Recommended Practice by Petroleum Equipment Institute, DRM is included at the request of the publisher, as it helps them protect their copyright by restricting file sharing.
Visit FileOpen to see the full list. Vent restriction devices ball float valves are no longer included as an option for installation. References to important new resources on corrosion in USTs storing diesel and ethanol fuel blends have been added to Appendix C. Minimize problems by tightly scheduling critical phases of the work, including: layout - paving removal and excavation - preparation of the base for the tanks - setting the tanks and other system components - backfilling and compaction to the top of the tanks - piping, electrical, and equipment installation - completion of backfilling - paving.
Factors that aggravate excavation problems include groundwater and surface water, unstable soil, adjacent backfilled areas, frost, presence of released product, and vibration.
Problems resulting from unstable soil or the infiltration of water may require the removal of tanks that are already set in order to clear fallen materials, or to Recommended Practices for Installation of Underground Liquid Storage Systems correct tank movement caused by settlement or flotation.
If such an event occurs, consult the tank manufacturer. An excavation that has caved-in will require additional backfill because the materials that have fallen into the excavation typically cannot be used as backfill. Excavation for under- ground tanks should be made with due care to avoid undermining foundations of existing structures. In the absence of local building codes or regulations, use the minimum distance to the base of adjacent structures or property lines depicted in Figure Additional dis- tances may be required to ensure that downward forces from loads carried by the foundations and supports are not transmitted to the tanks.
Clearance from existing structures. Care must be exercised to avoid undermining nearby struc- tures during construction or afterwards, permitting trans- fer offoundation loads onto the tank. The 45factor illustrated will accomplish this in most cases.
The total depth of the tank excavation is determined by the tank diameter, bedding thickness, hold-down pad if required , depth of cover, and slope and length of piping. Much of the tank's support is derived from compacted backfill material, which permits the downward forces present to be dissipated uniformly over a large area. To provide a firm foundation, set the tank on a bed of back- fill material I-foot thick that extends 1 foot beyond the ends and sides of the tank.
At least 2 feet of backfill is required between adjacent tanks and between tanks and excavation walls. Steel tank manufacturers specify that when a bottom hold-down pad is used under a steel tank, the thickness of bedding material between the tank and the pad can be reduced to 6 inches.
WARNING: Placement of a tank directly on a hold-down pad, on a pad smaller than the total tank area, or on intermediate supports saddles will cause uneven distribution of loads. This may contribute to structural failure and is never per- mitted.
In areas that are subject to vehicle traffic, cover may consist of at least 30 inches of compacted backfill and 6 inches of asphaltic concrete, or 18 inches of compacted backfill and 6 to 8 inches of reinforced concrete. Paving over tanks in traffic areas should extend at least 1 foot beyond the perimeter of the tank.
Comment At most facilities, fully loaded transports can be expect- ed to pass over the tank area. While some authorities require a minimum of 6 inches of reinforced concrete paving, we recommend at least 8 inches.
Cover in non-traffic areas should be at least 2-feet thick and con- sist of a minimum of 1 foot of backfill covered by filter fabric to prevent migration and a minimum of 1 foot of earth. An acceptable alternative is 1 foot of backfill mate- rial and at least 4 inches of reinforced concrete or 6 inch- es of asphalt paving. Maximum burial depths, measured from the top of the tank, are established by tank manufacturers and independent testing laboratories.
Seven feet is the standard maximum burial depth for fiberglass tanks. Consult with the fiberglass tank manu- facturer if a deeper burial depth is required. Five feet is the typical burial depth for teel tanks. The maximum burial depth for each steel tank is marked on the tank. The planned burial depth should not exceed the manufac- turer's recommendation. Excavated material that cannot be immediately removed from the site should be safely stockpiled at least 2 feet away from the edge of the tank excavation.
Unless approved for use as backfill, keep excavated materials separate from approved backfill materials and remove as soon as practical. PEl Recommended Practices All dimensions in inches. Depth of cover in areas subject to traffic. The majority of tanks are located in areas that are sub- ject to vehicle traffic. BacJqill helps dissipate traffic loads and offset buoyancy. Depth of cover in areas not subject to traffic. In areas that are not subject to traffic, cover requirements may be reduced, but this reduced depth of cover may not prevent flotation if groundwater or sur- face water enters the excavation.
