To separate steam at lower pressure, a flash vessel is used. This is a vertical vessel, as shown in Figure 7.8, with the inlet of high-pressure, high-temperature water located at about one third of the way up its height. The standard design of flash vessels requires that the diameter of the vessel is chosen so that the steam flows toward the top outlet connection at no more than about 3 m/s. This should ensure that any water droplets could fall through the steam in a contraflow, to the bottom of the vessel. Adequate height above the inlet is necessary to ensure separation. The separation is also facilitated by having the inlet projecting downward into the vessel. The water connection is sized to minimize the pressure drop from the vessel to the pump inlet to avoid cavitation.
High-pressure hot water inlet
Maximum steam velocity = 3 m/s
FIGURE 7.8 Flash vessel schematic diagram.