lesson 14 steam turbine protection

the major protective device on the steam turbine is the main steam stop valve if it becomes necessary to stop the turbine due to an abnormal operating condition the stop valve must be tripped closed in the case of a reheat turbine both the main steam stop valve and the reheat stop valve will be tripped simultaneously we have already learned that the stop valve is closed by actuation of the pilot relay or trip relay which drains oil pressure from the hydraulic system there may be several trip relays located on the hydraulic system actuated by different protection devices operation of any trip relay should cause the following action closure of main steam stop valve closure of reheat stop valve closure of steam admission valves positive closure of extraction line non-return valves trip open the generator breaker trip open the generator field circuit breaker and activation of alarms and Annunciation so what are these hazardous conditions that are considered severe enough to automatically trip the turbine well the most dangerous condition that must be avoided is that of overspeed and this could occur in the case of a load rejection for example similarly we would get the same result of the generator breaker was tripped open by mistake when the unit was loaded to about 50% capacity of course the turbine generator would immediately increase speed because at the instant of the breaker trip a very large quantity of steam is flowing through the turbine under normal circumstances the governor should be able to close the turbine control valve sufficiently to bring the speed close to normal within a few seconds however imagine what would happen if the governor failed to act properly or the power piston seized or some other defect prevented movement of the control valves in this case steam flow to the turbine would continue with that the speed would rapidly rise to two or three times normal in practice the speed would not get that far because centrifugal force acting on the mechanical permanent generator rotor would cause them to break up several instances have been reported of turbine blades and sectors of turbine wheels passing right through the turbine shell into the turbine room with the potential for injury to plant staff and damage to other equipment in order to prevent this possibility one and sometimes two quite separate over speed protection devices are installed and set to operate at 10 percent and 12 percent over speed each protection device operates a trip relay to close the stop valve and other protective action as already discussed here we see an example of a traditional mechanical overspeed device essentially it consists of a bolt inserted into the shaft as the shaft rotates the bolt tries to fly out by centrifugal force but this is restrained by a compression spring the spring is carefully adjusted so that the centrifugal force does overcome the spring compression at ten percent over speed the bolt been mechanically trips the latch which allows the trip relay to move to the trip position so draining off hydraulic oil and closing the stop valve you will remember that tripping the stop valve also actuates the positive closure of non-return valves on the extraction lines this is necessary to prevent entrained steam being drawn back into the turbine and aggravating the overspeed condition another common type of over speed relay uses electrical measurement of speed and an electrical circuit to initiate operation of the trip relay through its solenoid it is vital that these over speed trips be checked for operation at regular intervals and the actual tripping speed be recorded now this may be done when the unit is shot down or started up adjustment of the setting may have to be made from time to time this protection device is one of the most important on the term on a condensing turbine we expect pressure in the condenser and therefore of the turbine exhaust to be between one to three inches of mercury depending upon the actual cooling water system now imagine what would happen if the back pressure started to rise perhaps due to a large akin to the condenser or more likely a reduction in cooling water flow through the condenser now remember as the back pressure increases so does the density of steam at the turbine exhaust with the back pressure of 10 inches the density would be about nine times greater than the dendy at one inch back pressure this increase in density greatly increases the friction on the LP blading which is still turning at 3,600 rpm the result leads to overheating and possible damage to the low-pressure bleeding in order to prevent this situation of vacuum to ously monitors the back pressure and it is usually set to operate a trip relay when the back pressure reaches 10 inches of mercury the monitor will sound an alarm with a back pressure reaches 5 inches mercury so as to warn the operator to take action before a trip occurs while we're talking about back pressure of the turbine exhaust let's consider what would happen if during the startup we passed steam through the turbine before establishing cooling water flow the condenser well the steam entering the condenser would not condense and consequently would build up pressure eventually to such a level as to rupture the exhaust hood in order to prevent such an occurrence a large relief diaphragm is fitted to the exhaust hood this is normally held in place simply by the vacuum inside the pan that mus fear ik air pressure outside as soon as the pressure inside the hood rises above atmospheric the diaphragm acting as a relief valve to prevent any damage to the exhaust hood casing this diaphragm works quite independently of the mechanical devices and hydraulic mechanism that we have been discussing usually the arm the attention required is to make sure that the water seal around the edge of the diaphragm is not leaking at allowing air to enter the condenser another important protection device that is set to trip the thrust bearing failure monitor this equipment is fitted to precisely measure the position of the thrust collar in relation to its pedestal as long as the thrust bearing enough either side of the collar is operating correctly there will be no change in the relative position of the collar even when the turbine shell and the turbine rotor expand if the thrust bearing were to fail allowing axial movement of the rotor we could run into the serious problem of contact between the rotating and