Keep work areas clear of stockpiled materials. In areas with unstable soil, if per- sonnel are required to enter the excavation, slope or shore 8 excavation walls. Barricade work areas to protect both the public and installation personnel and to prevent acciden- tal damage from vehicles and equipment. Members of the installation crew should wear personal-protective equip- ment and have fIre extinguishers and fIrst-aid supplies on hand.
Comment Excavation safety requirements are defIned in U. Used underground tanks should be made safe before removal. Product retained in the tank and piping as well as released prod- uct should be recovered, removed, and disposed of in an approved manner. Monitor tanks frequently during removal because, even after the tanks have been com- pletely emptied, liquid can reenter the tank.
There is sig- nifIcant danger of fire or explosion due to the existence of vapors or the reentry of oxygen or previously released product into the tank. Careful placement and compaction of approved backfill materials is essential to protect under- ground tanks. Common deficiencies that adversely affect the structural integrity and coatings of tanks include: use of incolTect backfill material inadequate or improper placement or compaction of backfill materials rocks or debris left in the excavation voids under the lower quadrant of the tank failure to prevent migration of backfill materials.
After backfill is placed to the level of the top of the tank, add either the product to be stored or water as ballast until the piping is in place and backfilling and paving are complete. If product is used for ballast, do not fill the tank above 95 percent of tank capacity. If water is used for ballast, the tank may be filled complete- ly. When product is used for ballast, safeguard against Recommended Practices for Installation of Underground Liquid Storage Systems fire, product spills, leaks, accidents, and theft.
Secure all fill caps and pumps during unattended periods. Monitor product level frequently. Check local regulations before ballasting with product. During construction, provide adequate venting for prima- ry and interstitial tank spaces. In instances where tanks are to be ballasted before the backfill ' process is completed, follow the recommendations of the tank manufacturer.
The presence of water in a storage tank can promote internal corrosion and degrade fuel quality, especially if the tank is to contain ethanol- blended fuel. Install tanks to facilitate water removal. Backfill mate- rial should be a clean, well-granulated, free-flowing, noncorrosive, inert material e. Refer to the manuf"cturer's installation instructions for acceptable backfill material specifications and backfill procedures. Verify that backfill material is free of debris, rock, ice, snow, or organic material that could damage the tank or its coating and interfere with proper compaction of backfill materials.
Cover the bottom of the excavation with suitably graded and properly placed backfill material to a depth of at least 1 foot. If a hold-down pad is required, backfill material depth may be reduced to 6 inches. Carefully place backfill materials along the bottom quad- rant of the tank to ensure that the tank is securely and evenly snpported. Carefully place backfill around and over the tank to prevent damage to the tank or coating.
Continue backfilling the excavation using tank backfill material up to at least the top of the excavation or the site subgrade. If material other than the tank backfill is used to reach final subgrade, separate the two backfill materi- als with filter fabric. Backfill material should be a clean, well-granulated, free-flowing, noncorrosive, inert material. Standard backfill materials for fiberglass tanks are pea gravel and crushed rock.
Careful selection, placement, and compaction of backfill material is essential to prop- erly support and protect the tank and piping after instal- lation. Approved backfill material. Tank manufac- turers have approved several types of backfill materials, including sand, pea graw;l, and crushed rock.
The latter two are relatively self-compacting, reducing the need for manual or mechanical compaction. Refer to the manufacturer's installation instructions for backfill material specifications and back- fill procedures. Verify that backfill material is free of debris, rock, ice, snow, or organic material that could damage the tank and interfere with proper compaction of backfill materials.
PEl Recommended Practices] 5. Cover the bottom of the excavation or hold-down pad with a minimum of 1 foot of suitably graded and compacted backfill material.
To ensure that the bottom quadrant of the tank is fully and evenly supported, carefully place backfill materials along bottom, sides, and end caps of the tanks by hand shovel- ing and tamping.
Carefully place backfIll around and over the tank to protect the tank from damage. Continue back- filling the excavation using tank backfill material up to at least the top of the excavation or the site subgrade.
If mate- rial other than the tank backfill is used to reach final sub- grade, separate the two backfill materials with filter fabric.
Compact bedding and backfill mate- rials to ensure adequate support of the tank and to prevent movement or settlement. When sand is used, some man- ufacturers require a specified minimum-compaction den- sity. Place sand backfill into the excavation in to inch lifts and compact after each lift.
