THE ABOVE GROUND STORAGE TANK DEBATE

THE ABOVE GROUND STORAGE TANK DEBATE

BY MARK CHUBB

One of the principal aims of the fire code is to help make firefighters` jobs safer and more efficient. Since much of what the code requires is based on experiences, usually bad ones, this means that firefighters sometimes paid a significant price to get the code changed.

For decades, all model fire prevention codes prohibited the installation of aboveground storage tanks (ASTs) at service stations. Many firefighters probably have seen television film footage of the tremendous explosion in Kansas City in August 1959, which killed five firefighters and injured 64 other people. Others probably remember the August 1968 explosion in Kennedale, Texas, which took the lives of the chief and one firefighter from Mansfield, Texas, as well as one civilian; 28 others were injured. These incidents and others provoke vivid memories for many fire service professionals still active in fire code development. It is little wonder that ASTs generate a lot of concern among firefighters.

After these tragedies, codes were changed, and what had already become a common practice was made mandatory in many jurisdictions: Tanks at service stations were placed underground. Although methods of controlling the danger of AST failures were incorporated into the codes and standards governing their fabrication, installation, use, and maintenance, many people felt the fire risk was too great to permit aboveground storage tanks when underground tanks usually could be installed economically. Today, questions have reemerged concerning whether ASTs should be prohibited at service stations and, if they are not prohibited, what conditions should apply to their use. Concern for firefighter safety has become one of the central themes in the debate over the use of competing AST technologies.

WHAT BROUGHT THIS ISSUE TO THE SURFACE?

After 30 years of installing tanks underground, what seemed like an ideal solution to a fire problem became an environmental nightmare for many tank owners and their neighbors. Leaking underground storage tanks (LUST) threatened groundwater with contamination. In other cases, the contents of leaking tanks ended up in storm sewers and other underground structures where vapors could be ignited by unguarded ignition sources.

While the threat of fire-related tank failures had been controlled through installation of underground storage tanks for flammable and combustible liquids, the more insidious threat of corrosion-related failures had not. Although many tanks, especially those installed in recent years, were corrosion-resistant or equipped with corrosion protection features, many systems were either improperly designed for the conditions present, improperly installed, or simply left in place beyond their intended life spans. With tanks now out-of-sight, the need for maintenance and testing of tanks to ensure continued integrity was often out of mind. Reevaluation of underground storage tank practices was fed by growing awareness of the threat to groundwater and diminishing public tolerance of environmental risks.

By the mid 1980s, environmental concerns had stimulated a serious second look at the practice of installing tanks containing petroleum products underground. In 1988, the U.S. Environmental Protection Agency issued stringent new rules governing corrosion protection, secondary containment, and detection of leakage for underground storage tanks. The regulations, known as Technical Standards and Corrective Action Requirements for Owners and Operators of Underground Storage Tanks (40 CFR 280), prescribe financial responsibility requirements as well as tank installation and maintenance standards.

The new regulations placed new cost burdens on underground tank owners and operators. Naturally, the increased cost of underground installations led many owners to reconsider the feasibility of storing motor fuels in aboveground storage tanks. Interest in storage tanks was spurred by the belief that they would no longer be out-of-sight and could be inspected and maintained easily. In addition, federal regulations comparable to 40 CFR 280 do not exist for AST installations. Many tank owners have considered these factors significant cost-benefit advantages. With strong economic rather than technical arguments driving interest, this issue has inflamed passions more than almost any other matter before the model code community in recent years.

HOW SAFE ARE ASTs?

One of the principal questions before the voting representatives of the model fire code organizations has been, “Just how safe are aboveground storage tanks?” Unfortunately, this question is rather difficult to answer directly. Generally, agreement begins and ends with the premise that underground storage tanks are safer than aboveground storage tanks from a fire safety standpoint. But the relative benefits of different AST features and designs and the relative costs of ASTs compared with USTs remain in dispute.

Although no clear picture of which AST design is safer has emerged, examination of the causes of notable AST failures point to three significant factors that contribute to fires and catastrophic tank failures: tank overfills that produce large pools of fuel, secondary containment that exposes the tank to direct flame impingement when spilled fuel ignites, and inadequate or improper tank venting that either prevents expanding vapor from escaping quickly enough to relieve internal tank pressures or allows venting vapor to ignite and impinge on the shell of the venting or adjacent tanks.

