A Practical Primer for UST Piping Installations

Even if you pay for the best piping technology in the world, you can kiss those dollars goodbye if your pipes aren’t properly installed. In a review of proper piping installation steps, Andy Youngs offers strategies to ensure that you don’t get stuck burying someone else’s mistakes.

Close-up of construction workers demonstrating a proper method of coupling flexible piping.

Piping for underground storage tank (UST) systems has been the subject of substantial discussion and investment for fueling facility managers. In just the last year, millions of feet of underground piping were installed as tank owners rushed to meet the December 22, 1998 deadline for UST compliance with EPA regulations.

Piping for UST systems is generally made of rigid fiberglass-reinforced plastic (FRP) or flexible thermoplastic (flex pipe). Ducted systems have a flexible primary pipe inside of a flexible (or rigid) secondary (or tertiary) containment pipe, which allows removal of the primary pipe without digging up the containment pipe.

There are advantages and disadvantages for each type of system—rigid or flexible; ducted or direct buried. A number of hybrid systems have also emerged that seek to gain the advantages of both system types while minimizing any disadvantages.

In previous articles in PE&T during 1998, I discussed the technologies and technical factors used to select and specify piping types and suppliers. Therefore, these particular topics will not be discussed here (“What Specifiers Need to Look for in Flexible Piping,” May/June 1998; and “Technologies Used in Today’s Flexible Piping Systems,” July 1998).

Where to start
Regardless of the system specified, for whatever special features are desired, proper installation is absolutely essential. Without it, one cannot achieve an environmentally secure, long-lived, trouble-free UST piping system. In fact, specification of installation details (such as backfill type, piping slope, sealant type and use and installer certification requirements—to name a few—are just as important to the engineering plan of a site as is the piping type and supplier.

Fortunately, the engineer has access to readily available, qualified assistance in determining most of these details from the UST piping and piping system component manufacturers. Most major suppliers of UST piping and system components will supply the specifier with not only product specifications, but also with detailed installation instructions and installation specifications tailored to that supplier’s pipe and system components.

As warranty viability is usually tied to strict adherence to a supplier’s installation instructions, specification of these items becomes doubly important. However, information from the suppliers cannot cover every contingency that may be encountered in the field. Nor can this article. What is to be presented here is an overview of piping practices—a primer to be tempered by the instructions of the specific component being installed, and by the details of the site under construction.

The most likely point of problems or failure in any piping installation is at the joints. Here, an installer inspects the assembly of a two-inch coaxial 90-degree elbow.

A typical installation using flexible product pipe with a fiberglass vent pipe

Layout and flow rate
Every proper piping job starts with the piping layout. When provided with a sketch of the site, most pipe manufacturers, as a service or through a software program they give out, will provide a computer-generated site layout and bill of materials. Providing details such as unusual topography or grade, submersible pump size or dispenser model allows the supplier to provide the most complete and accurate response to a request for this service.

Product piping size and layout depend primarily on submersible pump size, number of dispensers and the length of the piping run. Two-inch pipe, whether flex or FRP, provides minimal flow restriction.

Thus, when using two-inch or larger piping, the restriction on nozzle output is usually the submersible pump. If a I HP submersible pump with a maximum output of 65 gallons per minute (gpm) is specified, even piping that is 10 inches in diameter will not get more than 5H gpm out of each nozzle when all nozzles of a six-dispenser site are running. So, in terms of flow rate, first look at the desired output, the number of nozzles and the specification of a submersible pump model that can meet or exceed that desired output.

When flex pipe was first introduced, there was a common belief that the absence of elbows, tees, etc., made a 1H-inch flex pipe equal to a nominal two-inch FRP pipe in flow. The inside diameter of 1H-inch flex pipe is 1.5 inches (area = 1.8 square inches), as compared with the inside diameter of two-inch FRP at 2.2 inches. (The inside diameter of a nominal two-inch pipe can be 2.2 inches because piping is actually defined by the outside diamater.)

The specifier must keep in mind that a 1H-inch flex pipe has only 47 percent of the cross-sectional area of a two-inch FRP pipe, and even with the improved flow due to the absence of elbows and tees, 1H-inch flex pipe does not have the same flow capability as two-inch FRP. That being said, two-inch FRP is usually well in excess of requirements, from a flow standpoint (if designed properly), for the most common UST site layouts used for service stations today, and 1H-inch flex pipe is generally sufficient for these sites.

