Which of the following is an incorrect cause for a pump delivering less than the expected discharge rate?

When analyzing the performance of a pump, factors that can lead to a reduced discharge rate generally involve impediments in the system or operational parameters that inhibit optimal function.

In the case of a discharge head being too low, it actually does not contribute to the pump delivering less than expected because a lower discharge head can facilitate easier flow. In fact, if the discharge head is minimal, the pump should theoretically be able to deliver a higher flow rate since it encounters less resistance in pushing the water against gravity. This contrasts sharply with other factors like motor speed, pump priming, and impeller conditions, which can significantly hinder the pump's ability to reach its designed discharge rate.

For instance, if the motor speed is too low, it would not generate sufficient energy to move the fluid at the desired rate. Similarly, a pump that is not primed cannot effectively begin moving water, resulting in diminished output. Additionally, if the impeller is clogged, this physical obstruction will severely limit flow as it restricts the amount of water that can be moved through the system.

Thus, while the other options can directly contribute to a lower discharge rate, a low discharge head does not inhibit flow and is, therefore, not a valid cause for decreased pump performance