Overfill is a significant concern for USTs and ASTs alike. However, when tanks are located aboveground, spilled fuel can easily pool around the tank, producing a direct fire exposure. Most model codes require minimal controls to prevent or control a spill from tank overfill. The secondary containment or diking required for ASTs ensures that the spilled fuel does not expose people, other property, or the environment to danger but in some cases may guarantee that the tank itself is directly exposed if the liquid is ignited. This leaves the tank vents to handle the most serious threat to the tank`s continued integrity. All tanks are equipped with normal vents and emergency relief vents. Normal vents permit vapors to be released from the tank due to filling, evaporation, and changes in ambient temperatures. Normal vents also prevent a vacuum from developing when contents are dispensed. Emergency relief vents are much larger and allow vapors produced by sudden or extreme heating of the tank contents from an exposure fire to escape without pressurizing the tank beyond its design limits.

Overfill controls. The primary protection against overfill involves determining the available capacity of the tank to be filled. All of the model codes require that tank capacity be determined before initiating transfer operations. Written transfer procedures are strongly encouraged. However, unlike USTs, not all model codes prescribe equipment requirements for preventing tank overfills. NFPA 30, Flammable and Combustible Liquids Code, requires equipment for USTs to shut off the flow of fuel automatically when the tank reaches 95 percent of its capacity and an audible alarm or flow restricter that will alert the operator when the tank reaches 90 percent of its capacity. The same controls are required by UFC (Uniform Fire CodeTM) Appendix II-F, 5.5, but similar requirements for ASTs are awaiting action in the other model codes.

Secondary containment. Containment of spilled liquid has long been a key tenet of the code. The tragic fire in Kansas City illustrated well the consequences of a flowing spill fire. Rather than concentrating on the fire exposure to the tanks, firefighters worked feverishly to control the running spill fire as it approached the service station building and roadway leading to the nearby Missouri River. Of course, the unfortunate effect of this strategy was to exacerbate the fire exposure to the unprotected horizontal tanks. In the absence of adequate emergency venting, the tanks eventually failed, releasing an immense fireball that engulfed firefighters and bystanders.

Diking is the most common form of secondary containment. This method uses noncombustible enclosures to ensure containment of the largest quantity of liquid that may be spilled from a single container or tank within the limits of the enclosure. The pooled liquid usually is exposed to the atmosphere and, therefore, ignition source control must be a high priority in managing a spill or release. When tanks are installed on structural supports within the enclosure, precautions must be taken to protect these structures from failure due to fire exposure. Another concern with elevated or vertical tanks is the possibility that, should a small leak occur, the elevation head from liquid within the tank could produce a stream capable of escaping the dike boundaries.

The most desirable form of secondary containment is remote impounding. Instead of collecting spilled liquids around the tank, remote impounding systems transport spills a safe distance away, where an ignition will not endanger the tank. However, due to the cost and relative complexity of this method, it is rarely employed for ASTs at service stations.

Integral secondary containment is becoming increasingly common. Many tanks incorporating double-wall designs have become available. In such designs, an outer tank creates an interstitial space around the inner liquid container to capture overflow and leaks. Leak detection equipment often is installed to monitor the interstice for leaking tank contents.

Obviously, managing spilled liquid within the secondary containment enclosure is among the most important priorities in controlling an incident involving a leaking tank. Some means for recovering liquid from the secondary containment enclosure must be available.

Tank vents. Emergency relief venting capacities are based on the anticipated rate of heat transfer from an exposure fire to the tank per unit area; the size of the tank and the area likely to be exposed; the time required to bring the tank contents to a boil; the time required to heat portions of the tank not in contact with liquid contents to the point where the metal will lose its strength; and the effect of drainage, insulation, or water in reducing the fire exposure or slowing heat transfer. NFPA 30A, Automotive and Marine Service Station Code, does not permit consideration of the value of insulation for purposes of reducing emergency relief vent capacities of ASTs at service stations.

SEPARATE, INSULATE, OR ISOLATE?

