1.0 Introduction

Improvements in power plant operating efficiency are the continuing goal of the electric utilities. Leakage of air or water into the plant’s main surface condenser adversely affects plant efficiency. The air–in–leakage reduces condenser vacuum and reduces the overall efficiency. For instance reduction of 1 mm Hg vacuum affects the heat rate by 2.25 kcal/kWh. Air–in-leakage also leads to high level of dissolved oxygen in feed water, resulting in accelerated deterioration of boiler and feed system.

Presently the air-in–leaks are detected by hydrostatic testing of condenser, i.e., by filling water one meter (approx.) above the tube nest during unit overhaul period. This test is basically off line test requiring sufficient time and does not cover the vacuum areas above the condenser tube nest such as low pressure cylinder (LPC) horizontal/vertical joints, LPC diaphragms, LPC turbine glands, L.P. heaters, etc. To overcome this problem and to have an on line air-in-leak detection system, gaseous tracer air-in-leak detection is found to be very effective technique and is widely accepted for on-line identification of condenser air-in-leakage. Use of helium gas has provided a highly sensitive and practical means of testing.

2.0 Principle

Gaseous tracer leak detection of any sealed container like a condenser requires that a pressure differential exist between the interior and exterior of the components being tested. The tracer gas is placed in the area of high pressure and migrates through leak paths to the lower pressure area.

3.0 Working And Test Set Up

Air-in-leakage testing is accomplished by drawing a sample of the air-steam mixture from vacuum pump/steam ejector exhaust by means of a portable vacuum pump (please refer to diagram in Annexure-A). Since moisture significantly affects the operation of the analyzer, complete removal of moisture from the sampled air-steam mixture is necessary. A detector probe is installed in the noncondensable stream of the sample. The test set-up may be checked to confirm that air is not leaking into the sampling system. The air leaking into the system will reduce the concentration of the helium in the sample. Further helium gas is released in the proximity of suspected condenser air-in-leak areas by an air gun, which is connected to a helium cylinder. The gas enters the leak and is propelled by the steam flow towards the condenser air removal section, where it is removed and exhausted. The sample of non-condensable mixture passes by the detector probe.