Many people have had trouble with electric fuel pumps being noisy, with reduced performance and reliability problems. They monkey around for months trying different things to remedy the situation. This comes from a lack of understanding of how a centrifical pump operates. Any pump operates on a flow curve. One can google "pump flow curve" and get a better understanding. If you try to run a pump off the left or right side of the design curve problems will arise. NPSH is the pressure that is available on the suction side. Whole volumes of professional papers have been written on the subject. Quite simply any time the pressure on the suction side is lower than the vapor pressure of the fluid pumped, vapor bubbles will form at the pump inlet. This causes the pump to cavitate. One can google "pump cavitation" and see the results. When a bubble is present it gets squeezed between the impeller and the volute, when this collapses the forces are tremendous.
