A fire hydrant is an above-ground connection that provides access to a water supply for the purpose of fighting fires. The water supply may be pressurized, as in the case of hydrants connected to water mains buried in the street, or unpressurized, as in the case of hydrants connected to nearby ponds or cisterns. Every hydrant has one or more outlets to which a fire hose may be connected. If the water supply is pressurized, the hydrant will also have one or more valves to regulate the water flow. In order to provide sufficient water for firefighting, hydrants are sized to provide a minimum flowrate of about 250 gallons per minute (945 liters per minute), although most hydrants can provide much more.
The need for fire hydrants developed with the advent of underground water systems. Prior to that time, water was obtained from easily accessible public wells or ponds. During the 1600s, London, England, began installing an underground water system using hollowed-out logs as pipes. When there was a fire, firefighters had to dig up the street and bore a hole in the wooden pipes. Later wooden plugs were inserted into pre-drilled holes at fixed intervals along the log pipes to make it easier for the fire-fighters to get water. This gave rise to the term fire plug, which is still sometimes used to refer to a hydrant.
As cities grew, so did their water systems. Larger systems meant increased pressures, and cast iron pipes were laid to replace the rotting wooden logs. When Philadelphia's new water system commenced operations in 1801, it not only served 63 houses and several breweries, but it also had 37 above-ground hydrants for fire protection. The first fire hydrant in New York City was installed in 1817 by George Smith, who was a fireman. He wisely located it in front of his own house on Frankfort Street.
Following the earthquake and fire that devastated San Francisco in 1906, the city installed an extensive emergency water system that is still in use. In addition to more than 7,500 hydrants connected to standard-pressure water mains, the system includes a reservoir and two tanks located on hills to supply nearly 1,400 high-pressure hydrants throughout the city. There are also two salt-water pumping stations to draw water from San Francisco Bay, plus five additional connections along the waterfront to allow the city's fireboats to pump into the hydrant system. As a final line of defense, the city has over 150 underground cisterns connected to unpressurized hydrants. Fire pumpers can connect a rigid suction hose to these hydrants and pull the water out of the cisterns by creating a vacuum.
Today, the size and location of fire hydrants in an area affect not only the degree of fire protection, but also the fire insurance rates. In many urban areas the lowly fire plug is all that stands between the first spark and a multi-million-dollar fire loss.
Types of Hydrants
There are two types of pressurized fire hydrants: wet-barrel and dry-barrel. In a wet-barrel design, the hydrant is connected directly to the pressurized water source. The upper section, or barrel, of the hydrant is always filled with water, and each outlet has its own valve with a stem that sticks out the side of the barrel. In a dry-barrel design, the hydrant is separated from the pressurized water source by a main valve in the lower section of the hydrant below ground. The upper section remains dry until the main valve is opened by means of a long stem that extends up through the top, or bonnet, of the hydrant. There are no valves on the outlets. Dry-barrel hydrants are usually used where winter temperatures fall below 32° F (0° C) to prevent the hydrant from freezing.
Unpressurized hydrants are always a drybarrel design. The upper section does not fill with water until the fire pumper applies a vacuum.
The hydrant barrel is usually molded in cast or ductile iron. Some iron wet-barrel hydrants have an epoxy coating on the inner surface to prevent corrosion. Other wet-barrel hydrants are molded in bronze. The hydrant bonnet is usually made from the same material as the barrel. The valve stem in a dry-barrel hydrant design is steel. The valve stems in a wet-barrel hydrant are usually made from silicon bronze.
The hydrant outlets are molded in bronze. If the barrel is cast or ductile iron, the bronze outlets are threaded into the barrel. If the barrel is bronze, the outlets are cast as part of the barrel. The outlet caps may be bronze, cast iron, or plastic.
Valve seats, seals, and gaskets are made from a variety of synthetic rubbers including styrene butadiene, chloroprene, urethane, and butadiene acrylonitrile. Fasteners may be zinc-plated steel or stainless steel.
Hydrants are given a coat of primer paint before they are shipped. When a hydrant is installed, the outer surface is coated with an exterior-grade paint.
