When two or more casings (or sections) are operated in series, the manufacturer usually furnishes two performance maps: one for each casing, and one showing overall casing performance. For determination of the surge volume, use the overall curve.
In most situations, it is desirable to have an individual anti-surge recycle line around each casing (or around each section of compression of compound casings). It is not practical for one anti-surge control to accommodate two casings or sections at operating conditions significantly removed from the rated point. In addition, the overall operating stability range can be improved because the anti-surge controls can be set for the stability range of each casing rather than the overall range for all casings.