ISO 10439-4:2015 specifies minimum requirements and gives recommendations for axial compressors, single-shaft, and integrally geared process centrifugal compressors and expander-compressors for special purpose applications that handle gas or process air in the petroleum, petrochemical, and natural gas industries. ISO 10439‑4:2015 specifies requirements for expander-compressors, in addition to the general requirements specified in ISO 10439‑1:2015.