Designing fire systems for flammable, combustible liquids
Specifying fire protection for the storage of flammable and combustible liquids in containers is critical.
- Develop an understanding of factors associated with the fire protection of flammable liquid storage.
- Learn about applicable codes and standards.
- Understand issues when working with approving agencies.
Loss history has repeatedly shown that improperly protected flammable and combustible liquids have resulted in catastrophic fires and large property losses. Unlike most other building occupancies that rely on multiple fire protection safeguards to prevent a large loss, there are no second chances with flammable liquid fires. Once the fire overwhelms the sprinkler system, it is uncontrolled. The fire department will be faced with a very challenging high heat-release fire. In most cases, fire department operations will focus on damage control by trying to protect other surrounding structures, rather than attempting to extinguish the fire.
One of the more notable events that defined the hazards and current protection needs of flammable liquid storage was the 1987 Sherwin-Williams distribution warehouse fire that completely destroyed a large sprinkler-protected warehouse. That fire became the impetus for extensive full-scale testing of various fire protection schemes during the late 1980s and 1990s. By 1996, both the NFPA and the FM Global standards were significantly revised to include the results of these tests.
The proper protection of flammable liquid storage is not an easy undertaking and, as the Sherwin-Williams fire showed, the mere presence of sprinklers in a building does not mean that adequate fire protection is provided. Such evaluations and the specification for a commensurate level of protection take significant experience, not only with the standards used, but also with sound fire protection principles and historical fire test data. A good level of fire safety comprises construction features, fire prevention principles, and fire control/suppression systems once a fire has started. This article focuses on the control/suppression portion.
Flammable versus combustible liquid
First, the liquid itself is not flammable, but its vapors are. The temperature at which sufficient vapors are released is known as the flash point, a key concept. Although an in-depth discussion on flash point is outside the scope of this article, the flash point is a general indication of how likely ignition is. In broad terms, the lower the flash point, the higher the fire risk. However, once a liquid ignites, the damage caused by flammable and combustible liquids is about the same.
To illustrate this concept, think of two products found in most residential garages: gasoline and motor oil. Which has the higher fire risk? Most would say the gasoline. But both liquids have similar heat release rates and when burning would cause approximately the same amount of damage, given identical quantities. The motor oil would just be more difficult to ignite.
In accordance with the definitions of NFPA 30: Flammable and Combustible Liquids Code, the International Fire Code (IFC), and OSHA, a flammable liquid has a flashpoint less than 100 F; in other words, it will burn at ambient temperatures. A combustible liquid has a flash point above 100 F; in other words, it will typically not burn at ambient temperatures and requires heating before burning. FM Global uses different definitions. In FM’s view, any combustible liquid that is heated will burn, thus it considers all liquids flammable, although this definition was recently changed to “ignitable.” FM divides water-miscible liquids into four different groups and applies higher protection requirements to liquids with a flash point lower than 200 F.
It is important to remember that these different classes are not an indicator of the potential amount of damage once they are involved in an uncontrolled fire. They are merely indicators of the relative hazard in terms of ignition potential and difficulty of fire extinguishment.
Codes and standards
Dept. of Transportation (DOT) and fire code requirements are two different standards with very different scopes. DOT standards are based on transporting hazardous materials over public highways and across state lines, typically without any type of fixed fire suppression. Generally, quantities are limited to a trailer. DOT regulations only apply when a vehicle leaves a private driveway, not for the storage of products at a facility. Because DOT has somewhat different definitions for flammable and combustible liquids, DOT labels are of little help in evaluating hazards and protection requirements for onsite storage.
The three main standards that provide guidance for protection include the IFC (currently the 2012 edition), NFPA 30 (currently the 2012 edition), and FM Global Data Sheets. OSHA defers to NFPA 30. There are some significant differences in protection philosophies in these standards. Foremost, businesses must comply with state and local codes, as required by law. For most parts of the United States, this means Chapters 27 and 34 of the IFC. However, such compliance may only satisfy the local fire prevention bureau and provides no guarantees that the building won’t burn to the ground. In addition to the local jurisdiction, the owners should satisfy themselves that their building and operation are adequately protected to ensure that a fire will not have a major impact on the company and its operations. Lastly, other stakeholders (i.e., insurance companies and lenders) need to be satisfied that all necessary precautions are met to prevent a major property and/or business interruption loss.
As a decision-maker in flammable/combustible storage protection, where do you begin? For starters, a call to the local fire prevention bureau is in order. A general discussion about the project with a fire marshal or plan checker will yield information such as applicable codes, any local amendments to the code, required forms, and other fire department requests such as permits and plan review procedures. The next call could be to the insurance carrier. Most carriers have a staff of technical experts who will gladly share their expertise, usually at no charge to their clients. Questions to ask the insurer include what standards the insurer uses so that a determination can be made whether the insurer’s standards exceed the jurisdictional requirements. Most of the time, they will.
While it would be most convenient to have one universal standard that everyone follows, the IFC unfortunately has not been updated to contain the latest protection criteria available and does not address many storage configurations. Therefore, most insurers base their protection criteria on the most current edition of NFPA 30. FM Global has its own standards that can exceed the requirements of NFPA 30. In general, it can be said that FM Global standards set forth the most restrictive protection requirements and are thus the most restrictive.
What information do I need?
The fire hazards posed by flammable liquid storage and the required level of protection depend on a variety of factors, including:
- Flash point
- Water miscibility and percent concentration
- Container type (i.e., metal, plastic, glass)
- Container size
- Storage arrangement (i.e., floor storage or rack)
- Storage height
- Ceiling height
- Specific gravity.
All of these factors are necessary to determine the protection requirements.
Basis of protection
The required sprinkler densities found in NFPA 30 and the FM Data Sheets are largely based on full-scale tests that have been conducted over the past four decades. These tests are cumbersome and expensive. Typically, they are sponsored by large companies that have a vested interest in a particular configuration and are conducted by research organizations and by insurance companies.
Although a significant number of tests have been performed (approximately 160 published to date), many storage configurations have not yet been tested. The protection criteria in NFPA 30 and the FM Data Sheets are based on the successful test results. No protection criteria are available for configurations that did not pass the full-scale test or that have not been tested. It is important to note that just because protection criteria do not exist for a particular arrangement does not necessarily mean that it cannot be protected; it just means that there are no known protection schemes.
When faced with a particular storage arrangement that has no corresponding protection criteria, it is helpful to research tests that were conducted under similar conditions. A test that concluded that the fire was not controlled could explain why no corresponding protection criteria are found in the standards. Videos of some tests can be found online. These provide good insight as to the mechanics of fire control with sprinklers and what constitutes passing or failing tests.