Consider the following statements regarding the operating characteristic curves of centrifugal pumps : 1. The input power curve will pass through the origin. 2. The head curve will have maximum value of head when the discharge is maximum. 3. The output power curve will start from origin. 4. The efficiency curve will start from origin. Which of the above statements are correct ?
- A1 and 2
- B3 and 4Correct
- C2 and 4
- D1 and 3
Solution
Concept
Operating characteristics of a centrifugal pump
Formula
Output power = ρ g Q H; η = output / input
Step-by-step solution
1. Input power at zero discharge (shut-off) is not zero because the impeller still churns the water, so the input power curve does not pass through the origin (statement 1 false). 2. The head is greatest at zero discharge and falls as discharge increases (statement 2 false). 3. Output power = ρgQH = 0 when Q = 0, so it starts from the origin (statement 3 true). 4. Efficiency is zero when Q = 0, so it starts from the origin (statement 4 true).
Explanation
At shut-off the pump delivers no water, so output power and efficiency are zero, but it still consumes some power. The head is maximum at zero discharge and decreases with flow.
Why the other options are wrong
Statements 1 and 2 are the common misconceptions, so the options with them are wrong.
Shortcut
Q = 0: head max, input power > 0, output = 0, efficiency = 0.
Common mistake
Thinking head is highest at maximum discharge.
Correct answer: B) 3 and 4