Check operating flow against the API 610 §6.1 preferred operating region, rated window and end-of-curve — plus heat-balance temperature rise and vendor MCSF margin
Understand the POR/AOR operating regions, why low-flow operation harms pumps, and thermal vs mechanical minimum flow
API 610 §6.1 defines the bands relative to BEP flow:
Aim to run inside the POR; the AOR is the absolute envelope, not a comfortable home.
There are two separate low-flow limits — the governing minimum is the larger:
API 610 §6.1 defines the preferred operating region (POR) as 70% to 120% of best-efficiency-point (BEP) flow — the band where a centrifugal pump runs with low vibration and good reliability. The allowable operating region (AOR) is wider and is set by the vendor, bounded by an upper vibration limit, temperature rise, or other mechanical considerations. Rated flow should fall within 80% to 110% of BEP, and the end-of-curve flow is 120% of BEP.
Minimum continuous flow has two parts. Minimum continuous thermal flow is the flow below which the liquid temperature rise across the pump exceeds an allowable limit — estimated from a heat balance, ΔT (°F) = H·(1/η − 1)/(778.16·cp). Minimum continuous stable flow (MCSF) is the hydraulic/mechanical limit below which recirculation causes unacceptable vibration; it is vendor-specified and rises with suction specific speed. The governing minimum is the larger of the two. API 610 gives no formula for MCSF.
Well below BEP, the impeller no longer matches the flow it was designed to pass. Suction and discharge recirculation form, producing high-energy eddies, pressure pulsations, cavitation-like damage, and elevated radial loads and vibration. At the same time the inefficiency heat is dumped into a smaller flow, so the liquid temperature rises and can flash. Sustained low-flow operation shortens seal, bearing, and impeller life — which is why a minimum-flow bypass or recirculation line is provided.