Steam turbines start-ups

1 TRANSACTIONS OF THE INSTITUTE OF FLUID-FLOW MACHINERYNo. 126, 2014, 169 198 MARIUSZ BANASZKIEWICZ Steam turbines start-upsThe Szewalski Institute of Fluid Flow Machinery, Polish Academy of Sciences,Energy Conversion Department, Fiszera 14, 80-231 Gda sk, PolandAbstractThe article presents a procedure and the chosen results of numerical computations of liquid flowparameters, which use the data from their measurements realized on the test stand, for themodel of formed suction intake with a rib, supplied by the screened open wet well. Comparativeanalysis of numerical computations and these determined bymeasurements, did concern thestandard flow acceptance Principles of Steam turbines start-upsSteam turbines are being started based on operating instructions developed byturbine manufacturers and constituting an integral part ofstart-up instructions ofa Steam unit.

2 The operating instructions describe the orderof executing start-upactions and provide the values of operating parameters thatshould be maintainedduring start-ups . Rigorous following the order of individual start-up phases andmaintaining the limit values of operating parameters is crucial from the point ofview of safety and reliability of turbine operation. The start-up phase is a particu-larly dangerous and complicated phase of Steam turbine operation as it consists ofstarting numerous equipment and auxiliary systems, and mechanical and thermalprocesses taking place have nonstationary nature ( , transient heating, varyingsteam flows, acceleration of rotors, vibrations, etc.). Theprinciples of correctconducting start-ups have been developed over the long-term operation of steamturbines and contain experience of turbine designers, constructors and basic phases can be distinguished in the process of Steam turbine unitstart: E-mail address: Banaszkiewicz turbine preparation to start-up, running-up to idle run and synchronization, preparation to start-up begins with barring the rotor by the turning turning gear operates as teeth gear, driven by an electrical (electrical turninggear, see Fig.)

3 1) or hydraulic (hydraulic turning gear) motor. Prior to start-upsturbine rotors must be thermally stabilised, and their deflection line must coincidewith the gravity deflection line. Rotor cooling down during standstill would beleading to its thermal bent due to faster cooling from the bottom, which wouldultimately lead to rubbing and damage of labyrinth to cold start, the turbo-set rotating system should berotated using turn-ing gear as long as possible. Particular care should be paid to the start-up withbarring time shorter than 72 h. In such a case the turbo-set will need prolongedheating-up at intermediate rotational speed. The heating time is limited by theachievement of proper dynamic state during continued turning gear can be switched off only when the highest temperature ofan high-pressure (HP) and intermediate pressure (IP) innercasing is lower than100oC.

4 If needed, the turning gear can be switched off for 15 min after a dayof barring, and after 2 days of barring for about 30 min on thecondition thatvacuum is broken and gland Steam supply is switched off. The maximum time ofturning gear standstill depends of the type of 1. Electrical turning to repeated switch on of the turning gear, turbine rotor must be man-ually turned by one revolution in order to check if rubbing has not taken rubbing is found it is necessary to wait until the rotor rotates freely and onlythen the turning gear can be repeatedly switched turbines start-ups171 Prior to switching on the turning gear, oil system is startedup. It is alwaysnecessary to check whether lube oil pumps has started and whether all bearingsare supplied with oil. Control of lube oil pressure and temperature, as well asrotor axial displacement and eccentricity is necessary.

5 The turning gear can beswitched on only when all jacking oil pumps are in starting the oil system and turning gear, the condensation system canbe started, that is cooling water, air ejection and condensate pumping , gland Steam system is started to prevent air in-leakage. Glandsteam must have appropriate temperature and pressure whichmust be controlledduring start-up. Prior to supplying Steam to the turbine , the Steam supplypipelines have to be drained and heated case of turbines with cooling water preheating and low-pressure (LP) by-pass, prior to supplying Steam the regeneration and dumpingsystems have to Run-up to idle run and synchronisationOne can distinguish three major types of start-ups : from a cold state when HP and IP inner casing temperature is lower orequal 170oC, from a warm start when HP and IP inner casing temperature is lower orequal 430oC, from a hot state when HP and IP inner casing temperature is greater state of a turbo-set usually occurs after a 6-day standstill.

