Basic Wastewater Operator Practice Exam

Assuming a constant suction head, how does the discharge of a centrifugal pump change with an increase in discharge head?

1. Will increase with a decrease in discharge head

2. Will decrease with a decrease in discharge head

3. Will decrease with an increase in discharge head

4. Will increase with an increase in discharge head

The correct answer is: Will decrease with an increase in discharge head

When considering the operation of a centrifugal pump, it is important to understand the relationship between discharge head and the discharge flow rate. The discharge head refers to the height to which the pump can raise water, while the suction head is the height of the water source relative to the pump. As the discharge head increases, the pump faces more resistance because it has to work against a higher elevation or pressure. This increased resistance leads to a decrease in the flow rate or discharge from the pump. Therefore, for a centrifugal pump, maintaining a constant suction head while increasing the discharge head results in a reduction in discharge. This principle is based on the pump's performance curve, which illustrates that as the head increases, the flow decreases. Each pump has a specific characteristic curve that shows how it will behave under different discharge head conditions. A higher discharge head means the pump is operating at a point where it can no longer maintain the same flow rate due to the increased energy loss from friction and gravity acting against the water being discharged. Thus, an increase in discharge head directly correlates with a decrease in discharge from a centrifugal pump under the assumption of a constant suction head.