What transient occurs when a valve closes suddenly on a pressurized line, and what parameter is used to estimate the severity?

• A. Water hammer; severity estimated by the resulting ΔP via Joukowsky equation and wave speed c
• B. Turbulent surge; severity estimated by the resulting friction factor
• C. Cavitation; severity estimated by NPSHr
• D. Siphon surge; severity estimated by velocity

Answer: (A)

When a valve closes suddenly on a pressurized line, the moving fluid is forced to stop quickly, creating a high-pressure surge known as water hammer or hydraulic shock. The severity of this surge is assessed by the pressure rise the pipe experiences, which is estimated with the Joukowsky equation: ΔP = ρ c ΔV. Here, ρ is the fluid density, c is the speed at which pressure waves travel through the pipe (the wave speed determined by the pipe’s elasticity and the fluid), and ΔV is the abrupt change in fluid velocity (often the initial flow velocity when the valve slams shut). If the valve closes rapidly and the initial velocity is large, the resulting ΔP can be substantial, making the surge more dangerous.

So the key idea is that the sudden closure creates a pressure wave, and the magnitude of that wave—and thus the severity of the transient—is captured by ΔP as given by the Joukowsky relation, with the wave speed c being a central factor. Other phenomena mentioned—like cavitation from low pressure or surges described by friction factors, or siphon-related effects—describe different situations or metrics and don’t apply to the standard sudden-closure transient in this way.