Repeat this process up to a level that is at least 60 percent of the ver- tical height of the tank. Some manufacturers require com- paction to subgrade. If mechanical compaction is employed, take care to protect the tank from damage.
Pea gravel and crushed rock are relatively self-compact- ing. Mechanical compaction. Take care in using mechanical compactors to prevent damage to the tank shell or coat 10 dimensions. Deflection in the tank's vertical diameter may be caused by improper bedding, voids in the backfill under the tank-bottom quadrant, or poor compaction of the backfill material at the tank sides.
Excessive deflec- tion indicates inadequate support or overtightening of anchor straps, which can cause structural damage or result in penetration of the tank bottom by suction stubs or submersible pumps. The amount of deflection of an installed tank should be measured to confirm the quality of backfilling and com- paction. Individual tank manufacturers establish the max- imum acceptable deflection.
If the measured deflection of an installed tank exceeds the limits established, consult the tank manufacturer. Filter fabrics are geotextiles de- signed to prevent movement of backfill materials while permitting water to pass through. They are intended for direct burial and, if properly selected, resist deterioration caused by both soil and the products commonly stored in underground tanks.
However, the composition, construc- tion, and mechanical properties of fabrics vary widely. Care in the selection of a suitable fabric is essential. Install filter fabric between the backfill and adjacent unstable soils, bogs, swampy areas, or landfills to prevent the backfill from migrating and thus diminishing the sup- port of the tank or paving. If dissimilar backfill materials, such as sand and pea gravel, are used in the same excava- tion, separate them with filter fabric to prevent the finer sand particles from migrating into voids between the pea gravel particles.
Filter fabric. Use filter fabric to separate baclifill materials from surrounding soil to prevent migra- tion and loss of support. Backfill and compaction.
Set tanks on a bed of backfill material i-foot thick. Take special care in placilig and com- pacting backfill materials under the lower quadrant of the tank. Provide support for manholes, street boxes, piping, and other components during construction. Do not use the tank to support cribbing, bracing, or blocking.
During backfilling, remove temporary supporting materials to prevent subsequent damage to the tank, piping, or equip- ment. Where installations are located in areas subject to high water tables or flooding, provision should be made to prevent tanks, either full or empty, from float- ing during a rise in water level - up to the established maximum flood stage.
During installation, tank vents and other openings that are not liquid tight should be extend- ed above the maximum flood-stage water level until pip- ing is complete. Comments The primary method of restraining tanks in areas subject to flooding is to increase the burial depth. In the absence of any tank-top sumps, normal backfill and paving on top of the tank provides adequate restraint if the burial depth is at least 60 percent of the tank diameter and the tank is 8 feet in diameter or less.
When increasing burial depth, exercise care not to exceed the maximum allowable burial depth for the tank. See Section 4. For purposes of cal- culating burial depth, each inch of reinforced con- crete above the tank can be considered equal to 1. Base the tank buoyancy calculation on worst-case conditions i. Refer to Appendix A for an example calculation. The diameter and capacity of the tank are the most significant factors that determine the buoyancy of a tank. Manway openings and tank-top containment sumps can also contribute to tank buoyancy forces.
The weight of backfill material and paving over the tank is the most significant factor offsetting tank buoyancy. Other factors offsetting buoyancy include the weight of the empty tank and attached equip- ment, anchors, and friction between the tank and backfill. Generally, requirements for anchorage are deter- mined by the conditions associated with each instal- lation. Dead load Buoyant and restraining forces. The weight of backfill and pavement over the tank is often sufficient to offset buoyancy and prevent flotation.
Additional anchoring may be required when a high water table is presentor flooding is anticipated. PEl Recommended Practices If soil conditions and the depth of the water table are unknown at the time of installation, consider the possibility that anchoring may be required and make contingency plans. Such plans should include having pumps, hoses, straps, cables, and other anchoring materials available. In areas with impervious soil, the infiltration of sur- face water into the tank excavation can fill the exca- vation with water, resulting in very strong buoyancy forces on the tank.
If a hold-down pad is used under the tank, the excavation should be deep enough to maintain the required burial depth for the tank. If deadmen anchors are used, the tank excavation should be large enough to permit placemerit of the deadmen out- side the tank diameter. Reduce the water level in the excavation to the lowest practical level during con- struction. If water ballast is used to sink a tank in a wet hole, the level of ballast in the tank should not exceed the level of water in the hole.
