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76 Cards in this Set
- Front
- Back
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Define prime mover |
Any devices designed to drive other machines |
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From where do internal combustion engines derive their energy |
From fuel which is burned in the engine itself |
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What sets Diesel engines apart from other internal combustion engines |
-Diesel engines use air alone to fill the cylinders during the intake stroke, they have no carbeurator system, but instead use a fuel injection system -the fuel is ignited solely by compression. Because of the increased temperature and compression these engines must have heavier construction to withstand the stress -all diesels are fuel injected, that is they have a metered flow of fuel into the combustion chamber |
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What are the two basic types of Diesel engines |
Four stroke and two stroke |
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Describe naturally aspirated and supercharged engines |
-in naturally aspirated engines, fresh air is drawn into the cylinder by a vacuum. The vacuum is created when the piston moves down the cylinder away from the head and combustion area -in a supercharged engine air is forced into the cylinder at higher than atmospheric pressure. This is done by means of a pump or closer similar to those used on two stroke engine |
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What are the compression ratios for Diesel engines |
Compression ratios refers to how many time the air is compressed from its original volume during the compression stroke The range of compression ratios for Diesel engines is 16:1 - 23:1 |
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Describe compression-combustion |
When air is compressed rapidly to a significantly smaller volume, the temperature of the air increases dramatically. When fuel is sprayed into the cylinders, the temperature and compression of the air cause the fuel air mixture to ignite and burn. The rapidly expanding mixture pushes the piston down in the cylinder |
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What are the methods of fuel injection |
Air injection - an external source of air forces a measured amount of fuel into the cylinders Mechanical injection - an injector pump forces fuel into the cylinders by applying a hydraulic force to the fuel (this is most used) |
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How are gasoline engines similar to diesel engines |
Can be 2 stroke or 4 stroke Can be naturally aspirated or supercharged and fuel injected |
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Describe how combustion is achieved with a gasoline engine |
Gas engines require that fuel and air be mixed together before ignition can take place. Once the piston reaches the top of its stroke on the combustion cycle, a spark plug fires. This causes the air fuel mixture to burn. Compression is not used for ignition gas engines are not subject to same stress and temps as diesel engines |
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What are the compression ratios with gasoline engines and how does that alter their construction |
6:1 to 9:1 or as high as 13:1 Gas engines can be lighter constructed than Diesel engines |
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How is fuel supplied to gasoline engines |
By a carbeurator or fuel injection systems |
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Compare high compression gas burning engines to both diesel and gasoline engines |
-use wither spark ignition or compression firing -they are similar to gas and Diesel engines by : they can be divided into 4 stroke or 2 stroke -they can be naturally aspirated or supercharged -they may differ in outward appearance but basic principles are the same |
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Describe the operating principles of four stroke cycle Diesel engines |
Four stroke cycle - four separate processes take place in two complete revolutions of the crankshaft -intake, compression, power, exhaust intake |
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Describe the operating principles of two stroke cycle Diesel engines with and without blowers |
In a two stroke cycle all operations intake compression power and exhaust are completed in two strokes and only one revolution of the crankshaft |
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Describe the purpose and operating principles of the following components |
A) crankshaft and journal bearings - crankshaft has two main functions, first is to support the pistons and relating parts, the second function is the most important- it transfers energy of motion -the crankshaft takes reciprocating motion and changes it to rotary motion -the crankshaft can also transfer the energy of reciprocating motion from the engine, to the energy of reciprocating motion for a compressor -the main journal bearings in a crankshaft along with bearing caps have 2 functions -hold the crankshaft in the block -provide lubrication for the crankshaft to turn in B) flywheel -crankshaft carries the flywheel which has 3 uses: -it provides inertia that carries the crankshaft through periods of no power stroke and also smooths out the power stroke -in automobile or truck use, the flywheel acts as a mounting surface for the clutch -it provides an ideal surface for a ring gear in which to mount a starter for starting the engine C)connecting rod and piston assembly- the connecting rod connects between the crankshaft throw and the piston-the piston houses the piston rings which seal against the cylinder wall D) cylinder head and valve assembly-the cylinder head is the top end of the compression - combustion chamber. The cylinder head assembly consists of valves, valve springs, rocker arms and rocker arm shaft. In some applications it includes a camshaft. The cylinder head also houses the intake and exhaust ports of the two stroke engine E) cam shaft with lifters and push rods- the cam shaft is a shaft with eccentric machined lobes called cams. It works by: 1)as the camshaft rotates the lobe lifts a lifter which in turn pushes on the putrid. The putrid then moves the putrid which in turn opens the valve. 