in gas turbine combined cycle power plants to extract maximum heat from the gas turbine exhaust >IP steam injected into gas turbine combustor to reduce temperature for NOx control.

describe blade cooling briefly

-air cooled blades to reduce metal temperature and increase life -sometimes water cooled

What type of power tubines do all gas turbines imploy?

-all gas turbines utilize power turbines of the impulse-reaction type, which is a combination of impulse and reaction

Explain Dummy Pistons

-an enlarged diameter section that is machined out of the rotor forging at the steam inlet end -piston surface exposed to steam pressure in High pressure end of turbine -exerts a force on the piston in the opposite direction of normal thrust force -a balance pipe is connected between the low pressure side of the dummy piston and an intermediate-pressure located along the turbine casing (since there is still a slight thrust towards exhaust end) -labyrinth seals used to seal around circumference of the piston

If what the waterwash is successful, what will the operator see?

-an increase in compressor discharge pressure -a decrease in turbine inlet Temperature

Explain mechanical overspeed trip systems

-an integral part of the governing system is that it prevents steam from entering the turbine, the speed becomes too dangerously excessive -generally located at the front end of the high pressure turbine shaft -designed to shut off steam supply to turbine -usually 10-12% above normal operating speed -centrifugal force overcomes spring tension, allows bolt to come out, hitting the trip linkage, shutting the trip valve

What is isochronous control?

-an isochronous governor regulates the turbine at a constant speed regardless of load -the speed droop is zero

Briefly describe bearing and lube oil system for gas turbines

-anti-friction bearings used for small units -journal/tilting pad for large units -separate bearings for thrust + radial load -stainless piping for corrosion -switches ; high oil T, low oil pressure, high filter diff. P

Why is ash undesirable in fuels?

-ash deposits drop ratings by 15% (not combustable)

What is compressor surge?

-at low RPM blades are not at correct angle and air seperation occurs -at start up, pressure rise is low, and compressor is trying to push air into a smaller volume and air chokes (called surge)

Where are the bearings located on a gas turbine?

-bearings will be located at both ends of the compressor and both ends of power turbine -thrust bearings positioned at inlet end of compressor and next to the power turbine bearing (1 for each shaft)

Describe the combined cycle

-better than using improvers -extract exhaust heat from gas turbine by means of a heat exchanger (HRSG) -produces steam for power/heat generation for a plant -exhaust temperature =600 C -cool combustor with secondary air -primary air for combustion (20%) -secondary air for cooling and dilution

Describe sealing in Reaction turbines

-blade sealing in reaction turbines is intended to retard steam leakage across the blade tips (delta P across blades) -knife edge design of sealing strips permits very close running clearances without the worry of significant damage if contact does occur (made of chrome-iron alloy)

Impulse Turbine Nozzles & Buckets (blades)

-blades (buckets) attached to the rotor, nozzles attached to casing -nozzles supply high velocity steam to the buckets, which occupy the entire circumference of the rotor wheel. The buckets convert the kinetic energy of the steam into mechanical energy, causing rotation.

What gas turbine lube oil maintenance is there?

-check for leaks (daily) -monitor pressure and temperature (daily) -check chip detectors when alarmed -topping oil reservoir (level control valve) -changing oil filters (diff. pressure) -cleaning cooler externally -oil sample analysis (oxidation) -calibration of instrumentations/test protective devices

Describe the anti-icing systems

-chunks of ice cause major damage to compressor -occurs if a certain combination of temperature and humidity occurs -anti-ice systems include: >bleed air from compressor and inject it to front of compressor >heating coils in air intake >feeding heated air from another source

Describe the can-annular combustor

-combine can and annular combustor -compact and strong -each can has its own ignitor and fuel lines -flame carried from one can to another

Describe the combustor and its purpose:

-combustion chamber used to heat the compressed air after it leaves the compressor and before it enters the power turbine -must have minimum delta P and produce minimum combustion impurities (to protect turbine blading) -20% of air enters combustor and mixes with fuel in flame tube -remaining 80% flows outside and serves as cooling air (some re-enters for dilution air) -combustor flame is 2100 C -diluted reduces combustion gas temperature to 1000 C to 1500 C -need diffusers to slow down air for proper combustion

Describe a single shaft in a gas turbine

-compressor and power turbine mounted on same shaft -load usually connected to same shaft (after turbine) -used for power generation where a constant speed is required -requires a large starting motor

Describe a dual shaft in a gas turbine

-compressor driven by a HP turbine (same shaft) -load driven on a second shaft by a LP turbine -purpose of HP turbine is to drive the compressor -purpose of the LP turbine is to drive the load -advantage is that greater operating flexibility can be achieved (load can vary at different speeds then compressor)

Describe the annular combustor

-consists of a single concentric flame located between an inner and outer casing -several fuel nozzles inject fuel into flame tube -compressed air fills space between casing and enters the flame tube through holes in tube walls

Explain what steam nozzles are made of, their shape and the two types.

