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Mechanical Engineering


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parsottam last won the day on October 22 2018

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    be mechanical
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    BVM, V V Nagar

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  1. The main functions of the governing system of a large turbine-generator unit used for electricity production via an extensive power network are: To contain the speed rise within acceptable limits should the unit become disconnected from the load. To control the steam valve positions (and hence the load generated) in response to signals from the operator, or from a separate station automatic control system. To control the initial run-up and synchronisation of the machine. To assist in matching the power generated to that demanded by responding to network frequency changes. The first of these functions is a vital one for the safety and availability of the plant. Consider a unit at full-load supplying a power network through its generator con nections. If these connections are opened, either by a power system fault or by the unit's own protection system, the steam flow at the instant of disconnection exceeds the steam flow necessary for steady state opera tion at 50 Hz (3000 r/min) by the amount necessary to generate full-load. The excess energy input must be reduced rapidly if an unacceptable overspeed is to be avoided. The governor performs this function by re sponding to the initial change in speed to close the steam valves. Separate overspeed trips (described in Section 3.5 of this chapter) are provided to guard against possible governor failure and ensure that the safety of the plant and personnel is always maintained. The governor supplements this safety function and, since it maintains the unit at the running speed, ensures the continuity of unit supplies from its own generator as well as the ability to reconnect the generator to the power system. The initial transient speed rise following such a load rejection, contained by the governor, is pri marily due to two factors: The stored energy of steam within the turbine and its associated pipework. The time taken by the turbine valves to close in response to the sensed overspeed. It is always well inside the overspeed trip setting and a full analysis is given in Heilbronn [1]. An electrical governing system for a typical turbine-generator with multiple steam admission paths comprises many elements, as depicted in Fig 2.1. Since it includes at least one closed-loop control function, the machine and network characteristics form an integral part of the system. The primary feedback is of turbine shaft speed which is usually measured by a toothed-wheel and probes at the HP end of the machine. This signal is processed by a modular electronic system, often mounted in a cubicle quite remote from the turbine, to form output signals which are directed back to each steam valve on the turbine. The processing is complex and is subject to detailed variations for each application; it generally includes the following: The speed/load characteristics of the machine when synchronised. A predetermined relationship between the high pressure (HP) and interceptor valve position. Facilities for operator control. Features to limit the maximum speed of the machine. Features to limit the output in the event of abnormal operating conditions. Features to permit routine proving and testing of the system. The above functions are described in detail in Sections 1.2 and 1.3 of this chapter. Fitted to each steam valve is a relay, whose function is to convert the low power electrical signal formed by the processing equipment into the movement of the steam valve. Since the mechanical forces involved are substantial (150 kN) and the time for full stroke may be less than 200 ms in the case of a load rejec tion, several stages of hydraulic amplification may be necessary. Conversely, in order to obtain fine control over load (or speed when unsynchronised), the gov erning system needs to be very sensitive and capable of moving the valves to within about 0.2% of the required position. The needs of high resolution and the ability to amplify small electrical signals, necessitates the use of precision hydraulic components with fine clearances. Although earlier mechanical/hydraulic governing systems shared the lubricating oil supply, adequate reliability of the precision systems is only assured by the use of a separate high quality fluid supply unit. Various configurations of valve relay and typical fluid supply unit characteristics are described later in this section. One of the features of an electrical governing system is that since the conversion to mechanical movement is made at the steam valve relay, all other interfaces are electrical. This facilitates connections to station automatic control systems, alarm systems, data process ing systems, switches and indicators both on the operator's desk and at the governor cubicle. All these other systems are closely associated with either the opera tion or maintenance of governing systems.
  2. HAZOP, which stands for hazard and operability study, is used to identify abnormalities in the working environment and pinpoint the root causes of the abnormalities. It deals with comprehensive and complex workplace operations, which, if malfunctions were to occur, could lead to significant injury or loss of life. HAZID stands for hazard identification. It is more of a general risk analysis tool designed to alert management to threats and hazards as early in the process as possible. The classification is made on the basis of probability and consequences. A HAZID study provides a qualitative analysis of a work site in order to determine its worker safety risk level. Both HAZID and HAZOP are risk analysis tools used in the workplace
  3. vibration is useful in many industrial applicable like material handing system, like screening..
  4. what is called positive displacement? where the term positive displacement is applicable? Is there is any other term for positive displacement? Where it is beneficial over there opposites? everyone to answer about above...
  5. What is special precautions required for selecting fire rated rolling shutters? what is the basis and material used for designing fire rated rolling shutters? Up to how much time it can withstand fire without damage?
  6. Why multi stage OH1 or OH2 type pump are not used in refinery, chemical and petro-chemical, fertilizer ans Oil & Gas industries?
  7. it is force lubrication system, you may check API 614 for shaft lubrication system to study in depth..
  8. I am looking for major difference in 2nd and 3rd edition of API 675. Can any buddy help me..
  9. In centrifugal compressor if suction pressure remain constant, discharge pressure also constant, no change in physical parameter of compressor and accessories and temperature is reduced what happen to the compressor capacity. what about volume flow is it remain constant or reduced? what about mass flow is it increased or the constant?
  10. Obvious it is 4 cylinder engine as specified in question it self.. This different arrangement are for the load balance is the prime...the animation is for cranks shaft outline.. if above is not correct, can you elaborate the crank shaft design with load distribution in details?
  11. Sequence of power delivered in a multi cylinder engine called firing order. It gives balance to engine and load is uniformly distributed in crankshaft. Proper firing order minimise vibration, smooth running and gives high life to the engine. Firing order for 4 stage engine may be 1-3-4-2, 1-2-4-3, 1-3-2-4, 1-4-3-2 depending on design of crank shaft and cylinder by designer.
  12. What are the different type of gear box? What are the different between general purpose and special purpose gearbox? what are the safety feature for gearbox in hazardous area?
  13. Pre-heat is required for the following reasons: (1) It lowers the cooling rate in the weld metal and base metal, producing a more ductile metallurgical structure with greater resistant to cracking. (2) The slower cooling rate provides an opportunity for any hydrogen that may be present to diffuse out harmlessly without causing cracking. (3) It reduces the shrinkage stresses in the weld and adjacent base metal, which is especially important in highly restrained joints. (4) It raises some steels above the temperature at which brittle fracture would occur in fabrication. Post heating is required to remove further residual stresses present in the weld pool after heat treatment process also. If post heating is not done, then residual stresses develops as cracks and propagates throughout, resulting in the failure of the material. Also Preheat/Post weld heat treatment can be used to help ensure specific mechanical properties, such as notch and toughness.
  14. In many over head crane with manual operation, chain drive is provided. In that one wheel is provided with chain and with help of chain one can rotate that wheel to move the crane accordingly... One more example, manual operated shutter of warehouse, where chain is provided to open and close the door as required. Sizing and selection of that chain, and chain wheel is based on applicable load.
  15. In case of coal based power plant, material handling system (supply of coal to boiler and waster management) is most common problem. Maintaining quality of water , Other routine problems like stream trap failure, operation of High pressure pump, maintaining the quality of fuel to maintain pressure and temperature in boiler etc..in case of steam turbine leakage of coil /tube bundle of heat exchanger , etc. There are many other problems which can be eliminated by preventive maintenance.
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