Pumps and appurtenances in water-supply engineering: Which one of the following statements is INCORRECT about common pump types and their principles of operation?
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AAir-lift pumps are commonly used to raise water from deep wells by lifting an air–water mixture.
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BJet (ejector) pumps are often used on small-diameter wells because the ejector can be placed down the well.
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CA hydraulic ram operates on the water-hammer principle to lift a portion of flow to a higher head.
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DRotary pumps use cams that rotate in opposite directions to move water.
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ENone of these.
Answer
Correct Answer: Rotary pumps use cams that rotate in opposite directions to move water.
Explanation
Introduction / Context:Pumps used in water-supply systems include air-lift pumps, jet (ejector) pumps, hydraulic rams, centrifugal pumps, and rotary positive-displacement pumps. Each type has a distinct principle, typical application, and installation constraint. This question checks recognition of correct operational principles and a common distractor about “cams” in rotary pumps.
Given Data / Assumptions:
- Deep and small-diameter wells often require special pump arrangements.
- Hydraulic ram devices use pressure surges (water hammer) to lift water.
- Rotary pumps generally use gears, lobes, vanes, or screws rather than “cams”.
Concept / Approach:Identify the statement that mischaracterizes a pump’s working mechanism. Air-lift and jet pumps move water by entrained air or jet-induced suction respectively; hydraulic rams exploit water hammer. Rotary pumps are positive-displacement units employing gears/lobes/vanes/screws, not opposing “cams”.
Step-by-Step Solution:
Check air-lift: compressed air reduces density in the riser, lifting water → statement is correct.Check jet pump: nozzle–venturi ejector down the well permits small-bore use → correct.Check hydraulic ram: uses water hammer and waste valve oscillation → correct.Check rotary pump: uses gears/lobes/vanes/screws; “cams rotating opposite” is not the accepted description → incorrect.Verification / Alternative check:Manufacturer catalogs and standard texts depict internal gear, lobe, vane, and screw pumps; none rely on counter-rotating “cams” as the primary displacement element in waterworks service.
Why Other Options Are Wrong:
- Option a: Describes air-lift principle accurately.
- Option b: Correctly notes small-well suitability of jet pumps.
- Option c: Properly cites water-hammer basis for hydraulic rams.
- Option e: Not applicable because one statement (d) is wrong.
Common Pitfalls:Confusing rotary positive-displacement internals with cam-and-follower mechanisms; mixing centrifugal and rotary PD concepts; assuming all deep-well pumping uses submersible centrifugal pumps.
Final Answer:Rotary pumps use cams that rotate in opposite directions to move water.