During full-load shop tests under natural gas, two multistage centrifugal compressors exhibited subsynchronous vibrations. Both of them are low-flow, high-pressure, high rotational speed compressors, and are fitted with tilting pad bearings and dry gas seals. A rotating stall problem was first eliminated by a modification of the diffuser geometry. Then, aerodynamic excitations caused the rotors to operate at their stability limit, and high vibration levels were observed at the first natural frequency. A complete rotordynamics analysis was performed in order to model precisely all the fluid–structure interactions. Modifications of the rotor designs were implemented, consisting in optimizing conveniently the bearing pads, replacing the toothed labyrinth seals of the balance pistons by damping honeycomb seals, fitting them with improved shunt hole systems. In addition, the dry gas seals were found to have been damaged, due to thermal effects, and further modifications were implemented to eliminate this problem. Final full load tests demonstrated a satisfactory behavior of both centrifugal compressors.
Centrifugal Compressor Testing Experience and Practice