While adding ballast, use lift- ing equipment only to keep the tank in position; the tank should be free to roll slightly. Carefully tend lifting cables to minimize the possibility of damage to the tank. Do not use cradles, beams, or timbers in the excavation. All methods of anchoring tanks use the weight of the backfill and paving on top o"f the tank to offset buoyancy forces.
However, for unstable soils, bedrock, large-diameter tanks, or extreme high groundwater conditions, a burial depth sufficient to offset buoyancy forces may not be feasible. In these instances, one of the following supplemental methods of restraint may be used.
Slab at grade. Buoyancy can be offset by adding weight on top of the tank by increasing the thick- ness and reinforcement of the concrete pad over the tank. The additional weight is limited to the weight differential between the submerged weight of con- crete and gravel. WARNING: When a grade slab is used for anchoring tanks, precautions must be taken to prevent tanks from floating if any part of the concrete must be removed.
Deadmen anchors. Deadmen anchors are convenient to use and reduce the amount of work required in the excavation. To be effective, deadmen anchors must be placed outside the tank diameter see Figure 6- 2 and extend the full length of the tank.
For long tanks, two deadmen of equal length may be butted end-to-end as long as there is a minimum of two anchor points per deadman section. Refer to the tank manufacturer's instructions for details on deadman construction. The weight of backfill on the deadmen anchors provides additional resistance to buoyancy forces acting on the tank. Reinforcing rods in the deadmen in this Figure are visible for illustration purposes.
Reinforcing rods should be completely enclosed in concrete. Bottom hold-down pad. A bottom hold-down pad usually consists of 8 inches of reinforced concrete that extends at least 18 inches beyond the tank sides and 1 foot beyond each end. This provides a firm foundation and offsets buoyancy by increasing the amount of backfill bearing down on the tank.
The thickness of the pad, the amount of concrete rein- forcement, and the number and size of anchor points must be calculated for each installation. Refer to the tank manufacturer's instructions for details on bottom hold-down pad construction.
WARNING: Never set a tank directly on a bot- tom hold-down pad; an adequate bed of backfill material must separate the tank and concrete. The bottom hold-down pad should never be shorter than the full length of the tank, other- wise, uneven stresses can develop, leading to structural failure.
The weight of backfill on the portions of the hold-down pad extending beyond the tank outline provides additional resistance to buoyancy forces acting on the tank. Isolating material is used to separate steel anchor straps from steel tanks. Anchor points should be securely attached to reinforcing rods firmly embedded in the concrete.
Tank straps are usually furnished by the tank manufacturer and should be installed according to the manufacturer's instmctions. Straps for steel tanks should be either nonmetallic or flat steel. The use of wire ropes or round bar is not acceptable. Metallic anchor straps should be heavier than required initially to provide a corrosion allowance during the expected life of the storage system. Electrically isolate straps for steel and composite tanks from the tank surface to ensure the proper operation of cOlTosion-protection systems.
Straps may be nonmetallic, fully encapsulated, or isolat- ed with material placed between the strap and the tank. Isolating materials must be non conductive, compatible with the materials stored, and suitable for use under- ground.
Isolating material should be at least 1I8-inch thick, wider than the strap, and should extend 1 foot below the widest part of the tank.
Distribute isolating material evenly, and test to ensure that components are electrically isolated after installation. If available, use materials provided by the tank manufacturer. Methods of attachment. Tank anchor straps should be firmly secured to anchor points with anchor bolts, turnbuckles, or wire cable and clamps. Secure anchor points to reinforcing rods embedded in the concrete. Straps should fit snugly before backfilling, but overtightening can damage the tank shell or coating.
Coat exposed metallic hardware with dielectric material to retard corrosion. Secure tank straps to anchor points with anchor bolts, turnbuckles, or wire cables and clamps. Refer to tank manufacturers' instruc- tions for the appropriate type and sizing of hardware used to connect anchor straps to anchor points embedded in the concrete.
Straps should fit snugly before backfilling. Take care to prevent damage from over tightening. Ensure that the straps and the tank surface are free of debris or burrs that might damage the isolating material or tank coating. Coat anchoring hardware with dielectric material. Anchoring hardware should be heavier than required ini- tially to provide a corrosion allowance during the expect- ed life of the storage system.
PEl Recommended Practices 7. Spill containment is intended to contain small releases of product that may result when the deliv- ery hose is disconnected from the fill pipe after a delivery. The purpose of overfill prevention is to stop the delivery of fuel into an underground tank before the tank is com- pletely full so that room is available in the tank to drain fuel contained in the delivery hose.
