Entrained liquid droplets must settle out of the vapor stream as the vapor moves upward in a vertical separator or horizontally through the vapor space of a horizontal separator. The designer must keep vapor velocity sufficiently low by providing enough cross-sectional area. The area needed is found by using a correlation of maximum allowable mass velocity of the vapor stream. The following equation (plotted in Figure 300-1) is used for knockout drums which protect compressors. It is known to be satisfactory when used with other design details, presented in later sections:
If the pressure is above 800 psig, the maximum allowable Gc is 90% of the value obtained from Equation 300-1.
Protection of a compressor is considered a critical service. For other services, use the following:
Vapor to fuel gas G = 1.5 Gc
Vapor to a distillation column G = 2.0 Gc
Vapor to a condenser G = 2.5 Gc
Minimizing entrainment is especially important in the design of high-pressure separators for hydrocracking and residuum desulfurization plants.
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