2)as the cam lobe turns the valve spring forces the valve to seat and seal the compression-combustion chamber (cylinder). The camshaft is driven by chain or by gear from the crankshaft. It travels at one half the speed of the crankshaft. F) engine block- is a stationary piece that holds other things in place. It also called the cylinder block. The block houses or supports all major and most auxiliary parts. The basic block is most often cast iron or cast aluminum. It is machined to accept cam shafts, crank shafts, cylinder sleeves, bearings and other engine pieces. They are webbed for strength and heat dissipation. It has no removable parts, one piece cast. Has 2 main pieces - the bedplate supporting the main bearing and crankshaft- and the upper section that houses the cylinders G) turbochargers - are blowers that are powered by exhaust waste gases of the engine. They are basically air pumps (compressors) |
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Compare the construction and components of two stroke gasoline engines to two stroke diesel engines |
A two stroke Diesel engine is very similar to a 2 stroke gas or high compression engine each has a: -crankshaft with main and crank throw bearings -connecting rod and piston assembly -head assembly |
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Compare super charger to turbochargers |
Superchargers are found in several (not all) 2 stroke engines. Superchargers perform the same function as turbochargers but in a different way. -turbochargers are driven by exhaust gases -superchargers are driven mechanically either by the engine or from a separate source such as a electric motor -another difference is that superchargers are rotary lobe, turbochargers are (turbine wheel0 |
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What procedures are followed for preventative maintenance with internal combustion engines |
Regular oil particle tests Regular oil changes Regular filter changes (air, oil and fuel) Daily checks for leaks and missing bolts Shutdown- disassembling engine to check for tolerances on parts (pistons, rings, bearings etc) |
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What is the source of energy for a steam turbine |
Thermal energy |
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Define MPa and MW |
MPa = megapascal - a million pascals MW - megawatt - a million watts |
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Define: |
A)turbine - a rotary engine or motor driven by water, steam or air B) nozzles - steam inlet C) steam chest - a housing with inlet steam nozzles D) diaphragm - a stationary element (held by turbine casing) housing the steam nozzles E) rotor - a rotating component attached to the tubing shaft F) blade - rotating elements (attached to the rotor or wheel) which the steam pushes against to turn the rotor G) stage - a section of a steam turbine made up of a stationary nozzle and a moving blade H) governor - a valve controlling the amount of steam admitted to the turbine I) extraction - bleeding off steam at intermediate pressures J) condenser - the component that condenses steam from the turbine, producing a vacuum at the exhaust of the turbine |
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Describe the principles of operation of a steam turbine |
-as steam passes through the nozzles, it drives the rotor as it passes over the blades. The direction of the steam is reversed in each stage. This results in the rotor turning. The force created by the steam jets on the blades produces the mechanical energy to turn the rotor. This in turn drives the shaft which may drive a generator |
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Describe axial flow and radial flow turbines |
Axial flow - the steam flows along the axis of the turbine shaft as it flows through the blading to the exhaust port Radial flow turbines are turbines in which the steam flow is perpendicular to or at a right angle to the turbine shaft as it passes through the turbine blading. In radial flow turbines the steam flow is reversed in every set of blades
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How is the turbine joined to the driven machine |
By a coupling |
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How is the flow of steam to the nozzles controlled; for more or less power on a small mechanical turbine |
In smaller mechanical drive turbines have manual valves which allow steam to be opened or closed to additional nozzles. In larger turbine generators the steam to each set of inlet nozzles can be opened or closed depending on the load on the generator. This is done using a governor system |
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How do nozzles convert pressure energy to velocity energy |
Steam nozzles (aka stationary blading) are housed in diaphragms. The nozzles are designed so that they converge towards the outlet. In other words, the inlet is larger than the outlet. This design causes the pressure to drop as the steam passes through the nozzles as the nozzles are stationary, the pressure energy is converted to an increase in steam velocity as it goes through the nozzles |
