Mar 2008

Handling Flammable Liquids

Using flammable liquids presents some unique and difficult fire protection challenges. Depending on the specific material, there is the possibility of generating vapors which can easily ignite – or explode. As a general rule, it is the vapors of the flammable liquid that actually ignite. Most flammable liquids produce vapors which are heavier than air; so as vapors are generated they tend to spread close to the floor, and can accumulate in low points – increasing the potential of eventually finding an ignition source. Should the liquid escape from its container, it too will spread, as you would expect any liquid to do. This expanse of liquid (pool) will have a large surface area which exacerbates the rate that ignitable vapors are formed. Once ignited, the vapors create heat which increases the rate of further vapor generation to feed the fire. When a flammable liquid is confined in a sealed container and exposed to heat, the pressure in the vapor space above the liquid increases. If the increase in pressure leads to container rupture, the sudden drop in pressure causes very rapid vaporization into an environment of elevated temperature, leading to a self ignition and the rather catastrophic phenomenon commonly revered to as “BLEVE” (burning or boiling liquid, expanding vapor explosion). By now, you probably have gotten the idea – you have to be extremely careful with flammable liquids.

Of course not all flammable liquids are created equal. Flammable liquids are defined in the Flammable and Combustible Liquids Code of the National Fire Protection Association (NFPA) as those with closed-cup flash points below 100 F and Reid vapor pressure not exceeding 40 psi at 100 F and are called Class I Liquids. Liquids with higher flash points are defined as Combustible liquids – considered somewhat less of a fire hazard. We will be limiting this discussion to the Class I liquids.

Within Class I are several categories and generally speaking these are (in order of descending fire hazard):

Class IA – flash point below 73 F, boiling point below 100 F.
Class IB – flash point below 73 F, boiling point 100 F or higher.
Class IC – flash point 73 F or higher, but below 100 F.

The boiling point is that liquid temperature where the liquid vapor pressure equals the surrounding atmospheric pressure – any higher causes liquid vapors to generate. The flash point is the liquid temperature at which sufficient vapor is given off to form an ignitable mixture with the air at the liquid surface. If you think about it you will notice that Class I liquids in a fairly normal room temperature environment present a vapor ignition hazard.

Some other characteristics that may be helpful in determining the extent of your hazard exposure are:

Specific gravity – liquids lighter than water will “float” which can make extinguishment with water based materials more difficult.
Evaporation rate – generally the lower the boiling point, the higher the vapor pressure and evaporation rate.
Solubility – liquids that are water soluble can be extinguished by dilution.

With this information you can figure out what your flammables consist of – of course, you may find you have materials in more than one category – in which case you should plan for the worst case, considering the different material hazards and the quantities of each you have.

There are several ways to determine what you may have on hand. Look for the container labels – flammable liquid identifiers are usually red with a “fire” symbol and appropriate wording. The NFPA diamond shape label usually identifies flammable liquids with a fire hazard rating of “3” or “4”. There are labels from other authorities such as the U.S. Department of Transportation. There is some difference between actual definitions for labeling requirements from one authority to another, so be careful. It is always a good idea to check Material Safety Data Sheets (MSDS) for specific product information. Alternatively, you can check for information using the CAS Number from the product label (CAS No is a unique numerical identifier for chemical compounds, mixtures, polymers, etc. assigned by the Chemical Abstracts Service of the American Chemical Society). The MSDS will typically provide specifics on most of the criteria we have mentioned – certainly flash and boiling points, specific gravity, vapor density, NFPA rating and a host of useful information and cautions. Once you have made your determinations, you have to consider how you store your materials, perhaps you dispense and mix as well, and get them to the point of end use. You want to do these things in a fire safe manner, and be prepared to react in the event something goes wrong.

Safe storage of flammable liquids starts with separation from the rest of your facility. Distance would be preferred but separation can also be achieved by construction features. For relatively smaller quantities on hand, there are other options. If you are a heavy user, with tank storage, truck or rail unloading, and closed piping and pumping systems for plant operations you will find you are beyond the scope of this article.

