"How Does Pump Suction Limit the Flow?"

Question 5: "How Does Pump Suction Limit the Flow?"

Answer:True, BEP is what a pump designed for, and it would be best if it operated there. However, since the actual operating point is an intersection between the pump curve and a system curve - the pump ends up operating all over its curve, because the system curve changes. Imagine a discharge valve slowly closing - the system curve (which looks like a parabola) will become steeper - and will intersect the pump curve at lower flow. Same for the opposite - if valve is opening - the system curve becomes "shallower", and will intersect the pump curve at higher flow. Intersection exactly at BEP is purely coincidental - if the discharge valve is set to make the system curve go right thru the BEP point at the pump curve.

Now, what happens if the valve opens wide enough to get the system curve intersect way past the BEP, at high flow? Keep in mind that a NPSHr curve also looks like a parabola with flow - it rises sharply at higher flow, past BEP. As it does, the NPSHR gets higher and, eventually, exceeds NPSA (available) - thus cavitation begins.

At low flow, cavitation is not a problem, but "other bad things" happen - the low flow becomes insufficient to "fill the impeller eye", and becomes sporadic, pulsing, etc. - causing pump vibrations, and even mechanical damage.