13.4.1 Separators - part 2
High velocity cyclone separator
The major problem with cyclone or centrifugal separators is the pressure drop produced by swirling the steam in a relatively large diameter drum and then straightening the flow at exit from the separator.
One method of reducing the pressure drop through the separator is by swirling the wet stem through a number of relatively small diameter tubes or cells arranged in a bundle in an enlarged section of cold reheat pipe. A typical separating cell and vertical assembly arrangement used in the Stein Industrie design is shown in Fig 2.101.
In common with the NEI-Parsons design of cyclone separator, a set of turning vanes at the cell inlet imparts a swirling motion to the steam/water mixture. Instead of a louvred drum, an annular skimmer slot is employed to collect the water and a small flow of extraction steam is allowed through the slot to improve moisture removal. Another set of guide vanes at the cell outlet straightens the flow and helps recover rotational kinetic energy. Since the flow does not depart significantly from the axial direction throughout the separation process, pressure drops are generally small. Also, because the cell bundle is not much larger than the diameter of the pipe on which it is fitted, the separator may be retrofitted into existing installations with the minimum of disturbance.
In tests carried out at Bugey II power station, the pressure drop for an inlet velocity of 50 m/s was about 150 mbar and the moisture content at outlet from the separator was 0.3% compared with 11.4% at inlet. However, 10% of the main steam flow was extracted to assist water removal.
Stein Industrie have also developed a horizontal high velocity separator, in which a portion of the steam from the bleed-off is passed into the central tube in the separating cell and recirculates through slots in this tube just downstream of the inlet vanes. The net leak-off flow is thereby reduced and good separation efficiencies have been reported from rig tests. The performance of this type of separator has not however been repeated on site, possibly because the drainage systems are less than adequate on site and also because of the difficulty, expressed above, of reproducing an an HP turbine exhaust water droplet size distribution in a test rig.