Rigid and Flexible Piping Systems Keep up with Tough Demands: Flexible Pipe Here to Stay

Today's underground piping systems at fueling facilities must hold up under harsh environmental conditions and vigilant testing and monitoring requirements. Yesteryear's single wall metal piping has been replaced by two basic types of double wall, non-metallic piping systems. In this article, OPW's Mike McCann tells about he technology of flexible piping and Environ's Kevin Struthers pitches in with a sidebar on coaxial fittings.

Technician inserts primary pipe into the access/containment pipe entering the dispenser sump. Courtesy of OPW Fueling Components.

Flexible pipe was introduced in the underground fueling system market more than ten years ago. Since then, it has gained significant market share.

Benefits
The benefits of flexible pipe are well documented. Continuous runs of secondarily contained flexible pipe snake from the submersible pumps throughout the dispenser islands. The only fittings necessary are the connections at the pumps and dispensers, where they are secondarily contained and accessible inside containment sumps. The need to bury piping joints in the backfill, one of the most notorious sources of underground fueling system leaks, is eliminated.

Another obvious benefit of flexible pipe is ease of installation. Since joints are not necessary for directional changes, much of the time and cost of measuring, cutting and fitting inherent in traditional plumbing operations is eliminated. For example, a six dispenser, three-tank site using secondarily contained flexible pipe can typically be completely piped and tested in one day, regardless of weather conditions.

Fewer joints also mean fewer chances for human error during installation, arguably one of the biggest causes of problems with any underground piping system. Some other piping systems are labor intensive and can pose joint-adhesion problems in cold or wet weather.

Some flexible piping systems allow access to the primary pipe without excavating. The primary pipe is inserted in an access/containment pipe and can be retracted for inspection, repair or replacement without breaking concrete. The entire underground piping system is accessible from the surface.

Technician inserts flexible pipe into dispenser sump. Courtesy of OPW Fueling Components.

Permeability
As with any product, flexible pipe has evolved over the years as more field experience has been gained. Much was learned about the behavior of various thermoplastics when used in this application. As a consequence, materials have improved since the early days of flex pipe to better accommodate the underground environment and the ever changing fuel and additive packages to be conveyed. A move is afoot to require all piping systems to meet more stringent permeability requirements. Also, operational issues are demanding that better materials be used.

Polymers
Primary flexible fuel pipes consist of multiple layers of extruded thermoplastics: an inner liner; a “tie” of multiple intermediate layers; and an outer cover. The materials of each layer have a specific function and must be carefully selected for compatibility with exotic fuel mixtures and the harsh underground conditions.

Because the surface is in constant contact with the product, the primary function of the piping liner is to provide long-term resistance to permeation by fuels and fuel additives. A lot of focus has been put on methyl tertiary butyl ether (MTBE) in recent months, as incidents of contaminated water supplies were reported. MTBE, while certainly problematic when released in the groundwater, does not present significant difficulties for most thermoplastics.

			Flexible piping and corrugated flexible duct pipe. Courtesy of Environ Products, Inc.
Several different types and sizes of sump entry boots for flexible piping systems. Courtesy of Environ Products, Inc.

Alcohol, MTBE’s heir apparent, has traditionally been the most difficult fuel constituent for thermoplastics to handle. Underwriters Laboratories has recognized this fact and is currently reviewing the permeation portion of UL Standard 971, Nonmetallic Underground Piping for Flammable Liquids, with an eye for tightening the requirement for all non-metallic fuel pipes.

As alcohol becomes increasingly the additive of choice for reformulated fuels, it has become even more apparent that premium liner materials will be required. Flouropolymers, such as Polyvinilidene Fluoride (PVDF), have a proven record of supreme chemical resistance to the most aggressive fuels and additives, including alcohol. Before the widespread introduction of alcohol fuels, Polyamides were the norm for pipe liners. Polyamides, however, provide much less permeation resistance than PVDF when exposed to ethanol and methanol. Another potential liner material that was touted briefly was an Aliphatic Polyketone. This material never lived up to its billing and the supplier has ceased production.

Cross section of flexible coaxial pipe. Courtesy of Environ Products, Inc.

Corrugated or smooth?
Another issue concerning pipe liners is the use of a corrugated versus a smooth-bore design. Corrugated flex pipes are more flexible than smooth bore pipes. The trade-offs to gain this added flexibility, however, are significant. A corrugated flow profile dramatically reduces flow capacity due to the frictional losses and turbulent flow caused by the corrugations. The corrugations also allow axial forces to act on the pipe resulting in lengthwise pipe growth when pressurized. Sealing end fittings to a corrugated profile is also more difficult, especially in the absence of a compressible layer.