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Why are shaft seals important |
-they add to the efficiency of the turbine -they add to the safety of the area around the turbine -they keep high pressure steam from leaking into the surrounding area -stop air from being admitted into the turbines |
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List and briefly describe the seals used on turbine shafts |
2 common seals are carbon seals and labyrinth seals Carbon seals - used on most smaller sized turbines using low to moderately high steam pressures 1.4 MPa to 4 MPa, carbon seals contain graphite and are self-lubricating. They are constructed in sections and held tightly to the shaft by springs Labyrinth seals - used exclusively on large high pressure turbine generators. These seals provide a complex path for the steam to follow in order to escape from the turbine casing. This ensures any steam escaping is at low pressure |
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Briefly describe back pressure turbines and backpressure/extraction turbines |
Back pressure turbines - use high pressure steam to drive the turbine and exhaust to a low pressure steam system. The exhaust steam from the turbine is under a back pressure. The low pressure exhaust steam system supplies steam required for different mill processes Back pressure/extracting turbines also use high pressure steam to drive the turbine and exhaust to a low pressure steam system. In addition, they allow steam to be extracted at intermediate pressures through extraction valves |
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How does the extraction of steam affect the turbines output power |
As the amount of extraction steam is increased the kilowatt output of the generator decreases |
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How is the intermediate pressure process steam prevented from backing up into the turbine |
The extraction steam line must be equipped with a check valve to ensure that no intermediate process steam is allowed to feed back into the turbine |
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Describe the operation of condensing turbines and extraction/condensing turbines |
-in a condensing turbine the high pressure steam is used to drive the turbine it exhausts to a vacuum produced by the condenser. The condenser condenses the steam from the turbine. When the volume of steam reduces, a vacuum is created at the exhaust of the turbine. A condensing turbine exhaust runs at a vacuum of 95kpa to 98 Kia. Condensing turbines have the largest pressure drop possible across a turbine making them the most efficient turbine uses fewer pounds of steam per kilowatt generated than other types of turbines Extraction/condensing turbine- similar to the backpressure extraction turbine. It feeds to both the intermediate pressure and low pressure steam systems from the extraction stages. The exhaust steam is fed to a condenser an extraction/condensing turbine can be designed to produce the maximum electrical load when the turbine is exhausting to the condenser, with no extraction. It can also be designed to produce the max electrical load with both the extraction steam systems open and some condensing taking place. This type of turbine generator is very good for starting up a mill where purchased power from an outside utility is limited or very expensive |
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List the rotating elements in turbine |
The turbine shaft The turbine wheels The attached turbine blades The blade shrouding The thrust bearings Bearings Couplings |
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Describe the operating principles of impulse and reaction blades |
Impulse blades are shaped so that the space between the blades does not allow any pressure drop in the steam as it passes through the turbine. All of the pressure drop occurs in the stationary blading or nozzles. All of the thrust acting on the rotor blades is from the change in momentum or impulse, of the steam as it changes direction in the blades. As can be seen the only opening between the blades is the same size as the outlet opening Reaction blades are sized and shaped to cause the pressure of the steam to drop as it passes through the blade. This produces an increase in steam velocity and this increase causes a reaction or back thrust on the blade as the steam exits the blade. This force is rather like the thrust caused by air flowing over an airplane wing. In these blades, the inlet opening between the blades is larger than the outlet opening
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How are the blades attached to the turbine shaft |
One of two ways -one is by a disk or wheel, the wheels are attached to the shaft and the blades are attached to the wheel -the other is drum type. The drum is a raised portion of the shaft or an enlargement of the shaft. In some designs the drums on large turbines are increased in diameter towards the low pressure end of the turbine |
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How are the turbine blades prevented from vibrating |
-the end of each blade is mounted securely in the wheel. The outer rim of the blade is held in place by a thin metal plate called a shrouding. In very large turbines tie wires are used instead of shrouding to join the blades together |
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Describe the types and accuracy of the turbine bearings |