-constructed of monel metal, formed over special dies -nickle-copper alloy with high tensile strength -shape of each nozzle is individually designed to ensure optimum expansion of the steam at the specified pressure and temperature. Passages that direct steam to the nozzle are located outside the casing (referred to as the steam chest) -2 designs: 1) convergent 2) convergent-divergent

Convergent Nozzles

-convergent shape of the nozzle causes the steam pressure to drop and velocity to increase -used for relatively small pressure drops (eddy currents if delta P is too high) -pressure drops to a specific exit pressure (called the critical pressure) -if less pressure is less than critical pressure, any extra energy achieved simply goes into the formation of eddy currents at the nozzle exit Pout/Pin = 0.577 wet steam Pout/Pin=0.55 Superheated steam

How does catalytic reduction for NOx also reduce SOx?

-copper oxide bath flue gas (SOx+NOx) -> CuO + SOx ->CuSO4 -CuSO4 is a catalytic that is then used to reduce NOx -90% SOx removed

Describe the can combustor

-cylindrical chamber in which combustion occurs -inner casing (flame tube) and outer casing -combustion is inside the inner casing with combustion and dilution air entering through holes from the outer casing -straight through and reverse flow

Describe inlet cooling for air

-decrease temperature, increasing power output (increased mass flow) -based on evaporative cooling >spray cooling water >fog cooling >special evaporative pads

Describe the Heavy Duty gas turbine

-designed for heavy industrial applications -larger and heavier than aero-derivative -suitable for base-load applications -more efficiency than aero-derivative -can use all fuels -external combusters

Describe the exhaust system

-directs hot turbine exhaust to a location that is safe for employees and other equipment -noise attenuation and silencers may be required to meet requirements -exhaust may incorporate heat exchangers for regeneration or combined cycle installation

What conditions are needed for the combustor

-duel fuel, switch during operation -need efficiency, low emissions, reliable + long life -sulphate and nitrogen cause corrosion + erosion -nickel or cobalt alloys for metal (resist corrosion and erosion) -low velocity combustion (diffuser needed) -vortex downstream of fuel nozzle for mixing pattern and combustion -most metal temperature limit is 1200 C

how is compressor cleaning accomplished?

-either by liquid or abrasive material

Describe pressure sensitive governors

-either control a steady back pressure at the exhaust or they extract steam from part way of the turbine and discharge steam at that point in a controlled pressure -usually a combination of speed and pressure control to assure relatively steady turbine operation

Explain electro-hydraulic governors

-electro-hydraulic governors use a combination of electronic and hydraulic controls. The turbine control console contains all the controls necessary for starting, accelerating, and loading the turbine, as well as for controlling the extraction steam flows and pressures if applicable -speed measure device (permanent magnet generator) -valve position is measured and fed back to the control console, providing more exact control. Provisions are made for on-line servicing of the computer circuit cards while turbine is carrying load -uses a hydraulic ram to position steam valve Serve-ram = uses a different high pressure hydraulic oil (not lube oil) at 8275-11030 kPa

How is off-line waterwash accomplished?

-engine stopped and cooled -engine ran at max crank speed with fuel ignitors and valves deactivated -once all wash fluid has been sprayed into intake, all disconnected lines are reconnect and unit is restarted -referred to as the crank-soak method

Describe the air intake system

-ensure clean air is provided -air filters installed -intake system more complicated if : i) cooling is needed or ii)icing conditions exist

Describe the regenerator

-exhaust heat used to increase T of air before combustion -increase efficiency by 15-20% -cost increased -increased pressure losses

Describe Axial-Flow Power turbines

-fewer stages in turbine compared to compressor -row of stationary blades called nozzles -row of rotating blades called buckets -nozzles: inc velocity (partial pressure drop) -moving blades: extract power with decrease in pressure and temperature

Describe the intake and exhaust considerations for gas turbines

-filters (remove air contaminants + foreign objects) -> Q+Q -smooth flow inside the compressor -cooling/heating systems for intake -P and T, Low delta P intake to minimize losses to increase efficiency -exhaust system provides a safe exit -silencer (reduce sound to acceptable limits) -heat recovery system

Temperature and pressure for lubricating oil

-flow more important than pressure for proper cooling -oil from the main line is 60-100kPa range -cooler outlet temperature is typically 43-49 C range >below this range, viscosity increases and flow will be affected >oil isn't cooled until it leaves the oil reservoir since the heat is used to evaporate water from oil >water vapour drawn off by small fan (vapour extraction)

What is the major cause of deterioration in gas turbine performance and why?

-fouling of the compressor blading -fouling results in decreased compressor efficiency which reduces overall thermal efficiency and maximum power -also results in compressor surging and acceleration problems

What is the most important component of the fuel gas system and why?

-fuel flow rate is measured and controlled by the fuel metering valve which is the most important component of the fuel gas system because it ensures the right amount of fuel is provided for the operating conditions

Describe the auxiliary systems considerations for gas turbines

-fuel gas system (P+T+Flow) and treatment -alternate fuel system -fuel treatment -lube oil system -hydraulic oil system -NOx control (steam injection) -anti-icing -water washing (compressor blades)

What does maintenance entitle for liquid fuels?

-fuel oil leaks (daily) -P and T (daily) and flow -cleaning centrifuges and replenishing chemicals (level) -changing filters (diff. press) -calibration and testing protective devices

Describe where Industrial and Commercial Gas Turbines are used

-gas turbines are used in a wide range of industrial applications such as electrical power generator drivers, gas compressor and large pump drivers -used to drive a generator to produce a relatively constant "base load" or as emergency/peak power generators -many commercial institutions use small gas turbines to produce power and heat -micro-turbines are used in smaller buildings

Lube oil can control?

