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How to reasonably start up and

shut down the steam turbine

Li ZhongyuanAbstract The reasonable methods of starting up and shutting down

steam turbine are a main subject for study to ensure turbogenerator economic efficiency and safety. All measures of reducing potential safetyhazards during the process of start-up and considering frugal operationare based on theoretical analysis and the experience, and with a purposeof reasonably shortening the start-up duration. This paper makes a broadexploration on optimizing start-up and shut-down of steam turbine ,andon the parameter control during rotor-driving and warming up periods,the start of pumps and flange bolt heating units, as well as the differentialexpansion control during the moment of shutdown,etc.

Keywords Steam turbine; start-up; shut-down

Starting up steam turbine is a heating process. The purpose of heatingis to keep each position of steam turbine obtaining sufficient preheating,decreasing temperature contrast of cylinder flange internal and externalwall, decreasing rotor surface and center temperature contrast, thus

minising metal internal stress, and making flange cylinder and rotor

expension,assuring internal activity clearance of steam turbine .Shutting down steam turbine is a reverse process.When we start up steam turbine, we often encounter the problem that

metal temperature raises quickly or slowly . To some extent, it affects thesafe operation of generating set, and causes that the network units can not

be connected to the grid . During the moment of shut-down , it oftenappears that the negative differential expansion constantly rises, until itincreases more than the alarm value, and sometimes approaches

protective value of tripping, which seriously affects the safe operation of the unit.

1 start-up of steam turbine1.1 Warming up the tube wall

Warming up the tube wall is on purposes of conducting heat evenlyand preventing excessive temperature rise, which causes the internal wallof Pipe tee has a significant thermal stress, so that pipeline and itsaccessory have a crack or deformation . Therefore, the low-pressure pipewarming must gradually increase steam pressure based on the speed of the metal wall temperature increase; In addition, the speed of the wall

temperature increase is related to the flow of steam pipe, and excessivesteam flow will heat pipe components intensely, so we should pay great

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attention to adjusting t he total steam valves or keeping drain-valve openin order to control the steam flow when we warm the low-pressure pipe.We should strictly control the speed of temperature and pressure rise.

1.2 start-up of pumps

We should postpone start-up time for saving the station-service power. The condensate Pump is an important auxiliary equipment, its power consumption is great. The start-up of the condensate pump can besaid to be a waste of electricity before sending steam into the seal, butwhen we send steam into the seal in order to creat vacuum and protect theheater, the pump should be run just-in-time. The generator cooling water

pumps, just starts before rushing to turn, makes the water get uniformlyinto generator rotator and stator.

Premature starting is meaningless, because generator coil cannot produce heat before operation with grid, it neednt be cooled. If we makethe water get into generator coil after rushing to turn, two unfavorablefactors will appear: first, rotator shaft seal filler will be damagedeasily;secondly,water gets into generator unevenly, that will makegenerator rotator produce larger centrifugal force.Therefore,generator cooling system should be operated at the moment before rushing to turn.

1.3 start-up of the heater Random start-up of the high and low pressure heater can make the

heater be uniformly heated. It is beneficial to prevent the outlet of copper tube from water leakage, and to prevent the heater from deformation

because of the larger thermal stress. The steam extraction pipeline thatconnects the heater joins low cylinder, if the heater starts up randomly,that means adding cylinder drainage point, and makes the temperaturedifference of upper and lower cylinder decrease.

But we maybe encounter the condition of the lower cylinder temperature is higher than the upper in the turbine start-up process.Taking Yumen Power Station 2# turbogeneration for example,Thiscondition often appears in the stage of low-load warming up, itenormously extends the warming up time, and the temperature maybeexceed specified value (50 ), in this time ,we should shut down theturbine, then restart it when the temperature goes down, and properly

prolong the warming-up time. To all appearances, the high and low pressure heater cannot be started up in this condition. So start-up of thehigh and low pressure heater should rather base on concrete conditionthan uniformly adopt the method of random start-up.

1.4 running of the flange bolts heating deviceThe proper running of flange bolts heating device is conducive to

control cylinders expansion, to reduce the differential expansion andimprove the rate of increase in load. The part that first contacts the steam,

its metal temperature will rise faster. and the metal temperature will rise,so the heat inside and outside of the cylinder flange is uneven and easily

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produce a greater thermal stress in the upper and lower cylinder, thecylinder flange inside and outside wall, flange and bolts. This kind of deformation causes to dynamic and static friction, and to damageequipments. So we should start the flange bolts heating device according

to cylinder and flange temperature condition.1.5 controlling the differential expansion

Because the differential expansion range is smaller( 1 .5 4mm ),iteasily appear that the differential expansion approaches specified value inthe turbine start-up process, we often adopt the method of extending thewarming up time or decreasing steam parameter to reduce he differentialexpansion. Apparently, the method of extending the warming up time isnot conducive to economical turbine start-up.

Practice proves that the method of adjusting the condenser vacuum tocontrol the differential expansion is easier. Specific method:openningdamage-vacuum valve can decrease vacuum effectively, the vacuumdecrease will affect the turbine inlet steam quantity and exhaust steamtemperature, nevertheless, exhaust steam temperature increase willimprove cylinder expansion, and this condition can effectively control the

positive direction rise of differential expansion, that means turbine shouldrush to turn by lower vacuum, this method can be beneficial to control thedifferential expansion and to warm up turbine.

2 shut-down of steam turbineShut-down process should be in strict accordance with the rules of

order to reduce the load, the load will be reduced with the speed about 5MW load each 5 min, because the load reduced quickly, which easilymakes the negative differential expansion become larger and turbinemetal temperature drop too quickly.

When turbine rotational speed attends zero, we should keep water quantity in condenser, to prevent cylinder exhaust steam temperature risesirregularly.

3 ConclusionsAny reasonable start-up or shut-down process is from practice. The

foregoing statement about optimizing operation is mainly focused on being reasonable to shorten the start-up time, reduce security risks of

running decommissioning and other aspects. Practice has proved that

these methods are effective.

Reference: Wang Zhiwei.Turbine operation technology [M]beijing;cepp ,2007.

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