Reciprocating pump — classification by displacement A reciprocating pump is also called a ________ displacement pump. Choose the correct term used in fluid machinery.

Introduction:
Pumps are broadly divided into positive displacement (PD) and rotodynamic categories. Recognizing which class a reciprocating pump belongs to is essential for understanding its flow characteristics, control methods, and protections.

Given Data / Assumptions:

• Piston/ plunger mechanism with suction and delivery valves.
• Finite swept volume per stroke; relatively low speed.

Concept / Approach:
Positive displacement machines transfer a nearly fixed volume of fluid each cycle irrespective of discharge pressure (within mechanical limits). Reciprocating pumps trap fluid in a chamber and displace it by piston motion, making them quintessential PD pumps. Consequently they require relief valves to prevent overpressure and often benefit from air vessels to smooth pulsating flow.

Step-by-Step Solution:

Verification / Alternative check:
Manufacturer curves for PD pumps show near-vertical head–flow characteristics: flow rate changes little with discharge pressure compared to centrifugal pumps.

Why Other Options Are Wrong:

• Negative/non-displacement/centrifugal: do not describe reciprocating mechanisms.
• Variable-inertia: not a standard category.

Common Pitfalls:
Operating PD pumps with closed discharge without relief valves; confusing their control logic with that of centrifugal pumps.

Final Answer: