Centrifugal Compressors Rerates and Retrofits – Capacity

While impellers and internal stationary components can be relocated and new ones added, the casing nozzle sizes are fixed. The maximum capacity that can be handled with a reasonable pressure drop is therefore dependent on the nozzle size and related to inlet gas velocity.

Inlet velocity is dependent on gas conditions, allowable noise levels, and inlet piping configurations. An acceptable rule-of-thumb is a maximum of 140 ft/sec for air or lighter gases and approximately 100 ft/sec for heavier hydrocarbons.

The actual inlet gas velocity can be calculated from:

Q = ACFM in ft3/minute at inlet pressure, temperature, Z, MW
D = inside diameter of the nozzle, in inches

If side load or compound inlets are involved, inlet gas velocity should be checked for all inlet connections.

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