Steam and water counterflow in a horizontal pipe 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)

Water hammer happens when a fast-moving liquid is forced to stop or change velocity inside a confined space, creating a high-pressure transient. In a steam–water system, a trapped steam pocket in a vertical section can be compressed or collapsed when pressurized water from a horizontal line pushes into that steam-filled pipe. The rapid intrusion and collapse of the steam pocket convert the momentum of the incoming water into a sharp pressure surge, producing the characteristic water hammer shock. This specific interaction—water entering a vertical steam-filled pipe and causing steam pocket collapse—best explains the cause in this scenario. The other descriptions don’t capture the decisive mechanism of a liquid slug colliding with and collapsing a steam pocket to generate the surge.