7.2 Cooling water leakage in condensers
The methods employed in British power stations for the detection of leakage of cooling water into the condenser, and of the consequent pollution of the condensate are:
- Condensate conductivity.
- Sodium contamination.
Location of condenser leakage is influenced by the following factors:
- CW supply, i.e., fresh, estuarine or seawater.
- Condenser design, i.e., underslung or pannier.
Experience has shown that the techniques used to locate the region of leakage in fresh water condensers are not sufficiently sensitive to locate leakages in seawater condensers. Also, the introduction of pannier type condensers for the 500 MW units meant that flooding the steamside with fluorescein solution, the most popular method of off-load detection with underslung types, was no longer possible. Underslung transverse condensers, however, have an advantage in that they can have up to six separate sections which can be isolated and monitored using condensate conductivity probes. This means that on-load location and elimination of leaks is considerably simplified, often without output loss.
The method of locating condenser CW leakage with a unit on-load has three separate stages:
- Location of the affected tubenest by condensate conductivity measurement (desirable conductivity of condensate approximately 0.1 /xS/cm).
- Location of the physical level of the leakage within the waterbox.
- Location of the leaking tube or joint.
The first stage is carried out using conductivity probes with the unit on-load. The tubenest causing the problem is identified by an increase in condensate conductivity. This is simplified in transverse underslung condensers, where multi-sectioning means that local increases in condensate conductivity will be significant at the location of the leak, and therefore easily detectable.
Having identified which condenser tubenest is at fault, the second stage is to determine the level of the leakage in that nest. This can be found either by altering the cooling water level in the waterbox and monitoring the condensate conductivity or by introducing a tracer gas into the waterbox, dropping the cooling water level, and monitoring at the air extraction plant for the appearance of the gas as the leak is uncovered.
It is interesting to note that these two methods are not suitable for all power stations, e.g., the former method is satisfactory at sea-cooled stations, but, it is unsuitable for freshwater-cooled stations where the latter method is adopted.
These procedures are adequate for narrowing the field of search to within two or three rows of tubes, but stage three provides techniques which will precisely locate the source of the leak.
The techniques used include:
- Fluorescein method; the steam side of the condenser is flooded with a fluorescein solution.
- Foam or film methods; the condenser tubeplates are covered with a foam or thin film material whilst a vacuum is maintained on the steam side.
- Bubbler devices; air bubbling through a jar of water is used to detect a tube leak.
- Tracer gas methods; a halogen gas detector is situated at the air extraction equipment and the suspect tubes sprayed with gas.
- Flame and smoke techniques; the flame from a taper (or smoke) is used to indicate a flow of air into a leaking tube.
- Ultrasonics; the noise generated by air passing through a leak is detected.
The sensitivity and application of these techniques are discussed below.