The bearings at each end of the turbine are journal bearings. They allow the rotating element to roseate @ high speeds writhin the casing of the turbine. They carry the entire weight of the rotating element. Because of low clearances in turbines the bearing tolerances must be very exact to ensure moving parts do not come into contact with stationary parts Thrust bearings- the thrust created by steam acting on the rotor is counteracted by the thrust collar or thrust bearing. The thrust collar can be a machined portion of the shaft riding on thrust plates to stop any axial movement of the rotor, or it can be a collar which is keyed to the shaft |
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How is the shaft steam - seal system started up and maintained |
1) the steam is fed to the lower pressure section of the high pressure seals and to the low pressure seals 2) as the turbine generator comes up to speed, steam from the turbine fill the labyrinth seals 3) the pressure controller to the seals closes as the pressure from the seals exceeds the pressure controller set point 4) once the turbine - generator is running the first stage of the high pressure seal exhausts steam to the intermediate pressure steam extraction line 5)in the next stage, steam exhausts to the low-pressure steam extraction line 6) the final stages and the low pressure stage seals exhaust to the shaft seal condenser. The condenser creates a vacuum on the system to ensure no leakage to the surrounding area of the turbine takes place 7) the condensate from the shaft seal condenser goes to the turbine main condenser intercooler 8) a vacuum pump is used to exhaust any non condensable gases from the shaft seal condenser |
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Describe the operating principles of a condenser and a hot well |
-a condenser is a shell and tube type heat exchanger -the cooling water enters one end of the condenser head and is passed through the condenser tubes. The condenser may operate with one pass or two passes of the cooling water through the tubes. The water passes are controlled by a baffle in the feed end head. This directs the water to the appropriate sections of the tube the exhaust of the turbine feeds to the shell body of the condenser. As it passes over the water cooled tubes, it is cooled and condensed. This causes a vacuum on the steam exhaust of the turbine. The condensed steam then falls to the condenser hot well. The hot well is a storage tank for the condensed steam or condensate. The temp of condensate is just below the boiling point of water. In the hot well any non-condensable gases are removed from the exhaust steam. This is done by a steam ejector or a small vacuum pump |
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Why and how is the cooling water resused |
The cooling water is heated as it condenses the steam. The heater cooling water may be sent to a cooling tower or to a heat exchanger where the water is recoiled and then reused in the condenser. This system is used where fresh water is at a premium. The cooling water in a closed recirculating system must be chemically treated to ensure that there is no growth of algae or bacteria in the cooling system. These growths fowl up the system and prevent proper heat transfer. |
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How are control valves operated on a turbine |
-can be manual or automatic or a combo of both -can be motor driven, hydraulically driven or electronically controlled. They can limit flow to one direction only -can be quick opening, slow opening, quick closing or slow closing |
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Describe each of the following |
A)main stop valve - the main steam stop valve is the first valve of the turbine generator system. This is a manual valve off the high pressure header. It is often motor driven, normally from an open/close selection station at the valve. The purpose of the valve is to allow isolation from the steam system for shutdowns and maintenance on the turbine. The main valve is equipped with a small bypass line and valve to warm up and pressurize the steam line to the turbine. It is usually a globe valve or a parallel sliding gate valve as they give a positive shutoff and the seats are not prone to scoring B) exhaust valve - the exhaust valve is situated on the low pressure steam header. In the extraction line of an extraction turbine it is situated on the intermediate - pressure header. They are used to isolate the turbine from these steam systems for shutdowns or maintenance. These valves are always manual. Can be globe, wedge, gate or parallel sliding gate valves. Wedge gate valves are cheaper and because of the lower pressures the seats of the valves are not likely to score C) throttle valves - in smaller mechanical drive turbines the throttle valve is a manually operated globe valve or a parallel sliding gate valve. Throttle valves are used to manually open the steam to the turbine for: -warming the turbine up -bringing the turbine up to speed until the governor valve takes over the speed control D) emergency trip valve activation-are designed to safeguard the turbine and prevent it from becoming damaged. The trip valve can be activated by: -turbine over speed -operator action -low oil pressure -high bearing temperature -low or high inlet steam pressure to the turbine -high vibration E) governor valves -controls the steam to the steam chest, controlling the speed of the turbine. Once the generator is online the governor valve controls the electrical output of the turbine generator F) extraction valves- to increase the amount of extraction steam the extraction valve is closed off. This limits the amount of steam allowed to flow to the low pressure end of the turbine and forces more steam out of the extraction valve. Can be manually or automatically controlled. They can be operated by steam flow control, by pressure control of the intermediate pressure steam system or by electrical load control of the turbine generator. G) non return valves on extraction lines- because the exhaust (back) pressure steam system and the extraction steam systems are under pressure the extraction lines from the turbine are equipped with non return valves. These isolate the steam header when the extraction valve is closed. This prevents reverse flow when: -Starting up or shutting down and extraction turbine -running with the extraction valve wide open and little or no extraction is taking place -if all sections of the turbine do not receive steam the casing and rotor may be damaged |