Typically a detached storage area should be 50 to 75 ft. or more from your other facilities. The minimum safe distance has to be determined by the extent of hazard (fire or fire/explosion) and quantities stored. Consideration of the value of the inventory and the detached structure, as well as the affect on operations in the event of a loss are factors you can consider when deciding on the extent of fire suppression. This could range from fire extinguishers to fire hydrant availability to automatic fire suppression systems. Other possible location options are structures outside of your facility exterior walls or on the inside of an exterior wall. Least desirable is construction of a room within your plant interior space. Try your utmost to use ground level – basements and upper floors are generally frowned upon without extraordinary features included. At best the specific hazards have to be evaluated to determine the necessary fire ratings of construction, possible explosion pressure relief and pressure resistant walls, type of fire protection/suppression. The construction may have to be enhanced to contain flowing liquids should an accidental release occur. This usually entails watertight floors/walls in part, curbs or trapped drains at doorways, trapped drains for the enclosure. Heat should be indirect, steam, hot water or something similar. If you use the cut off room for dispensing or mixing of flammable liquids continuous exhaust ventilation to a safe location should be designed to keep vapor concentration below 25% of the Lower Explosive Limit (more correctly the lower flammable limit). This can be done mechanically or with natural ventilation but calculations or air sampling or minimum exhaust rate is needed to prove the ventilation is sufficient. Exhaust ventilation is usually provided near the floor since most flammable vapors are heavier than air. If mechanical systems are used, it would be advisable to have them alarmed for failure. The room has to be evaluated electrically also – for potential vapors under both normal and emergency situations. This evaluation will determine where National Electrical Code requirements for Class I, Division 1 or 2 classified areas exist, and the appropriate electrical equipment and wiring. When considering fire protection, it is unlikely that automatic sprinkler protection can be excluded from flammable liquid areas inside or adjacent to your facility. The design is highly dependant on storage configuration and would be site specific. As a general rule, the designs, whatever they may be specifically, will be at least Extra Hazard design or the equivalent. The lack of fixed protection systems places limits on the quantity of flammable liquids that can be stored – usually up to a maximum of 3,000 gallons.

If you have relatively small amounts of flammable liquids you can store and dispense in other locations. You are limited to 25 gallons of Class IA liquids, 120 gallons of Class IB and IC in aggregate. More than this can be stored up to the quantities needed for one continuous 24 hour period. You can store (no dispensing) 660 gallon of IB and IC liquids per separate pile in a general warehouse if the existing sprinkler system meets certain strict design minimums and subject to a number of other restrictions. There is also the option of using Listed/Approved Storage Cabinets. With general area sprinkler systems provided you can put groups of six cabinets together in one area and can continue to do this in other locations at least 100 ft. away (if there is no sprinkler protection you are limited to half the number of cabinets per group). Each cabinet is limited to 120 gallons of storage. Cabinets are provided with knock-outs for connecting vents. They are not required to be vented and the knock-out should be left in place. Should you decide to vent, they have to be at an exterior wall for venting directly to outside.

There are a number of other things you should consider for safe storage and dispensing of flammable liquids. Most of the equipment mentioned here is listed/approved for flammable liquid dispensing. Metal containers are by far preferred to plastic. Glass has limited use for small (one gallon) quantities. Containers should be kept covered when not in use to minimize vapor generation. Drums should be provided with a safety bung. This is a device designed to attach to the 2-inch diameter opening at the top of a drum communicating with the vapor space in the drum. Should the drum be exposed to fire, the bung prevents bursting of the drum through emergency venting capability, which relieves internal drum pressure and allows flammable vapors to burn outside the drum. A flame arrester in the bung prevents the flames from igniting vapors inside the drum. They also minimize the transfer of vapor from the drum to the surrounding atmosphere by remaining in the closed position when the drum is not actively being used. During dispensing, provisions should be available for grounding and bonding for static electricity control. There is a detailed discussion specific to this on the Risk Logic Web Site – the Monthly Technical Article from October 2005. Manual dispensing from drum pumps on upright drums is preferred to motorized pumping or gravity dispensing from drums placed on-side. It is faster to stop in an emergency and you can never turn off gravity. When using gravity feed, use a spring loaded to close safety valve. If using a hose, the termination should be the same spring loaded valve. Also a manual or fusible link to close valve, should be used as nearby the drum discharge as possible. The hose should be compatible with the material being dispensed. Frequent inspection for deterioration is important. Dispense into safety receptacles. Use safety drip pans where needed and waste containers designed for oily rags and trash.

Employees involved with flammable liquids should receive appropriate training on the hazards and processes and emergency actions. Fire extinguishers with a “B” rating should be available – stand alone or in combination with multipurpose units. An emergency response fire plan should be in place as well as a preventative maintenance plan. Maintain good housekeeping and perform frequent self inspections of the area and equipment used for flammable liquids operations.

A lot of engineering and design must be considered to achieve a safe flammable liquids operation. To meet the highest level of fire safety, Risk Logic can help. Feel free to contact us with any questions or problems for which you need assistance.