Horizontal separators are most efficient when large volumes of liquid are involved. They are also generally preferred for three-phase separation applications. In a horizontal separator, shown in Fig. 2-10. The fluid enters the separator and hits an inlet diverter, causing a sudden change in momentum. The initial gross separation of liquid and vapor occurs at the inlet diverter. The force of gravity causes the liquid droplets to fall out of the gas stream to the bottom of the vessel, where it is collected. The liquid collection section provides the retention time required to let entrained gas evolve out of the oil and rise to the vapor space and reach a state of “equilibrium.” It also provides a surge volume, if necessary, to handle intermittent slugs of liquid. The liquid leaves the vessel through the liquid dump valve.
Gas and oil mist flow over the inlet diverter and then horizontally through the gravity settling section above the liquid. As the gas flows through this section, small droplets of liquid that were entrained in the gas and not separated by the inlet diverter are separated out by gravity and fall to the gas-liquid interface. Some of the drops are of such a small diameter that they are not easily separated in the gravity settling section. Before the gas leaves the vessel, it passes through a coalescing section or mist extractor. This section uses elements of vanes, wire mesh, or plates to provide a large amount of surface area used to coalesce and remove the very small droplets of liquid in one final separation before the gas leaves the vessel. Normally, horizontal separators are operated half full of liquid to maximize the surface area of the gas-liquid interface.
Horizontal separators have certain advantages with respect to gravity separation performance in that the liquid droplets or gas bubbles are moving perpendicular to the bulk phase velocity, rather than directly against it as in vertical flow, which makes separation easier. The level controller and liquid dump valve operate the same as in a vertical separator. Pressure and level are maintained as in a vertical separator. Horizontal separators are smaller and thus less expensive than a vertical separator for a given gas and liquid flow rate. Horizontal separators are commonly used in flow streams with high gas-liquid ratios and foaming crude. Typical L/D ratios for horizontal separators normally fall in the range of 2.5–5.