Eq 7-8 can also be used for calculating the gas capacity of horizontal separators. However, some modifications are required to reflect the fact that in the gas gravity separation section of a horizontal separator, the liquid droplets are falling perpendicular to the gas flow rather than in direct opposition as occurs in a vertical separator. This makes it easier to separate droplets in a horizontal vessel. Partially offsetting this advantage is the fact that in a horizontal separator, the liquid gravity separation section is occupying part of the vessel cross section, leaving reduced area for gas flow.

In calculating the gas capacity of horizontal separators, the cross-sectional area of that portion of the vessel occupied by liquid (at maximum level) is subtracted from the total vessel cross-sectional area. Typical horizontal separator designs will have the normal liquid level at the half-full point. Values of K for horizontal separators from API 12J are given in Fig. 7-19.

There is some disagreement as to how K should vary with separator length. The API 12J recommendation is shown in Fig. 7-19. Many separators are greater than 10 feet in length, with some reaching 50 feet or more. The relationship shown in Fig. 7-19 for adjusting for length will give K factors greater than 1 ft/sec for large separators. These higher values of K for large (long) horizontal separators are generally considered to be overly optimistic. In practice, K = 0.5 ft/sec is normally used as an upper limit for horizontal separators equipped with wire-mesh mist extractors. Separators equipped with vanetype or cyclonic mist extractors may utilize higher K values than those for mesh pads.

The same general principles as discussed for vertical separators apply for horizontal separators with mist extractors in high liquid loading applications. For a horizontal separator, mesh pad and cyclonic type mist extractors will normally be installed horizontally with vertical upflow, while vane pack may be installed horizontally, vertically, or sometimes in a veepattern. Additionally, in a horizontal separator, the liquid droplets are settling perpendicular to the gas flow which makes separation easier. For these reasons, the approach of derating the mist extractor K to calculate the cross-sectional area of the gas gravity section is not as straightforward as for a vertical vessel. Typically the required cross-sectional area of the gas gravity section of a horizontal separator is sized based on droplet settling theory. The procedure is similar to that discussed previously for separators without mist extractors. For vessels with mesh pad mist extractors a typical droplet size for design is 150 microns. For separators equipped with vanetype or cyclonic mist extractors, a larger drop size may be appropriate, which may allow for a smaller vessel. The vessel manufacturer should be consulted.

In calculating the gas capacity of horizontal separators, the cross-sectional area of that portion of the vessel occupied by liquid (at maximum level) is subtracted from the total vessel cross-sectional area