The collapse of Tacoma Narrows Bridge is a popular illustration of the disastrous effect of resonance in real world. When resonance the bridge experienced large amplitude vibration without bounds as time passed. Resonance occurs when excitation is equal to the natural frequency of a system. Centrifugal pump can have resonance too and the study of rotor dynamics will consider this vibration effect. Typically, analysis will look at the shaft stiffness of bearing. Critical speed is the speed when centrifugal pump will experience resonance. As such, critical speeds are determined and analysed. It should be noted that this critical speed analysis deals with the mechanical performance of a centrifugal pump. It does not however consider the hydraulic side of the pump. The hydraulic performance will consider another two speeds namely impeller specific speed & suction specific speed.
Impeller specific speed is a dimensionless number. This index is used to relate impeller type & hydraulic performance. The values of impeller specific speed come in range where various impellers are classified. This table showing values of specific speed and impeller type can be found in many publications. This tabulation is convenient for designer to meet various hydraulic conditions. For example, impeller with low specific speed typically used for high head & low NPSHR. On the other hand, impeller with high specific speed typically used for low head & high NPSHR. In both cases, NPSHR stands for net positive suction head required.
Suction specific speed is also an index like impeller specific speed. This index is an important consideration for suction piping layout. This suction specific index comes in a range. Typically, higher index number indicates lower net positive suction head requirements. Suction conditions are essential for centrifugal pump operations. Many hydraulic problems in centrifugal pump have its roots in suction.