Lecture note 2009

Lecture note 2009 - Royal Institute of Technology, Energy...

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Unformatted text preview: Royal Institute of Technology, Energy Department, Stockholm, Sweden Three Lectures on turbines by Professor Torsten Strand, Ass. Professor, Energy Department (formerly Siemens Industrial Turbomachinery, Finspong, Sweden) torsten.strand@finspong.com The three lectures are Operation and control of industrial gas turbine power plants [1] Introduction to gas turbine technology and gas turbine operation Security of electricity production [3] Introduction to methodology for designing power plants for high availability Turbo‐machinery in a sustainable world [4] Scenarios for the use of turbo machinery operating with zero emissions or on renewable energy ‘fuels’ Below you can find notes going with the power point presentations. Operation and control of industrial gas turbines History The first gas turbine that produced positive power was put in operation in Christiania (Oslo) in Norway 1903 by the inventor Ægidius Elling [2]. Gas turbine development went on slowly until World War II when jet engines were first developed. Industrial gas turbines were used as reserve and peak power until mid 1970‐ties when gas turbines successfully were installed for baseline power production in Saudi Arabia. Since then gas turbine have become widely used in many different applications. There are turbines from 10kW up to 500MW. The industrial GT market is nowadays dominated by a few major companies; General Electric, Siemens, Alstom, Mitsubishi HI, Rolls Royce and Solar Turbines. In Sweden Siemens Industrial Turbomachinery in Finspong produces gas turbines in the power range 15‐60MW. Volvo Aero manufactures parts to a variety of jet engines (Rolls‐Royce, GE, Pratt&Whitney). Most companies manufacture the turbines and equip them with the auxiliary package but there are several separate packagers for GE and Rolls Royce machines. GE turbine technology is licensed to number of manufacturers around the world. Industrial gas turbines are used in a lot of different applications: power plants on and off shore, heat and power cogeneration plants in industries and municipalities (district heating), drives for compressors in oil & gas industry (pipelines, gas compression, LNG plants), marine propulsion etc. The gas turbine or Brayton cycle The gas turbine is a build up as a very compact and complete energy converter. The core engine is composed of a compressor, a combustor and a turbine. The turbine drives the compressor and the driven equipment, most often a generator. In many medium size gas turbines, 10‐40MW, the turbine is divided in two. The high pressure turbine is driving the compressor and the low pressure or power turbine is driving a generator or a compressor often via a gear box. The engine is always delivered as a complete plant with all the necessary systems for operation. The main ones are the lubrication oil system and fuel systems, but there are a lot more. To run the turbine, the driven equipment and all the auxiliary system a fairly advanced control system is included. The gas turbine is called an ‘air breathing engine’ mainly because it uses a lot of air as working medium. The air is compressed in the compressor to the turbine inlet pressure, which can be 1500‐2500 kPa for industrial machines and up to 5000kPa for a jet engine. The air temperature is increased during the compression up to around 350‐600°C depending on pressure. In the combustion chamber fuel, nowadays mostly natural gas, sometimes diesel oil, is burned using only a part of the available oxygen in the air. The hot combustion gas is then expanding in the turbine delivering power to the compressor and the drive shaft. The turbine inlet temperature is high, in newer turbines in the range of 1400°C. The material in the turbine vanes (stator blades) and rotor blades is super alloys (Ni‐based alloys with high temperature capability; strength and oxidation resistance) but they can withstand maximal temperatures of around 900°C. So these high temperature blades have to be cooled. Part of the compressor air is used for cooling. For a turbine with 1400°C around 20‐25% of the compressor air is used for turbine cooling, which is not so good for the turbine efficiency. The higher the turbine inlet pressure and temperature the better performance (specific power output and efficiency), but it could be still better if less cooling air could be used. The gas turbine cycle is easily described in the t‐s diagram and the thermodynamic calculations are straightforward. The gas turbine thermodynamics can be described with a few equations: compression, heat release at constant pressure, expansion combined with a flow balance and an energy balance. The compressor and turbine thermodynamic efficiencies are in the range of 85‐92 % and the combustion efficiency 99.9% The performance is characterised by very high specific power but not so high efficiency in simple cycle, 35‐40%. A lot of heat is going out in the stack. So therefore a lot of gas turbines have an exhaust heat recovery unit, mostly producing steam for processes but also for steam turbines (combined cycle). The combined cycle efficiency can reach 60% in large plants and the total use of the fuel energy in a CHP (captive heat and power) plant can be 93%. Gas Turbine operation A gas turbine plant is always fully automatic. Usually there is no