-governor -fuel valves -VSVs -IGVs -bleed valve controllers

Tandem compouding

-have the shaft of the LP and HP turbines coupled together -LP turbine coupled to load -power of both applied to load

What are the advantages of a closed cycle?

-higher pressures can be used (higher densities are possible) -working fluid is clean (no corrosion or erosion) -working fluid need not support combustion, may be chosen for its superior thermodynamic properties -cheaper fuel may be used in air heater

Describe Split Casing

-horizontally split casing is the most common -easily facilitate assembly and inspection -casing made of relatively thick metal to withstand high P and T -wall thickness is gradually made thinner as you approach exhaust -for a given operating condition, metal must have sufficient strength, resist erosion and deformation -casing joints made steam tight by carefully machined finished flange faces (no gaskets) -Dowel pins used to secure and align flange joint and bolt holes -boring mills used to machine inside the casing for grooves for diaphragms

Describe Water Seals

-if a positive, leakproof seal is required, a water seal is installed -consists of an impeller on the turbine shaft, which rotates in a casing filled with water -water is thrown out from the impeller and forms a leak-proof water barrier -mainly applied to low pressure glands to guard against air infiltration -may also be applied to final seal for high pressure and IP glands -cannot operate properly at low speeds so you need gland sealing at start up -supplied with clean, cool condensate from CEP discharge

Reaction Turbine Operating Principle

-if the moving blades of a turbine are shaped so that steam expands and the pressure drops as it passes through then a reaction force will be produced in the blades.

Reheat turbines

-in some turbines, the steam losses enough heat energy in the HP stage -to maintain maximum efficiency and avoid condensation, entire steam flow is extracted and reheated to original temperature in an external heat exchanger, then returned to the turbine.

How does air leaks cause inefficiencies in condensers?

-increases delta T between exhaust steam and condensate -loss of vacuum caused by air flow in -monitor air flow meters -usually caused by faulty air ejectors

Where does differential expansion occurring and how to eliminate it?

-it occurs between fixed and moving parts -its eliminated by: >casing thinner towards LP end >double casing >flange heating >max clearance at start up, min clearance during operation

Describe the basic shape of the reaction blades:

-large entrance angle (90 degrees) and small discharge angle -leading edge is rounded (compared to sharp leading edge on impulse) -long, tapered trailing edge with convex side almost straight -looks like a tadpole

What are the advantages of the annular combustor

-light weight -less air flow restriction -compact -even distribution of combustion

Describe Sentinal valves

-located on the highest point of the casing -main purpose is to alert operator that casing pressure abnormally high (relieves steam to ATM, producing high pitched whistle) -both visual and audible warning of high pressure -useful application in start up (if exhaust block valve is not opened)

Describe the electronic overspeed trip system

-magnetic speed sensors are mounted to the turbine casing; as the gear teeth pass the sensors, magnetic induction generates an AC voltage that is read by an Electronic Control Module (ECM) which contains pulse counting sensors. Module converts the electronic pulse signals to RPMs -if speed reaches overspeed setpoint, action is initiated to shut the emergency stop valve -to avoid nuisance trips, most electronic systems use three magnetic pickups and trips occur once 2 sensors agree trip speed has been reached.

What is the main purpose(s) of a flexible coupling?

-main purpose is to absorb differential expansion (thermal) -also: joins two shafts together shaft deflection dampens vibration/noise axial movement minor misalignment

Describe the natural gas fuel characteristics

-mainly methane (sweet, sour=sulphur) -best fuel for gas turbines -promotes efficient combustion -lowest emissions -engine life longest -free of liquid contaminants (often heated)

Why do we have casing drainage?

-maximum allowable wetness of exhaust steam is generally 14%. -shape of casing allows for free water to drain to the condenser -special drainage grooves also arranged in casing to help remove water more effectively

Describe Thrust adjusting gear

-maximum efficiency of reaction turbine is dependant on maintaining the very close clearances between stationary and moving blades -this adjust gear allows manual control over shaft position -during start-up, thrust block adjusted so it pushes against a stop in the direction of the exhaust (max clearances) which avoids any danger of rubbing while turbine temperature increase and stabilizes. -when turbine is loaded and temperature is stabilized, thrust block adjusted for minimum clearance (therefore increased blade efficiency)

Why is there a lower bearing temperature in heavy duty units?

-more mass flow (flow + quantity)

Describe the axial compressor

-move air axially along shaft -casing gets gradually smaller, causes velocity to increase -fixed blades between each rotating blade tends to slow the air down and raise the pressure -compressor is similar to reaction turbine (in reverse) -high rotational stresses -most used and is multistaged -compression ratio 1.1:1 -> 1.4:1 -max ratio is 30:1 -> 40:1 -discharge pressure of 3000kPa -rotors: INC P, INC V -stators (diffusers) INC P, DEC. V

Impulse turbine blade summary:

-no delta P (less axial thrust) -HP blades = less stress than IP/LP -HP blades = higher thermal stress (short and hot) -little blade sealing, shrouding guides steam through moving blades -shaft sealing across diaphragms (delta P) = labryinth

What type of materials are employed in the power turbine?

-nozzles are cast with nickel based super alloys >inconel, vidment, waspalloy, hastelloy -turbine blading made of heat resisting, steel forged and machined to shape (hollow blading for cooling) -rotors are usually made of disc that are bolted or welded together -1st stage of blading is the highest Temperature (950 C) >use Inconel -2nd stage of blading is used >use nimonic

What is the most effective way of cleaning compressor blading?

