Which of the following is a cause for reduced pump discharge?

• A. Inlet Valve Stuck
• B. High Ambient Humidity
• C. Motor Speed Too High
• D. Discharge Head Too High

Answer: (D)

When a pump has to push fluid against a higher discharge head, the flow it can deliver decreases. The discharge head is the pressure or height the pump must overcome in the discharge piping, including static lift and friction losses. On a pump curve, increasing head results in lower flow, so too much head means reduced pump discharge.

In a wastewater system, this happens if the discharge line is long, has many bends, rises uphill, or has valves restricting flow—any of these raise the head the pump must overcome, trimming the amount pumped per minute.

Other factors are less directly related to discharge. An inlet valve stuck can limit how much water enters the pump, reducing discharge as a suction-side issue, not because the discharge head is high. High ambient humidity doesn’t change hydraulic head, and motor speed being too high would more often increase discharge (until limits or protection engage), rather than reduce it directly.