There are two types of compressors, defined by either an axial or radial casing construction. Figure 200-8 illustrates this construction, referred in the API 617 Standards as:
• axial, or horizontally split
• radial, or vertically split
API 617 (Centrifugal Compressors) requires the use of the vertically-split casings when the partial pressure of hydrogen exceeds 200 psi.
Other factors which influence the horizontal/vertical split decision include the absolute operating pressure of the service and ease of maintenance for a particular plant layout.
The top half of the horizontally-split casing (Figure 200-9) is removed to access the internals. The stationary diaphragms are installed individually in the top and bottom half of the casing. Main process connections may be located either in the top or bottom half.
The horizontally-split down-connected casing has the advantage of allowing removal of the top half for access to the rotor without requiring removal of major process piping.
Vertically-split or barrel compressors have a complete cylindrical outer casing. The stationary diaphragms are assembled around the rotor to make up an inner casing, and installed inside the outer casing as a unit, contained by heads or end closures at each end. Some later designs hold the heads in place by use of shear rings (Figure 200-10).
On the vertically-split casing, maintenance of the rotor and other internal parts (other than bearings and shaft-end seals) involves removal of at least one head, withdrawal of the inner casing from the outer pressure containing casing, and then dismantling of the inner casing to expose the rotor (Figure 200-11). The inner casing and rotor can be removed from either the up- or down-connected vertically split outer casing without disturbing process piping.
Both the horizontally and vertically-split casing designs allow removal of bearings and shaft-end seals for maintenance without disassembly of major casing components.
Figure 200-12 gives a comparison of pressure vs. capacity for multistage horizontally-and vertically-split casing construction. The size/rating comparisons are general. Specific pressure/capacity ranges and casing configurations vary between manufacturers.