-off-line water wash

Describe the turbine casings

-often referred to as the shell -contains nozzles, stationary blades, diaphragms, bearing cases, and all external components. -most operate under high P and high T conditions -must resist distortion -design must ensure that there is very small running clearances of the blading and labyrinth glands remain within allowances, and that the alignment of the rotor remains true.

What are the components of a gas turbine lubrication system?

-oil reservoir (quantity) -filters for cleaning (quality) -pumps (pressure) -coolers (temperature)

Velocity and Pressure Stages

-on very large impulse turbines, its common to employ another arrangement of velocity-pressure compounding, in which a velocity compounded first stage is followed by a second pressure compounded stage -allows for higher steam pressures to be used and obtain greater work and efficiency from the energy available in the steam.

Describe the Power turbine design:

-power turbine extracts power from the hot gases supplied from the combustion section by expanding the gases and decreasing the pressure and temperature ~50-60% of power used to drive compressor, remaining for load or generator -operate at a very high temperature, with high blade loads and large rotational stresses -blade life depends on: i) fuel, ii) material, iii) operation, iv) load, v) temperature

Reaction Turbine Pressure-Velocity Profile

-pressure can be seen to decrease across both the stationary blades and the moving blades -absolute velocity increases in the stationary blades, but decreases in moving blades -distinguishing feature: pressure drop across moving blades => delta P across inlet and outlet of moving blades

Describe the building and enclosure considerations for gas turbines

-protection against environment -reduce noise -protection for personnel -gas detectors -ventilation -fire suppression systems (CO2/halon)

Explain blade shrouding

-provides support, prevents leakage, helps steam direction -in LP section, support more important then sealing -use lacing wires for extra support or alone -inlet stages have high T, exhaust low T and high rotational stresses -shrouding can be stainless or low carbon content steel -has to be strong, corrosion resistant and erosion resistant -can be forged, machined or welded

Describe the efficiency and rating of gas turbines

-rated by power produced at output shaft (load) -standard conditions are 15 C, sea level, natural gas with 60% relative humidity -thermal efficiency = rated power and fuel energy rate (mass flow x LHV) -n=Pout/Fuel energy rate -35-40% (simple cycle) -HRSG=60-70% efficiency

Explain Drum Rotors on turbines

-reaction blades have pressure drop across them (large axial thrust) -reaction turbines MUST use drum rotors (eliminates surface areas that could cause thrust) -fairly light, rotor consists of disc welded together at their circumference to form a semi-hollow drum -greater resistance to centrifugal forces

Describe the steam injection systems and the purpose of them

-reduces NOx by dropping combustion temperature -water/steam must be pure (corrosion prevention) -water/steam can be injected into combustor or can be premixed with liquid fuels -main disadvantages is that thermal efficiency decreases due to energy transfer to the water or steam. Also an increase in combustion activity and pulsation (reduces life of components) >increase in power output however -practical limit for NOx = 25ppm (nat gas) or 42ppm (liq fuels)

Describe Pressure-Velocity Compounding

-reduces both steam pressure and steam velocity in stages -consists of two or more velocity-compounded sections in series -main advantage: high steam pressures can be utilized to obtain relatively low rotational speeds

Describe Pressure Compounding

-reduces steam and blade velocity by causing the steam pressure to drop in two or more stages -rows of moving blades are separated from each other by partitions or diaphragms, into which the nozzles are set Advantages: -increased efficiency due to decreased frictional losses -reduced centrifugal forces -for a given rotational speed, they can be designed for a wide range of inlet steam conditions

Describe velocity Compounding

-reduces steam velocity in stages -1 set of nozzles, 2 sets of rotating blades, 1 set of stationary blades -total pressure drop occurs in the inlet nozzle, then remains constant -stationary blades only redirect steam (no losses) -reducing velocity in 2 stages results in the speed of the rotating blade being one half the speed of the simple single stage impulse turbine -2nd set of rotating blade approximately twice the size of the 1st -larger size allows for same impulse force to be applied to second set.

Compounded Turbines

-refers to the arrangement of two separate turbines in which the exhaust steam from one becomes the inlet steam for the other -steam flow through the turbines are in series -used in relatively large systems -two arrangements: cross-compounding and tandem compounding

Describe the Reheater

-reheat combustion gases between expansion stages -increases energy content of gases (increased thermal efficiency) -less air must be compressed to do same amount of work -use a second combustor (excess air from HP turbine exhaust)

What does the routine maintenance entitle for gas systems?

-relatively maintenance free -checking for leaks (daily) -monitoring P and T (daily) -fuel filters (diff. press) -calibration and testing protective devices

Describe Catalytic Reduction for NOx reduction

-remove NOx from exhaust -add ammonia (NH3) to flue gas then catalyst -NH3 + NOx -> N2 + H2O -catalyst = titanium oxide or vanadium oxide -removes 90% of NOx 4NO + 4NH3 + O2 -> 4N2 + 6H2O 2NO2 + 4NH3 + O2 -> 3N2 + 6H2O

Extraction Turbine

-remove moisture through lines -used primarily in process plants where steam from the turbine may be used at a specific pressure for process or heating purposes and this pressure is available at some point between the turbine inlet and exhaust. -a controlled amount of steam is extracted at one or more points, to provide the pressure required by the user.

