API 610 interpretation, head, performance curve, shut-off head, 110%
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I seek expert interpretation of clause b): If parallel operation is specified and the pumps are not individually flow-controlled, the following is required: a) the pump head curves shall be continuously rising to shutoff; b) the head rise from the rated point to shutoff shall be at least 10%; c) the head values of the pumps at any given flow within the preferred operating range shall be within 3% of each other for pumps with a discharge size greater than 3 inches (80 mm). NOTE: The above requirements are not necessary for pumps equipped with individual adjustable speed drives (ASDs) that are controlled by individual flow measurements
Some senior engineers interpret this as a way to facilitate on-site operation by allowing flow rate to be determined based on outlet pressure; others say it is done to leave a margin in case the head pressure decreases over time. Both explanations seem reasonable