The basic design and construction of pressurized fire hydrants in the United States are defined by the American Water Works Association (AWWA), which sets general standards for hydrant size, operating pressure, number of outlets, and other requirements. Unpressurized hydrants may be the same design as the pressurized hydrants within a city or fire district in order to maintain commonality, or they may be a simple capped pipe design with no valves.
The main body of the hydrant is called the barrel or upper standpipe. It may consist of a single piece or it may be made in two pieces. If it is made in two pieces, the upper portion with the outlets is called the head and the lower portion is called the spool. This terminology is not exact and varies from one manufacturer to another, as well as from one city to another.
The hydrant outlets usually have male National Standard Threads (NST) to mate with fire hose couplings. The smaller outlets, sometimes called the hose nozzles or connections, are 2.5-inch NST. The larger out-lets, sometimes called the steamer nozzles or connections, are 4-inch or 4.5-inch NST. The outlet caps are secured to the hydrant body with short lengths of chain. The terms hose connection and steamer connection date back to the 1800s. Before the advent of modern fire apparatus, minor fires were often fought by connecting a single hose line directly to the smaller outlet on a pressurized hydrant. If the fire was larger, a steam-powered pumper, called a steamer, took water from the larger hydrant outlet and pumped it into several hose lines.
The hydrant valves are actuated by turning metal stems. The portion of each stem that protrudes from the exterior of the hydrant is pentagonal shaped and is called the operating nut. This five-sided nut requires a special wrench to turn and helps prevent unauthorized use. On some hydrants the operating nut is a separate piece that slips over the stem. This allows the nut to be replaced if it becomes worn from use.
Some dry-barrel hydrants include a break-away feature to allow easy repair if the hydrant is struck by a vehicle. This design includes a breaker ring on the barrel of the hydrant near the ground and a breakable coupling on the valve stem inside the hydrant. When struck, the upper barrel and stem snap free without disturbing the under-ground piping or valve.
Although the basic components of all fire hydrants are similar, the shape of hydrants varies from one manufacturer to another. Some hydrants have the classical round body with a domed bonnet. Others have square or hexagonal bodies. Some areas that are undergoing urban renewal have hydrants that are low and modern looking.
Making a fire hydrant is primarily a metal-casting process, and most hydrant companies are metal foundries that specialize in manufacturing a variety of municipal water works components.
Here is a typical sequence of operations for manufacturing a wet-barrel fire hydrant.
Forming the molds
- 1 The outer surface of a mold is formed by a piece called the pattern. To make a hydrant pattern, the hydrant's outer shape is generated in three dimensions on a computer. This data is fed into a stereo lithography machine, which uses laser beams to harden liquid plastic into the shape of the hydrant. This hardened plastic piece is used to make multiple copies of left and right pattern halves out of rigid polyurethane.
- 2 The inner surface of a mold is formed by a piece called the core. To make a hydrant core, the hydrant's inner shape is machined into two halves of a block of aluminum or cast iron to form a cavity. The two halves are clamped together, and the cavity is filled with a mixture of sand and a plastic polymer. When the block of aluminum or cast iron is heated gently, the polymer hardens the sand to form the core. The block is then opened, and the core is removed. This process is repeated to make multiple cores.
Casting the barrel
- 3 When a production run of hydrants is O ready to start, the patterns and cores are brought to the mold-making machine. The left and right patterns are pressed into the two halves of a mold filled with sand to form impressions in the shape of the outer surface of the hydrant. Molding sand is a special mixture that holds its shape without crumbling. The hardened sand core is then carefully laid on its side and held with short spacers to form a cavity between the core and the impression in one of the mold halves. The other half of the mold is put in place over the core and the mold is clamped together. This process is repeated for each hydrant.
- 4 Molten metal is poured into each mold through an inlet passage called a gate. Pouring continues until the metal starts to rise through outlet on the opposite side called a riser. As the molten metal hardens, it cooks the polymer in the core sand. This raises the temperature of the polymer far beyond its initial setting point and causes it to break down and allow the sand to become loose again.
- 5 After the casting has completely hardened, the mold is split apart and the core sand is dumped out. The casting is placed in a horizontal cylinder filled with small metal pellets and tumbled to remove any small bits of metal or molding sand that may have adhered to the casting.
