Tips for Choosing In-Cabinet Fire Suppression
Virtually every laboratory has areas with elevated fire risks, with fume hoods being a primary concern. The presence of ignition sources, such as hot plates and Bunsen burners, the use of pyrophoric materials and the inherent volatility of the various chemicals and compounds that are commonly found in fume hoods all add up to a serious fire risk.
Traditionally, fire protection for laboratory fume hoods has been limited to a code-mandated alarm system, which does nothing to suppress a fire, and an overhead water sprinkler system. Overhead sprinklers can do a good job of containing a fire, but they can also damage property and equipment as much as—or more than—the fire itself.
If laboratories rely on sprinklers for fire protection, they must be aware that when a fire occurs in a fume hood, the heat and flames follow the airflow through the cabinet and out the exhaust, which greatly reduces the ability of an overhead sprinkler system to react. By the time a fire grows large enough to overcome the airflow and activate the sprinklers, the amount of damage could be staggering.
Even a short delay in the detection and suppression of a fume hood fire can lead to serious losses and threaten life and property, which is why in-cabinet fire protection has become a popular option. The ability to quickly detect and suppress a fire inside a fume hood, right at its source, can reduce a major incident into a minor event. However, the speed and direction of the airflow inside a fume cabinet makes detection by traditional methods difficult.
Single-point fire detectors, such as glass bulbs and fusible links, are typically mounted in-cabinet above the workspace. These types of detectors may or may not sense the route the heat is following. Optical-type detectors that “see” the energy given off by a fire can be effective in fume hoods, but are extremely expensive. Various other types of detectors are available, but very few are suitable for in-cabinet applications.
One practical and cost-effective type of in-cabinet fire detection is linear heat detection. Linear heat detectors are available in both electric and pneumatic designs and provide multi-point detection at any point along their length. Linear heat detectors can be installed behind the fume hood baffles and across the exhaust duct opening, which provides fast and reliable detection of heat and flames.
Once a fire has been detected it should be suppressed using a fire-extinguishing agent appropriate for the types of chemicals used in the fume hood. Unfortunately, no one extinguishing agent is suitable for all applications. In fact, applying the wrong agent can actually cause a fire to worsen. Several commercially available fire-suppressing agents are available to protect fume hoods, with each having advantages and disadvantages. These extinguishing agents include dry chemical powders, carbon dioxide, foam and clean fire-suppressing agents (Halon replacements).
For most fume hood applications, multi-purpose (ABC) dry chemical agent is the fire suppressant of choice. Dry chemical is widely accepted as the most effective agent available for extinguishing flammable liquid fires. It is recommended for fume hood applications because it won’t react negatively with most chemicals.
An alternative to dry chemical powder is carbon dioxide, which is electrically non-conductive and effective on Class A, B and C fires. Carbon dioxide is desirable for fume hood applications because it won’t harm sensitive equipment and will suppress a fire without leaving behind any residue. Note, however, that carbon dioxide may interact negatively with some laboratory chemicals.
If laboratories have concerns about which fire-extinguishing agent is best, they can refer to the Material Safety Data Sheet (MSDS) for the material(s) being used in the fume hood. This will likely dictate, or at least help narrow, the choice for best extinguishing agent.
The extinguishing agent must be discharged long enough to offset the amount of the air/agent mixture flowing through the fume hood and out the exhaust. Therefore, the discharge of the agent must be forceful and targeted to cover the entire volume of the fume hood including the airflow.
In addition to quick and reliable detection and suppression of a fire, a good in-cabinet fire detection and suppression system should also be capable of shutting off the gas and electric supplies, sounding an alarm and closing the fume hood sash (where applicable). As per NFPA 45, the system shouldn’t shut off the extraction.