permanent operation crew on site. The operation is controlled by a PC in a remote control room (the main control room of a paper mill, starch plant, rubber plant, district heating plant etc). The site has normally a daily inspection scheme (½ an hours/day) and a maintenance program stretching over some 40000 hours (five years) of operation. The operation can be divided in the start up, normal operation on load and shut down. The shut down can be normal or drastic; a trip of the electrical grid or by some plant failure or a forced shut down by some failure indication. The gas turbine control The gas turbine control system is composed of a number of computers (usually 3‐4) that communicates with o the measurement equipment on the gas turbine, the driven equipment and the auxiliary systems o the drivers for servo motors o the operator room PC o the service centre via internet The tasks of the control system is basically to automatically operate the gas turbine but also to protect it (for long life) and its environment (fuel explosions, mechanical failures) and to keep track on the remaining life of critical parts to predict need for maintenance. The value of this part of a gas turbine plants is around 10%, so it is significant. The logic of the control system is based on the thoughts of the persons doing the thermo‐, aero‐ and rotor dynamics during the design phase, prescribing what has to be measured, at what rate and with what accuracy. A control specialist is involved to find the computerized solutions, feed backs and time delaysand programmers doing the final work. Start procedure The start procedure is controlled by a sequencing program. Normally a gas turbine plant has to be ventilated by air for some time, often around 5 min, to be sure that there are no explosive gases in the flow path. During that time when the engine is running on ventilation speed, driven by the electric start motor all other systems are activated and checked for accurate performance. Then the engine is accelerated up to ignition speed, the spark plug or torch flame is activated and fuel is injected in the combustor. Now fuel is ramped up in a preset way and the rotor is speeding up partly by turbine power partly by the electric motor to idle conditions. Here a power plant unit is synchronised to the net and the acceleration continues up to a prescribed load or to full load. The acceleration is normally done at a fairly low ramp in order to reduce thermal stresses in the components. Quick start up is possible, but with reduced component life. Some reserve units can be started very quickly and be on load in a couple of minutes in order to pick up load at a power failure. Operation During operation the control system has the task to control speed (50 or 60 Hz) and the load. For a long time this was done by fast analog controllers, but today the systems are all digital. Operating at full load means that the engine is running against a limitation, which can be different depending on site conditions. Normally the limitation is turbine inlet temperature in order to protect the turbine for over heating and assure a prescribed life time. However during very high ambient temperature the turbine exit temperature may get too high and at very low ambient temperature the power output (generator power limit, gear box strength or shaft torque) or the rotor speed may be too high. Shut down Normal shut down is done slowly to reduce thermal stresses. But quick stops, trips and load rejection have to be handled. A trip for an internal failure signal or from the electrical net must be followed by an immediate fuel shut off to prevent explosions and over‐speed of the rotors. Rotors are generally designed to take some 25‐30% over speed (rotor burst or blade loss), but normally the control system is required to keep the over speed < 10‐ 14%. A load shedding is a sudden reduction of load without shutting down. It is a tricky control situation in which fuel is reduced to a very low flow rate but still keeping combustion alive in order to take up load at a lower output. Combustion control Most gas turbines have today Dry Low NOx combustion systems of the Lean Premix type. In these systems fuel and a surplus of air is premixed before combustion and the flame is burning at a relatively low temperature rather close to the lean flame extinction limit. The operating window between bad combustion (indicated by unburned fuel products typically CO) and high NOx is small. Many different control strategies are used by different manufacturers. Almost all includes a pilot flame which helps to stabilize the main flame even at very lean conditions. A single shaft gas turbine with a modern compressor often has the capability to rather closely adjust the air flow to the fuel flow by the variable inlet guide vanes, but that is not possible with the twin shaft machines. Fuel staging, air bypass, air bleed off are then tricks to be used. The operating stations The gas turbine operation is done from a PC, very often situated away from the site, in which one can find a number of pages providing necessary information: the start up sequence step by step, the valve operations in the fuel system, log sheet for all relevant data for performance evaluation etc. A commissioning engineer can operate the unit from the small signal handling module, using a mobile PC. It is possible to access to control system via internet from a service centre. The service personnel