Question

Which of the following statement related to pump Head versus Discharge characteristic curve is NOT true ?

• A. The head produced by a pump in general steadily decreases with increasing pump discharge
• B. The head generated by the pump when discharge is zero (pump is operating against a closed valve) is called shut-off head
• C. Shut-off head is used to design piping on the discharge side of the pump, so that it is able to withstand the shut-off head when the discharge valve is closed
• D. Pump efficiency is maximum at the shut-off head

Hint:

At shut-off head, the pump is running but no water is discharged ($Q = 0$). Since no work is done on the water, the efficiency is zero.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
A pump characteristic curve (H-Q curve) describes the relationship between the total head ($H$) developed by the pump and the rate of flow or discharge ($Q$).
Detailed Explanation:
Let us evaluate each of the statements:
- Option (A): Head steadily decreases with increasing discharge: This is a true characteristic of centrifugal pumps. As flow increases, the head developed decreases.
- Option (B): Shut-off head at zero discharge: This is true. The maximum head developed by a pump when the outlet valve is completely closed ($Q = 0$) is the shut-off head.
- Option (C): Shut-off head used for piping design: This is true. Since the shut-off head represents the maximum pressure a pump can generate, downstream piping must be designed to withstand this pressure safely.
- Option (D): Pump efficiency is maximum at the shut-off head: This is incorrect. At the shut-off head, the discharge $Q = 0$. Since water power ($P_w$) is proportional to discharge ($P_w \propto Q \cdot H$), the output power at shut-off is exactly zero. Therefore, pump efficiency is $0\%$ at the shut-off head. Maximum efficiency is achieved at the Best Efficiency Point (BEP) along the operating range.

Step 2: Final Answer:
The statement that is not true is Option (D).