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Posted by Harold Kestenholz - hydronic.net on May 22, 2002 at 14:27:36:
In Reply to: Boiler feeding line Water hammering posted by BoilingMan on May 22, 2002 at 11:37:51:
A water hammer arrestor is for water-to-water use and would do little for this steam situation as there would be no gas pocket to absorb the shock.
Steam hammer is from cool water introduced into steam, the collapsing steam bubbles produce a hammer-like slug of non-compressible water toward the space the steam pocket took up before collapse.
Look to the temperature of the line entering the boiler; if the feed line is getting near to boiler temperature while water sits in it, steam in the line will collapse from the colder water entry. Water preheating to near the boiler water temperature can reduce hammer.
Water hammer can forge mechanical valves into new shapes from the hydraulic shock. If the feed is heated over the temperature of the boiler water, and released from a higher-pressure pump feed area, the water can flash to steam as it enters the lower-pressure boiler and cause hammer.
Pre-heated water entering the pump can flash to steam as the pump lowers the incoming water pressure, causing cavitation and water hammer, taking the vanes off the pump. Piston pumps are less prone to this problem as they isolate the high and low pressures on the different sides of the isolating valves. Determine the cause of the hammer.