Flow analysis software has been applied to service station piping layouts with reasonable success by many of the piping manufacturers. My experience with the use of flow software, and the validation of the flow models thus generated, has been generally positive. This experience has led to a “rule-of thumb” that a single series run of 1H-inch flex pipe can be used for up to four dispensers. A six-dispenser layout normally requires two runs of 1H-inch flex pipe, or a looped system (see Figure 1).

Looped systems optimize flow to all nozzles during high usage, and have been very successful. In addition, by placing ball valves in looped systems, a person can work on sections of the system without disrupting any dispenser other than the one being worked on.

Figure 1:Looped piping systems optimize nozzle flow during heavy use.

Historically, high-flow systems such as truck stops have used three-inch diameter piping. Computer flow analysis and field experience have shown that in most applications, use of the larger diameter piping is warranted only for trunk lines leading from the tank to the first dispensing unit or branch point, and that smaller diameter piping can be used thereafter. Traditionally, FRP piping layouts have been done in parallel (Figure 2). Use of sumps, especially with flex pipe, have brought about the use of serial layouts (Figure 3), which are the norm for flex pipe. Sumps are used increasingly for FRP piping. Serial layouts, or hybrid parallel/serial layouts, usually minimize pipe and fitting use and, therefore, minimize cost.

Figure 2: Typical parallel layout.

Figure 3: Typical series layouts: A) Using crossovers to maintain dispenser product order. B) Using crossed flex connectors in dispenser sumps to maintain product order.

Crossovers: Pipe layouts often require one pipe to cross over the path of another pipe. With FRP, piping crossovers are easily accomplished and are done routinely. However, in my opinion, these piping crossovers are to be avoided wherever possible because they make it more difficult to maintain proper slope throughout the whole system.

Also, if not properly separated and isolated with backfill, the pipes can point load against each other in a way that is likely to cause failure down the road. Where crossovers cannot be avoided, ensure that the pipes are separated by a minimum of three inches of approved backfill (Figure 4). Crossovers often result from the desire to maintain uniform product order in multi-product dispensers. In series layouts, most crossovers of this type can be avoided by crossing the product lines in the dispenser sump, using flexible connectors for this purpose (Figure 5). Specifiers are encouraged to specify all-metal, fire-rated flexible connectors for sump applications, to ensure compliance with fire codes. Interpretation of these fire codes can vary by locality. The slight added expense of a fire-rated flex connector can make the approval process easier with the local inspector.

Figure 4: Where crossovers are unavoidable, place three inches of approved backfill between pipes.

Figure 5: Product order can be maintained and piping crossovers avoided by use of flexible connectors in the tank sump.

Slope: Piping layouts are also affected by the need to maintain proper pipe slope. Piping is installed at a minimum slope of one percent (J-inch per lineal foot) to accomplish the following: (1) to allow drainage and monitoring of secondary containment interstice; (2) prevent formation of vapor pockets in piping systems; and (3) to allow suction piping to drain back to the tank in the event of a breach in the system.

Vapor recover piping should be installed with a two percent (G-inch per lineal foot) slope. Figure 6 illustrates the minimum burial depth requirements of the piping as required to maintain proper slope on a conventional installation.

Trenches and bedding
Manufacturers’ requirements differ somewhat; but in general, trenches should be cut with generous radii, and deep enough to allow six inches of approved backfill to bed the trench prior to pipe installation. It is important to cut the corners of the trenches with a radius rather than sharply. (This is primarily for flex pipe, but doesn’t hurt in a rigid pipe application, either.) Spacing between any two objects in the trench, including between piping or conduit and the trench wall, should be a minimum of four inches.

Figure 6: Minimum burial depth requirements to maintain one percent slope (Note, depths for two percent slope for vapor recovery piping would be 12 inches, 18 inches, 24 inches, 30 inches and 42 inches.)

Backfill: Backfill should be clean, washed sand or pea gravel. Crushed stone of certain types, without sharp edges and washed, may be acceptable to some piping manufacturers. Backfill should be smooth, so as not to damage pipe and component surfaces. Washing of backfill is critical, to prevent fines which could disrupt initial compaction and later erode, causing voids, pipe movement and uneven settling of the piping system.

Materials used for backfill must provide sufficient soil modulus to give enough haunch support to transfer the downward force of vertical surface loads into equal radial load on the pipe (Figure 7). Sand must always be uniformly compacted when used as backfill.

Installation order: Most installers prefer to establish grade, set island forms and set dispenser sumps prior to installation of the sump entry fitting. Be sure to set dispenser sumps properly so that shear valves will be located as required for effective operation in the event of an impact.

Figure 7: Proper backfill provides haunch support needed to transmit vertical surface loads to uniform radial loads on pipe.

