Pressurized water entering a vertical steam-filled pipe (steam pocket collapse) is described. Which option represents the cause of water hammer for this scenario?

• A. Pressurized water entering a vertical steam-filled pipe (steam pocket collapse)
• B. Cause of Water Hammer
• C. Steam and water counterflow in a horizontal pipe
• D. Hot water entering a low pressure line

Answer: (A)

When a vertical pipe that is filled with steam has a slug of pressurized water entering, the pressure surge comes from the collapse of the steam pocket as water displaces it. Water is nearly incompressible, while the steam pocket can rapidly condense and vanish. As the incoming water front encounters the steam, the steam cannot be pushed out quickly enough and the pocket collapses, sending a abrupt pressure pulse through the line—the classic water-hammer transient. The energy stored in the steam and the momentum change of the incoming water combine to create a high-pressure wave that travels along the pipe.

This scenario is specific to a steam-filled vertical line with a steam pocket, whereas the other options describe different situations (for example, two-phase flow interactions in a horizontal pipe or a thermal transient from hot water entering a low-pressure line) that do not capture the steam-pocket collapse mechanism driving this water hammer.