Hydraulic Machines MCQ with answer and solution

Concept

Air vessels provide a continuous flow at a uniform rate by absorbing fluctuations and reducing the inertia/acceleration effects of the liquid column in the pipes.

Formula

Acceleration head (h_a) = (L/g) × (A/a) × ω²r cos(θ)

Step-by-step solution

1. Identify the function of an air vessel in a reciprocating pump: it acts as a reservoir for energy and fluid. 2. Recognize that without an air vessel, the fluid must accelerate and decelerate in the entire length of the pipe during every stroke. 3. Understand that the air vessel limits this acceleration to only the short length of pipe between the cylinder and the vessel. 4. Conclude that this minimizes flow fluctuations and the acceleration head.

Explanation

Air vessels are closed chambers containing compressed air in the upper part and liquid in the lower part, connected to the suction and delivery pipes. They act as a buffer to store liquid during the high-velocity part of the stroke and release it during the low-velocity part, which maintains a near-constant flow rate and significantly reduces the acceleration head. This reduction in acceleration head allows the pump to run at higher speeds without the risk of separation or cavitation.

Why the other options are wrong

A: While they contain compressed air, their primary engineering purpose is flow regulation, not storage of air for other uses. B: They do not directly reduce the coefficient of friction; they reduce the work lost to friction by maintaining a constant velocity (since friction loss is proportional to v²). D: While they reduce work done against friction and acceleration, 'increasing efficiency' is a secondary result of the primary function of minimizing fluctuations.

Shortcut

Air vessels = Constant flow + Less acceleration head.

Common mistake

Confusing air vessels with air relief valves or thinking they are primarily for increasing pressure rather than smoothing flow.