A hydraulic rotating machinery system that is configured to control a process fluid of a process, said process fluid having widely varying conditions of service, the HRM system comprising: a controller; an HRM cluster comprising a plurality of HRMs (102, 202) having variable operating speeds, each of the HRMs (102, 202) being a pump, a turbine, or a hybrid pump/turbine, the operating speeds of the HRMs (102, 202) being controlled by the controller; an HRM plumbing system, the HRMs (102, 202) being interconnectable by the HRM plumbing system to form a flow path through which the process fluid can flow from an inlet of the HRM plumbing system to an outlet of the HRM plumbing system; a plurality of valves included in the HRM plumbing system, the controller being able to actuate the valves so as to control a selection of the HRMs (102, 202) that are included in the flow path and an arrangement in which the HRMs (102, 202) of the selection are included in the flow path; wherein the controller is further able to switch interconnections between four of the HRMs in the cluster such that, for all possible combinations of two selected HRMs of the four HRMs, the controller is able to actuate the valves so as to connect the two selected HRMs in series while the other two HRMs of the four HRMs are bypassed; and non-transient media cooperative with the controller, the non-transient media containing instructions that, when executed by the controller, cause the controller to: accept information regarding at least one of a status of the process and the COS of the process fluid; and according to said information, control the operating speeds of the HRMs (102, 202) and the selection and arrangement of the HRMs (102, 202) in the flow path so as to continuously satisfy at least one requirement of the process while ensuring that the HRMs (102, 202) in the flow path operate substantially at their optimal hydraulic efficiency points over said widely varying COS of the process fluid. a controller; an HRM cluster comprising a plurality of HRMs (102, 202) having variable operating speeds, each of the HRMs (102, 202) being a pump, a turbine, or a hybrid pump/turbine, the operating speeds of the HRMs (102, 202) being controlled by the controller; an HRM plumbing system, the HRMs (102, 202) being interconnectable by the HRM plumbing system to form a flow path through which the process fluid can flow from an inlet of the HRM plumbing system to an outlet of the HRM plumbing system; a plurality of valves included in the HRM plumbing system, the controller being able to actuate the valves so as to control a selection of the HRMs (102, 202) that are included in the flow path and an arrangement in which the HRMs (102, 202) of the selection are included in the flow path; wherein the controller is further able to switch interconnections between four of the HRMs in the cluster such that, for all possible combinations of two selected HRMs of the four HRMs, the controller is able to actuate the valves so as to connect the two selected HRMs in series while the other two HRMs of the four HRMs are bypassed; and non-transient media cooperative with the controller, the non-transient media containing instructions that, when executed by the controller, cause the controller to: accept information regarding at least one of a status of the process and the COS of the process fluid; and according to said information, control the operating speeds of the HRMs (102, 202) and the selection and arrangement of the HRMs (102, 202) in the flow path so as to continuously satisfy at least one requirement of the process while ensuring that the HRMs (102, 202) in the flow path operate substantially at their optimal hydraulic efficiency points over said widely varying COS of the process fluid. accept information regarding at least one of a status of the process and the COS of the process fluid; and according to said information, control the operating speeds of the HRMs (102, 202) and the selection and arrangement of the HRMs (102, 202) in the flow path so as to continuously satisfy at least one requirement of the process while ensuring that the HRMs (102, 202) in the flow path operate substantially at their optimal hydraulic efficiency points over said widely varying COS of the process fluid.