Next, tanks should be set and tank sumps installed. Once this is done, proper slope requirements will dictate where to install the bulkhead fittings. (The terms bulkhead fittings, entry boot, flexible entry boot, pipe entry boot and conduit entry boot are used essentially interchangeably in the petroleum equipment industry nomenclature. All refer to a fitting used to provide a method to seal the entry of a pipe or conduit into a sump in a fluid-tight manner.)

Bulkhead fittings: The installation of bulkhead fittings is one of the most critical areas of installation. Proper installation of bulkheads prevents water intrusion into the sump during the life of the installation. Use of an appropriate sealant should be specified, and installers should take special care to tighten components to exact manufacturers tightening torque instructions (see Special Piping Considerations). Also important in bulkhead fitting installation is the use of the exact size of drill bit or hole saw required.

Hole saws should always be used for the larger holes, especially with threaded or single-hole style bulkhead fittings, as the bulkhead fitting must seal that hole. Any lack of circularity or “wallowing-out” of drilled holes can make it difficult to install the bulkhead fitting in a completely secure manner.

Bedding the trench: Once bulkhead fittings are installed, the next step is to bed the trench, so that piping is laid with complete support, on the proper slope. This is especially important with flex pipe, and with vapor recovery pipe, to prevent traps due to sags in longer runs.

With rigid pipe, as an alternative, pipe supports can be used to provide proper support to prevent sags. Where vapor recovery piping is installed at a different slope than the product pipe, but in the same trench, the trench should be bedded for the vapor recovery piping. Then, that piping should be installed and tested, and the trench then bedded to the level required for the product piping.

Once the trench is properly bedded, the piping is installed in accordance with manufacturer’s instructions. Again, the best resource for specifications on how to properly install each brand of piping comes from the manufacturers themselves.

Installation and testing
It is essential to keep all components clean and free of dirt and debris, especially in installing the newer coaxial fitting systems, which are now available for both FRP and flexible piping. The O-ring seals on these systems must be kept clean during installation for them to function properly.

The most likely point of problems or failure in any piping installation is at the joints. Joint and fitting installation should be done with the greatest care and precision, using precise instructions from the supplier. Use of proper torque is critical in any tightening activity with fittings, especially with coaxial fittings. Once the piping is installed, it is tested. Primary piping, secondary piping and sumps should all be tested for integrity, both before and after backfill.

Use of proper torque is critical in any tightening activity with fittings, especially with coaxial fittings. Pictured is a coaxial pipe assembly in a dispenser sump with secondary closure.

Testing before backfill makes it a lot easier to find and fix a problem than after the pipe is covered up with tons of backfill. However, the installation should also be tested after backfill, to ensure that none of the components were damaged or misaligned during backfill, causing leak potential.

Test to manufacturer’s instructions, which are normally three to 10 psi for secondary piping; 50-60 psi for primary piping; and about two to three feet of head pressure for sumps and entry fittings.

Many contractors leave the test pressure on during backfill, which saves the step of re-pressurizing. Also critical at this stage is testing tank vent and Stage II vapor recovery lines (see Special Piping Considerations).

Testing of sump and sump penetration integrity is not always done, but should be done as part of the installation specification. The most common method is to fill the installed sump and components with water, and monitor the level of the water for some period, usually overnight, to see if any water leaks out. With proper component selection and installation, that will be the last water that sump will ever see.

Some manufacturers have introduced sump systems that can be tested via vacuum after installation. This is a much tougher test to pass than the water test, but not easily done with most standard systems on the market.

Special installation concerns
FRP and flexible systems each have some special characteristics that require caution during installation. The following list is by no means exhaustive, nor should it be used to compare one type to the other. The information is here for the specifier who has chosen one of these systems, and wishes to see it installed properly.

1. FRP piping installations require adhesive bonding (gluing) of the primary, and sometimes secondary, piping joints. To ensure that this bonding process is as uniform and effective as possible, it is important to keep all surfaces clean and free of moisture and solvents or oils. Tapering tools specific to the manufacturer should be used to be sure that the pipe taper fits the taper of the fitting.

2. Installation of FRP piping requires different approaches for cold or hot weather. This is a frequently overlooked fact during installations at which extreme temperature swings are experienced during the course of a day.

3. Flexible piping installations require care that the piping is not damaged via over-bending, or kinking of the pipe. Flexible piping is easily installed in cold weather, the low temperatures serving only to stiffen the pipe, and perhaps make fitting installation slightly more difficult. Flexible piping is often installed in ducted systems. This requires careful measuring to ensure that the pipe length, especially with swaged fittings, is not too short or too long when inserted in the ducting.

