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Water hammer. What is it, how can it be prevented.

Water hammer. What is it, how can it be prevented. Life threatening water hammer in a steam system is generally "condensation induced water hammer".  This occurs when steam becomes totally enveloped in sub-cooled condensate which has not been drained from the system.  The steam, once surrounded,  can "implosively" condense leaving a vacuum that condensate, pushed by system pressure, rushes in to fill.  The resulting collision of condensate with itself or a system component  can generate over-pressures due to the impact of well over 1000 psi--the magnitude depends on how fast the condensate gets moving as it accelerates into the void.  The surest way to guard against water hammer in a steam system is to make sure condensate is not able to collect and cool in a steam system. 

Use these helpful hints from www.kirsner.org to understand this important safety phenomena

1. High pressure steam in contact with sub cooled condensate is an unstable and potentially explosive mixture.
2. Don't admit steam into a line filled with sub cooled condensate. In fact, always be wary of admitting steam to any cold steam line if you cannot be absolutely certain that the line's been completely drained.
3. Allowing sub cooled condensate to flow into a steam filled line is more dangerous than admitting steam into a line with sub cooled condensate.
4. If you suspect a pressurized steam line is filled with sub cooled condensate, don't attempt to drain the condensate. Shut the steam off first, then drain the condensate. If you do open a drain, and the line hammers, close it and get the steam off. The line may continue to hammer until you get the steam off.
5. A mixture of steam above sub cooled condensate can sit dormant in an isolated steam line like a loaded gun awaiting a triggering event. Opening a valve to admit steam or opening a bleeder to drain condensate can trigger an event. Don't let yourself or those you supervise inadvertently pull that trigger without first making sure the gun is unloaded.

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