Question

Which of the following statement is NOT true with regard to centrifugal pump?

• A. The capacity of pump varies directly as the speed
• B. Pressure head varies as square of the speed
• C. Cavitation can be avoided by decreasing the pressure on the suction side of the pump
• D. The efficiency of pump decreases as the viscosity of liquid increases

Hint:

Centrifugal Pump Affinity Laws & Cavitation:
$Q \propto N$, $H \propto N^2$, Power $\propto N^3$.
Cavitation is prevented by INCREASING SUCTION PRESSURE (Higher NPSH).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Centrifugal pump affinity laws and cavitation physics: pump affinity laws establish $Q \propto N$, $H \propto N^2$, and $P \propto N^3$; cavitation occurs when suction pressure drops below liquid vapor pressure ($P_{ ext{suction}} \le P_v$), so cavitation is avoided by INCREASING suction pressure (or increasing NPSH), not decreasing it.
Key Formula or Approach:
\[ \text{Cavitation Condition: } P_{\text{suction}} \le P_{\text{vapor}} \implies \mathbf{\text{Prevented by INCREASING Suction Pressure } (NPSH_a > NPSH_r)} \]

Step 2: Detailed Explanation:
Evaluating the operational characteristics and affinity laws of centrifugal sanitary milk pumps:
1. Affinity Law 1 (A): Volumetric discharge capacity varies directly with impeller rotational speed ($Q \propto N$). (True).
2. Affinity Law 2 (B): Developed pressure head varies with the square of impeller speed ($H \propto N^2$). (True).
3. Effect of Viscosity (D): High liquid viscosity increases hydraulic disk friction, reducing pumping efficiency and head. (True).
4. Cavitation Physics (C): Cavitation occurs when static pressure at the pump suction drops below the saturated vapor pressure ($P_v$) of the liquid, causing flashing of boiling vapor bubbles.
- To prevent and eliminate cavitation, the operator must INCREASE the pressure on the suction side (raise suction tank level, eliminate suction line restrictions, lower liquid temperature).
- Decreasing suction pressure directly causes catastrophic cavitation. Therefore, Statement C is NOT true.

Step 3: Final Answer:
Thus, 'Cavitation can be avoided by decreasing the pressure on the suction side' is NOT true, matching option (C).