The compressor stability range is discussed in connection with performance curves and surge in Section220. This is very important.
In addition to performance stability, a satisfactory margin must be maintained between the operating speed range and the critical speeds of both the compressor and driver.
Although API 617 defines these required margins, the following can be used as a general guideline:
• lateral critical—should not fall in the range from 15% below any operating speed to 20% above the maximum continuous speed.
• torsional criticals—(complete train) no torsional critical should fall in the range from 10% below any operating speed to 10% above maximum continuous speed.