SUB : ROLLING OF TURBINE THROUGH IPT DURING GENERATOR DRY OUT (UNITS HAVING HP-LP BY PASS) A.INTRODUCTION : The turbine is required to be run at 3000 rpm on no load, for very long duration, when generator dryout operation is being carried out. Such no load operation may be required for about 30-40 hours or some times even more than this Working of turbine under such abnormal conditions, causes the exhaust hood temperature to rise beyond the permitted limit. In order to control the rise of exhaust hood temperature, it is recommended to roll the turbine through IP cylinder, thereby increasing steam flow through IP & LP cylinders The HP cylinder is kept under vacuum under such operation. By adopting this method of rolling the turbine from cold state for generator dryout it has been foreseen that the exhaust hood temperature can be kept within the permitted limit. B.INSTRUCTIONS : These instructions are valid only for the units which have HP/LP bypass stations. Prior to steam rolling the unit. relevant protections and interlocks should be got checked. The other systems like oil system. Condensate system vacuum system etc, should be put into service as per the normal procedure given in operation manual. For starting the turbine from the cold state, the following instructions to be followed : 1.Close tightly the bypass valves of NRV in the cold reheat line, so as to stop the entry of the steam in the HP cylinder when HP bypass station is put into operation. 2.Open both the HP cylinder evacuation valves, so that the HP cylinder is kept under vacuum throughout the running of the set. TECHNICAL CIRCULARS3.Close fully the main stop valves and their bypass valves. 4.Open the drains of main steam pipe lines 5.Fire the boiler and raise the steam parameters by opening the start up vent. When 50C superheat is available, the main steam lines are heated up to a temperature of about 250C. The pressure before MSV under such condition shall be around 15-20 kg/cm2. 6.When the vacuum in the condenser is deeper than 540 mm of HgC, open the HP-LP bypass station and heat up the cold reheat and hot reheat lines up to I.V. The drains of HRH and CRH should be kept open. These pipe lines may be heated up to a temperature of about 150 C. 7.The pressure controller of the L.P. bypass station should be brought to at about 0 5 to 0.6 kg/cm2. So as to maintain this pressure during heating of these transfer pipes lines of IPC. 8.Heat the IP transfer pipe lines to 150C by opening the IV by 15-20 mm The control valves should be kept closed and the valves in the drain lines of these transfer pipe should be kept full open. 9.During the process of heating of the IPC transfer pipe lines, care should be taken so that the barring gear may not get disengaged. To avoid disengagement of barring gear, the steam pressure in HRH line should be controlled and if required may be lowered. 10.H.P. casing and H.P. transfer pipe line should not be heated up prior to rolling. The cylinder may get heated by a little due to the churning of the turbine rotor blades during rolling. 11.When CRH, HRH and IP transfer pipe lines are heated up to the specified temperature(150C), then the turbine can be steam rolled through IP admission, keeping HP cylinder in evacuated condition. 12. Before steam rolling check the following a)Eccentricity of rotor is within the permissible limit. b)Metal temperature of hot reheat lines and transfer lines of IPT c)Metal temperature of IV d)Metal temperature of IPC in the steam admission 7one. e) Differential expansions of HP, IP & LP rotors. 13.The main steam parameters before steam rolling shall be Pressure-20-25 Kg/cm2. Temperature-250-280C. 14.Open both the condensate spray valves of steam throw device. 15.Set the pressure set point of LP bypass station at 2-3 kg/cm2 and temperature set point be set at 100C. The operation of HP bypass station may he kept on manual while its temperature control loop must be put on auto with set value at 250C. In case the auto control loops of bypass station have not been commissioned, then the manual control (with close monitoring) can be done. 16.Open the HP bypass station. The flow of steam shall thus be established from boiler to the condenser through HP-LP bypass station. The pressure controller of LP bypass station shall keep the pressure in the hot reheat line at the set value of 2-3 kg/cm2. The temperature of steam before IV shall be around 300C. 17.With the help of speeder gear, open the IV & ESV. Turbine shall start rolling through IP cylinder, Check and ensure that barring gear gets disengaged as the rotor speed is more then 34 rpm. 18.Further movement of the speeder gear in anticlockwise direction shall open the control valves of IP turbine and speed shall rise as the flow through IP & LP turbine increases. The L.P. bypass station shall start closing.The pressure in the hot reheat line shall be maintained of 2-3 kg/cm2 In case it drops, adjust the position of HP bypass valves in such a way so that the pressure in HRH line is maintained. 19.Raise the speed to 500 rpm and carryout the inspection of the turbine Check the bearing vibrations seal inteferences, lub. oil temperature, bearing babbit temperature, axial shift differential expansions of the rotors. Constantly watch the heating up of i.p. cylinder and differential temperature of left and right flanges of cylinder. See that the metal temperature of IPC and transfer pipes have increasing trend, Soak the set at 500 rpm for about 30 minuts. 20. Raise the speed to 12OO rpm by turning the control gear in anticlockwise direction thereby further opening the control valves of IPT. Soak the set at this speed for about 25-20 minutes. 21. Raise the speed to 3000 rpm. without pause by further opening the control valves of IPT. Watch the differential expansions of the rotors The parameters of steam in hot reheat line before IV shall be around. 22. Hold the set at 3000 rpm for the drying out of generator. Keep a constant watch over the differential expansions of the rotors, exhaust hood temperature of LPC, vibrations of the bearings, and steam parameters. 23. Throughout such operation, the HP rotor is churning under vacuum and hence the metal temperature of HP cylinder should be constantly monitored. 24. The steam temperature before. IV in the HRH line can be controlled either by giving attemperation in the superheater or by injecting cooling water in HP bypass station. CONCLUSION : By running the unit at 3000 rpm in such a way, it has been observed that the LPC exhaust hood temperature can be kept below 80C, during generator dry out process. Close monitoring of important parameters and high operational skill is essential for such operation. The temperature rise of the exhaust hood of LPC can be controlled by reducing the inlet steam temperature, improving the vacuum, and to some extent by increasing the flow of cooling condensate spray in the steam throw off device.