Explain mechanical governors

-rotation of the turbine shaft causes the flyweights to pivot outwards (due to centrifugal force) -greater the speed, the greater the centrifugal force, the more outward the flyweights move -movement of the flyweights causes movement of the governor rod, which, through level and fulcum, causes movement of the governor valve -disadvantage is that these have high speed-droop (10%) -not suitable for large machines where control must be extremely accurate

Explain Disc Rotors on Turbines

-rotors of an impulse turbine generally consists of relatively small diameter shaft, with thin, large discs (wheels) mounted on the shaft -moving blades attached around the circumference of theses discs -no pressure drop across moving blades of an impulse turbine, there is no axial thrust created across discs -2 methods of producing disc rotors: >individual discs >solid forging of a solid rotor

Summary of reaction turbine blades:

-sealing more important because there is a delta pressure across blades --more important in HP section -shrouds have a knife edge, fixed blades have double edge -radial fins machined into shroud

Cross Compounding

-separate turbines, each with its own shaft and attached to its own load

Why must we have shaft sealing?

-shaft sealing must be provided to prevent steam leakage where shaft extends through the casing -must also be provided to prevent leakage (ingress) of ambient air into casing (if it was condensing turbine)

Describe 3-D blading

-shape not symmetrical (change from parallel sides to twisted or bowed) -helps distribute steam more evenly across the blades -low vortices => less turbulence and better efficiencies

Explain the design of a impulse rotating blade

-sharp leading and trailing edge, similar to a banana shape -steam direction is changed when it strikes the blade, which causes the blade to spin (impulse force)

What is the advantage of centrifugal compressors?

-simplicity -strength -short overall length

Single flow vs Double flow

-single flow turbine is one in which steam flows in one direction only between inlet and exhaust -in a double flow turbine: steam flows in two directions. blading is designed so that rotation created by both sets are in the same direction >exhaust from both ends join together outside the casing and is divided to a common, low pressure user >one advantage is the elimination of axial thrust (counter-acted) >large number of stages can be used without excessively large disc or casing

How does a loss of vacuum cause inefficiencies in condensers?

-slime and mud on water side -partial pressures from uncondensed steam -cooling water outlet is low -effects efficiency, exhaust temperature increases so does condensate temperature =>no subcooling

Explain governor droop

-speed droop is the change in speed that results from an increase in load -an ideal governor will have the ability to maintain a constant speed at any load (mechanical losses that prevent achievement of this speed control) -the difference b/w no load and full load speed, expressed as a percentage is called the droop of a governor =[Speed(no load)-Speed(Full load)]/Speed (no load) x100% Governors with low droop are more sensitive to load changes and generally have more accurate control than those with high droop

Describe the Lean Premix Combustion method of NOx reduction

-stages -premix fuel with air before combustion ex/ 30% of air mixed with fuel T=2260C 70% added at later stage to burner then change to 60% mix with fuel T=1590C -> NOx formation prevented

Describe Speed Reduction Gear Sets

-steam turbines often operate at speeds higher than that required by driven machine -reduction gear set must be used to provide speed required at driven shaft -gears are set at an angle (helical gears) which reduce noise and vibrations

What are the disadvantages of a closed cycle?

-supply of cooling water is needed -complex -size increases -cost is higher

Describe the centrifugal Compressor

-takes air into the "eye" of the rotor -air is accelerated to high velocity by blades -diffuser converts velocity into pressure energy (increased cross-sectional area) -multistage capability -9:1 compression ratio

Describe extraction governing

-the control system is complicated since they must allow changes in the turbine load without affecting the steam extraction -they must allow changes in the quantity of steam extracted without affecting the turbine output

What happens to the life of a gas turbine with the quality of fuel?

-the life of a gas turbine is reduced as the quality of fuel decreases. Maintenance cost increase as well

What is the purpose of shrouding in an impulse turbine?

-the purpose of shrouding is mainly to strengthen the assembly and stabilize the ends of the blades. -This allows for longer blades and greater clearances

What are the effects on inlet air temperature?

-the temperature of the air entering the gas turbine has a great impact or operation and performance -as temperature decreases, density increases and mass air flow increases -power produced is proportional to mass flow -gas turbine will produce more power at lower ambient temperatures

How does thermal differences cause inefficiencies in condensers?

-thermal differences between exhaust steam temperature and cooling water outlet which is usually caused by fouled heat exchanger

Why must turbine casing be heavily insulated?

-thermal shock -dont loose T to environment -water damage -create comfort/safety

Explain mechanical hydraulic governors

-these have a pilot valve and hydraulic amplifier -removes direct linkage of flyweights to the governor valve -flyweights position an oil pilot valve that admits high-pressure oil to a piston that moves the governor valve -advantage is that mechanical losses of the governor are greatly reduced -droop of this governor is almost zero

Describe aero-derivative gas turbines

-they are adapted from aircraft (jet) engines -thrust producing component removed and replaced with a power turbine -longer and addition compressor stages -low weight -easily maintained (maximum availability) -fast starting -shorter life than heavy duty gas turbines -more efficiency than reciprocating engines with same power rating -able to switch fuels

What other sealing do condensing turbines use?