Corrugated or smooth?
Another issue concerning pipe liners is the use of a corrugated versus a smooth-bore design. Corrugated flex pipes are more flexible than smooth bore pipes. The trade-offs to gain this added flexibility, however, are significant. A corrugated flow profile dramatically reduces flow capacity due to the frictional losses and turbulent flow caused by the corrugations. The corrugations also allow axial forces to act on the pipe resulting in lengthwise pipe growth when pressurized. Sealing end fittings to a corrugated profile is also more difficult, especially in the absence of a compressible layer.

Flex pipe tie layers
The tie layer or layers reside between the pipe liner and the outer cover and should be physically bonded or “tied” to the liner and outer cover. Bonded construction eliminates the possibility of liquid migration within the layers of the pipe.

Flexible coaxial pipe and corrugated flexible duct pipe enter sump wall. Courtesy of Environ Products, Inc.

The key function of the tie layer is to provide resiliency and flexibility. Premium grade urethanes are recognized as the tie layer material of choice throughout the flexible hydraulic hose industry. These materials exhibit unparalleled resistance to stress cracking, an important consideration since flexible pipes are inherently designed to be installed under stress. The tie layer also provides the all-important compressible base of material onto which end fittings can be permanently and reliably attached.

Outer cover
The pipe’s outer cover—which is exposed to the underground environment—must be resilient and not be susceptible to environmental or microbial degradation. Higher grade Polyamides fit this bill. While not the best selection for a liner material because of their inferior permeability resistance to alcohol, these Polyamides do not degrade when exposed to microbes found in groundwater and do not crack when immersed in petroleum fuels.

Various Polyethylenes have also been tried as fillers in flex pipe tie layers and even as primary pipe covers. These inexpensive materials, however, can be subject to chemical growth as well as environmental stress, cracking when exposed to hydrocarbons. Underwriters Laboratories is also contemplating incorporating additional tests to the UL 971 standard to address these issues.

Outlook
Flexible piping systems provide reliable long-term conveyance and containment of petroleum fuels. Eliminating buried joints enhances the integrity of the fueling system. Flexible pipe also offers significant advantages in installation and can provide access to the entire piping system without excavation. Marginal materials will be designed out of the market as requirements become more stringent and as operational demands dictate. The advancing technology will continue to keep up with the tough demands on petroleum piping systems.

Coaxial Couplings and Fittings
by Kevin Struthers

Coaxial couplings and fittings are a double wall design with primary and secondary fluid paths integrated into them (couplings are attached to the end of flexible coaxial piping which connects into coaxial fittings). When a coaxial coupling is installed on flexible pipe with a coupling machine, the secondary fluid path of the pipe is automatically connected to the secondary fluid path of the coupling. This eliminates the need for test boots and jumper tubes required by other coupling and fitting systems, saving time and money and eliminating at least two joints.

Another important feature of the coaxial system is the coaxial fitting’s double o-ring axial primary seal. A liquid-tight seal is created by simply inserting the coupling into the fitting. The function of the coupling nut is to keep the coupling engaged. The tightness of the coupling nut does not affect the quality of the seal; it is ensured by the seal and fitting design.

The coaxial couplings and fittings have a non-metallic exterior that makes them suitable for burial. This permits greater flexibility in station piping layouts. For example, it allows the use of a shallow dispenser pan in all dispenser installations because site-specific slope requirements can be handled underground with a coaxial tee fitting and a pipe riser.

Design concepts were developed using solid modeling software that allowed fluid flow-path evaluations and injection-molding viability analyses. The final design concepts were machined from plastic bar stock and evaluated against design criteria and Underwriters Laboratories (UL) standards. Production grade samples were injection molded from an advanced glass-reinforced thermoplastic resin.

These samples were subjected to a year-long design verification test program. The test program verified compatibility with the wide range of petroleum products, alcohol and alcohol mixtures. The coaxial couplings and fittings were also subjected to impact and loading tests to ensure that they could handle the rigors of installation and direct burial. The coaxial couplings and fittings were also tested to ensure that they could withstand the surge pressures generated by the sudden starts and stops of typical pump and dispenser use. The coaxial couplings and fittings were subjected to more than 1,000,000 consecutive 150 psi pressure surges at room temperature. The test was repeated with the test chamber temperature lowered below freezing and then repeated again with the temperature elevated. After the coaxial couplings and fittings passed this test program, they were sent to Underwriters Laboratories to complete the third party listing process and were installed in select test sites.

Environ Products introduced coaxial couplings and fittings in 1998 as an enhancement to its GeoFlex ® piping system. The coaxial system makes it easier and more cost effective to install a double wall piping system.

Kevin Struthers is Vice President of Engineering and Manufacturing for Environ Products, Inc. He can be reached at kstruthers@envproduct.com

Discuss