Sloped demister

On many boiler steam drums, there is a sloped demister to suppress entrainment and hence improve steam quality. However, if this demister becomes partly fouled with hardness deposits, it probably does more harm than good. In such cases, I would probably discard the demister rather than replace it. If you feel you have been doing a good job on level control in the boiler, but steam quality is still poor, you should consider this possibility. 

Entrainment of high TDS boiler feedwater into steam from a boiler is promoted by excessive pH. Caustic is often added to the feedwater to reduce corrosion. However, if the pH increases much above 10.5, then the boiler tends to foam and hardness deposits are carried over into the steam. Hardness deposits degrade the performance of steam ejectors, steam superheating coils, and steam turbines. 

George Gibson The Basics of Phosphate - PH Boiler Water Treatment, Laboratories, Inc. Power Engineering, February 1978. 

British Standards Institution, Recommendations for Treatment of Water for Land Boilers, BSI BS 2486 1978. 

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To further improve the steam quality, the experienced design engineer places a dome, as shown in Fig. 15.4, on top of the kettle. The dome will have a sloped demister mesh pad. A demister acts to coalesce small droplets of water into larger droplets. The larger droplets easily settle out of the flowing steam. The demister is sloped, to promote liquid drainage from the surface of the pad. 

Effectiveness of Internals in Controlling foam. Originally, the separators contained limited internals—an inlet disengaging device, surge baffles, an anti-foam baffle, and a demister at the outlet. After severe foaming occurred, however, a bank of parallel plates (Dixon plates) was filled in each separator on one of the Ninian and one of the Brent trains The plates provide a large surface area and are sloped so that the separated oil flows dow n to the surface of the liquid. The installation is illustrated m Ftg S. The parallel plates arc 4 in. apart and extend from near the top of the vessel in the vapor space down to the normal liquid level. 

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