Parallel Pumps: Add Flow at a Common Head

Two centrifugal pumps in parallel need a common-head comparison and a combined system assessment.

Combine Matching Curve Points

KSB explains that parallel pump flow is the sum of individual flows at the same head. The actual combined operating point comes from the intersection of that combined pump curve with the system curve. Adding two unrelated rated-flow numbers omits the common-head condition.

Retain individual curve identities and the shared-route model. A second pump can change the common flow and therefore the route loss. The resulting head can differ from the head observed with one pump, so twice one observed rate is not automatic. KSB primary engineering reference.

Illustrative Addition

Suppose two invented curves each supply 8 m³/h at a common head of 15 m. Their combined curve contains 16 m³/h at 15 m. If the hypothetical system requires 19 m at 16 m³/h, that combined point is not the system intersection.

The example shows how curve addition differs from delivered-flow prediction. It supplies no proposal for an HM tank installation. To find the actual combined operating point, the engineer needs the complete relevant curves and system cases, not only these isolated values.

Review Shared and Individual Conditions

Ask the designer how single-pump and combined operation are covered, including curve compatibility and the selected isolation or non-return arrangement. Keep control and installation responsibilities with the approved design; this educational comparison is not a start-up sequence.

Record the common-head additions and the predicted intersections separately in the worksheet. A proposal should show which demand cases need parallel duty and which use one unit. That makes peak-capacity claims reviewable while preserving the actual system assumptions. Relevant HM context: Pharmaceutical Storage Tank.