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List the various types of governors found on steam turbines |
Mechanical governors Hydraulic governors Electronic hydraulic governors |
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Describe the operating principles of |
A) mechanical governors - if the turbine speeds up the ball weights are forced out by the centrifugal force. This closes the governor valve, slowing down the turbine -if the turbine slows down, the balls of the flyweight drop. This opens up the governor valve, speeding up the turbine. B) mechanical-hydraulic governors - used primarily on smaller turbine generators. With these types of governors the flyweight ball movement controls an oil valve on the oil system. This valve controls the oil pressure to the steam governor valve. It works by -if the turbine speeds up, the fly weight ball closes the oil to the governor valve and opens the drain from the governor valve. This reduces the hydraulic oil pressure on the governor valve allowing the spring on the governor valve to close the valve. Less steam is then admitted and this slows down the turbine -if the turbine slows down, the action of the flyweight opens the oil valve to the governor valve and closes off the drain. This increases the pressure on the governor valve, more steam is admitted to the turbine, speeding it up C) hydraulic governors- use the oil pressure from the turbine main oil pump to control the governor valve. The main oil pump delivers high pressure oil for the governor control loops. It also delivers low pressure oil via a pressure reducing valve, to the bearing lubrication system. In all of the hydraulic systems the oil is returned to the oil reservoir, both from the governor and the bearings. If the turbine slows down, the oil pressure drops. This opens the governor valve to admit more steam to the turbine, speeding it up. This is done by: 1) the lower pressure in the servomotor causes the high pressure oil to be opened to the top of the drive piston of the servo motor. It also causes the return drain to be open on the bottom 2) this moves the servomotor piston down opening the governor steam valve. This speeds up the turbine D) electronic and electro-hydraulic governors - use an electronic speed signal form the turbine shaft to control the turbine speed. With these types of governors the mail oil pumpand aux oil pump are constant speed, constant pressure pumps. These are usually mounted on the oil reservoir. The oil pumps supply high pressure oil for the governor and low pressure oil for the bearings 1) the electronic control takes an electronic signal from the speed pickup on the turbine shaft 2) it analyses the speed and puts out an electronic signal to the torque motor 3)the torque motor controls the oil pressure to the governor servo motor 4) the servomotor controls the oil pressure to its drive piston 5) this opens and closes the governor steam valve as required |
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Describe the operating principles of a |
A) gas turbine - similar to a steam turbine except that hot gases rather than steam expand through the turbine. Major diffference between steam and gas turbine is that the gas turbine has a compressor B) simple open cycle turbine 1) air is drawn into the compressor where it is compressed to several atmospheres and the discharged into the combustor 2) the combustor ignites fuel, which heats the compressed air 3) from the combustor the heated gases are allowed to expand through the power turbine 4) once the energy has been spent, the heated air is close to atmospheric pressure again and is exhausted back to the outside atmosphere C) complex cycle - 1)air is drawn into a low pressure compressor 2) from the low pressure compressor the air flows through an intercooler into a high pressure compressor. The air entering the compressors should be relatively cooler than the air in the turbine 3) from the high pressure compressor, the air flows through primary combustor 4) the air then expands through the high pressure turbine and enters a secondary combustor 5) this superheated air is now allowed to expand through the power turbine. It exhausts into the low pressure turbine which drives the low pressure compressor 6) the spent air is now allowed to exhaust through a recuperator to the atmosphere. The recuperator recovers heat form the spent gases D) closed cycle turbine - the cycle follows the same path as the previous cycles however the working medium is not released back to the atmosphere but is used over and over. Because the cycle is closed a small auxiliary compressor is needed to either charge or discharge the system |
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Describe the operating principles of a |