- 6 The cast gates and risers are cut off with an abrasive cut-off saw, and are returned to the furnace. The cast barrel is ground with a handheld power grinder to remove any rough surfaces.
- 7 If the hydrant has a two-piece barrel, the / head and spool are cast, ground, and finished separately. If the hydrant is made from cast or ductile iron, the outlets are cast, ground, and finished separately in bronze.
Machining the barrel and valves
- 8 The entire hydrant is fixed lengthwise in a lathe, and shallow concentric grooves are cut into the face of the lower flange. This allows the flange to seal against a gasket when the hydrant is mounted. The flange bolt holes may be drilled at this point or they may be drilled just before shipment.
- 9 If the barrel is a two-piece design, the lower portion of the head has National Pipe Taper (NPT) threads cut on the inside and the upper portion of the spool has NPT threads cut on the outside to allow the two pieces to be joined. The head is drilled and tapped on one side in the area of the NPT threads to hold a locking set screw.
- 10 The hydrant—or the head, if it is a two-piece design—is repositioned cross-ways in a lathe along the centerline of the larger outlet. A rotating piece, called a fixture, clamps the hydrant in place and provides a counterbalance as the hydrant is spun. The lathe bevels the inner surface of the barrel around the outlet opening to provide a smooth seating surface for the valve disc. The opening for the valve stem insert is drilled and threaded. Finally the outlet or outlet opening is threaded. This process is repeated for each of the outlets.
- 11 The valve stems, valve stem inserts, and valve disc holders are machined, and threaded separately.
Assembling the hydrant
- 12 Starting with the upper valve, an oring seal is placed over the valve stem, and the stem is threaded into the stem insert. The inside end of the stem is pushed through the stem insert opening, and the disc holder, rubber disc, and locking nuts are reached up inside the barrel, threaded onto the stem, and locked in place with a set screw. The stem insert is then threaded into the barrel, and the replaceable operating nut is slipped over the outside end of the stem and held in place with a nut. This process is repeated for each of the valves.
- 13 If the barrel is a two-piece design, an oring is slipped over the threaded portion of the spool and the assembled head is screwed down to seal against the oring. The threads are locked in place by a set screw.
Testing the hydrant
- 14 The AWWA standards require that bronze hydrants be rated at 150 psi (1,034 kPa), and ductile iron hydrants be rated at 250 psi (1,723 kPa). Each hydrant is filled with water and pressurized to twice the rated pressure to check for leaks.
Preparing for shipment
- 15 After the hydrant is pressure tested, the outlet caps and chains are attached, a plastic protector is slipped over the bottom flange, and the exterior of the hydrant barrel is given a coat of primer paint.
All incoming material is inspected to ensure it meets the required specifications. This includes spectrographic analysis of the raw materials used to make the castings. The moisture content of the molding sand is critical to the casting process, and it is checked before every casting run. When a run of castings is machined, the first piece is checked for proper dimensions before the remainder of the castings is machined.
It is unlikely that the fire hydrant will disappear from the urban landscape anytime in the near future. Water is still the most cost-effective fire suppressant, and the hydrant is still the most cost-effective way to provide a ready supply of water. If anything, the fire hydrant will gain importance as fire departments and taxpayers alike realize that strategically placed, high-capacity hydrants can significantly reduce fire insurance rates.
Where to Learn More
NFPA 291: Fire Flow Testing and Marking of Hydrants. National Fire Protection Association, 1995.
NFPA 1231: Water Supplies for Suburban and Rural Fire Fighting. National Fire Protection Association, 1993.
Long, Germaine R. "Fire Plugs with Personality." Firehouse (June 1977): 36-37, 59.
Stevens, Larry H. "Water Works: Get the Most Out of Your Hydrants." Firefighter's News (August/September 1996): 32-33, 35-39.
