Procedure of vacuum leak detection in KWU steam turbines by steampressurisation 1. Introduction Air ingress in condenser, more than the design value, is not desirable. It decreases the condenser vacuum. Efficiency of the machine reduces with increase of air ingress. This article spells out the procedure of vacuum leak detection in KWU steam turbines by steam pressurisation. 2. Factors responsible for low vacuum The factors are : i) Air ingress through leaking joints. ii) Insufficient cooling water flow. iii) Fouling of cooling water tubes of condenser. iv) etc. Malfunctioning of vacuum-pulling equipment e.g. vacuum pump, ejectors v) Excessive thermal loading of condenser due to leaking drain valves and HP/LP by pass. 3. Various methods employed for checking air ingress The various methods are: i) By filling water in the system, or popularly called as "Vacuum Tightness Test". ii) Candle flame test. iii) Shock pulse measurement. iv) "Freon gas" leak detection. v) Steam pressurisation. In KWU turbines, vacuum leak detection is done by filling water up to one metre above the top row of condenser cooling water tubes (Fig. 1). This procedure is called vacuum tightness test. During this test, CW side of the condenser is kept completely empty. Fig. 1 Vacuum Tightness test in condenser . c) On Condenser • Condenser springs are locked. • Close drain valves of alternate drains of HRH & CRH lines connection to unit flash tank. The following sections bring out the procedure of detecting leakage points in the above-mentioned undetected area by steam pressurisation. Previously. Starting device is kept at 0%. iii) Steam pressurisation of vacuum system. Fill the water in the system up to one meter above the tube nest. This instrument is currently not manufactured due to environmental degradation by the Freon gas.1 Preparation of Thermal Scheme. Steam Pressurisation of vacuum system Steam pressurisation of vacuum system is done is an absolutely cold machine and piping system. It involves the following three stages : i) Prepare the system for vacuum tightness. a polythene tube is used. . This procedure is to be adopted during commissioning as well. remain. ii) Preparation of thermal scheme of turbine for steam pressurisation. "FREON GAS" detection instrument was employed for the same. 4. and readiness of Turbine and its Auxiliary Equipment for Performing Steam Pressurisation a) For Pipeline of Live Steam of HPC • Close test valve of ESV’S. Mark the level of water in the tube. • Close valves of HP & LP by pass system and their spray lines. • Ensure water boxes of circulating water are completely drained. as is done in case of water-fill test. undetected. 4. if any. Attend to the leakages if any fall of level is noticed. Check for any fall of level in 24 hours. Normally.In this procedure. Level in the condenser is raised one metre above the top most layer of condenser tubes. to check the level of water. which will remain under vacuum even at a load of 80 to 100% on the machine. The purpose of this test is to check leakages in those areas. Note: Helium gas leak detection equipment is under development. leakage points one meter above the tube nest. b) All Pipelines & IPC • Close test valve of IVs. however. Vacuum breaker valve is closed. • Close the valve in the air exhaust line of main ejectors. Keep seal steam valve in closed position and ready for operation. • All vacuum gauges in the system are to be isolated. and switch off power supply to its motor.d) On Ejectors/Vacuum Pump • Isolate steam-supply lines to ejectors. • Close drip line from HP heater to condenser. g) Arrangement for Steam Supply • Keep leak-off valve in locked open condition. e) On Regenerative High Pressure Heaters and Deaerator • Close extraction valves on HP heaters and deaerator. free from insulation and property illuminated. • Put TG-set on barring gear.0 kg/cm2 on exhaust part of turbine for measuring pressure in condenser. • Switch on oil supply system of turbine. exhaust-hood temperature gauge is to be replaced with gauge having a range of 0 to 200 °C. • Close air line valve to starting ejectors/vacuum pumps. Surface temperature of the area under test is likely to be more than / equal to 100 °C. • Install pressure gauge with a range of 0 to 1. h) Area to be Tested by Steam Pressurisation • The area should be approachable by additional scaffolding. . f) On Turbine • Atmospheric-relief diaphragms on the exhaust part of LP are to be in operational condition • Switch on all turbovisory instruments. Drain line connection of seal steam header is to be diverted from condenser flash tank to atmosphere. • Ensure readiness of supply of steam to LP exhaust from Auxiliary PRDS. • L.P. exhaust-hood temperature.100% on the machine. • Keep a watch on the operational parameters of steam turbine.6 hours for the pressure to start building up. • At an exhaust-hood pressure of 0. Special attention may be paid to the areas not covered by "Vacuum Tightness Test" and which also remain under vacuum even at a load of 80 .g. start the steam pressurisation of vacuum system. axial shift.10 to 0. Following are the suggested areas : • Parting plane of IP turbine • Diffuser section joint connection of L. for which the following should be done : • Manually open the steam supply valves supplying steam to the LP exhaust. differential expansion of HP. Pressure : 5 kg / cm2 After noting down the initial readings of the turbine supervisory system.150 °C.20 kg/cm2. further pressurisation should be stopped by cutting off the steam supply to turbine.2 Process of Steam Pressurisation Steam parameters (saturated) are : Temperature : 120 . start checking the leakage points. IP & LP cylinders. e. It takes around 4 . and record the readings. and raise the pressure slowly. turbine • LP cylinder girder joint • Atmospheric relief diaphragms • Neck welding of condenser with LP • Drain for LP inner guide plate diffuser • Condenser dome corners welding • Dome valve welding (water box side) with condenser • Condenser main holes • Turbine flash boxes bellows . It any of the turbine parameters is going out of limits. exhaust-hood pressure. Locate the leakage points.4.P. W. pumps .Not in operation iv) Gland steam exhauster v) Condenser completely drained on C. LP by pass rack and vacuum pressure switches rack • LP by pass downstream connection to condenser. Note : Status of equipment should be as follows : i) C.Not in operation Schematic of Steam Pressurisation Process is shown in Fig 2.Not in operation . till no leaks are observed.• Vacuum line to speed control rack. welding joint with condenser • LP gland boxes joint • Instruments tapping point on the above mentioned areas • TD BFP exhaust steam connection up to its isolating values • Various flange connections with IP casing Repeat the steam pressurisation process after attending to the leakage points. . box side and to be on jacks.Not in operation ii) Condensate extraction pumps iii) Boiler feed pumps . .W. Seal steam header drain is diverted from flash box to atmosphere.5. . • Water manometer may be used to measure the condenser pressure during the test. This is expected. as pressure starts building up in the condenser steam space.2 kg/cm2 (g). Feedback/Additional Information • Turbine rotor may come to rest even if turning gear valve is open. Lock the leak-off valve in open position and power supply to its motors switched off. • Alternate source of steam supply to condenser is via steam supply header. • Steam supply to condenser steam space is regulated by seal steam supply valve. • It takes roughly 6-8 hrs for condenser to start getting pressurised to a value of 0. • Turbine and its system need to be absolutely cold. . Insulation is to be removed.• Proper illumination and approach are to be ensured for areas under test.