Question

Given below are two statements:
Statement I: For radial flow impellers, brake power (BP) generally increases from a non-zero value to a peak and then declines slightly as discharge (Q) increases from zero.
Statement II: For mixed flow impellers, brake power (BP) increases steadily from a non-zero value as discharge (Q) increases.
In light of the above statements, choose the correct answer from the options given below:

• A. Both Statement I and Statement II are true
• B. Both Statement I and Statement II are false
• C. Statement I is true but Statement II is false
• D. Statement I is false but Statement II is true

Hint:

To remember this difference:
- Radial flow pumps should be started with the delivery valve closed to minimize starting load (lowest power at $Q=0$).
- Axial/mixed flow pumps must be started with the delivery valve open to avoid overloading the motor (highest power at $Q=0$).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Centrifugal pumps utilize different impeller designs (radial flow, mixed flow, or axial flow) depending on the required discharge and head.
Each impeller design has unique characteristic curves showing how head, brake power (BP), and efficiency change as the discharge rate ($Q$) increases.

Step 2: Detailed Explanation:
Let us analyze both statements:
1. Statement I: For radial flow impellers, brake power (BP) generally increases from a non-zero value to a peak and then declines slightly as discharge (Q) increases from zero.
This statement is correct.
For radial flow pumps, the brake power curve starts at a minimum non-zero value at zero discharge (known as the shut-off head).
As the discharge ($Q$) increases, the power consumption rises to a maximum near the best efficiency point (BEP).
Beyond this point, the power curve plateaus and declines slightly as the pump reaches its maximum free-discharge capacity.
Therefore, Statement I is true.
2. Statement II: For mixed flow impellers, brake power (BP) increases steadily from a non-zero value as discharge (Q) increases.
This statement is incorrect.
For mixed flow and axial flow impellers, the brake power characteristic curve is quite different.
In these pumps, the brake power is actually at its maximum at zero discharge (shut-off) and decreases as the discharge increases.
Operating these pumps at shut-off or very low flow rates can overload the motor.
Therefore, the power consumption does not increase steadily with discharge.
Thus, Statement II is false.

Step 3: Final Answer:
Statement I is true because radial flow pumps consume more power as discharge increases, but Statement II is false because mixed flow pumps consume the most power at zero discharge.
Hence, the correct option is (C).