Question:
Which of the following two statements are not true for Positive-Displacement Pumps ?
(A) Positive-displacement pumps demonstrate high discharge pressures and low flow rates
(B) The output flow is proportional to pump speed.
(C) Such pump cannot be used for metering applications
(D) Relief valve must not be provided to handle Very high head pressures
Choose the correct answer from the options given below :
Which of the following two statements are not true for Positive-Displacement Pumps ?
(A) Positive-displacement pumps demonstrate high discharge pressures and low flow rates
(B) The output flow is proportional to pump speed.
(C) Such pump cannot be used for metering applications
(D) Relief valve must not be provided to handle Very high head pressures
Choose the correct answer from the options given below :
(A) Positive-displacement pumps demonstrate high discharge pressures and low flow rates
(B) The output flow is proportional to pump speed.
(C) Such pump cannot be used for metering applications
(D) Relief valve must not be provided to handle Very high head pressures
Choose the correct answer from the options given below :
Show Hint
Never close the discharge valve of an operating positive-displacement pump.
Unlike centrifugal pumps, which simply stall or churn the fluid, a closed discharge valve on a PD pump will quickly cause mechanical damage due to extreme pressure buildup.
Unlike centrifugal pumps, which simply stall or churn the fluid, a closed discharge valve on a PD pump will quickly cause mechanical damage due to extreme pressure buildup.
Updated On: Jul 21, 2026
- (A) and (C)
- (B) and (C)
- (A) and (D)
- (C) and (D)
Show Solution
Verified By Collegedunia
The Correct Option is D
Solution and Explanation
Step 1: Understanding the Concept:
Positive-displacement (PD) pumps operate by trapping a fixed volume of fluid and forcing it through a discharge pipe.
Their operating characteristics are fundamentally different from those of dynamic (centrifugal) pumps.
Detailed Explanation:
Let us evaluate each statement to identify those that are not true:
- Statement (A): PD pumps are characterized by their ability to generate very high discharge pressures while delivering relatively low, constant flow rates. This is true.
- Statement (B): In a PD pump, the volume of fluid displaced per revolution is constant. Therefore, the discharge flow rate is directly proportional to the rotational speed of the pump. This is true.
- Statement (C): Because their flow rate is directly proportional to operating speed and largely independent of discharge pressure, PD pumps are highly accurate. This makes them ideal for metering and dosing applications. The statement claims they cannot be used for this purpose, which is not true.
- Statement (D): If a discharge valve is closed while a PD pump is running, the pump will continue to force fluid into the line, causing pressure to rise exponentially. To prevent catastrophic failure of pipes or the pump motor, a pressure relief valve must always be installed on the discharge line. The statement claims a relief valve must not be provided, which is not true.
Since statements (C) and (D) are not true, Option (D) is the correct choice.
Step 2: Final Answer:
The statements that are not true are (C) and (D).
Positive-displacement (PD) pumps operate by trapping a fixed volume of fluid and forcing it through a discharge pipe.
Their operating characteristics are fundamentally different from those of dynamic (centrifugal) pumps.
Detailed Explanation:
Let us evaluate each statement to identify those that are not true:
- Statement (A): PD pumps are characterized by their ability to generate very high discharge pressures while delivering relatively low, constant flow rates. This is true.
- Statement (B): In a PD pump, the volume of fluid displaced per revolution is constant. Therefore, the discharge flow rate is directly proportional to the rotational speed of the pump. This is true.
- Statement (C): Because their flow rate is directly proportional to operating speed and largely independent of discharge pressure, PD pumps are highly accurate. This makes them ideal for metering and dosing applications. The statement claims they cannot be used for this purpose, which is not true.
- Statement (D): If a discharge valve is closed while a PD pump is running, the pump will continue to force fluid into the line, causing pressure to rise exponentially. To prevent catastrophic failure of pipes or the pump motor, a pressure relief valve must always be installed on the discharge line. The statement claims a relief valve must not be provided, which is not true.
Since statements (C) and (D) are not true, Option (D) is the correct choice.
Step 2: Final Answer:
The statements that are not true are (C) and (D).
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