| In areas where aggressive soils are encountered, the use of a polyethylene tube or sheet encasement has been proven to provide highly effective and economical protection. The protection against corrosion provided by loose polyethylene is different in several ways and should not be confused with coatings applied directly to the barrel of the pipe. The most significant difference is its ability to protect without creation of concentration cells at holidays. Also, since the encasement is applied when the pipe is actually put in the ground, coating damage due to shipping, handling, etc., is minimized.
The uniform environment created by the loose encasement may be invaded by groundwater. Water inside the polyethylene tubing initially bears the characteristics of the soil environment and corrosion may start. But, within a short period of time, initial oxidation depletes the oxygen supply in the water, and other electro-chemical corrosion reactions also progress to completion. At this point, a state of chemical equilibrium is reached.
Since the first field installation of polyethylene wrap on gray iron pipe in 1958, installations have been made in corrosive soils throughout the world. The polyethylene encasement procedure developed in the United States has been adopted by several other countries, and an International Standard for Polyethylene Sleeving (ISO 8180) has been developed.
Research by the Ductile Iron Pipe Research Association at several corrosive test sites has verified that polyethylene encasement provides a high degree of protection and results in minimal and generally insignificant exterior surface corrosion of either ductile iron pipe or gray iron pipe thus protected. These findings have been confirmed by the results of numerous investigations of field installations.
Field tests have also indicated that the dielectric capability of polyethylene provides shielding for ductile and gray iron pipe against stray current at most levels encountered in the field.
Because polyethylene encasement is a passive method of protecting ductile iron pipe in aggressive soils, it can effect greater savings than cathodic protection systems which require continual monitoring, maintenance, and trained personnel. For protection in the isolated areas of aggressive soils, ANSI/AWWA C105/A21.5 covers materials and installation procedures for polyethylene encasement of underground installations of ductile iron piping for water and other liquids.
Polyethylene wrap in tube or sheet form for piping encasement is manufactured of virgin polyethylene material conforming to the requirements of ANSI/ASTM Standard Specification D1248. The specified minimum nominal thickness is 200 microns. Material and installation methods are all in accordance with the requirements of AWWA C105. |
