The term 'case' may be misleading. Typical centrifugal and gear type pumps usually have a flange or connection for suction and discharge piping and this forms the hydraulic passage for the fluid. The suction bell is used for submerged suction to guide the flow into the impeller or where the end of suction pipe needs requires it to eliminate the edge effect of the pipe.
If you post more specific details, such as type of pump, you can get better responses. Afterall, only our bosses should expect us engineers to read minds. Keep the wheels on the ground
Bob
showshine@aol.com