6 The differencebetween the above types of start-up consists in the fact thatin the former casesteam supplied to the turbine has greater than casing metal temperature, while inthe latter two cases, depending on the boiler characteristics, Steam temperaturecan be lower or higher than the casing cold starts usually take place after long standstills, hence in such casesinspection of turbine and equipment according to their instructions is Steam parameters required for cold startLive and reheat Steam temperature must be at least 50oC higher than the sat-uration temperature and for instance for Steam pressure 5 MPa live Steam tem-perature should be min. start-up for a cold state should be conducted with stabilised livesteam parameters which for a high power output unit with Steam parameters535oC/18 MPa typically assume:172M. Banaszkiewicz live Steam :p= 5 MPa,T= 350oC 425oC, reheat Steam :p= ,T= 350oC open HP and IP stop valves, condenser vacuum should be at least MPaabs.

7 Typical recommended condenser vacuum is on the level MPa abs,however it strongly depends on the cooling conditions. It isnecessary to check ifdifferential expansion and shaft eccentricity are within the allowable limits. Wheneccentricity exceeds the allowable value, it is necessary to wait turning the shafton the turning gear and with open by-pass until shaft eccentricity is lower thanthe limit HP and IP stop valves can be opened, with control valves closed. In theturbine governor the target rotational speed 3000 rpm is setas well as run-up rate(for instance 75 rev/min2). In case the target speed is set on the value betweenthe turning gear speed and 3000 rpm, one should avoid settings in the range ofcritical speeds. Prior to automatic run-up it is always necessary to check on lowspeed the rotor dynamic behaviour, temperature distribution in oil system andbearings, as well as auxiliary systems operation.

8 When the proper condition ofthe turbine is confirmed, the automatic run-up can be startedwhich in turn canbe terminated any time. In case an attempt is made to stop run-up in the rangeof critical speeds, turbine run-up will be stopped only whenthe critical range hasbeen passed, as it is forbidden to stop run-up in the range of critical speeds ofturbine and generator start-up can be done through all its cylinders simultaneously ( ,HP, IP and LP) or with by-passing some of them ( , HP) in order to ensurebetter start-up conditions. For instance, for 360 MW turbines , depending on theHP inner casing temperature, one can distinguish two modes HP and IP valvescontrol: metal temperature<200oC idle run load is mainly covered by HP andLP turbines , metal temperature>200oC idle run load is mainly covered by HP run-up it is necessary to monitor vibrations and indications of HPand IP temperature probes.

9 When excessive vibrations occur, the turbine speedshould be lowered and kept constant until vibrations are within the permissiblelimits and continue with running-up. To reduce excessive vibrations the turbinespeed must be kept constant for a longer period of time. Typically, such a stopis done at 1000 rpm. If during run-up no excessive vibrationsoccur it is notnecessary to heat-up the machine on intermediate speed. During running-up it isimportant to pass through the critical speeds with high rate(600 800 rev/min2).The permissible value of rotor eccentricity below 600 rpm istypically equal toSteam turbines start-ups173200 mp p. One the threshold is exceeded, automatic trip of turbine takes 600 rpm rotor relative vibrations should be monitored. The real values ofcritical speeds should be measured at first start-up, included in turbine operatinginstructions and programmed in the range of rotational speed 0 3000 rpm high power output turbo-setshave a couple of critical speeds.

10 Theoretical values of critical speeds can be asfollows: I frequency: generator rotor 940 rpm, LP rotor 1490 rpm, HP rotor 1900 rpm, IP rotor 2230 rpm, II frequency: generator rotor 2580 running-up with maximum rate can be done when temperature probessignals are> 40% or<+80%. Once these threshold values are exceeded, therunning-up rate should be lowered so as to at values 50% and+100% it is equal0 ( turbine run-up terminated).At 3000 rpm auxiliary oil pump can be switched-off. After the pump has beenstopped, the jacking oil pumps are switched-off automatically. During start-upthe operation of LP turbine should be supervised with special emphasis on: Steam temperature downstream the penultimate stage, partial vacuum break at turbine shutdown, protection against condenser pressure parameters before turbine depend on its thermal statewhich is deter-mined by standstill duration.