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Research on utilization of a frequency converter and sensorless methods in the drive system diagnostics has been mostly limited to the motor and to the frequency converter itself. However, it has been shown that a frequency converter also allows the sensorless estimation of the operational state of a centrifugal pump providing new opportunities for the diagnostics and control of pumping systems. In this paper, a novel hybrid estimation method for the centrifugal pump operational state is proposed. It is based on the identification of the process by using those flow rate and head estimates produced by QP-curve-based method, which are known to be relatively accurate. After the identification phase, the process model can be used together with the estimates of the frequency converter for the sensorless operational state estimation of the pump. The method is evaluated by laboratory measurements, and its applicability to industrial pumping and fan systems is also discussed.