A) single shaft turbine - on a single shaft turbine, all elements are on the same shaft as the compressor. They are useful for constant speed applications as a generator. Also because the control systems are simple, their response to the needs of the driven element is much more positive (that is quicker to respond) B) two shaft turbines- in this type of turbine the work performed by the high pressure turbine is used strictly for the compressor. The low pressure turbine is used to drive the load. The advantage of this arrangement is that the low pressure turbine can be set at one speed while the compressor and high pressure turbine fluctuate according to load. This application can be reversed so the low pressure turbine can be allowed to fluctuate with the load while the speed of the high pressure turbine remains constant. Main disadvantage of this configuration is that it needs a sophisticated control system |
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What types of compressors are used with gas turbines and which type is more efficient |
Axial flow - most efficient Radial centrifugal flow Axial centrifugal flow |
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List the turbine auxiliary systems |
-main oil pumps -Auxiliary, emergency and jacking oil pumps -turning gear -gland seal system |
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How is the main oil pump driven on a turbine generator |
Can be a turbine shaft driven pump or a stand alone electrically driven pump |
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What is the purpose of the auxiliary oil pump |
-it feeds into the high pressure side of the system, it ensures lubrication to avoid damage to the bearings in the event of a park trip of all systems |
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How is the temperature of the oil going to the bearings controlled |
-bearing temperature is controlled by controlling the temperature of the lubrication oil with an inline oil cooler -to raise the temp, the flow of cooling water is cut back -to lower the temp, the flow of cooling water is increased |
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What is the desired temperature range for oil leaving the bearings in a turbine-generator |
55C-66C (130F-150F) |
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What is the reason for turning a turbine generator until completely cooled and how is this accomplished |
-the distance between bearings is generally large, this puts a large stress on their shafts. If the turbine is down for a long time the generator shaft can become bowed, to prevent this turbine generators are turned until they are completely cooled - this process is called barring |
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What is the purpose of jacking pumps |
In order to supply a higher oil pressure than the lubricating system to lift the shaft off the bearings |
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How are jacking pumps used |
1) start the jacking pump to lift the turbine and generator shafts off of the bearing 2) then start the turning gear 3) once the rotating element is turning slowly, start the auxiliary oil pump and shut down the jacking pump. At operating speeds, the pressure of the jacking pump is too high and causes excessive oil leakage from the bearings |
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What pre start checks should be done on a turbine generator |
1) the turbine - generating lubrication piping system is properly connected, complete and activated 2) coupling giraffes are replaced and securely in place 3) covers are on and secured 4) insulation of turbine components is replaced 5) cooling water piping system is connected, complete and activated 6) the electrical system is in order and ready to run 7) casing drain piping is connected & complete 8) steam lines to and from the turbine are clear of locks and tags 9) auxiliary equip is ready to run 10) the area is clean, with no debris, or tools left lying around. Oil spills are cleaned up |
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Briefly describe the following |
A) combustor - combustion takes place in the combustor. This component of the gas turbine must operate in a wide range of applications from no load to full load conditions. Combustion worlds on the principle of flame propagation (once the combustor is ignited the fuel burns continually from the preceding fuel. There are four styles cannular, annular, can type and straight through Combustors have no moving parts B)igniter- is normally a spark ignition system in which high energy is discharged from a capacitor to the ignition lead and through to the igniter plug. This process is very similar to the spark ignition system of an automobile engine. (The igniter is only used during startup cycles) C) atomizer or vaporizer - there are 2 basic ways of injecting fuel into the combustor -the first is to use an atomizer which sprays a fine mist into the chamber. The fuel is burned in this form. This is the most usual system -the second way is through vaporizing the fuel. Fuel is metered into a tube which is surrounded by combustion gases. The hot tube and hot air evaporate the fuel. The fuel mixture is discharged into the combustion chamber where it is mixed with more air and is ignited D) flame tubes - is the hottest part of the system and generally is the first to wear out. It has perforations to allow a secondary flow of air to flow through the walls of the flame tube thus cooling the hot primary air in the flame |
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How are electric motor frames constructed |
