13.4.1 Separators - part 1
In the NEI-Parsons design of separator, shown in Fig 2.100, water droplets are recovered from the steam by centrifugal action. The vessels are conveniently located in the cold reheat pipes from the HP turbines, generally one vessel per pipe.
The wet steam is passed from a normally horizontal inlet pipe through a diverging conical channel to a row of swirler blades. The swirler blade ring imparts tangential velocity, or swirl, to the steam and helps to agglomerate the water droplets. The water films forming on the aerofoil surfaces of the swirler blades are subsequently detached as coarse droplets by the main steam flow. The droplets are then centrifuged by the steam flow and deposited onto the inner surfaces of a louvred liner which removes and partitions the free water from the main steam flow.
The louvred liner is provided with a series of narrow axial apertures uniformly distributed around its periphery to entrain the water droplets. Separated water films then cascade around the outer surfaces of the louvred liner. To encourage the separated water to flow over to the louvres, steam is extracted from the separator vessel and passed to a heater in the feed train. The separated water is also passed from the vessel, via drain pipes, to a convenient point in the regenerative feed train.
In order to ensure that steam leaves the separator with the minimum of swirl, anti-swirl vanes are fitted into the outlet nozzle. These vanes ensure that a low energy axial flow of steam enters the reheater, minimising the risk of disturbance to the reheater tube bundles.
The performance of cyclone separators is very dependent on the droplet size of the water entering the vessel. Very high efficiencies (of the order of 98%) can be achieved with a wetness of, say, 12% if the droplets are relatively coarse, but efficiency falls considerably if the separator is presented with steam of the same wetness with droplets in the sub-micro metre range. Tests to determine separator efficiency, using superheated steam artificially wetted in a spray-type desuperheater, generally give optimistic efficiencies since the large water drops produced by the desuperheater are easily separated from the steam. Measurements taken on site on practical installations have confirmed separation efficiencies in excess of 95% for an inlet wetness of 12%, and 93% at very high moisture contents above 20%.
A major factor in achieving these high efficiencies is the growth in droplet size from a mean diameter of about 10 microns at HP turbine exhaust to about 120 microns in the transfer pipe to the separator. Further agglomeration of water films on the surface of the swirler blades results in droplets of about 240 micron diameter being centrifuged to the louvred drum.
Erosion and corrosion are minimised by employing suitably resistant materials in areas subject to impingement. At the separator inlet, where moderate steam velocities and water droplet sizes generally prevail, 2.25%Cr l%Mo steel is used for the inlet pipe and cone pieces. In the main steam space, where velocities and droplet sizes are greater, the swirler blades, louvres and internal cladding on the pressure vessel are of 12% Cr low carbon steel. The basic pressure vessel is manufactured from carbon steel.