7.1.1 Locating air leaks
Air leakage can occur in any part of the condensing system which operates near or below atmospheric pressure, and is often caused by failure of joints, at flanges, and also by weld and material failures. When the turbine is off-load, detection of air leakage is made in some parts of the system by flooding with water. Water seepage from sections normally under vacuum indicate leakage areas.
When the turbine is on-load, however, different techniques must be employed. These include: audible methods (if the leak is large enough and the background noise low); visible means, by observing the action of a lighted taper when in the proximity of air flowing towards a leakage point; and the spraying of suspect areas of plant with a tracer gas which is easily detected by careful monitoring of the air extraction equipment. The last method is the most common technique employed today.
The tracer gas detectors generally used in power stations are the RILO, AEI and LEYBOLD types. These are known as halogen leak detectors as they are capable of detecting low concentrations of halogenated compounds in an air stream. For minimum toxicity and maximim sensitivity the halogenated compounds chlorofluorocarbon and dichlorofluoro-methane (trade names Freon 12, Isceon 12 or Arcton 12) are used.
Of the leak detectors mentioned above, only the LEYBOLD equipment has a detector probe which can be immersed in the gas to be tested. This equipment is used by installing the probe in the air extraction pipe from the condenser, whereas the other detectors sample from the air extraction system discharge.
Most leakage points are found at flanges and joints, and are usually small cracks or holes. These can normally be sealed with any of a number of proprietary metallic resin or rubber-based sealing compounds.