The frame of an electric motor is made of laminated silicon steel and is often enclosed in a housing. The purpose of the frame is to support the windings and to provide the magnetic circuit. There are 3 types of frames: -the open frame has end bells so air can circulate -the semi closed has a screen in one end bell to prevent debris from being sucked into the motor -enclosed frames are often used in explosion proof motors |
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How are frame sizes classed and why is this important to a millwright |
Frame sizes are classed by diameter Within each diameter size there are a few standard lengths. Frame sizes are used to obtain frame dimensions such as the overall length and the diameter of the motor, and distances between mounting bolts. They also indicate shaft length, diameter and height. |
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Describe DC motor construction including field poles, armature assembly, commutator, brush holders, tension springs, brushes and bearngs |
Direct current is a flow of electric current in one direction only. It may remain substantially constant in value. DC motors convert this electrical energy into mechanical energy. Field poles - usually made of laminated silicon steel. They support the field windings and act as a magnetic path between the frame and armature Armature assembly- is made of laminated steel which is part of the magnetic circuit. It supports the armature windings in slots. The speed and voltage of the machine determine the size of the wire and the number of windings Commutator - is made from copper bars that surround the shaft, upon which the brushes ride. The bars are insulated from the shaft and from each other with mica or some other insulating compound. The function of the commutator is to provide a means to feed the external electrical supply into the rotating coils Brush holders, tension springs and brushes - brush holders support the carbon brushes and the tension springs provide pressure on the brushes so as to provide an electrical connection between the commutator and the brush. Bearings- support the weight of the armature and provide a surface of minimal resistance to rotating motion |
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Describe the characteristics of the following DC motors |
A) series connected field motors - the series motor is one in which the field windings are connected in series with the armature. The same current which flows through the armature conductors also establishes the magnetic field. The speed of the motor can be controlled by changing the applied voltage or by increasing or decreasing the resistance of the field winding. Therefore as the load is increased the speed of the motor decreases. This give the motor a very high starting torque with a high no-load speed. The advantage of this DC’s motors high starting torque is that it allows battery operation, which makes it portable. Such applications include cordless drills, screwdrivers etc. Industrial applications include traction motors in diesel locomotives, forklifts etc |
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Shunt connected |
Jjdsk |
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What stresses do gas turbines have to endure |
Stresses from heating and loading |
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How are the turbine blades cooled by convection or by transpirataion |
-convection cooling uses compressor air from the intermediate pressure zone on the compressor -transpiration cooling - uses a watery vapour (steam) to cool the blades of the turbine. It is very effective but requires some form of reheat system for the creation of wet steam. Under most circumstances, this is not possible |
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How are turbine blades shaped? |
Turbine blades begin at the root as impulse blading and turn into reaction blading further out |
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Describe a radial inward flow (centripetal) rotor turbine |
-Radial inflow turbines are used for small turbines because the axial turbine tolerances cannot always be reduced proportionately from larger machines- -the compressed gases enter from the outside of the rotor and exit from the center |
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What types of bearings and seals are used on gas turbines |
-the rotors sit in pairs of bearings (either journal or rolling type) one of the pair incorporates an thrust bearing assembly. -in larger gas turbines, labyrinth seals are sued as shaft seals on the compressor and the turbine |
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List the protection system on a gas system |
-engine overspeed (governor) -temperature -low oil pressure -high oil temp -flame detection -vibration -fuel pressure |
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Briefly describe the gas turbine governors and lubricating system |
The governor provides automatic regulation for both starting and normal operation of the gas turbine. These controls can be very complex depending upon degree of automation, environment or even the operating characteristics of the turbine. This system can be adapted to either single or two shaft turbines Lubrication systems - the lubrication system supplies oil to the bearings and gears of the turbine. Large turbines require lubrication pumps which are driven separately from the turbine. This is so that maximum oil pressure can always be ensured during startup, operation and shutdown. Small turbines have integral oil pumps |
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Describe the characteristics of the following DC motors |