4. Flexible piping should be installed with some slack, or waviness, in the piping run. This is especially important when installing the piping in hot weather. In hot weather, piping can reach upwards of 120 degrees F in the trenches, yet will cool to 50 degrees F or less once backfilled, and after colder weather ensues. Due to the relatively high rate of thermal expansion and contraction of flexible piping (as compared to steel and FRP piping), piping runs installed in hot weather will contract as they cool, leading to problems with the installation unless slack is placed into the piping runs. In one flex pipe installation in the desert, in which there was an 80 degree F temperature swing between noon and midnight, the contraction of the pipe pulled entry fittings loose and dislocated impact valves. Just a little slack in the piping run would have prevented this.

5. Perhaps the most important thing to specify for piping installations is that they be performed by installers who are factory-trained, authorized or certified by the piping and component manufacturers. The pipe manufacturers have just as much at stake as do the specifiers in ensuring a proper, secure and long-lived installation. Their training and installation programs should reflect this, and use of trained and certified installers should ensure that the product is installed properly.

Figure 8: Drop-out Tank

Figure 9: Common areas for sealant use: A) Sump adapter seams B) Pipe entry fittings C) Sump seams D) Conduit entry fitting

Special Piping Considerations

Specifying techniques Specification of the correct piping and component type and manufacturers is only half of the battle for ensuring the best available UST piping system for any application. Specification of installation techniques; accessories such as sealant; and the requirement for installer certification are essential to providing the most secure installation for the site. The best resources the specifier has in this regard are the pipe and accessory manufacturers and preferred local contractors. Area contractors will be aware of requirements and details specific to the area of the site, and can be of great assistance in developing a proper engineering plan.

Vent and vapor recovery piping
Piping slope is important to meet NFPA standards, allow secondary containment drainage and monitoring, and to prevent formation of vapor pockets in product piping. Over the years, however, many installers have paid little heed to the one percent (J-inch per lineal foot) slope requirements for product piping, without incurring a great deal of negative consequence. Tank vent lines, for which slope is recommended at a minimum of two percent (G-inch per lineal foot), are usually short, with a great deal of rise, and so are easily sloped to required levels.

But with the advent of Stage II vapor recovery, failure to maintain proper slope became problematical in nearly every instance. Without proper slope, flow and blockage tests fail, and vapor recovery systems don’t function as intended. Even rigid FRP piping, in longer runs, can sag if not properly installed, causing condensate blockages and vapor recovery system failure.

Recommended slope for tank vent lines and Stage II vapor recovery lines is two percent or G-inch per lineal foot. This is twice the slope of product piping. Assume a four-dispenser site, with dispensers 25 feet apart, and the first dispenser 100 feet from the tanks. A series piping layout would have 175 feet of piping, requiring piping slope of 44 inches. Factoring in a 12-inch minimum pipe burial depth, this brings the total piping slope to 56 inches, or nearly five feet below grade.

With most tanks buried at a depth of 36 inches to 48 inches, two solutions can be applied to ensure that proper piping slope is maintained. One is the use of a creative piping layout that limits vapor recovery piping length to that which achieves the two percent slope. In this instance, with the tanks 100 feet from the nearest dispenser, a tank burial depth of 48 inches would be required as well as extra trenching and pipe. A better solution is the use of a dropout tank (Figure 8).

Dropout tanks are placed at the lowest point in the vapor recovery piping system, and create an evacuable condensate trap. This trap is continually drained by the venturi action of the submersible pump. Dropout tanks can be field constructed using FRP piping, or are commercially available from several suppliers. Use of dropout tanks allows for maintenance of recommended piping slope, thus ensuring a clear path for the flow of vapor back to the ullage of the tank.

The recommended two percent slope for vapor recovery piping is not required by regulations. Current regulations generally require only that the vent and vapor recovery piping meet the one percent slope requirements of product piping. This leads many installers to run the Stage II vapor recovery piping in the same trench with the product piping, at a one percentage slope.

Unfortunately, with only a one percent slope, it takes very little installation error, or ground movement after installation, to create a trap in the system, even with use of rigid piping for Stage II vapor recovery lines. The resulting deterioration in flow capability, or even complete blockage in some cases, then requires expensive excavation to restore the system to proper working functionality.

Specifiers are encouraged to require a two percent minimum slope for tank vent and Stage II vapor recovery piping. It is also an excellent idea to require flow and blockage tests after backfill and concrete/asphalt surfacing; in fact, the practice is often required by local regulators to ensure viability of the Stage II system. Flow and blockage testing

Discuss