Thermoplastic Flexible Piping in Underground Tank Systems
The purpose of this fact sheet is to provide underground storage tank (UST) owners, operators, and installers with information on problems associated with flexible piping systems and give specific guidance on Oil Control Program (OCP) recommendations to follow to return the UST facility to compliance when a problem is encountered. In general, polyethylene materials are subject to expansion when they come in direct contact with petroleum. However, polyethylene material is not used as the primary fuel barrier, but rather only as part of the outer wall of the flexible piping system. OCP is aware of several instances where flexible piping has expanded and swelled and no longer performs as installed or intended. Several southern states have also reported various related problems, not all of which have been observed in Maryland. In spite of this, all problems will be discussed herein to provide as much detail as possible.
In January 2003, OCP, in association with the Maryland Ad Hoc Committee on Oil, met with manufacturers of flexible piping systems, including Environ Products, Total Containment, Advanced Polymer Technology, and OPW, to better understand the issued related to their products. Manufacturers described varying experiences with the expansion problem that ranged from none at all to isolated occurrences. Flexible piping owners are encouraged to seek more information directly from their manufacturer.
The Nature of the Problem
The expansion rate of polyethylene upon its contact with petroleum product depends on a number of factors, including, but not limited to, the production grade of the polyethylene, the volume of the petroleum product making direct contact with the material and the duration of the contact. The purpose of the outer wall of flexible piping is to channel any released product from the primary portion of the pipe to the containment sump on the top of the tank where it will be detected and removed in a timely manner. If fuel is allowed to sit in these areas, then swelling problems can occur. It is very important that no fuel be allowed to remain in the containment sump.
Also, swelling of the piping might be caused by a small release, such as that caused by a leaky fuel filter, in the dispenser area. If the released fuel gets between the inner and outer wall of the piping it will begin to gradually flow towards the tank top. Depending on the volume of the leak, it could take some time before the product is detected at the tank. It is probable that swelling could occur in the piping without product being observed in the containment sump.
Finally, the outer wall of piping can come in contact with petroleum that was previously released into the environment either in liquid product form or vapor form. It is probable that swelling could occur under these conditions, although in theory, a high water table would most likely also be present in these conditions. The swelling response of polyethylene to the vapor phase is much less understood but should be considered in any evaluation.
Once swelling of the flexible piping begins, the extra force generated on the piping can result in other negative impacts on components of and inside the containment sump as described in the section below.
Identifying the Problem
While it is likely that most flexible piping systems will not experience significant problems, OCP urges UST owners, operators, and installers to closely inspect their piping systems now and continue to periodically inspect them on a monthly basis. OCP encourages visual inspection of the piping by opening dispenser cabinets and submersible pump man ways.
Visible evidence that might indicate the integrity of the piping system is compromised includes:
- The ells, tees, riser pipes and flex connectors found within the sumps where the piping is terminated may be twisted, over-stressed, or pushed out of normal alignment.
- The pipe may be over-bent within the tank sump or it may be folding over on itself (kinked).
- The outer jacket of double-walled (coaxial) piping may be extended over the metallic ferrule of the pipe coupling.
- The outer jacket of coaxial piping may be splitting as it attempts to grow over the metallic coupling.
- The pipe may be swelling and appear to be bulging or "ballooned."
- The pipe may be wrinkling or it may be sticky/spongy and softer than it was originally.
- The outer walls of the primary pipe and/or the secondary jacket may be cracking.
- The rubber boots that are installed in the walls of the containment sumps may be stretched or torn.
- The donut that make up part of the boot of some pipe systems may be dislodged or the clamps may not be in place.
- The rubber "test" boots that are installed at the pipe terminations of some coaxial pipe systems may appear to be compressed or distorted.
- The metallic ferrules that are part of some pipe system couplings may be cracked.
- Piping manufactured prior to 1994 that is yellow in color may be delaminating and a fungus/microbial growth may be attacking the outer walls of the pipe.
Actions to Take Upon Discovering Problem
Two phone calls should be made within two hours of discovering swelled piping. The first call should be to the piping manufacturer so they can assist the system owner in determining the appropriate course of action concerning the swelled piping. The second call should be to the OCP inspector at (410) 537-3442. In some circumstances, the piping will be permitted to remain in operation while the investigation proceeds. In others, it is possible that OCP will require removal of the piping from service until the swelling is under control and the integrity of the piping is not in question. OCP will require that swelled piping meet all manufacturers' installation requirements in order for the piping to remain in operation.
In theory, if the source of the petroleum that is in contact with the flexible piping is removed the piping will cease expanding and in time will return to its approximate original dimensions. OCP has yet to see this approach demonstrated in practice, and, at present, is withholding judgment on whether a repair rather than total replacement is an appropriate action to discovering a swelled flexible piping system. At present, a case-by-case review is indicated.
In all cases where the swelled flexible piping is authorized to remain in the ground following any corrective actions, the system owner should receive a written statement from the manufacturer of the impact of any recommendation actions on the piping warranty. If the manufacturer of the flexible piping system recommends a course of action allowing the piping to remain in the ground, then it is reasonable for OCP to expect the warranty for that piping to be unaffected and to remain in full force. The most likely circumstance that could force the removal from service and replacement is that the piping has swelled so much that it has exceed the maximum bend radius in the containment sump and failure of the piping could be imminent.
The piping material and its performance and product is evaluated by the Underwriters Laboratories (UL). Currently UL is working to revise the code that governs how flex pipe systems are manufactures. The revised code should ensure that all materials used in flexible piping systems would not experience these same problems. Until this new generation of piping becomes available, it is very important that UST systems owners and operators continue to inspect the piping in the tank top containment sump and below the dispenser for signs of change.
Please ensure that automatic line leak detectors are functioning properly and that required inventory control and monthly leak detection is being conducted. Consideration should be given to continuous monitoring of the piping system by installing electronic sensors in the piping sumps. Properly functioning electronic sensors can help ensure that the presence of fuel within the sumps will be promptly detected.
If you believe your flexible piping system may be experiencing any of the above problems, please take action immediately as described. If you need additional assistance, please contact the Oil Control Program (OCP) at (410) 537-3442.