-they use gland sealing to ensure air cannot leak into condenser since they are at a low pressure (~10-20 kPa) -stops air infiltration and allows a vacuum to be established.

Chemical treatment of liquid fuels for gas turbines and why do we do it?

-treatment varies with the type of fuel and may include centrifuges, filters, de-watering, and chemical treatment -sodium, potassium and vanadium are detrimental to the power turbine as they cause rapid corrosion. -sodium/potassium removed by water washing -add magnesium to reduce vanadium

When are turbine blading replaced?

-tubine blades replaced at fixed intervals (75000 to 100 000 hours to avoid creep and failure -ceramic components are used to allow a significant increase in temperature allowances

How is on-line waterwash accomplished?

-turbine brought to idle and cooled -waterwash fluid injected -engine returned to operating conditions -used to increase time between off-line washes

Explain turbine thrust and common ways to reduce/eliminate it

-turbines are anchored at the exhaust so thrust is towards inlet of turbine -even those the delta P across moving blades in an impulse turbine is negligible, there is still a small thrust -thrust in reaction turbines is much more severe -one method of elimination is the double flow turbine exhaust (common in LP sections) -other methods include: >thrust bearings >dummy pistons >thrust adjusting gear

Different lube oil systems in small, medium and large turbine systems

-turbines must be provided with lubricating systems that will ensure a reliable supply of lubricating oil is going to all moving parts -small turbines (<150 kW) normally provided with ring-oiled bearings -medium sized turbines may have ring-oiled bearings and a circulating oil system -large turbines have oil-circulating systems

What are some methods to classify turbines?

-type of blading -type of compounding -direction of steam -exhaust condition -steam flow to/from turbine

Convergent-Divergent Nozzles

-used for larger pressure drops -pressure at the nozzle throat is the critical pressure, then pressure continues to drop in the divergent section -velocity increases across entire nozzle -produces required steam velocity without producing eddy currents

Describe Double Casing

-used for very high steam pressure applications -high pressure applied to the inner casing -exhaust steam directed to outer casing -pressure differential reduced as well as temperature differential -thermal stresses on casing and flanges greatly reduced

Describe bypass or overloading governing

-used in both impulse and reaction turbines -consists of two throttling valves -one on the inlet of the 1st stage and the other at the inlet located downstream of the first few stages -purpose of the second inlet point is to allow the turbine to maintain speed while producing extra power, during high load or overload conditions

Describe Labyrinth Seals

-used in large turbines for: i)shaft/casing ii)shaft/diaphragm iii)dummy piston/casing -consists of several rings separated by small space and surround the casing openings through which the shaft passes. -each ring is tapered toward the outer periphery, forming a sharp edge with a minute clearance to the shaft(or casing) -sharp edge ensures better sealing and provides less rubbing surface if shaft is slightly eccentric -with several rings in succession, steam has a difficult time passing through the small clearances.

Describe Carbon Ring Seals

-used on small, single staged turbine -each ring formed by three segments, bolted together and held together by a garter spring around the circumference. -spring allows radial movement, but prevents axial movement -rings susceptible to wear, causing leakage, but relatively easy to replace -limited to shafts <150mm diameter (heat caused by rings along shaft is detrimental) -each carbon ring fitted with a key way, so a key in the bearing house prevents the carbon rings from rotating along shaft -intended for shaft/casing sealing

Describe nozzle governing

-used only in impulse turbines -maintains a set turbine speed by regulating the flow of steam to inlet nozzles, which then direct the steam to the turbine blades =>bar lifts and cam-lift systems >length of nozzle valve stems vary, as bar moves up or down, it closes or open nozzles as required

Explain Barring gears (or turning gears)

-used to overcome the difficulty that sagging or hogging has when starting the turbine -it turns the shaft at a low speed via electric motor (1% of operating RPM) -turbine is warmed up on the barring gear for a specific time to ensure even warming and avoid vibrations -when turbine comes offline, it turns on barring gear for a specific time to ensure casing and rotor cools evenly

Non-Condensing (back-pressure)

-used when exhaust steam can be used for a process or heating purposes -primarily used in process plants -regulating station used to maintain exhaust at a constant pressure -turbine can also have bleed points (back-pressure, bleeder turbine)

Condensing Turbine

-used when there is no process use for the exhaust steam and/or maximum power is required from a limited amount of available steam -low exhaust pressure allows turbine to exhaust maximum energy from steam -exhaust steam below atmospheric pressure in a condenser (condensed by cooling) -LP allows greater delta P to do more work

Describe back pressuring governing

-uses a pressure sensing element on the exhaust steam line -set point on controller adjusts positions of inlet steam throttle valve -commonly used on condensing turbines -also used on systems used for heating purposes as pressure needs to remain constant to ensure good heat control -the efficiency of backpressure turbine is high since there is no exhaust steam losses -the disadvantage of this system is that power output is completely dependant on the demand for process steam.

Describe the intercooler

-uses cooling water -inlet air cooled between compressor stages -isothermal compression (const. T) takes less work than adiabatic(no heat removal) compression -specific volume is reduced (smaller sized machine) -benefits decrease as pressure ratio increases (more losses through intercooler because you need a smaller pressure to get through the intercooler)

What is the source on contamination in compressors?

