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Water as an Extinguishing Agent (it doesn’t always work!)

April 5, 2013

Water ExtinguisherFire departments use lots of water to fight fires.  The majority of fires can be extinguished with water.  As one alert citizen pointed you a while back, water isn’t always the best choice for putting a fire out.

There are several different classes of fires, and each is extinguished a little differently.

Class A fires are the average combustibles that you would find in a dumpster or garbage can.  Wood, paper, cardboard and textiles are fuels that generate Class A fires.  Water is a great extinguishing agent for these fires.  Most people think we’re drowning the fire with water and starving it for oxygen.  That’s not really the case.  Water absorbs a tremendous amount of heat, especially as it turns to steam.  Normally you extinguish Class A fires with water by removing the heat and putting the fire out.

Class B fires involve flammable liquids like gasoline, diesel fuel, alcohol and molten plastics.  These fuels are liquids and flammable vapors are given off at the liquid surface.  It is these vapors that burn, not the liquid.  If I leave the surface of the liquid pool calm and placid, the surface area is the smallest it can be.  If I stir that liquid surface up, I increase the surface area, the amount of flammable vapors released, and the amount of fire.  That’s one reason you don’t pour water on stove-top cooking oil fires.  Almost all successful Class B extinguishment is accomplished by starving the fire for oxygen.  On your stove-top you cover the cooking pot with the lid and starve flames for oxygen.  We cover gasoline spills with foam to exclude oxygen and extinguishing the fire.

Class C fires involve electrical current and you must approach these fires carefully!  If you extinguish the fire while electricity is still supplied, the fire will re-ignite.  Furthermore, water will conduct electricity from the source to you (electrocution).  That’s why you disconnect power before fighting a Class C fire.  An ABC fire extinguisher helps as that powder won’t conduct electricity to you.

Class D fires involve combustible metals.  Not all metals look metallic.  This designation comes from chemists.  Some metals are powders or amorphous lumps.  The problem with Class D fires lies in how the metal behaves with water.  Some metals explode on contact with water (sodium).  Some will burn even hotter when you apply water (magnesium).  In some cases, water will have no effect.  Factories using combustible metals are familiar with the characteristics of their product and have fire extinguishing agents on hand that address the specific idiosyncrasies of the product being machined.

Class K fires came up in the 1990s.  Fire experience and laboratory testing showed that use of vegetable oils with high auto-ignition temperatures was becoming more commonplace.  Standard Class B extinguishing systems were not keeping the fire suppressed for a long enough time, and the oils were re-igniting after initial extinguishment.  In 1998 the National Fire Protection Association changed the fire extinguisher standard creating Class K fire distinction.  Water, and many Class B extinguishing agents, won’t work on Class K fires.  Today commercial kitchens use special Class K fire extinguishers.

We spend a significant amount of time training on proper extinguishing agents and techniques.  At home, you need to know that water should not be used on stove-top cooking fires, flammable liquids in the garage, or on burning energized equipment.  The first step for ALL home fires is to call 911 and get us headed your way first (we really don’t mind!).  For cooking fires, cover the pot and shut off the burner.  For electrical fires, kill the power first, then fight the fire.  And for flammable liquids, use a dry chemical fire extinguisher.  If the fire is too big, remember to get the people out.  There are no things in your house that are more important than the people.

We hope you never have to fight a fire in your home.  But if you do, be sure you do it right.  If you’ve got questions, give me a call at 466-4602, or e-mail me at dbleeker@scfd9.org, or post your question here on this page.

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From → Fire Prevention

2 Comments
  1. really nice post and very interesting explaining style. I am really impressed

Trackbacks & Pingbacks

  1. How to Use a Fire Extinguisher | The Washington Surveying & Rating Bureau

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