The 4 Most Common Pipe Materials for Underground Storage Tanks and the Pros and Cons of Each
Tank piping is buried with limited access at the UST sump, dispenser sump or transition sump, making it the one component of Underground Storage Tank (UST) fueling systems that is most often overlooked during maintenance and inspections. Modern tank piping is designed to safely transfer fuel from storage tanks to fuel dispensers, emergency backup generators, furnaces and boilers, so it’s important to choose the right piping material for your project.
Many older fuel storage systems were installed with steel piping. In 1988, the EPA determined steel was susceptible to rust and corrosion, and prohibited unprotected steel from being installed in new UST systems. Two materials used to replace steel pipe in the 90’s also had high failure rates. These were flexible thermoplastic and HDPE Piping. Manufacturers have addressed shortcomings of early UST piping products by replacing them with pipe that is UL listed for fuel compatibility and resists corrosion both internally and externally.
Today, the most common reasons for piping failures, as tracked by the Californian Water Board, are improper installation and poor design. Common failure points in UST piping occur at the entry to sumps, in secondary pipes and coupling connections. The most important safeguard is the EPA requirement for double-wall piping. When a primary fuel line fails, secondary piping systems contain the leak and send fuel to the tank sump, where a sensor triggers an alarm at the tank monitoring system console. In this article, we will compare the 4 most common piping materials installed in USTs today, and discuss the pros and cons of each.
The Pros and Cons of Flexible Fuel Pipe for Underground Storage Tanks
Flexible fuel pipe is one of three types of piping that are used in underground storage tank installations. Not all flexible piping was created equal. Some flexible fuel pipe manufactured in the early 90’s used polyurethane coated secondary piping which was incompatible with MTBE. When MTBE was combined with water, and came in contact with polyurethane, it caused fungal decay. A compromised pipe would crack and flake. In other cases, where the pipe was exposed to fuel for extended periods, the pipe would swell and create kinks. Total Containment, inc.’s Enviroflex Model 1500/1501/2500 product piping, manufactured prior to October 1994, is of particular concern.
Current flexible fuel pipe is UL971 listed to January 2014 standards and biofuel compatible (E85 and Biodiesel). It is easy to work with in any weather or site configuration and can be used in suction or pressure applications. It is available in large rolls so service station contractors can install it using continuous runs and it features a 3 foot bend radius. All connections to the UST and dispenser are made within the sump and the pipe can be disconnected for repairs and replaced if needed. This is accomplished by installing flex pipe within a 4" dual layer thermoplastic corrugated chase pipe.
One advantage that flex pipe has over other style pipe is that there are very few installation requirements. Flex pipe can be installed in extreme hot or cold, wind or snow and in shorter times than other pipes. The main installation precaution is to install the pipe with sufficient slack. Flexible piping installed in hot weather will contract when cooled after backfilling. Flexible pipe can kink or bend if over-flexed (especially in cold weather) so care should be taken to maintain the manufacturer recommended bend radius.
The Pros and Cons of Semi-Rigid Pipe for Underground Storage Tanks
The second type of pipe is a semi-rigid coaxial or fusion welded pipe consisting of a High Density Polyethylene (HDPE) outer layer and Ethylene vinyl alcohol (EVOH) resin inner layer. It has some of the characteristics of flexible pipe in that it comes on rolls but uses fittings to connect joints. The components are bonded using electrofusion welds providing watertight containment with zero leak paths. Semi-rigid coaxial are also welded to dispenser and tank sumps so there is no chance of the joint breaking down over time and allowing water in the sump. The pipe is welded to the entry boot and the boot is welded to the sump wall. This is the biggest advantage semi-rigid pipe has over flexible pipe. Semi-rigid pipe is available in lengths up to 165 feet and is very durable.
According to Franklin Fueling Systems, a semi-rigid pipe manufacturer, there are no failures in underground fuel pipe systems and no loss into the ground in over 30 years. HDPE has well proven resistance to: stress cracking, puncture, scratch, impact, microbial and rodent attack. A disadvantage of semi-rigid pipe is the cure time of the pipe weld. Standard cure time in ideal operating conditions is 20 minutes. In wet or cold weather the cure time will increase. There is also a reduced bend radius of the pipe in cold weather.