"Fire Hydrant." How Products Are Made. . Encyclopedia.com. (October 20, 2017). http://www.encyclopedia.com/manufacturing/news-wires-white-papers-and-books/fire-hydrant
"Fire Hydrant." How Products Are Made. . Retrieved October 20, 2017 from Encyclopedia.com: http://www.encyclopedia.com/manufacturing/news-wires-white-papers-and-books/fire-hydrant
The term fire hose refers to several different types of hose specifically designed for use in fighting fires. The most common one consists of one or more outer layers of woven fabric with an inner layer of rubber. It is usually manufactured in 50 ft (15.3 m) lengths with threaded metal connections on each end. Unlike other hoses, most fire hose is designed to be stored flat to minimize the space required. For example, the average fire pumper in the United States can carry 1,200 ft (366 m) of 2.5 in (64 mm) diameter fabric-covered, rubber-lined hose in a space about the size of a king-size bed.
The earliest recorded use of fire hose was in ancient Greece. According to the Greek author Apollodorus, one end of an ox's intestine was attached to a bladder filled with water. When the bladder was pressed, the water was forced through the long ox gut and was directed "to high places exposed to fiery darts."
The forerunner of the modern fire hose was invented in 1672 in Amsterdam, Netherlands, by Nicholas and Jan van der Heiden (Heides). Their discharge hose was made of leather with tightly sewn seams. Brass fittings were attached to each end to allow several sections to be coupled together. In 1698, they made a suction hose of heavy sailcloth coated with paint or cement to make it watertight. The hose was reinforced with internal metal rings to prevent it from collapsing under a vacuum.
Early leather hoses leaked badly, and their sewn seams were prone to rupture under pressure. The first riveted leather hose was developed in 1808 in Philadelphia by a group of volunteer firefighters. Their hose had seams held together by 20-30 metal rivets per foot (65-100 rivets per meter) to eliminate leaks. Two members of the group patented this design in 1817 and began manufacturing it. Although woven cotton and linen hoses were also introduced in the early 1800s, and rubber-coated hoses were introduced in 1827, none of these designs was developed enough to replace riveted leather hose until about the 1870s.
Modern fire hoses use a variety of natural and synthetic fabrics and elastomers in their construction. These materials allow the hoses to be stored wet without rotting and to resist the damaging effects of exposure to sunlight and chemicals. Modern hoses are also lighter weight than older designs, and this has helped reduce the physical strain on firefighters.
Types and Sizes of Fire Hose
There are several types of hose designed specifically for the fire service. Those designed to operate under positive pressure are called discharge hoses. They include attack hose, supply hose, relay hose, forestry hose, and booster hose. Those designed to operate under negative pressure are called suction hoses.
Attack hose is a fabric-covered, flexible hose used to bring water from the fire pumper to the nozzle. This hose ranges in nominal inside diameter from 1.5 in (38 mm) to 3.0 in (76 mm) and is designed to operate at pressures up to about 400 psi (2,760 kPa). The standard length is 50 ft (15.3 m).
Supply and relay hoses are large-diameter, fabric-covered, flexible hoses used to bring water from a distant hydrant to the fire pumper or to relay water from one pumper to another over a long distance. These hoses range in nominal inside diameter from 3.5 in (89 mm) to 5.0 in (127 mm). They are designed to operate at pressures up to about 300 psi (2,070 kPa) for the smaller diameters and up to 200 psi (1,380 kPa) for the larger diameters. The standard length is 100 ft (30.6 m).
Forestry hose is a fabric-covered, flexible hose used to fight fires in grass, brush, and trees where a lightweight hose is needed in order to maneuver it over steep or rough terrain. Forestry hose comes in 1.0 in (25 mm) and 1.5 in (38 mm) nominal inside diameters and is designed to operate at pressures up to about 450 psi (3,105 kPa). The standard length is 100 ft (30.6 m).
Booster hose is a rubber-covered, thick-walled, flexible hose used to fight small fires. It retains its round cross-section when it is not under pressure and is usually carried on a reel on the fire pumper, rather than being stored flat. Booster hose comes in 0.75 in (19 mm) and 1.0 in (25 mm) nominal inside diameters and is designed to operate at pressures up to 800 psi (5,520 kPa). The standard length is 100 ft (30.6 m).
