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Nylon Liner: Permeation Resistance

Posted by Brett Boozer on Nov 20, 2020 1:31:07 AM

Hydrocarbon Transport Options

For decades, carbon steel was an industry standard for hydrocarbon transport, offering great physical strength, and ease of fabrication.

Unfortunately, steel is highly susceptible to both internal and external corrosion, supporting oxidation in air, soil, and water. Carbon dioxide, hydrogen sulfide and free water, along with a corrosive soup of microorganisms, have long posed challenges to corrosion management. (api.org) In response, many operators have adopted chemical inhibition programs, but the added costs of chemicals, injection equipment, periodic maintenance and inspections must be tallied against any advantage they bring.

Unlike steel, traditional polypipe is highly resistant to corrosion. This, combined with its low cost and high availability, has even made traditional polypipe a popular liner material for aging steel pipelines. It is relatively inexpensive, flexible, lightweight and far easier to install. Readily available in large coils, traditional polypipe also minimizes the number of joints and fittings required in any given installation. However, the Achilles heel of traditional polypipe is a vulnerability to chemical permeation.

TRADITIONAL POLYPIPE’s Permeation Problems

At the risk of oversimplifying complex chemistry, permeation is exactly what it sounds like - the tendency of one substance to move through another. In the case of polyethylene, gas and liquid hydrocarbon compounds will eventually be driven into and through the plastic pipe wall by the pressures and temperatures inside. This process expands and thus weakens the pipe, making it more susceptible to permeation and leading to early-life failure.

Density of the pipe and the molecular length of the attendant hydrocarbon compounds are factors that determine the severity of the problem. Denser pipe is naturally more resistant to permeation, thus the preference of high-density over low-density polyethylene.

At the other end of the equation, short-chained or aromatic hydrocarbons are much more easily driven through the traditional polypipe wall. Other factors influencing the rate of permeation include the pressure differential between the inside and outside of the pipe, temperature, available surface area and the type and volume of hydrocarbon compound in question.

Against this chemical onslaught, traditional polypipe has limited resistance, significantly swelling upon exposure, a phenomenon that is accompanied by loss of strength. It has been estimated that as much as 30 percent of all plastic component failures is due to environmental stress cracking (ESC).[1] Traditional polypipe pipe experiencing permeation can also expand up to 10 percent in volume, a problem exacerbated by internal pressure. Think of a balloon expanding, only in this analogy, the pipe itself is the inside of the balloon. When this expansion occurs, the pipeline can neither be repaired nor reconnected.

Permeation of traditional polypipe is so well-known and well-documented within the industry, that it has become systematically integrated into the spec process. Any traditional polypipe system that is designed to accommodate more than a two-percent content of liquid hydrocarbons must be de-rated by 50 percent.

 

Liabilities

Since liners are not designed for pressure, the space between the pipe and the liner must be minimized; when the gas volume in the annulus expands, the subsequent pressure can otherwise cause the liner to collapse. This particular problem requires that the annulus be frequently vented, and because traditional polypipe comes in long spools or sections and tie-ins aren’t always an option, repairs often mean replacing the entire line.

Additionally, due to the chemicals involved, no one wants a spill or leak. A hydrogen sulfide release can result in a health, safety or environmental event. In the case of a gas distribution line in which there are large amounts of condensates or aromatics, a permeation-induced pipe failure can result in natural gas being released. Obviously, many of these materials are flammable - a containment failure can be costly in terms of both assets, liabilities and human life.

Permeation Solutions

To prevent permeation, you have to consider the density of the polymer. In the case of short-chain or aromatic hydrocarbons, the molecular density is very low, which means the hydrocarbons can more easily pass through the plastic wall. Long-chain polyamides, often referred to as nylons, are much denser and chemically stable than traditional polypipe. As a result of their chemical resistance, long-chain polyamides have for decades been successfully used in offshore flexible flow lines and risers.

In 2015, polyamide-6 (PA6) and polyamide-12 (PA12) products were developed specifically as liners for gathering pipelines, serving as a barrier between the flowing hydrocarbons and the host pipe.

Ultimately, this problem has been addressed by composite pipes with a nylon liner, which can be engineered to address specific application requirements - from temperature, to pressure, to permeation - in a single, bound tubular.

Composite pipe by Baker Hughes features a unique design that includes an inner nylon liner for ultra-low permeation. For this liner, there are actually three different ultra-low permeation options: a nylon PA6 liner, which provides a strong chemical barrier for a limited range of applications at a lower price point; a nylon PA12 liner, which provides a stronger chemical barrier and is the only approved nylon for standalone gas piping systems; and a polyphenylene sulfide (PPS) liner, which provides the strongest chemical barrier. These options are resistant to H2S, CO2, bacteria and harsh chemicals; sustain no paraffin buildup; are fully piggable and suitable to hot oil. Featuring the lightest weight and longest lengths among spoolable pipe solutions, Thermoflex pipe can be installed with less manpower in less time.

Finally, with any of these inner liner options, no pressure de-rating is necessary, even at higher temperatures.

 

[1] (H. F. Mark. Encyclopedia of Polymers Science and Technology – 3rd Ed. Vol 12. John Miley & Sons Inc. 2004)

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