The underflow purity coefficient of a de-oiling hydrocyclone operating at a fixed split ratio is limited by the existence of a finite turndown ratio during excursion in the feed rate. The purpose of this paper is to use advanced computational fluid dynamic methods to identify a hydrodynamic reason for this phenomenon. The results of the simulation show that as the feed Reynolds number increases beyond a critical value, a redistribution of the angular momentum within the hydrocyclone decreases the distance of the zero axial gradient of radial pressure gradient, (d(dP/dr)/dz)(r=0) = 0, from the vortex finder, which thereby shortens the length of reverse flow core. The breakdown of reverse flow core at a high feed Reynolds number causes a catastrophic drop in the separation efficiency and possesses a finite turndown ratio. This insight offers guidance on how to maintain separation performance during an excursion in the feed rate. (C) 2017 Elsevier B.V. All rights reserved.