Given historical fire safety concerns and the hazard posed by a possible AST failure at a service station in a potentially densely populated area, tank designers and regulators have struggled over which approach incorporating these features provides the most reasonable protection from tank failure. At present, three technologies seem to be competing for dominance in the aboveground tank marketplace. The simplest and most common type of AST is the unprotected steel shell tank. Fire-resistive and protected tanks, usually of double-wall design with an outer layer of insulating material such as concrete, have become quite popular in recent years. Finally, tanks in special enclosures or vaults have also been recognized as a solution to the fire protection challenges associated with storing flammable or combustible liquids aboveground. These three approaches could be characterized as separate, insulate, and isolate, respectively.

Separate. Unprotected steel shell tanks must be manufactured and tested to UL (Underwriters Laboratories, Inc.) 142 or a comparable engineering standard. These standards govern the quality of materials, fabrication techniques, and structural stability of the tank design. Installation and maintenance of these tanks is governed by the nationally recognized model fire codes NFPA 30 and NFPA 30A. NFPA 30A requires unprotected steel shell tanks to be located well away from dispensers, significant structures, property lines of adjacent land that can be built upon, public highways, and railroads (see Table 1). Obviously, this means that a substantial lot is required for a service station employing ASTs. Separation is intended to minimize the possible exposures to the tank as well as from the tank. The chances of ignition are minimized by keeping operations that may produce ignition sources a safe distance from the tank. So, too, the possibilities of exposure to physical and mechanical hazards that can damage the tank shell may be minimized by separation from buildings, highways, roadways, and other hazards. Should a fire occur, these same distances make it easier to confine the scope of the incident to the fire-involved tank.

Insulate. Insulated tanks must be manufactured and tested to UL 142 and UL 2085 or equivalent engineering and testing standards (see Table 2). To meet these standards, tanks must provide a specified degree of thermal protection to tank contents when exposed to a two-hour pool fire test. This test is intended to replicate a severe fire exposure from a flammable or combustible liquid spill fire. (The purpose of this test, like all other fire tests, is not to predict performance in an actual fire but to compare the performance of similar products and materials under uniform testing conditions.) In addition, tanks must be tested to determine their resistance to vehicular impact and, optionally, bullet penetration. These tests measure the ability of the tank to withstand physical and mechanical hazards that could lead to a tank failure. Because of the degree of physical and thermal protection, these tanks may be installed with reduced separation distances to structures, property lines, highways, and railways per NFPA 30A and each of the nationally recognized model fire prevention codes (see Table 1).

Isolate. When separation of tanks from exposures by suitable distances alone is impractical, isolation of the tank in a vault or special enclosure is an alternative. Unprotected steel shell tanks or insulated tanks may be installed in this manner; however, these approaches most commonly are used in conjunction with unprotected steel shell tanks. Spill control or containment is provided by the vault or special enclosure itself. The term “special enclosure,” as used in NFPA 30A, Section 2.2, refers to a six-inch (152 mm)-thick reinforced concrete tank enclosure in or under a service station building. Vaults described in NFPA 30A, Section 2-4.4, are above- or below-grade structures designed and used for no other purpose than to enclose a single tank itself. Like special enclosures, vaults separate tanks from adjacent buildings, spaces, other tanks, and processes by six-inch (152 mm)-thick reinforced concrete walls and floor/ceiling or roof/ceiling assemblies. Such assemblies generally are considered to possess fire resistance comparable to an assembly with a fire resistance rating of approximately three hours, determined in accordance with ASTM (American Society for Testing and Materials) E 119. The BOCA® (Building Officials and Code Administrators International) National Fire Prevention Code (1993) specifically recognizes special enclosures and vaults with two-hour fire resistance-rated separation in lieu of specifying six inches (152 mm) of reinforced concrete. Table 3 describes other important features of vaults and special enclosures.

WHERE ARE WE GOING?

After several years of debate, the use of aboveground storage tanks at service stations has become accepted once again by the nationally recognized model fire codes (see Table 4). Concerns about fire safety have not been left behind. Many safeguards have been put in place to ensure that there are no significant dangers to the public or firefighters.

The requirements of these codes have not diminished the demand for ASTs, but they have ensured that such installations are limited to facilities where appropriate controls can best be implemented and maintained. For instance, although NFPA 30A permits unprotected steel shell tanks, a large parcel of land is required to meet separation distance criteria specified in the code. The National Code and the Standard Code have smaller separation distance criteria than those in NFPA 30A but restrict the use of ASTs to private service stations where it is assumed personnel will be better trained in the maintenance and operation of the fuel dispensing system. The Uniform Fire CodeTM restricts ASTs to special enclosures except when local governing authorities adopt special requirements for protected ASTs found in Appendix II-F.

