Permeation of organic compounds through ductile iron pipe gaskets

Abstract

<p>Ductile iron (DI) pipes have been used for the conveyance of drinking water in drinking water distribution systems over the past several decades. It has been estimated that almost half of all new water mains installed in North America are DI pipes. Although DI pipe itself is resistant to chemical permeation, the polymeric gaskets that join and seal the pipe segments are reported to be susceptible to permeation by organic contaminants. Pipe-drum, diffusion cell experiments, and numerical simulations were conducted in this research to obtain a faster mean to evaluate possible permeations through DI gaskets.</p> <p>Of the five types of gasket materials tested using the gravimetric sorption test, ethylene-propylene-diene monomer (EPDM) had the highest sorption of gasoline, while fluoroelastomer rubber (FKM) exhibited very low sorption of gasoline. The sorption test results suggested that the least to most resistance to permeation of premium gasoline for the five gasket materials were EPDM, styrene-butadiene rubber (SBR), chloroprene rubber (CR; neoprene), acrylonitrile butadiene rubber (NBR), and FKM. A typical gasket was found to be made of two portions, the heel and the bulb, of the same polymer but different formulation. Gravimetric sorption tests suggested that the heel portion of all gaskets may be more resistant to permeation than the bulb making it the limiting step for permeation of organic compounds in gasoline.</p> <p>Pipe-drum experiments showed that SBR gasket had the highest permeation rates of benzene, toluene, ethylbenzene, and xylenes (BTEX), followed by CR, EPDM, and NBR. With regards to threats to drinking water under water stagnation conditions in the pipe, the 5 μg/L maximum contaminant level (MCL) for benzene will likely be exceeded during an 8-hour stagnation period for SBR gaskets in contact with free-product premium gasoline. NBR gaskets were found to be sufficiently resistant to permeation by benzene or other BTEX compounds in gasoline and the benzene concentration is unlikely to exceed EPA MCLs. Assessment based on data from the pipe-drum experiments suggested that when there is flow of water in the pipe, benzene and other BTEX compounds in gasoline will not exceed EPA MCLs.</p> <p>A diffusion cell device was developed to obtain diffusion coefficients of BTEX compounds for various gasket materials under controlled conditions. Using curve fitting of the permeation data by numerical modeling, the diffusion coefficients of BTEX compounds through SBR and NBR gasket materials was found to range from 10^-7 to 10^-8 cm²/s. The steady-state permeation rates were found to correlate in a linear relationship with thickness while the diffusion coefficients were found to be invariable to the thickness of the polymer tested (2 mm to 5 mm). The diffusion cell provided a rapid, inexpensive, and relatively well-controlled means to study permeation of polymeric gasket materials for DI pipes and the data obtained were used to model benzene permeation of the pipe-drum experiments.</p> <p>The permeation of benzene through a 4-inch SBR gasket of a pipe joint was modeled using Multiphysics diffusion module. The simulations showed that the heel portions as well as part of the bulb portions of a gasket were likely to be in contact with the contaminants. Model simulation predicted that a 4-inch SBR gasket under hydrostatic pressure would permeate more organic chemicals than a pipe without hydrostatic pressure, posing greater risk to organic chemical permeations. Increase in the length/size of the bulb portion of a 4-inch SBR gasket by compression or swelling from 10% to 30%, reduced the permeated mass of benzene by about 29% to 71% within 150 days of exposure to gasoline.</p> <p>In summary, SBR and NBR gaskets are compatible with any level of gasoline contamination in groundwater. NBR gasket is the most effective choice when a gasket material resistant to gasoline is desired. Diffusion cell experiments in combination with numerical simulations can be used in evaluating possible BTEX permeations effectively. Gasket exposure area and its orientation in the socket after pipe joint assembly are likely to affect permeation path and permeated mass of contaminants. Results from this study can be used as a basis for crisis management for DI pipes exposed to gasoline and for development of a better gasket to improve the reliability of infrastructure of development of water distribution system.</p>