In reciprocating pumps used in fluid mechanics and hydraulic machines, air vessels are fitted on the suction and delivery sides to improve performance under unsteady flow. Which primary effect best explains why air vessels are provided on reciprocating pumps?

Introduction:
Reciprocating pumps create pulsating flow because the piston accelerates and decelerates water each stroke. Air vessels are classic attachments that mitigate the adverse effects of this unsteadiness. Understanding their principal function is essential for sizing, power estimation, and cavitation avoidance.

Given Data / Assumptions:

• Single- or double-acting reciprocating pump with suction and delivery pipes.
• Water is essentially incompressible; trapped air in the vessel is compressible.
• Objective: reduce unsteady inertial effects in the pipeline.

Concept / Approach:
During piston motion, the water column in pipes undergoes acceleration and deceleration, generating an acceleration head h_a proportional to (l/gd) * (dv/dt). An air vessel provides a compressible cushion that absorbs and supplies flow during transients, flattening velocity fluctuations and thereby reducing the required acceleration of the water column and the corresponding acceleration head.

Step-by-Step Solution:
1) Without an air vessel: high dv/dt → large h_a → high instantaneous power and pressure swings.2) With an air vessel: part of the stroke volume is briefly stored/released in the vessel → lower dv/dt in long pipes.3) Reduced dv/dt ⇒ reduced h_a in both suction and delivery lines.4) Consequences: smoother discharge, reduced risk of separation/cavitation on suction, and power savings.

Verification / Alternative check:
Power input peaks diminish when h_a is reduced; measured pressure traces show attenuated pulsations with an air vessel installed.

Why Other Options Are Wrong:
Smoothen the flow: true as an outcome but not the fundamental targeted head term; the more precise primary effect is reducing acceleration head.

Reduce suction head: static suction head is geometric; air vessels do not change sump level.

Increase delivery head: they do not increase head; they reduce pulsations.

Increase pump speed without cavitation risk: speed limits still depend on NPSH and valve dynamics.

Common Pitfalls:
Confusing acceleration head with friction head; assuming air vessels change static levels rather than transient components.

Final Answer:
reduce acceleration head