stationary blades therefore it is essentially the first sign of thrust collar moving failure of the thrust bearing the trip relay be activated so as to close the stop love and bring the turbine to a halt another condition which will cause the turbine stop valve to trip is loss of bloom oil pressure if lube oil is not available to provide lubrication to an operating machine the bearings will rapidly overheat and fail with possible disastrous consequences for other components of the turbine and generator bloom oil is monitored continuously and if pressure falls or or a very low value a trip relay will be actuated to trip the turbine note that this relay will also prevent the opening of the turbine stop valve if lube oil circulation has not all even established during startup in the case of hydraulic oil and ultimately fitted to indicate low pressure but a trip really a is not always provided this is because the hydraulic equipment is designed to operate in a fail-safe manner if the hydraulic oil pressure fails both the stop valve and control valve will be closed by spring pressure in their respective power cylinders in addition to the protective devices mentioned the operator has the option to trip the unit manually but a Lille fir which is usually located on the turbine pedestal on most units it is necessary for the operator to reset the trip mechanism manually at the turbine pedestal after any trip has occurred the operator can all remotely from the control room this is simply a remotely operated solenoid which operates things this solenoid trip may also be activated by certain generator and boiler protection for example if an internal fault developed inside the generator it would be necessary to trip the generator breaker and the fuels so the turbine stop valve to bring the generator to a stop as soon as possible similarly if a fault on the boiler was serious enough to cause a boiler trip then the turbine stop valve would also need to be tripped at the same time otherwise we would be pulling steam and possibly water out of a shutdown boiler summarizing then let's list the major because that are installed on most steam turbines main and reheat stops valves and associated hydraulic turret relays positive closing non-return valves on extraction steam lines over speed trip low vacuum trip thrust bearing failure trip loss of lube oil pressure trip manual trip lever on the front pedestal remote solenoid trip by operator push button and generator or boiler protection inter tripping it is vital that all protection devices be tested regularly to ensure or that they will actually operate when required in some plants the turbine generator may remain on load for minute hi in this case some form of simulated tests may be carried out for example partial operation of the over speed trip is actuated but actual tripping of the turbine is blocked an extremely important test maneuver is the exercising of the turbine stop valves the test circuit allows the stop valve to partially closed and then open again this test is performed daily in many plants and the objective is to make sure that the stop valve does actually move and not remain frozen in the open position as has actually occurred on some occasions all of the protection devices in the old no use whatsoever if the stop valve failed to close when commanded for a similar reason the positive closing non-return valves on the extraction lines are also exercised daily or at some other regular interval make sure that you are thoroughly familiar with the established procedure for testing protective equipment on your turbines also using the information that we have presented here make sure that you know the function and location of all protective devices on your particular turbines and note that there are certain items of equipment that could be considered protective but they do not actually stop valve and shut down the turbine pressure D loading device is often connected to the hydraulic control valve system the objective of the deep loader is to protect the turbine against falling steam pressure from the boiler for example a defect on the boiler auxiliary plant may reduce steam output from the boiler but as the turbine is not aware of this it will continue to pull the same flow out of the boiler with the consequence that the pressure Falls and will continue to fall as long as this condition prevails the danger is that with falling pressure during the water level in the boiler drum will rise substantially due to swell and this in turn may lead to carryover that is water passing along with the steam into the turbine to prevent this possibility the D loading device monitors the steam pressure if this falls below 80 percent of nominal the D loader starts to close the control valves to maintain the steam pressure at 80 percent of course this will reduce the output of the turbine generator but this is far preferable to damaging the turbine during startup of the unit which may well take place at low steam pressure the D loading device is bypassed and only brought into service when the unit is on load and boiler pressure is normal most of the turbine operations are performed remotely from the central control room but a regular local inspection by a roving operator in traditional control rooms the operator works at a large hood switching controls indicating lights indicating instruments and recording charts for the that is boiler turbine generator and auxiliaries the annunciator panel usually located above provides indication of any alarm conditions near the plant instant Asians and in fact in many converted plants both control systems are employed and in this case the operator works at a console with three P or four CRTs providing indications arms and the means for remote operation referring to turbine operation the indications we are most interested in are steam conditions that is pressure and temperature vacuum usually depended upon certain water conditions lube oil temperatures varying vibration and mechanical condition of the turbine as indicated by the supervisory instruments now in this module we have only been able to cover the most common features of turbine operation using this information as a guide you must make sure to thoroughly learn and become familiar with your own term no doubt each of your machines will have their own idiosyncrasies which become well-known to the experienced operator please switch off the

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