These devices are required to isolate the compressor for the same reasons they are required for reciprocating compressors.
Discharge Check Valve (Each Stage)
In reciprocating compressors the compressor valves themselves act as check valves, preventing backflow from high-pressure stages to lower pressure stages. Multistage centrifugal compressors require check valves on each stage to isolate each surge loop, as well as to prevent backflow
during unusual operating conditions.
The compressor can operate at any point on the performance curve, For the maximum value of suction pressure, the pressure rise across the machine at the surge control point must be less than the system pressure rating. If not, a relief valve should be installed.
Suction Shut-down Bypass (Purge) Valve
The suction shut-down bypass valve is used to purge the piping system of air prior to compressor start-up. This valve is small to prevent high gas purge rates from spinning the impellers.
Discharge Coolers and Suction Scrubbers
These items are required for the reasons discussed under reciprocating compressors.