can check the performance, see the operating statistics and even operate the unit (which is generally not done from responsibility reasons). Many small problems can normally be sorted out by this possibility and sometimes even critical situations can be avoided. The data logger There is also a log system in which all data is saved. To the log system different modules can be attached for analysing trends in performance, bearing vibrations, emissions etc. If a sudden shut down occurs the log data very often can help to tell why and during tuning of the different operating modes the log data are very valuable. The most common module is the bearing vibration analyser. Deviations from the normal vibrations patterns are the most sensitive signs of faults in the engine. In many countries there are NOx regulations which requires some sort of emission data logging. In Sweden there is a ‘zero‐tax’ on NOx emissions; if you are below the mean you are paid, if you are above you have to pay. Compressor washing There is a performance deterioration trend in a gas turbine. Due to the high air flow, dust particles enter the compressor in spite of the inlet filters, causing depositions on blade and vane surfaces. The flow capability goes down and the friction losses increases with lower performance as a result. The rate of deterioration is very different between sites and there is usually an economic optimum for when to wash. Real compressor washing has to be done with the engine shut down and somewhat cooled down, running at low speed. The compressor is ‘soaked’ with water with detergent and rinsed carefully. This procedure takes some 4‐5 hours and gives almost a full performance recovery. Many operators try to prolong the time between such stops by using on line washing by just spraying water into the compressor inlet. The first very important blades and vanes can be cleaned in this way while the dirt is moved downstream in the compressor. The power recovery is limited, but the method can be economically sound. There is a Swedish company that successfully provide systems for on line washing. Conclusions In this lecture you have got a very light introduction to gas turbine technology without going deeply into the basic thermodynamics of the Brayton cycle. You have got an introduction of the build up of a gas turbine plant. The basics for operation and control has been discussed: the start up schedule, the full load limitations, the different stops and the combustion control. References [1] Operation and control of industrial gas turbines Power point presentation 2009 by Torsten Strand [2] Centenary of the first gas turbine to get net power output: a tribute to Ægidius Elling Lars Erik Bakken, Kristin Jordal, Syverud and Timot Veer. ASME GT2003‐38069 Control questions: 1. Explain the differences between the different turbine inlet temperatures! 2. Which are the full load limitations and when/why do they appear? 3. What is the maximum blade material temperature and how can it be kept at acceptable level in the hot section of the turbine? Security of electricity production This lecture is about the design philosophies and methods for achieving a reliable power plant with high availability. Introduction Secure electric delivery to the end customer involves a long chain of activities; the main ones are the fuel chain, the energy converter (gas turbine plant in our case) and the electric distribution system. The fuel questions will be discussed in the next lecture. The electric distribution systems In this lecture this part will not be expanded. It is just pointed out that by the interconnection of the electric grids over continents, power demand is even out but also the risk for a total black out has increased. The reason is mainly that the large nuclear power plants have very strict security systems. A small disturbance on the grid can cause a plant to trip out, causing a little larger disturbance by which more plants are tripping out etc. With many large plants out, the frequency variation can be so large that most plants go down and there is a blackout. In Sweden we have had a few total blackouts but during the last 10 years the problems have been local; trees falling on airborne power lines due to hurricanes or snow storms. The critical air power lines are now being replaced by ground cables. The power plant The dominating sales point for a power plant is ‘availability’. A power plant must be Available when needed otherwise the owner loses money and reputation and the customers are frustrated (an industry looses production, some plants can be difficult to start up after a failure etc). There are a few important statistical values used to characterise the plant performance • reliability • availability • mean time between failure • mean time to repair • forced outage rate Typical availability values are 91‐99% for turbine plants. The values can be influenced by a number of activities starting already at the design of the turbine and the turbine package. Unavailability hours The unavailability consists of the planned outage hours for service and maintenance and the unplanned stops due to failures. The planned outages depends on a the prescribed maintenance schedule which is a strategical decision depending on what is acceptable or competitive on the market place and what is technically feasible. Secure design methods It is then up to the design teams to realize the maintenance scheme, that is to