Consider the following statements regarding positive-displacement pumps : 1. Positive-displacement pump is better able to handle shear sensitive liquids. 2. A well-sealed positive-displacement pump can create a significant vacuum pressure at its inlet, even when dry. 3. The rotor(s) of a positive displacement pump run at higher speeds than the rotor (impeller) of a dynamic pump at similar loads. Which of the above statements are correct ?
- A1 and 2 onlyCorrect
- B1 and 3 only
- C2 and 3 only
- D1, 2 and 3
Solution
Concept
Features of positive displacement pumps
Step-by-step solution
1. They handle shear-sensitive liquids gently because they run slowly (true). 2. A well-sealed pump is self-priming and can create a vacuum at its inlet even when dry (true). 3. They run at lower speeds than dynamic pumps (statement false).
Explanation
Positive displacement pumps move fluid in fixed volumes at low speed, so they are suitable for viscous and shear-sensitive liquids and are self-priming.
Why the other options are wrong
Statement 3 reverses the speed relation.
Shortcut
PD pump: slow, self-priming.
Common mistake
Thinking PD pumps run faster.
Correct answer: A) 1 and 2 only