Reading a Pump Head Capacity Curve
Every Kohler pump spec sheet includes a performance curve: a graph plotting flow rate (GPH or GPM) on the horizontal axis against total dynamic head (TDH) in feet on the vertical axis. The curve shows flow rate at every possible head condition. A pump rated at 6,200 GPH at 0-foot head may only deliver 2,800 GPH at 20-foot TDH -- that is its real-world output when pumping up to an elevation 20 feet above the suction point. Always identify your total dynamic head (static head + friction losses in suction and discharge hose) before selecting a pump, then read the flow rate from the curve at that TDH value.
Calculating Suction Lift and Why It Limits Performance
Suction lift is how far below the pump the water source sits. At sea level, atmospheric pressure supports a theoretical maximum water column of 33 feet; in practice, centrifugal pumps lose prime and cavitate above 25 feet of suction lift due to friction losses and vapor pressure. At 5,000 feet elevation, the maximum practical suction lift drops to about 18 to 20 feet. For Kohler pump installations near or above 3,000 feet elevation, subtract 1 foot of suction capacity for every 1,000 feet of altitude above sea level from the rated specifications.
Engine HP and Its Relationship to Pump Curve
Engine HP determines where on the pump curve you can sustain continuous operation. A pump head that requires 8 HP at peak efficiency cannot run at that point without overloading a 7 HP engine -- the engine will de-rate under load and either run rough or stall. Kohler matches engine and pump head deliberately; the KP6200 pairs the 9.5 HP CH395 with a pump head whose peak efficiency point falls at 7.5 HP demanded, providing a 25 percent power margin for altitude de-rating, hot ambient temperatures, and aging engine performance.
Centrifugal vs. Trash Pump: Impeller and Volute Differences
Centrifugal pump impellers are closed (shrouded on both sides) for maximum efficiency with clean water; tight clearances between the impeller vanes and volute walls maximize pressure-to-flow conversion but bind immediately if a pebble or leaf enters. Trash pump impellers are open or semi-open -- the vanes are unshrouded and have wider clearances -- allowing solids up to 3/4 inch to pass through without binding. Efficiency is lower (you lose 5 to 10 percent of flow for the same engine HP) but reliability in dirty-water conditions is dramatically better. If there is any chance the source water contains debris, specify a trash pump head from the start.