Hot Water Explodes Out of Manway

I just put down the phone. Two pipe fitters were burned opening the demineralizer drum shown in Fig. 44.3. My client wanted to understand what happened. It s a common incident. 

The water-filled drum had been depressured by venting to the atmosphere. The drum had been drained to a level just below the manway. The manway was carefully and slowly unbolted and removed. 

Nothing happened for a few minutes. Suddenly the hot water boiled 

This happens because the drum is not at a uniform temperature. The water in the drum is at its bubble point or saturated liquid temperature throughout the drum. But because the water in the lower portion of the drum is at a higher pressure due to the static head of water, its saturation temperature is also higher than the water at the top of the drum. Also, the cooler water at the top of the drum is a little denser than the hotter water at the bottom. 

If something happens to disturb this delicate balance, the hot water at the bottom will move toward the top. As the hotter water moves up, it loses a bit of head pressure and starts to vaporize. The bubbles of steam stir up the vessel s contents and accelerate boiling and steam evolution. The sudden generation of steam pushes the hot water out the open manway. 

If something happens to disturb this delicate balance, the hot water at the bottom will move toward the top. As the hotter water 

Honestly, this explanation is my best guess. But what I m not guessing at is that this incident is not unusual. It s safer to drain saturated water levels well below manways before they are opened. I have accidentally drained 180°F heavy naphtha on my gloved hand without injury. A similar careless incident with 180°F water resulted in a trip to the infirmary. Excluding fire, hot water is more hazardous to personnel than hydrocarbon liquids of the same temperature. Treat hot water with care and respect. 

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