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Mechanical Engineering

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  2. 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.
  3. 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
  4. vibration is useful in many industrial applicable like material handing system, like screening..
  5. 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...
  6. 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?
  7. cavitation occurs at pump inlets due to lower pressure at suction (lower NPSH or dissolved gas in liquid). where is vapour lock is in piping system due to not providing the venting arrangement.
  8. Triple-volute centrifugal pump can handle up to 20 percent entrained air, where a typical centrifugal pump can only deal with air entrainment levels of 5 to 8 percent. Along with entrained air, cavitation is a top candidate for causing pump problems. Cavitation occurs when the pump’s internal pressures are lower than the vapor pressure of the liquid which results in rapid vapor formation within the pump which collapse as the liquid is swept into the higher pressure regions of the pump. The cavitation effect may cause material damage to the impeller and possibly casing, which is resultant of the sudden formation and implosion of vapor bubbles. The frequencies recorded of cavitation “hammering” are from 1,000 cycles per second up to 25,000 cycles per second and the resultant damage is generally termed pitting. The noise (sand, gravel, rumbling) heard outside the pump during cavitation, is caused by the collapse of the vapor bubbles. The energy expended in accelerating the liquid to high velocity in filling the void left by the bubble is a loss, and causes the drop in head associated with cavitation. The loss in capacity is the result of pumping a mixture of vapor and liquid instead of liquid. Even a slight amount of cavitation will reduce the capacity significantly. Entrained air gets into a pump, the lower-pressure bubbles become larger. If an air bubble gets big enough to cover the impeller eye, the pump becomes airbound. A generality to keep in mind when evaluating entrained air vs. cavitation is this: 1) If it’s entrained air, the liquid entering the pump already has liquid and air. In the pump it’s liquid and air. And it comes out liquid and air. 2) With most traditional cavitation, the liquid coming into the pump is fully liquid. As soon as it hits the inlet of the pump, it starts to vaporize and comes out as liquid. Only about 75 percent of cavitation creates pump noise but the material damage is always there. In cavitation, the pitting damage on the impeller. Three rules of thumb for determining whether cavitation is causing pump performance problems: 1) Throttle the discharge and check the noise – Throttle back the pump on the discharge (not the suction) to lower the flow rate. If the pump noise goes away, it’s about an 80 percent probability that cavitation is the problem. 2) Raise the liquid level and check for noise – If you raise the liquid level in the supply tank and the pump noise goes away, it’s about an 80 percent probability of cavitation. 3) Cool the liquid and check for noise – If the process liquid is normally at 200°F and you cool it to 180°F and the pump noise goes away, it’s probably cavitation.
  9. Vibration is usefull in many cases, conditions is only controlled vibrations..explore the case where vibration is useful and in what way.. Example: 1. Vibration is usefull in all musical instruments, music is created beacuse of controlled vibration. Without vibration music is not possible. Explore more benifits of vibration in industries.

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