BRAKE HORSEPOWER CURVES FOR VARIOUS SPECIFIC SPEED IMPELLERS B038
Take a look at the following chart and you can see why pump people recommend starting low specific speed pumps with the discharge valve throttled or shut, and why they recommend starting high specific speed pumps with the discharge valve open.
Although these starting methods will save power, you should be aware that in both cases you are causing high radial loads on the impeller.
These radial loads cause shaft deflection and are a major cause of premature mechanical seal failure at pump start-up.
See: shaft deflection for more information about this subject.