Selecting Overfill Prevention Equipment
For Underground Tanks

Factors to consider before making the installation

By Marcel Moreau

Deliveries to underground tanks typically rely on gravity to move the product and tight-fill connections to prevent spillage. Be very careful when selecting an overfill prevention device if other delivery techniques are used.

In the “old days,” overfill prevention for underground tanks was not something that most tank owners, delivery personnel or tank installers thought much about. Rather than overfill prevention, the more usual approach was overfill planning. In a day when dirt was a common backfill material, sand or gravel was sometimes placed around the fill pipe to facilitate the drainage of product when the inevitable product spill during a delivery occurred.

While we have come a long way in improving fuel delivery practices, the day-to-day implementation of overfill prevention leaves much to be desired. In part, this is because many smaller tanks, non-retail tanks and non-motor fuel tanks utilize fuel delivery procedures that are not compatible with commonly used overfill prevention devices. The purpose of this article is to help raise awareness of what the appropriate overfill prevention device choices are for various types of tanks and raise some warning flags about situations that are particularly hazardous. Please note that the discussion in this article is limited to underground tanks.

Who Is Responsible for Selecting Overfill Prevention Devices?
PEI RP100, Recommended Practices for Installation of Underground Liquid Storage Systems, places responsibility for selecting and installing an appropriate overfill prevention device on the storage system designer and installer:

Designers and installers need to be aware of the various pitfalls of overfill prevention devices in order to install equipment that is compatible with the underground tank delivery procedures that will be used on a specific tank.

All flow shutoff devices should only be used with gravity deliveries and tightfill connections.

What Delivery Procedures are Used to Fill Underground Tanks?
When most of us picture a fuel delivery, we see a large tractor-trailer and a driver connecting a 4-inch diameter hose that is 10 to 20 feet long to a below-grade fill pipe using a large fill adaptor that clamps tightly to the tank fill pipe. Fuel flows from the truck to the tank by gravity; there are no pumps or meters involved. This delivery method is known as a tight-fill, gravity delivery. This is the traditional method of delivery to large, retail motor fuel underground tanks, but it is not the only method of delivery in common use.

The other commonly used delivery method typically involves a bobtail truck and hose that is about 1.25 inches in diameter and 150 feet long stored on a reel on the truck. There is a meter on the truck that measures the amount of fuel delivered and a pump to move the fuel through the meter and the hose. At the end of the hose is a nozzle that may attach directly to a fill pipe (tight-fill connection) or may be attached to a length of pipe that is inserted into the tank fill pipe (loose-fill connection). This delivery method is known variously as a pumped, metered or “wet hose” delivery. This last term derives from the fact that the hose remains full of product at all times.

Factors to Consider
Knowing the delivery method is a requirement for selecting an appropriate overfill prevention device. The following list contains factors that should be considered prior to selection:

Pumped vs. Gravity Delivery — Shutoff valves designed to be installed in the drop tubes of underground tanks are not designed to withstand the pressures produced by the positive displacement pumps used in pumped fuel deliveries. Ball float valves are also incompatible with pumped deliveries because the tank will be severely over-pressurized and may rupture if the ball float valve closes during a pumped delivery. Shutoff and ball float valves should only be used with gravity deliveries.

Ball float valves are not recommended for overfill prevention.

Tight vs. Loose Fill — Shutoff valves and ball float valves are designed for use with tight fill deliveries. If either of these devices operates with a loose fill delivery, a spill of product will be the likely result.

Straight vs. Remote Fill — Both ball floats and shutoff devices can cause product to flow up the straight portion of the fill pipe when fuel is delivered through a remote fill. RP100 warns against the use of ball float valves and remote fills. A disastrous overfill and subsequent fire that killed five people in Biloxi, Mississippi, in 1998 was caused in part by the use of a ball float valve and a remote fill.