-usually dust, salt and other airborne particles that are not trapped by air filters

Impulse Turbine Pressure - Velocity profile

-velocity increases sharply in nozzle, drops in blades -pressure drop in the nozzle, remains constant through blades (distinguishing feature of the impulse turbine)

Describe H2O injection for NOx reduction

-water or steam injected into combustor -NOx reduced to 225ppm from 150-200 ppm (natural gas) -inject H2O into fuel, reduced 15%, lowers Temperature -high quality water must be used (no impurities)

Difference between sagging and hogging on turbine shafts?

-when a turbine is left cold and at a stand still, weight of the rotor will tend to cause the rotor to bend slight downward between the support bearings (sagging) -if left at a standstill while hot, lower half of rotor cools faster then upper half, so rotor bends upwards due to differential expansion (hogging)

Describe an open cycle

-working fluid (air) is drawn in from atmosphere and discharged back to atmosphere -simple -efficiency is low

Describe a closed cycle

-working fluid contained and recirculated with a system -increases net power output -increases thermal efficiency of plant

What happens when natural frequency equals rotational frequency? Where do we often operate out turbine?

-you get critical speed -operate between first and second critical speed

How to counteract surge?

1) Bleed valves (dump air=maintain flow) -exit 1st stage and or 2nd stage -close bleed valves at certain RPM 2)Variable Geometry -IGVs =inlet guide vanes -VSV =variable stator vanes -rotate to optimize air flow -IGV close at start up -open to correct angle @ certain RPM -VSVs close to open position from 80-92% RPM (80 is closed, 92 is full open)

What are the 5 types of blade cooling and how do they work?

1) Film -air enters blade and exits small holes in blade 2) Transpiraton - air exits blade via pores (much smaller holes) all over blades 3)Convection - Air makes many passes from blade hub + tip 4) Impingement - high speed air jet blasted on to leading edge (hottest part) 5) Water cooling - preheated H2O enters blade hub and exits tip as steam.

What are the two factors that determine efficiency and power?

1) compression ratio: -increased compression ratio = higher cycle efficiency and greater output power 2)turbine inlet temperature -increased turbine inlet temperature has same effect (more work done by the same amount of air)

What are the three types of liquid fuels used in gas turbines?

1) distillates -kerosene (no treatment) 2)blended heavy distillates + low ash crudes -some treatment 3)residuals + heavy ash crudes -cleaning and treatment

What are the 2 issues with reaction turbines?

1) due to delta P in moving blades, steam tends to leak around edges and tips of blades (blade clearances must be minimal) 2)unbalanced thrust on rotor due to delta P (in direction of steam flow) (thrust must be balanced)

What are the 2 disadvantages of the simple impulse turbine?

1) extremely large centrifugal forces on the buckets, due to the high rotational speeds 2) large friction losses, due to the high velocity of the steam

what three staging methods are used to resolve the high centrifugal forces and damage caused to the equipment?

1) pressure compounding 2) velocity compounding 3)pressure velocity compounding

What are the four methods of NOx reduction?

1)H2O injection 2)Low NOx emission control 3)Catalytic Reduction 4)Lean Premix Combustion

Steps that happen when RPM increase in a mechanical hydraulic governor?

1)RPM Increase 2)Flyweights move out 3)plunger moves out 4)opens drains 5)oil pressure decreases 6)hydraulic cylinder pressure decreases 7)pivot drops and pilot valve moves down 8)throttling cylinder pressure decreases and starts to close 9) turbine slows down

What are the four considerations for gas turbine installation?

1)building and enclosures 2)intake and exhaust 3)Auxiliary system 4)reducing gears - between turbine and load

What are the 2 types of compressors used in a gas turbine and where is each one used?

1)centrifugal 2)axial -use a starter motor till it is self-sufficient -in small gas turbines, centrifugal compressors are normally used with a combination of several axial stages -majority of large gas turbines use multi-stage axial compressor -compressor used 2/3 of fuel burned -mass flow constant

Describe the steps and flow of air through a gas turbine:

1)compressor draws in ambient air and compresses it to 3000kPa and 600 C 2)air enters a combustor where fuel is continuously burned to heat air and produce combustion gases -flame=2100 C -secondary air is used for cooling + dilution -exhaust temperature reduced to 1000 C 3)hot gases enter turbine where it expands to 600 C, creating rotational energy.

What is the 3 purposes for secondary air?

1)stabilize flame 2)complete combustion 3)cools combustion gases

Describe briefly the oil circulating system

>oil is drawn from a reservoir and delivered at full pump discharge pressure (350-500kPa) to a header, which supplies oil to the governing and control mechanics. >oil from the header is reduced to 55-103kPa and flows through a cooler to another header, which supplied bearing and other parts requiring lubrication.

What are the advantages and disadvantages of the can combustor?

ADV: non-uniform outlet temperature minimized and higher pressure available for firing DISADV: multiple ignitors and fuel lines needed, space, and flow resistance

What must be done for the flanges of casings?

Flanges must be very thick, so they heat up more slowly then the rest of the casing. Need flange heating to avoid possible distortions and leaks (steam or electrical probes)

What is absolute expansion?

How much a rotor expands over time and how much the casing expands over time

What are the 3 cycle improvements

I)regenerators II)Intercoolers III)Reheater

Impulse turbine staging

In an impulse turbine in order for the steam to transfer all its kinetic energy to the moving blades, steam must leave the moving blade at the lowest possible velocity. The lowest velocity occurs when the blade velocity is equal to half the steam velocity. -> get high rotational speeds (high centrifugal forces) ->mechanical damage ->high frictional losses

What does the heat rate express?

