Page 169 - Compression Machinery for Oil and Gas

Integrally Geared Compressors Chapter 4 157

FIG. 4.18 IGC with cast and fabricated casings. (Courtesy Mitsubishi Heavy Industries.)

In addition to defining the aerodynamic flow path, the casings are also the
structural element that contains pressure and controls operating gaps in close-
clearance areas—for example, eye seals (covered impellers) or shroud-to-blade
tip clearance (open impellers). This is a distinct difference with barrel-type
inline compressors, for example, which are able to separate the functions of
the flow path components (bundle diaphragms) and the pressure-containing
component (main barrel casing). So, in IGCs, minimizing deflection due to
internal pressure and external loads is an important aspect to the case design.
A critical area in a volute is the tongue—where the small and large areas of
the flow path intersect. For aerodynamic reasons, this region has a relatively
sharp corner, which creates a large stress concentration under pressure loads
(Fig. 4.19). Usually, there is a trade-off that needs to be made between aerody-
namic performance of the volute and allowable stresses related to the radius in
the tongue region.

FIG. 4.19 Finite element analysis (FEA) simulation of volute demonstrating stress concentration
in tongue region.