Pumps control valves, relationship

The relationship I ll now describe between a control valve and a pump applies to a constant speed motor-driven centrifugal pump. This covers more than 90 percent of the systems we encounter. 

All manipulated variables which are direct inputs to the process are flows or functions of flow. (Cascade control loops are excluded from the above, with the exception of flow loops, because the output of the primary controller is not a process input.) The process cannot be manipulated without changing the position of valves or dampers or the speed of pumps or compressors, all of which affect flow. The relationships between flow and controller output vary greatly, available pressure drop being a significant factor. The fact remains that in order to bring about control of any variable, in any process, flow must be made to change. From this definition, flow may be considered both a controlled and a manipulated variable-this coincidence was discussed in Chap. 3. 

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