Centrifugal compressor and method of modifying centrifugal compressor

A method of modifying a multistage centrifugal compressor compressing working gas, the centrifugal compressor including a casing, a rotor that includes a rotary shaft and a plurality of impellers, and a diaphragm that defines a gas flow path including a return flow path causing the working gas discharged from the impellers to be sucked into the impellers on corresponding next stages, the casing including, in advance, a liquid injection path penetrating through the casing at a position corresponding to a return bend that causes the working gas to flow into the return flow path, the method comprising:

• providing an internal path serving as a path through which liquid supplied to the liquid injection path passes, in the diaphragm at a position corresponding to the return bend;
• providing, in the diaphragm along a circumferential direction, a chamber that includes a plurality of injection portions injecting the liquid introduced through the internal path, to a vicinity of a discharge port of the corresponding impeller from different positions in the circumferential direction;
• accommodating, in the casing, the diaphragm provided with the internal path and the chamber;
• and
• inserting a liquid supply pipe that is supplied with the liquid, into the liquid injection path and the internal path from outside of the casing.