Finally, review the speed based on impeller stress and compressor critical speeds.
Impeller stress levels are related to the impeller tip-speed as discussed in Section240. While the maximum allowable tip speeds vary with manufacturer, impeller design, and material, a good rule-of-thumb for impellers with backward leaning blades is 900 ft/sec maximum tip velocity.
Determine impeller tip speed by:
Maintain the following critical speed separation margins:
• Any critical speed at least 20% below any operating speed
• Any critical speed at least 20% above maximum continuous speed
Revamping of the rotor may have some effect on critical speeds; however, ignore this effect for the initial feasibility estimate.