Besides fire safety requirements, efforts have been underway in Congress since 1993 to enact federal legislation requiring AST owners to comply with requirements comparable to those applicable to USTs (40 CFR 280). Bills were introduced in the U.S. House of Representatives (H.R. 1360) and Senate (S. 588) to enact what is entitled the Safe Aboveground Storage Tank Act. Several national and state fire service organizations, including the International Association of Fire Chiefs and the International Association of Fire Fighters, endorsed the legislation. Although hearings were held in the last session of the 103rd Congress, this matter has received little attention during the 104th Congress.

Continued debate concerning the installation and use of ASTs is likely. A thorough understanding of existing code requirements and protective features will aid the fire service by ensuring that disasters like those in Kansas City and Kennedale never occur again. n

This article presents an overview of requirements found in nationally recognized model codes and standards. The opinions of the author do not represent the official positions of the code-writing organizations, nor do they constitute an official interpretation of the intent or applicability of any of the code requirements discussed in the article. All decisions concerning the intent and applicability of these codes are the responsibility of the codes` users and are subject to the judgments of the authorities having jurisdiction. Local requirements may vary from nationally recognized codes and standards. The author is not responsible for any errors or omissions resulting from reliance on the technical content of this article.

References

1. BOCA® National Fire Prevention Code, 9th ed., 1993, Section F-3207.0. Building Officials and Code Administrators International, Inc., Country Club Hills, Ill.

2. EPA Technical Standards and Corrective Action Requirements for Owners and Operators of Underground Storage Tanks, 1988, Code of Federal Regulations, Title 40, Part 280. U.S. Government Printing Office.

3. H.R. 1360/S 588: Safe Aboveground Storage Tank Act of 1993. 103rd Congress, First Session.

4. Uniform Fire CodeTM, 1994, Section 5205 and Appendix II-F. International Fire Code Institute, Whittier, Calif.

5. NFPA 30, Flammable and Combustible Liquids Code, 1993. National Fire Protection Association, Quincy, Mass.

6. NFPA 30A, Automotive and Marine Service Station Code, 1993. National Fire Protection Association.

7. NFPA 325M, Fire Hazard Properties of Flammable Liquids, Gases, and Volatile Solids, 1991. National Fire Protection Association.

8. NFPA 326, Safe Entry of Underground Storage Tanks, 1993. National Fire Protection Association.

9. NFPA 327, Cleaning and Safeguarding Small Tanks and Containers, 1993. National Fire Protection Association.

10. Standard Fire Prevention Code©, 1994. Southern Building Code Congress International, Inc.

11. SwRI 93-01, Testing Requirements for Protected Aboveground Flammable Liquids/Fuel Storage Tanks. 1993. Southwest Research Institute, San Antonio, Texas.

12. UL 2085, Standard for Safety for Insulated Aboveground Tanks for Flammable and Combustible Liquids, 1994. Underwriters Laboratories, Northbrook, Ill.

Unprotected steel shell tanks without required secondary containment, vehicle impact protection, or separation from adjacent building. (Photos by author.)

Unprotected steel shell tank containing Grade K-1 kerosene. The location of the tank and dispenser violate the separation distance requirements in NFPA 30A. With the exception of NFPA 30A, the nationally recognized model codes prohibit unprotected steel shell tanks at public service stations.

(Top) Aboveground storage tanks surrounded by a security fence at a coastal marina to prevent tampering and provide limited physical protection. (Bottom) Unprotected steel shell tanks inside the security fence at the marina depicted above. Note the secondary containment with rain shields provided around the base of each tank as well as the emergency vent atop each tank.

MARK CHUBB is fire code coordinator for the Southeastern and Southwestern Associations of Fire Chiefs, where he works with Southern Building Code Congress International, Inc. (SBCCI) managing the development, promotion, and interpretation of the Standard Fire Prevention Code©. He has an associate`s degree in fire science from the Community College of the Air Force and a bachelor`s degree in fire science and urban studies from the University of Maryland. Chubb is a corporate member of the Institution of Fire Engineers and an editorial advisory board member of Fire Engineering.