Question 67 QMED01 - Junior Engineer

The Correct Answer is D ### Explanation for Option D (Correct Answer) **Option D: almost completely closed** is correct because of the fundamental purpose of cushioning valves in reciprocating pumps, particularly when operating at high speeds. 1. **Purpose of Cushioning Valves:** Cushioning valves (often referred to as air chambers or surge chambers in different contexts, or sometimes specifically regulating the amount of air/fluid trapped at the end of a stroke) are used to mitigate hydraulic shock and control the inertia forces of the fluid, especially when the direction of motion reverses (at the end of the discharge and suction strokes). 2. **Effect of Speed:** When a reciprocating pump operates at its **maximum speed**, the piston's acceleration and deceleration forces are maximized. These high inertia forces require rapid reversal of the fluid. 3. **The Need for Restriction:** If the cushioning valves were wide open, the clearance volume (or the volume of the trapped fluid/air) would be too large, leading to severe pressure fluctuations and instability at high cycle rates. To maintain volumetric efficiency and ensure the pump rapidly transitions between strokes without significant delay or excessive "slippage," the clearance volume must be minimized. By closing the cushioning valves (leaving them only slightly open if adjustable), the inertia effects are controlled by the system's design geometry rather than excessive reliance on external damping, ensuring the pump maintains its intended flow characteristics and prevents cavitation or major pressure spikes caused by poor timing at high speeds. Essentially, high speed demands a tight, rapid cycle, which is achieved by minimizing the clearance volume accessed via these valves. ### Explanation of Incorrect Options **A) wide opened:** If the cushioning valves were wide opened at maximum speed, the effective clearance volume would be significantly increased. This increased volume would severely reduce the volumetric efficiency, lead to delayed pressure buildup (since more time is needed to compress the larger volume), and often result in unstable operation and excessive fluid "slippage" at the high operating frequency. **B) 3/4 opened:** While less detrimental than fully wide opened, having the valves 3/4 opened still represents a large increase in clearance volume relative to the pump's cylinder volume. This setting would still lead to noticeable reductions in efficiency and inadequate control of inertia forces necessary for reliable high-speed operation. **C) half opened:** Similar to option B, half opened still provides too large an effective clearance volume for operation at maximum design speed. High-speed operation requires the tightest practical cycle to maintain pressure head and flow stability, necessitating minimization of the clearance volume, which is achieved by closing the cushioning valves.