Device for operating multiple centrifugal pumps, comprising • a communication interface (I/O) for receiving at least one input information on a state of operation of the centrifugal pumps and for transmitting at least one output information to one or multiple driving units of the centrifugal pumps, where the at least one output information reflects a reference value (m) for the number of centrifugal pumps to be operated in parallel, • a data storage unit for storing at least one additional information, • a processing unit for generating the at least one output information from the at least one input information and the additional information, wherein • the communication interface (I/O) is adapted to receive as input information an instantaneous pressure drop (H_i) across and an instantaneous flow rate per pump (Q_i) or speed (n_i) of the centrifugal pumps which are currently operated in parallel, • the data storage unit is adapted to store as additional information an actual number (m_a) of currently operated centrifugal pumps and a rated speed (n_r) of the centrifugal pumps, characterized in that • the centrifugal pumps are of the same type and size and are operated at the same speed, • the processing unit is arranged ∘ to determine from the input information and the additional information an instantaneous efficiency (E_i), a first expected efficiency (E_e1) under the assumption that the actual number (m_a) is reduced by one, and a second expected efficiency (E_e2) under the assumption that the actual number (m_a) is increased by one, and ∘ to generate the reference value (m) depending on which of the instantaneous (E_i) or first expected or second expected efficiencies (E_e1, E_e2) has the highest value, and ∘ to determine an updated reference value (m_u) for the number of centrifugal pumps to be operated in parallel by repeating above determinations after having successfully changed the number of centrifugal pumps operated in parallel to the reference value (m). • a communication interface (I/O) for receiving at least one input information on a state of operation of the centrifugal pumps and for transmitting at least one output information to one or multiple driving units of the centrifugal pumps, where the at least one output information reflects a reference value (m) for the number of centrifugal pumps to be operated in parallel, • a data storage unit for storing at least one additional information, • a processing unit for generating the at least one output information from the at least one input information and the additional information, wherein • the communication interface (I/O) is adapted to receive as input information an instantaneous pressure drop (H_i) across and an instantaneous flow rate per pump (Q_i) or speed (n_i) of the centrifugal pumps which are currently operated in parallel, • the data storage unit is adapted to store as additional information an actual number (m_a) of currently operated centrifugal pumps and a rated speed (n_r) of the centrifugal pumps, characterized in that • the centrifugal pumps are of the same type and size and are operated at the same speed, • the processing unit is arranged ∘ to determine from the input information and the additional information an instantaneous efficiency (E_i), a first expected efficiency (E_e1) under the assumption that the actual number (m_a) is reduced by one, and a second expected efficiency (E_e2) under the assumption that the actual number (m_a) is increased by one, and ∘ to generate the reference value (m) depending on which of the instantaneous (E_i) or first expected or second expected efficiencies (E_e1, E_e2) has the highest value, and ∘ to determine an updated reference value (m_u) for the number of centrifugal pumps to be operated in parallel by repeating above determinations after having successfully changed the number of centrifugal pumps operated in parallel to the reference value (m). ∘ to determine from the input information and the additional information an instantaneous efficiency (E_i), a first expected efficiency (E_e1) under the assumption that the actual number (m_a) is reduced by one, and a second expected efficiency (E_e2) under the assumption that the actual number (m_a) is increased by one, and ∘ to generate the reference value (m) depending on which of the instantaneous (E_i) or first expected or second expected efficiencies (E_e1, E_e2) has the highest value, and ∘ to determine an updated reference value (m_u) for the number of centrifugal pumps to be operated in parallel by repeating above determinations after having successfully changed the number of centrifugal pumps operated in parallel to the reference value (m).