It is equal to the amount of joules required to produce 1kWh of electrical energy (MJ/kWh)

What material and for what operating conditions are casings made of?

LP Section (<230 C) = cast iron (>2%carbon) IP Section (up to 430 C) = cast carbon (<2% carbon) HP section (>550 C) = cast alloys (chromium/molybdenum) Really high T = more chromium to resist creep deformation

What must be run after shut down?

Lube oil remains on for even cooling

What do we want to protect in gas turbines?

Protect against: -turbine overspeed -exhaust gas temperature high -vibratons -oil and bearing high temperature -fuel gas pressure (low/high) -delta P high (filters)

What are scavenge lines and pumps used for?

Scavenge Lines (coming from bearing sump) have temperature elements -scavenge pump and chip detectors in bearing sump with alarms -scavenge pump--> to filter-->cooler-->then tank

Why do thrust bearings run the hottest?

They have two loads: Radial and axial loads (140 F)

What are steam turbine governors used for?

Turbine governing systems control the steam flow to hold the speed of the turbine constant with varying loads, or to hold the pressure constant with varying demands for process steam. -Turbine governors can be considered as falling into two classes: 1) those responsive to speed changes 2)those responsive to pressure changes

Explain the steps that occur in a mechanical overspeed trip device with hydraulic governor system

When turbine speed increases to trip setting (usually 110% of operating speed) following occurs: -centrifugal forces overcome the bolt spring tension -bolt moves to trip position and strikes tripping lever -unlatches relay rod -main spring moves relay to tripped position -piston A opens the stop valve oil-port to drain -piston B closes off high-pressure oil inlet port

What can be added to a mechanical hydraulic governor to make the speed droop zero?

a hydraulic cylinder called a servo or servomotor will change the speed droop to zero

What are the 2 basic types of gas turbines?

a) aero-derivative b) heavy duty

What is included in supervisory equipment?

a) eccentricity-centre point of shaft constant (if misaligned, bearings and sealing effected) b)differential expansion (clearances=efficiency/damage) c)vibrations d)speed (RPM) e) Generator output f)bearing T

What type of metal is used to combat high temperatures?

add nickel and ceramic coatings to protect against high temperature

What does efficient combustion lead to?

high carbon dioxide, low carbon monoxide, low NOx and low SOx

What are the three types of combustors?

i)annular ii) can iii) can-annular

What are the 3 major components of a gas turbine and a brief description of their purpose?

i)compressor section: -supplies high pressure air for combustion and mass flow ii)Combustion section: -to burn a fuel and produce hot combustion gases iii)power turbine section: -convert the energy created by the hot, high pressure gases into rotational power (impulse-reaction combo)

what are the three methods used in speed sensitive governors?

i)nozzle governing ii)throttle governing iii)bypass or overload governing

Why must steam turbines temperature be maintained at designed value?

i)prevent damage (high T) from over heating and expansion ii)prevent condensation (low T) which causes erosion on blades and corrosion iii)warranty $$ 1000 F +/- 1 degree (follow trends)

What ways are turbine blading protected against water damage?

i)stellite blading (protects against erosion) ii)reheat between stages iii)superheat iv)bleed/extraction points v)drains vi)insulation

What 4 causes create condenser inefficiencies?

i)thermal difference ii)loss of vacuum iii)air leaks iv)cooling water systems

What must the waterwash fluid/material be?

it must be very clean and confirm to quality standards specified by manufacturer

Explain Jacking Oil systems

large turbines with heavy rotors equipped with jacking oil pump which supplies the lower part of the bearing with oil at approximately 10 000kPa, which lifts the shaft and supplies lubricating oil -lifts it a fraction of a millimeter >no metal to metal contact during start up of barring gear and reduces load on electrical motor >shuts off when barring gear disengages >also used when turbine is shutdown with barring gear on for cooling period

What are compressor blades made of and why?

low to med/high temperatures due to heat of compression -600 C on exit -stainless steel, alloy, or titanium compressor blading used -resist corrosion and erosion : ceramic coatings are also applied to increase efficiency and decrease the amount of corrosion and erosion that can occur

What happens when you turn the handwheel clockwise on the servo of a mechanical hydraulic governor?

moves the sleeve, exposing less oil drains in the servo cylinder (plunger hiding them), allow the pressure to increase under the hydraulic cylinder, allowing more oil to go under pilot valve, pushing the throttling valve open (more steam=more speed)

Whats another way of cleaning the compressor?

on-line washing and it is not as effective but still viable if downtime not acceptable

The speed sensitive governor is a ___________ controller and may be in the form of? (3)

proportional-action controller a)mechanical b)mechanical-hydraulic c)electronic-hydraulic

How does the cooling water system cause inefficiencies in condensers?

reduced vacuum if flow is slow -pump amps are low if flow is low -low flow means more delta T between cooling water in and cooling water out

What should be equal when condensers are new?

steam exhaust, condensate and cooling water outlet should be close to equal temperatures

Topping Turbines

this is a back pressure turbine with the specific purpose of taking steam from a high pressure header and reducing it to a lower pressure that is used by a process or by turbines that require lower inlet pressures. -it can provide lower pressure steam, while also driving an electrical generator (electrical generation is a by product)