TIME FOR LEAK FREE WATER SYSTEMS
At a Leak in a Water Pipe and At Some Gasketed Joints -
Contaminated Water Can Enter the Pipe
News that the water in Del Rio, Texas was contaminated from the Rio Grande River flood waters (August 23-26, 1998 water was trucked in to Del Rio) made me wonder about the source of contamination. In a leak tight system piping system the only source of contamination would logically be the water reservoir or the water well in which case the contamination could be controlled or eliminated. If the treatment plant or well is shut down from the flood, then a leak tight piping system would contain a substantial amount of good water for drinking and hygiene purposes.
On the other hand, if the piping system has water leaks then contaminated water around the pipe can get in the pipe through these leaks. All it takes is a slightly higher pressure on the outside of the pipe than on the water pressure on the inside. A sufficient external pressure on water pipe could easily occur in a town with flood waters several feet deep if the water pressure is low in the pipe.
Another potential source of inflow of contaminated water into the pipe is at gasketed joints of the type used on bell type sockets. These gaskets, shown in the sketch below are designed to keep water in the pipe but not out of the pipe. As the water pressure increases the in the pipe the V-shaped gasket attached to the inside of the bell joint presses tighter against the outside pipe wall However, if the water pressure on the outside of the pipe is greater than the pressure on the inside of the pipe, then the V-shaped gasket will collapse against the bell and contaminated water will enter the pipe.
Properly designed mechanical joints for plastic pipe exert a positive pressure on the seal so that it presses against the outside surface of the pipe without help from internal pressure. A seal is then maintained regardless of which whether pressure is higher inside or outside the pipe. (Gas companies use this type of mechanical joint as they do not want water in their gas pipes).
There are two ASTM specifications for plastic pipe with the type of joint shown below. D3139 covers pressure pipe joints such as would be used in the water system. D3212 covers plastic drain and sewer pipe joints. Both specifications require a vacuum test with the pipe in a deflected position to guarantee that the condition discussed above does not occur. The test for drain and sewer pipe is more severe than that for water pipe. Go figure. This gives me cause for concern.
Of course the people of Del Rio only know that the water got contaminated. The public has to believe that our utilities are the best they can be and accept bad water as another major inconvenience caused by the flood. However, a polyethylene piping system using state of the art pipe joints would not have become contaminated unless bad water was pumped into the system at the source.
Please give me your thoughts on this subject.
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