Pump “Run Out”
End of the line….
When a pump manufacturer’s published curve stops at the “run out” point (Max Flow at the right side) that is the end of the curve, there is NO more flow and surely no more head. There is a good reason the curve stops where it does, so don’t be surprised when the pump does not operate there. This is the main reason experienced pump people will avoid selections near “run out”.
Imagine the pump curve at that point becomes a straight line pointed straight down at the X axis. This is true regardless of the manufacturer. The reasons will vary slightly with specific speed but, the short answer, the pump is probably cavitating and/or stalling at that point regardless of the NPSHA.
Back Pull Out Design
Some history: The original ANSI pumps of circa 1960 were designed purposely with back pull design. This design concept was used because when the system process changed, the maintenance staff could pull the existing impeller out and replace it with a different impeller. The replacement would be of a different diameter and or vane number so the pump could operate as close to B.E.P. (Best Efficiency Point) as possible for the revised system condition. It was even conceived that impellers could be changed per shift or daily.
In reality, this feature was almost never used for that purpose. I love the idea of running a pump as close as possible to its B.E.P., but I am sure today’s reduced maintenance and engineering groups have better things to do with their time. These days, some of these issues can be covered by the use of variable speed drives.
– The Summit Pump Team
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