If a shutoff device is used with a remote fill, a “trap door” or similar device must be used beneath the fill adaptor on the straight portion of the fill pipe to prevent fuel from shooting up out of the straight fill when the shutoff valve closes. A standard fill cap is not sufficient to contain the pressure created at the straight fill when the shutoff valve closes.

Submerged Turbine vs. Suction Pump — Ball float valves should not be used with suction pumps because the pressure created in the tank when the ball float closes can push fuel out of the air eliminator vent on the suction pump at the pump island.

Two-Point vs. Coaxial Stage I Vapor Recovery — Recent rules requiring Stage I vapor recovery nationwide at facilities with monthly throughputs of more than 100,000 gallons mean that many tanks are likely to have Stage I vapor recovery added to them. Though not the preferred method of vapor recovery, coaxial Stage I is a much easier retrofit than two-point vapor recovery in most cases. Adding coaxial Stage I to a tank with a ball float valve for overfill prevention completely bypasses the ball float valve and renders it ineffective.

Large vs. Small Tanks — It is unlikely that a tractor trailer transport is going to drive up to fill a 1,000-gallon tank at a mom-and-pop retail facility or a farm tank. Deliveries to smaller tanks are most often made using bobtail trucks using pumped deliveries. Depending on the delivery company's policies and the class of fuel delivered, a loose-fill connection may also be used rather than a tight fill.

Overfill alarms are the most versatile overfill prevention device. They can be used with gravity or pumped deliveries as well as tight- or loose-fill connections.

Storage vs. Supply Tanks — Storage tanks contain fuel that will be dispensed into another tank or container (e.g., a vehicle fuel tank). Supply tanks provide fuel to a stationary device like a boiler or a generator. Supply tanks tend to be smaller than storage tanks, so they are more likely to receive fuel from a bobtail truck than a tractor-trailer. But even large supply tanks may receive pumped deliveries. For example, generator tanks are usually kept reasonably full, which means that fuel is much more likely to be delivered from a bobtail truck than a tractor-trailer.

Retail vs. Fleet Operation — Retail fuel purchasers in this country are generally very trusting. They trust that the amount of fuel metered into the delivery truck is in fact the amount of fuel that flows out of the truck into their underground tank. There is no meter or other reliable verification process typically used when a tractor-trailer delivers a load of fuel. Many fleet operations, especially government or related operations (e.g., schools, military, postal service) are not so trusting. They specify in their fuel delivery contracts that fuel will be metered into the tank. Introducing a meter into the delivery process most often means that a pump will also be used. As a result, even a very large tank might receive pumped deliveries from a tractor-trailer rig outfitted with a meter and pump.

A Warning About Ball Floats
RP100 contains six different warnings concerning the use of ball float valves. In light of all these warnings, the RP100 Committee discussed whether all mention of ball float valves should just be deleted from the document. After some discussion, it was decided not to delete the discussion of ball float valves because this was the only published document that described the limitations of ball float valves, and these limitations were important for the industry to know. To clarify their opinion about ball float valves, the Committee included a recommendation against the use of ball float valves:

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 reason, ball float valves are not recommended.(RP100, Section 7.3.3 (emphasis added))

Situations where a device is generally acceptable are indicated with a check mark. Situations where a device is not suitable are indicated with an X. The triangle symbol indicates that site-specific factors must be considered in the selection of an overfill prevention device. RP100 contains a general recommendation against the use of ball float valves, so no check marks are indicated for ball float valves.

Making the Right Choice
Choosing what type of overfill prevention device to use in a specific storage tank is not a decision that should be taken lightly. An inappropriate choice can result in substantial spills, tank ruptures and even death. Be sure to consider the generic factors listed here, as well as any facility-specific factors and investigate how deliveries will be made to an underground storage tank before selecting and installing an overfill prevention device.

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The PEI Journal • First Quarter 2009 • Volume 3, No. 1 • Contents are Copyright © Data Key Communications, Inc.
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