• Temperature rise of the liquid -- This is usually established as 15°F and results in a very low limit.  However, if a pump operates at shut off, it could overheat badly.
• Radial hydraulic thrust on impellers -- This is most serious with single volute pumps and, even at flow rates as high as 50% of BEP could cause reduced bearing life, excessive shaft deflection, seal failures, impeller rubbing and shaft breakage.
• Flow re-circulation in the pump impeller -- This can also occur below 50% of BEP causing noise, vibration, cavitation and mechanical damage.
• Total head characteristic curve – Some pump curves droop toward shut off, and some VTP curves show a dip in the curve.  Operation in such regions should be avoided.

There is no standard which establishes precise limits for minimum flow in pumps, but ANSI/HI 9.6.3-1997 Centrifugal and Vertical Pumps – Allowable Operating Region discusses all of the factors involved and provides recommendations for the “Preferred Operating Region.”