The Pros and Cons of Fiberglass Reinforced Plastic (FRP) Pipe for Underground Storage Tanks
The third type is made of Fiberglass Reinforced Plastic (FRP) and is manufactured in two styles, LCX and Red Thread. LCX coaxial fiberglass pipe provides secondary containment within a small interstitial space. This rigid style of pipe comes in lengths of 20 to 40 feet. Each pipe joint is joined with a bonded clamshell fittings that can be either a straight, tee, 45 or 90 degree.
The second type of FRP pipe is the pipe-in-a-pipe system or single-wall fiberglass over single-wall fiberglass. This style is installed with either 3” diameter piping over 2” diameter piping or 4” over 3”. According to Ameron International, a manufacturer of fiberglass systems, this style of pipe costs 63% less than 2” coaxial flexible pipe and 46% less than 1½” coaxial flexible pipe.
The disadvantages of coaxial and pipe-in-a-pipe systems are the relatively short pipe runs, and joints at the end of each pipe length resulting in systems which are labor intensive to install. Each joint must prepared by sanding the pipe end and coupling before gluing the components together. A clam shell joint is then placed over the coupling and bolted together. The clam shell increases the mechanical strength of the joint and provides secondary containment continuity around the spliced area. Joints are glued using a 2 part epoxy which can take up to 8 hours to cure in cold weather. Gluing joints in wet weather is not recommended and each joint must be kept clean so that dirt does not contaminate the joint when it is glued.
A benefit of FRP piping is longevity and life cycle cost. Ameron provides a 30-year warranty against water intrusion into tank sumps which means UST operators utilizing FRP systems no longer have to dispose of water from sumps. A dry sump also means less corrosion or damage to UST piping, pumps and sensors. Another advantage of FRP is hydraulic efficiency due to smooth wall bore characteristics and larger diameter couplings (e.g., elbows, connectors and tees) that do not restrict the pipe ID. Hydraulic efficiency allows engineers to specify smaller diameter pipe which reduces material cost. Rigid pipe installations provide a constant slope from tank to dispenser and the inherent strength of fiberglass allows for higher operating pressures than flexible or semi-rigid pipe. According to the Fiberglass Tank and Pipe Institute, over 150 million feet have been successfully installed since the piping was UL-971 listed in 1968.
Pre-insulated pipe is another product that is installed in UST systems. Buildings that run their furnaces or boilers using #6 heating oil require insulated double-wall piping to maintain the viscosity of the fuel. The fuel can be heated within the UST, inside a boiler or within the pipe using a heat trace. Pre-insulated pipe is ideal for this application because the piping has a temperature range of 32 degrees F to 750 degrees F. Heat loss is a function of flow in the line so, the greater the flow rate, the lower the temperature drop. Insulated piping systems reduce the heat lost per gallon through the line.
Pre-insulated pipe combines the strength of steel with the corrosion resistance of fiberglass. There is no need for cathodic protection because the outer wall of the piping is gel coated and jacketed with 100 mils of fiberglass cladding. Wool insulation between the jacket and outer pipe wall allows the system to pass a boiling test certification. Steam heating systems use pre-insulated pipe as transmission lines for carrying fluids between buildings.
Typical applications include:
- Maintaining viscosity for efficient fluid flow
- Keeping fluids above their freezing point
- Reheating high viscosity fluids such as oils, resins and chemicals
A disadvantage of pre-insulated pipe is the time required for installation. Each steel pipe must be welded before a joint can be sealed. Insulation must be kept dry during installation so installers are required to monitor changes in weather. Open trenches must be covered or dewatered when groundwater is discovered during excavation.. It is important to choose the correct backfill material to prevent damage to the pipe coating or fiberglass jacket. Material costs of pre-insulated pipe compared to uninsulated pipe are typically %50 higher.
Underground storage tank piping is an integral part of every fuel delivery system. Inspection and maintenance in older systems can be complicated by limited access to buried fuel lines. Systems installed prior to UL specifications to fuel handling may include materials that were later proven to be ineffective in preventing leaks long term. New installations have a variety of double walled options, choosing the right pipe for your underground storage tank will depend upon the application and installation requirements.