CONSTRUCTION OF CENTRIFUGAL COMPRESSOR
The main constructional features of a centrifugal compressor are:
- Balance drum
- Guide vanes
Just as a pipe holds gas during transportation, some type of enclosure must house the parts which actually cause a rise in pressure in a centrifugal compressor. A simple casing contains a single impeller and is generally found in low pressure applications. This type of casing is easy for repair and maintenance entry. Usually a centre cover, on being unbolted, reveals the impeller and single bearing.
A single centrifugal impeller is limited in raising pressure. For example, if a gas has an initial inlet pressure of 2 psig, a single impeller compressor would raise that pressure to about 5 psig.
If a higher pressure is needed, several impellers can be placed in series, within the casing, each taking as suction, the discharge of the previous impeller. This arrangement of a series of impellers is called a multi-stage compressor. The casing must totally enclose all these impellers.
Horizontally Split Casing
With this type of casing, the top half can be easily removed to get to the working parts. The casing is built to split apart at the horizontal centre line, the top and bottom halves being held together by bolts and positioned by dowel pins. By lifting the upper casing, all compressor internals, such as the rotor shaft with its impellers, labyrinths, seals, bearings, and diaphragms can be seen for inspection and repair. See Figure 19.
These casings can be manufactured several ways. Intake, discharge, and/or side-load connections may be placed pointing up, sideways, or down. Side load nozzles allow intake or removal of gas at pressures between the inlet and the discharge. Several casings may be driven by one driver, the shaft being extended through both ends of the casing where necessary. This arrangement is commonly called a tandem system.
Materials used for horizontally split casings are determined by stress, temperatures, and corrosion. For normal temperature ranges -20° F to 400° F, grey cast iron, cast ductile iron, or cast steel of varying grades are used. API Standard 617 requires steel casings for the following conditions:
¤ Gas pressure above 250 psig.
¤ Flammable or toxic gas at a pressure over 75 psig.
¤ Gas gaining compression heat above 450° F.
¤ Flammable or toxic gas gaining compression heat above 350° F.
Casing drains can be installed at the bottom of each impeller volute. These are simple
tapped flanged, or threaded opening connected to a gate valve. The casing flanges on a
horizontally split compressor have a pressure limitation of about 700 psig. Lower pressure limitations apply to extra large units, or those handling hydrogen, which leaks through the horizontal gasket.
Vertically Split Casing
A vertically split type casing (a “barrel” type casing) is used for higher pressure services. It is a forged or cast steel cylinder made to withstand higher discharge pressures. The bolts holding the end plate to the casing form a vertical line.
The cylindrical, symmetrical casing design provides high strength. Changes in metal dimensions caused by temperature rise are also symmetrical and relatively equal, eliminating large stresses and distortion encountered in other types of compressor casing. Heavy forged end covers, or plates, can be made for both ends or just one, depending upon the design.
VERTICALLY SPLIT CASING