Suction hose, sometimes called hard suction, is usually a rubber-covered, semi-rigid hose with internal metal reinforcements. It is used to suck water out of unpressurized sources, such as ponds or rivers, by means of a vacuum. Suction hose ranges in nominal inside diameter from 2.5 in (64 mm) to 6.0 in (152 mm). The standard length is 10 ft (3.1 m).
Another suction hose, called a soft suction, is actually a short length of fabric-covered, flexible discharge hose used to connect the fire pumper suction inlet with a pressurized hydrant. It is not a true suction hose as it cannot withstand a negative pressure.
In the past, cotton was the most common natural fiber used in fire hoses, but most modern hoses use a synthetic fiber like polyester or nylon filament. The synthetic fibers provide additional strength and better resistance to abrasion. The fiber yarns may be dyed various colors or may be left natural.
Coatings and liners include synthetic rubbers such as styrene butadiene, ethylene propylene, chloroprene, polyurethane, and nitrile butadiene. These compounds provide various degrees of resistance to chemicals, temperature, ozone, ultraviolet (UV) radiation, mold, mildew, and abrasion. Different coatings and liners are chosen for specific applications.
Hard suction hose consists of multiple layers of rubber and woven fabric encapsulating an internal helix of steel wire. Some very flexible hard suction hose uses a thin polyvinyl chloride cover with a polyvinyl chloride plastic helix.
Hose connections may be made from brass, although hardened aluminum connections are more frequently specified because of their lightweight.
A fabric-covered fire hose has one or more layers of woven fabric as a reinforcement material. A hose with one layer is called single jacket hose and is used where light-weight is important or where the hose is expected to have infrequent service. A forestry hose is single jacket for light-weight. An industrial fire hose is single jacket because it sees infrequent use. A hose with two layers is called a double jacket hose and is used where weight is not as critical and where the hose is expected to have frequent, sometimes harsh use, as in urban fire service.
A jacketed hose is usually lined with a thin-walled extruded tube of rubber or another elastomer material that is bonded to the inside of the hose. This prevents the water from seeping through the hose jacket. Some forestry hose is made with a perforated rubber liner to allow it to "weep" a little water through the jacket as a protection against embers that might otherwise burn the hose.
Another type of fabric hose construction is called through-the-weave extrusion. In this design a single fabric jacket is fed through a rubber extruder. The extruder coats both the inside and outside of the fabric with a rubber compound to form both an inner liner and an outer coating at the same time. The extruder forces the rubber into and through the jacket weave to form an interlocking bond. This construction produces a lighter weight hose and is primarily used for larger-diameter supply hoses.
Fire hose is usually manufactured in a plant that specializes in providing hose products to municipal, industrial, and forestry fire departments. Here is a typical sequence of operations used to manufacture a double jacket, rubber-lined fire hose.
Preparing the yarn
- 1 There are two different fiber yarns that are woven together to form a hose jacket. The yarns that run lengthwise down the hose are called warp yarns and are usually made from spun polyester or filament nylon. They form the inner and outer surfaces of the jacket and provide abrasion resistance for the hose. The yarns that are wound in a tight spiral around the circumference of the hose are called the filler yarns and are made from filament polyester. They are trapped between the crisscrossing warp yarns and
provide strength to resist the internal water pressure.
The spun polyester warp yarns are specially prepared by a yarn manufacturer and are shipped to the hose plant. No further preparation is needed.
- 2 The continuous filament polyester fibers are gathered together in a bundle of 7-15 fibers and are twisted on a twister frame to form filler yarns. The plied and twisted yarn is then wound onto a spool called a filler bobbin.
Weaving the jackets
- 3 The warp yarns are staged on a creel, which will feed them lengthwise down through a circular loom. Two filler bobbins with the filler yarn are put in place in the loom.
- 4 As the loom starts, the filler bobbins wind the filler yarn in a circle through the warp yarns. As soon as the bobbins pass, the loom crisscrosses each pair of adjacent warp yarns to trap the filler yarn between them. This weaving process continues at a high speed as the lower end of the Pjacket is slowly drawn down through the loom, and the bobbins continue to wrap the filler yarns around the circumference of the jacket in a tight spiral. The woven jacket is wound flat on a take-up reel.
