The vessel diameter is dictated by the desired mass velocity through the vessel. The recommended allowable mass velocity takes into account the terminal, or free settling velocity of aerosol particles at predetermined operating conditions. A K factor is used to modify the velocity calculation for such factors as extreme pressures (high or vacuum), suspended solids content, reduced surface tension, entrainment loading and viscosity. The value of K in the formula for calculating maximum allowable velocity depends on the nature of the gas contaminants and whether mist elimination devices are to be used. Optimum velocity factored by K value is calculated using the following Souders Brown equation:
Empirical data suggests that when sizing the minimum inside diameter of a basic vertical gravity separator (no impingement device), the following K values should be used:
K = 0.11 or less Gas streams containing high liquid loading with no impingement device
K = 0.15 – 0.17 Gas streams containing low liquid loading with no impingement device
These values were largely determined through experiments with a perforated plate column where tray spacing and liquid seal depths were varied to observe the effect on K value. The K value was observed to level off between 0.17 to 0.19 at a disengaging space of 28 inches, suggesting a reasonable value for simple gravity separation without the aid of impingement devices.
These values are consistent with the Scrubber Services Classes ‘A’, ‘B’, and ‘C’ as described in API Specification 11P – Specification for Packaged Reciprocating Compressors for Oil and Gas Production Services, however, slightly more conservative. In the interest of maximizing compressor reliability, it is preferred to design on the conservative side to provide a degree of process capacity contingency.