In the past, the lack of spill-containment and overfill-prevention equipment has often resulted in environmental contamination. Spill containment is usually achieved by installing a liquid-tight container, usually referred to as a "spill-containment manhole," around the underground tank fill pipe.
Spill-containment manholes may also be used at Stage I vapor-recovery risers and at automatic tank-gauge risers. When specifying and installing spill-containment man- holes, the following three factors should be considered. Drainage of accumulated liquids. Spill-contain- ment manholes are typically equipped with a bot- tom drain valve that allows accumulated liquids to be drained into the undergrqund tank.
Dirt and debris commonly prevent these drain valves from sealing completely. If water entry into the tank via the drain valve is a concern, the valve can be removed and replaced with a liquid-tight plug.
Some spill-containment manholes are equipped with a small hand pump that can be used to remove accumulated liquids. Exclusion of water. To prevent water from enter- ing into grade-level spill-containment manholes, slope concrete away from the manhole. To prevent the transfer of stress to the underground tank as a result of the differential movement between the concrete pad and the tank, the spill-containment manhole should provide flex- ibility between the fill pipe and the spill-contain- ment manhole assembly.
When installing spill-containment manholes, take care to properly backfill beneath and around the manhole to provide proper support. Three types of overfill-pre- vention devices are commonly used: alarms, flow shut- off devices, and vent-restriction devices. Overfill- 14 4 - 6 in. Spill-containment manhole. Spill-contain- ment manholes contain small spills during the uncou- pling of delivery hoses. Care in installation and maintenance is necessary to minimize the infiltration of surface and subsurface water.
Consult the authority having jurisdiction to deter- mine the level at which the overfill device should operate. Consult with the storage system owner to determine the delivery procedures that will be used to fill the tank.
Select and install an overfill device that will be compati- ble with the anticipated delivery procedures. Alarms consist of an external sig- naling device that is typically connected to an auto- matic tank-gauging system. To be effective, the alarm should: provide visual and audible signals to the delivery person be located in close proximity to where the delivery person stands during the delivery be clearly labeled as a "tank overfill alarm" so that delivery personnel will recognize the device as an overfill alarm.
Flow shut-off devices, often called "flapper valves," are installed in the fill pipe of underground tanks and automati- cally stop the flow of product into the tank during a Recommended Practices for Installation of Underground Liquid Storage Systems delivery. Mter the main valve closes, various bypass mechanisms allow the contents of the delivery hose to be drained into the tank.
In order to operate prop- erly, shut-off devices should be installed according to the manufacturer's instructions, particularly with regard to attaching the shut-off device to the drop tube and attaching the drop tube to the fill pipe.
Shut-off devices that are designed for use with underground tanks should only be used with gravi- ty deliveries and where there are liquid-tight con- nections between the delivery hose and the fill pipe. In a remote-fill installation, the gauge riser above the flow shut-off device must be properly sealed or else product will pour from the gauge opening when the shut-off device closes.
WARNING: Do not install flow shut-off devices on tanks equipped with remote-fill pipes and a gauge riser directly above the flow shut-off device unless a specially designed fit- ting e. H a trap door or equivalent device is not installed in the gauge riser, a properly tightened, threaded pipe cap must be installed in the riser above the flow shut-off device and a separate gauge riser installed to permit manual gauging of the tank.
Vent-restric- tion devices, often referred to as "float-vent valves" or "ball-float valves," are installed inside the under- ground tank just below the vent opening. If, after considering all of the warnings listed below, a ball- float valve is installed, it must be installed in an extractable fitting to allow access for inspection, maintenance, and tightness testing.
When a ball-float valve operates as designed, the tank becomes pressurized, creating a hazardous condition. The pressure is most often relieved by releasing flammable vapors at grade. For this rea- son, ball-float valves are not recommended. WARNING: Vent-restriction devices must not be installed in storage systems where there is any possibility of a pumped delivery into the tank.
When an overfill occurs in conjunction with a pumped delivery and a tight-fill con- nection, the tank becomes severely over-pres- surized and may rupture as a result.
H the vent-restric- tion device closes during a delivery when the gauge opening is uncapped, product will pour from the gauge opening onto the ground. The manual, which contains 33 line drawings, is completely indexed. View the RP Table of Contents. Skip to main content.
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