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You are here: Experts > Cities/Towns > New York > New York > New York Steam Pipes
Follow-Ups to Answer from Expert John V. Barbieri
Dave Bullock writes on 2007-07-20 12:49:10
When it all shakes out, I doubt the age of the steam main will be determined to be a significant factor. It's been my experience steam line breaks of this nature are usually caused by condensation induced water hammer. It is my opinion this line ruptured from the same cause.
The most likely scenario is one in which the condensation (water)inside the steam main is not being removed by the steam traps as fast as it was accumulating.
I suspect the root cause of this failure was a submerged steam line. The steam line was located underground. The vault in which the line lies probably became flooded with water, perhaps from rain water given recent weather in the northeast.
When a steam line becomes submerged, the insulation obviously becomes soaked. Sometimes the insulation falls off the pipe, other times it does not, still the outcome is the same. The insulation is rendered ineffective.
When steam comes in contact with the inner pipe wall, it gives up heat and condenses back into water. This water is called condensate.
Steam traps are provided at low points on the piping system to remove the accumulated condensate. Steam system designers usually size the traps to provide a safety factor of 5 to 8 times the condensation rate of an insulated steam pipe.
In the case of a submerged steam line, condensation is taking place at a far greater rate than normal. Without the benefit of insulation, steam heat is lost through the pipe wall into the surrounding water at a terrific rate. As heat is lost, the steam condenses back into water.
The amount of condensate generated by a submerged steam line can easily exceed the capacity of the steam traps to remove the water. In effect, the steam line slowly floods with water over a period of time.
At some point, the water level inside the pipe rises enough that a wave is formed by flowing steam still traveling down the pipe. This wave forms a water slug that completely blocks off the inside of the pipe.
Steam pressure upstream of the water slug pushes on it, while at the same time, the steam bubble downstream from the slug condenses forming a vacuum which imparts a "pull" effect on the slug. The combined push-pull effect greatly accelerates the water slug traveling down the pipe.
Steam systems typically flow at 50 to 60 MPH. The combination of push and pull effects produced by condensation induced water-hammer can accelerate the water slug to 100 MPH or greater. At the higher speed, the shock created when the water slug slams into a valve or an elbow (sharp turn)can exceed the burst rating of valves, fittings and piping.
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