design components with acceptable life and maintainability. One way to secure a design is to use FMEA (Failure Mode Effect Analysis), a systematic way to analyse a product, a component or just a part. It is used by many gas turbine manufacturers. Redundancy Redundancy is a word for a lot of practical measures that can be used for avoiding a failure, e.g. a plant trip, or reducing the effect of a failure. The most common way of redundancy is to make a backup, e.g. two valves instead of one. To build in redundancy is one very important way to improve availability and avoid serious problems. Redundancy is used on all levels in the energy supply systems. The electric grids are fed from different directions, reserve power e.g. fast starting gas turbines are installed, district heating systemshave storage tanks, industries have backup boilers etc. There are several standards e.g. the API (American Petroleum Institute), Gasuni, TUV standards that prescribe redundancy of gas turbine power plant systems e.g. the lubrication system which must have three pumps operating on different supplies in order to prevent bearing failure. The turbine OEMs (Original Equipment Manufacturer) generally offers different levels of redundancy on most systems. Many unnecessary trips occur due to signal errors into the control system, so now the rule is very often 2 out of 3; if 2 signals out of 3, sometimes 3 out of 3, give the same indication action is taken, alarm, load drop, shut down or trip depending on the seriousness of the signal. It is not always easy to provide three fully independent signal chains. In the control system there are often at least two parallel computers that are checking each other. In the combustion system it is normally required that there must be two independent flame detectors, which must be very fast. A third fast system is not so easy to install. Extreme redundancy schemes are implemented within the oil&gas industry. On off shore platforms very often three gas turbines are installed; each of them can deliver all necessary power. Two are in operation running at ½ loads, one is in stand by. If one unit trips the other one takes the full load and the stand by unit starts. The same goes for compressor stations along pipelines. Floating Production Storage Offload ships for oil production often have at least two propulsion units. Component life There are different views of component life. The traditional is that the component after some service time has reached a limit (creep, corrosion, oxidation, low or high cycle fatigue). Most parts in a gas turbine have very long life <100 000hrs, but some parts have a limited life. The hot section components are subject to creep, low cycle fatigue and oxidation. All companies have their design criteria e.g. 40 000 hrs creep life, 2000 cycle fatigue life and 20 000 hrs oxidation life of blade coatings for industrial turbines. Jet engines and aero derivates (Stationary units derived from jet engines by some modifications) have generally shorter creep life but better LCF life (many starts/stops). Studies of all components for air craft engines have revealed that there are several different life patterns. It was found that quite few components, 2% (probably the hot parts) followed the above pattern. Quite a few parts have long indefinite life, but the dominating pattern(72%) was ‘infant mortality’. New parts tend to have faults! Is that a result of present trends in manufacturing control? If you are following the prescribed manufacturing process (which includes control measures) according to ISO4000 the end product will be OK! No final control before delivery or control of incoming goods is supposed to be necessary! Classification of plants regarding requirements on reliability and availability Highest reliability is required on plants with risk for personal injuries (nuclear power, air flights, marine propulsion…..), but also for plants were stops can cause high primary or secondary costs (primary cost; loss of sale of power, secondary loss; stop in connected plants, e.g. paper mills, oil platforms) Service and Maintenance For small and midsize gas turbines there are two service concepts available; on site service and repair or engine change out. By the last concept it is possible to reach above 98% availability even with a not too reliable unit, if the exchange process is well organised. On site service and maintenance is cheaper and can very often be done when the units are out of service for planned reasons (district heating unit not in operation summertime, main complex stopped during vacation….) A major overhaul is however by preference done at a service centre where spare parts, special tools and experienced personnel are readily available. Summary In this lecture different views on and ways how to influence reliability and availability have been discussed. Redundancy, systematic risk analysis of new designs, component reliability and maintenance concepts have been demonstrated as common and effective ways to improve availability. [3] Security of electric supply 2009. Power point presentation by Torsten Strand Control Questions: 1. Define availability and what levels are normal within for a gas turbine power plant 2. What is and Equivalent Operating Hour? Explain why a start is supposed to be equivalent to many operating hours. 3. Try to use FMEA on some component or product close to you! ...
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