A) series connected field motors - the series motor is one in which the field windings are connected in series with the armature. The same current which flows through the armature conductors also establishes the magnetic field. The speed of the motor can be controlled by changing the applied voltage or by increasing or decreasing the resistance of the field winding. Therefore as the load is increased the speed of the motor decreases. This give the motor a very high starting torque with a high no-load speed. The advantage of this DC’s motors high starting torque is that it allows battery operation, which makes it portable. Such applications include cordless drills, screwdrivers etc. Industrial applications include traction motors in diesel locomotives, forklifts etc B) shunt connected field motors - this motor type has its field windings connected in parallel (shunt) with the armature winding. This motor has a low starting torque but has good speed regulation. Shunt motors are widely used in applications requiring adjustable speed control. Speed can be varied in two ways. The speed can be varied by changing the magnitude of the speed of current C) compound field motors - these combine the characteristics of the shunt motor and series motor by incorporating both series and parallel windings. These types of motors are variable speed. They work well with load that require constant speed but have high peaks needing high torque. Applications include reciprocating equip such as pumps and compressors D) permanent magnet motors- use permanent magnets to produce the necessary magnetic field. As a result, these motors have similar torque-speed characteristics to a shunt connected field motor. Permanent magnet motors usually have ceramic magnets which resist demagnetization and are much smaller than conventional magnets. These motors have a wide variety of uses from small toys to aerospace applications |
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What are the operating principles of electric motors |
They are divided in two groups: DC direct current motors AC alternating current motors The interaction of electricity and magnetism is referred to as electromagnetism and its principles are used in generators and in motors |
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Describe the design characteristics of the pillowing three phase AC motors |
A) squirrel cage induction motors - there are no physical connections between the rotor and the stator. Hence the magnetic field must be induced into the rotor a squirrel cage induction motor has: -a frame that provides a magnetic circuit and supports the stator windings -stator windings -a squirrel cage rotor -end bells B) wound rotor motor - the difference between wound rotor motors and squirrel cage motors is in their rotor design. The wound rotor has insulated windings. These motors are used widely throughout industry to drive fans, compressors, machine tools, and process pumps C) synchronous motors - in these motors, speed is synchronized between the rotor and the rotating magnetic field. That is the speed of the motor is directly proportional to the frequency of the operating current. Because these motors need assistance in starting, squirrel cage windings are placed into the pole faces. The Poles stick out toward the rotor and are connected to slip rings on the shaft. |
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Describe the design characteristics of the following single phase AC motors |
A) split phase motors - use an auxiliary winding on the starter to help start the motor. These motors are fractional HP units. They can use either wound or squirrel cage rotors although squirrel cage is the most widely used. These motors are found in small pumps. They consist of a housing, an armature, a stator and a centrifugal switch inside the housing B) capacitor start motors - these motors are either fractional HP or integral HP as high as 15 HP. This motor has a similar construction to the split phase except that there is a capacitor on the starter winding. The capacitor acts to provide a high starting torque. Capacitor started motors utilize the capacitor only while starting. It is disconnected once the motor reaches 3/4 speed. This is done by a centrifugal switch which opens to bypass the capacitor in the circuit. They are used in applications where a high starting torque is required such as conveyors, pumps and reciprocating compressors C) permanent split capacitor motors- keep the capacitor operating at all times. The permanent split capacitor does not have a centrifugal switch. These motors have a low starting torque and a high rated torque. These motors are ideal for grinders and sanders D) repulsion motors - can be divided into several groups such as repulsion start, induction run motors; repulsion motors; and repulsion start induction motors. Despite the diversity of theses repulsion motors their basic construction is similar. These motors have the same basic construction: -stators are all similar to split phase -rotor is slotted with windings and a commutator -bearings support the armature -brushes ride on the armature |
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List the information on the name plate |
Horsepower rating Phase Frame size Volts Amps RPM Bearing numbers for both ends Cycle Model Code Class of insulation Temperature rating Service factor |
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What are the advantages and disadvantages of series connected field motors |
Advantages Suited to heavy starting duty where severe loads are expected Small size for comparable HP motors makes them ideal for high torque loads Disadvantages Requires load to limit speed Speed adjustment is limited Limited application to traction and lifting |
