the net generated head of the pump and its draught tube.

Flow measurement is much more difficult. Until recently the only reliable method was the so-called ‘isotope injection’ method where the dilution of a radioactive isotope of tritium is measured as it passes through the system. The isotope is injected through special pipes into the pump draught tube and is then thoroughly mixed with the water as it passes through the pump.

Another method which is gaining favour is the multi­ beam time-of-flight ultrasonic method. This method is cheaper than the radioactive method and much quicker to use. The disadvantage is that it relies on being able to fit a number of transducers around the circum­ ference of the discharge pipe, constraining the pump­ house designer to provide a length of straight pipe about 0.6 diameters long and preferably more than 3 diameters away from obstructions such as junctions or valves. This seems a small requirement but in practice it is often difficult to find. Where the discharge manifold is in concrete, the isotope method is usually used.

bay area. It is preferable, however, that consideration is given to obtaining a water supply from the CW outlet system and the elcctrochlorinator power consumption/ output performance improves with warmer waler. Figure 2.74 shows the basic stages of the electro­ chlorination process.

A detraining tank facility is provided in the solution circuit to permit release of hydrogen gas produced by electrolysis, and for this reason special consideration is given to the location of the plant. An open enviroii- r ment is chosen and the same factors apply as those

relevant to on-site hydrogen generation plants.

For inland stations, consideration is given to bulk storage in the form of storage tanks, together with the dosing pumps, etc. The size of the installation may vary depending on dosing needs, the fitting of condenser mechanical cleaning equipment, etc.

The sodium hypochlorite solution from the electro­ chlorination or bulk storage plant is'dosed into the CW system in the same controlled fashion as chlorine.

Ail installations are provided with road access.

19 Chlorination plant

Chlorination plants, although still in evidence in some power stations, are being phased out throughout the CEGB. Although not relevant to modern practice, a brief outline of the chlorination plant is given here to cover those stations where such plants are still in existence.

Chlorine is a poisonous gas afid stringent safety measures are always adopted by the specialist contrac­ tors engaged in this work. Chlorine in the liquid state is stored in bulk tank installations, refilled from road or rail tankers. The liquid chlorine is fed to electric evaporators and then in gaseous form to chlorinators which produce the chlorine solution.

The chlorination plant is housed in a weatherproof building, with clear access in and out for personnel. The building must be provided with forced ventilation via a plenum duct at floor level. The chlorine dosing solution is piped to the injection points in the CW system in rubber-lined pipes, the dose rate and periods being programmed through a master clock.

The product used to replace chlorine is a sodium hypochlorite solution, produced on site in the ease of coastal stations by utilising a seawater electrolysis plant, and by being purchased as a commercial product by inland stations and stored in bulk. Inland stations could also employ an electrolysis plant, but would have to provide a salt water feedstock by means of a brine producing plant.

For coastal stations the electrochlorination plant is housed in a dutch barn type structure adjacent to the CW pumphouse, seawater being taken from the fore-

FROM CW SYSTEM

TO CW SYSTEM

Fig. 2.74 Electrochlorination process