Vacuum Leak Test In 270 Tg

Procedure of vacuum leak detection in KWU steam turbines by steam pressurisation 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

In this procedure, however, leakage points one meter above the tube nest, remain, undetected. Previously, "FREON GAS" detection instrument was employed for the same. This instrument is currently not manufactured due to environmental degradation by the Freon gas. The following sections bring out the procedure of detecting leakage points in the above-mentioned undetected area by steam pressurisation. The purpose of this test is to check leakages in those areas, which will remain under vacuum even at a load of 80 to 100% on the machine. This procedure is to be adopted during commissioning as well. Note: Helium gas leak detection equipment is under development. 4.

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. Fill the water in the system up to one meter above the tube nest. Normally, to check the level of water, a polythene tube is used. Mark the level of water in the tube. Check for any fall of level in 24 hours. Attend to the leakages if any fall of level is noticed. ii)

Preparation of thermal scheme of turbine for steam pressurisation.

iii)

Steam pressurisation of vacuum system.

4.1 Preparation of Thermal Scheme, 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. Starting device is kept at 0%.

b)

All Pipelines & IPC

•

Close test valve of IVs.

• Close drain valves of alternate drains of HRH & CRH lines connection to unit flash tank, if any. •

Close valves of HP & LP by pass system and their spray lines.

c)

On Condenser

• Condenser springs are locked, as is done in case of water-fill test. Level in the condenser is raised one metre above the top most layer of condenser tubes. •

Ensure water boxes of circulating water are completely drained.

d)

On Ejectors/Vacuum Pump

•

Isolate steam-supply lines to ejectors.

• Close air line valve to starting ejectors/vacuum pumps. Vacuum breaker valve is closed. •

Close the valve in the air exhaust line of main ejectors.

e)

On Regenerative High Pressure Heaters and Deaerator

•

Close extraction valves on HP heaters and deaerator.

•

Close drip line from HP heater to condenser.

f)

On Turbine

• Atmospheric-relief diaphragms on the exhaust part of LP are to be in operational condition •

Switch on all turbovisory instruments.

• Install pressure gauge with a range of 0 to 1.0 kg/cm2 on exhaust part of turbine for measuring pressure in condenser. •

All vacuum gauges in the system are to be isolated.

• L.P. exhaust-hood temperature gauge is to be replaced with gauge having a range of 0 to 200 °C. •

Switch on oil supply system of turbine.

•

Put TG-set on barring gear.

•

Ensure readiness of supply of steam to LP exhaust from Auxiliary PRDS.

g)

Arrangement for Steam Supply

• Keep leak-off valve in locked open condition, and switch off power supply to its motor. Keep seal steam valve in closed position and ready for operation. Drain line connection of seal steam header is to be diverted from condenser flash tank to atmosphere. h)

Area to be Tested by Steam Pressurisation

• The area should be approachable by additional scaffolding, free from insulation and property illuminated. Surface temperature of the area under test is likely to be more than / equal to 100 °C.

4.2

Process of Steam Pressurisation

Steam parameters (saturated) are : Temperature : 120 - 150 °C; Pressure : 5 kg / cm2 After noting down the initial readings of the turbine supervisory system, start the steam pressurisation of vacuum system, for which the following should be done : • Manually open the steam supply valves supplying steam to the LP exhaust, and raise the pressure slowly. It takes around 4 - 6 hours for the pressure to start building up. • Keep a watch on the operational parameters of steam turbine, e.g. differential expansion of HP, IP & LP cylinders, axial shift, exhaust-hood temperature, exhaust-hood pressure, and record the readings. • At an exhaust-hood pressure of 0.10 to 0.20 kg/cm2, start checking the leakage points. It any of the turbine parameters is going out of limits, further pressurisation should be stopped by cutting off the steam supply to turbine. Locate the leakage points. 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 - 100% on the machine. Following are the suggested areas : •

Parting plane of IP turbine

•

Diffuser section joint connection of L.P. 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

• Vacuum line to speed control rack, LP by pass rack and vacuum pressure switches rack • LP by pass downstream connection to condenser, 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, till no leaks are observed. Note : Status of equipment should be as follows : i)

C.W. pumps - Not in operation

ii)

Condensate extraction pumps

iii)

Boiler feed pumps - Not in operation

iv)

Gland steam exhauster

v)

Condenser completely drained on C.W. box side and to be on jacks.

- Not in operation

- Not in operation

Schematic of Steam Pressurisation Process is shown in Fig 2.

5.

Feedback/Additional Information

• Turbine rotor may come to rest even if turning gear valve is open, as pressure starts building up in the condenser steam space. This is expected. • Alternate source of steam supply to condenser is via steam supply header. Lock the leak-off valve in open position and power supply to its motors switched off. Seal steam header drain is diverted from flash box to atmosphere. • Steam supply to condenser steam space is regulated by seal steam supply valve. • Water manometer may be used to measure the condenser pressure during the test. • It takes roughly 6-8 hrs for condenser to start getting pressurised to a value of 0.2 kg/cm2 (g). •

Turbine and its system need to be absolutely cold.

• Proper illumination and approach are to be ensured for areas under test. Insulation is to be removed.