- 5 The inner and outer jackets are woven separately. The inner jacket is woven to a slightly smaller diameter so that it will fit inside the outer jacket. Depending on the expected demand, several thousand feet of jacket may be woven at one time. After an inspection, the two jackets are placed in storage.
- 6 If the outer jacket is to be coated, it is drawn through a dip tank filled with the coating material and then passed through an oven where the coating is dried and cured.
Extruding the liner
- 7 Blocks of softened, sticky, uncured rubber are fed into an extruder. The extruder warms the rubber and presses it out through an opening between an inner and outer solid circular piece to form a tubular liner.
- 8 The rubber liner is then heated in an oven where it undergoes a chemical reaction called vulcanizing, or curing. This makes the rubber strong and pliable.
- 9 The cured liner passes through a machine called a rubber calendar, which forms a thin sheet of uncured rubber and wraps it around the outside of the liner.
Forming the hose
- 10 The jackets and liner are cut to the desired length. The inner jacket is inserted into the outer jacket, followed by the liner.
- 11 A steam connection is attached to each end of the assembled hose, and pressurized steam is injected into the hose. This makes the liner swell against the inner jacket and causes the thin sheet of uncured rubber to vulcanize and bond the liner to the inner jacket.
- 12 The metal end connections, or couplings, are attached to the hose. The outer portion of each coupling is slipped over the outer jacket and an inner ring is inserted into the rubber liner. A tool called an expansion mandrel is placed inside the hose and expands the ring. This squeezes the jackets and liner between the ring and serrations on the outer portion of the coupling to form a seal all the way around the hose.
Pressure testing the hose
- 13 Standards set by the National Fire Protection Association require that each length of new double jacket, rubber-lined attack hose must be pressure tested to 600 psi (4,140 kPa), but most manufacturers test to 800 psi (5,520 kPa). Subsequent to delivery, the hose is tested annually to 400 psi (2,760 kPa) by the fire department. While the hose is under pressure, it is inspected for leaks and to determine that the couplings are firmly attached.
- 14 After testing the hose is drained, dried, rolled, and shipped to the customer.
In addition to the final pressure testing, each hose is subjected to a variety of inspections and tests at each stage of manufacturer. Some of these inspections and tests include visual inspections, ozone resistance tests, accelerated aging tests, adhesion tests of the bond between the liner and inner jacket, determination of the amount of hose twist under pressure, dimensional checks, and many more.
The trend in fire hose construction over the last 20 years has been to the use of lighter, stronger, lower maintenance materials. This trend is expected to continue in the future as new materials and manufacturing methods evolve.
One result of this trend has been the introduction of lightweight supply hoses in diameters never possible before. Hoses up to 12 in (30.5 cm) in diameter with pressure ratings up to 150 psi (1,035 kPa) are now available. These hoses are expected to find applications in large-scale industrial fire-fighting, as well as in disaster relief efforts and military operations.
Where to Learn More
NFPA 1961: Fire Hose. National Fire Protection Association, 1997.
NFPA 1963: Fire Hose Connections. National Fire Protection Association, 1993.
Goldwater, Sam and Robert F. Nelson. "Large-Diameter Super Aquaduct Flexible Pipeline Applications in the Fire Service." Fire Engineering (April 1997): 147-149.
"Fire Hose." How Products Are Made. . Encyclopedia.com. (October 20, 2017). http://www.encyclopedia.com/manufacturing/news-wires-white-papers-and-books/fire-hose
"Fire Hose." How Products Are Made. . Retrieved October 20, 2017 from Encyclopedia.com: http://www.encyclopedia.com/manufacturing/news-wires-white-papers-and-books/fire-hose
The hand-held fire extinguisher is simply a pressure vessel from which is expelled a material (or agent) to put out a fire. The agent acts upon the chemistry of the fire by removing one or more of the three elements necessary to maintain fire—commonly referred to as the fire triangle. The three sides of the fire triangle are fuel, heat, and oxygen. The agent acts to remove the heat by cooling the fuel or to produce a barrier between the fuel and the oxygen supply in the surrounding air. Once the fire triangle is broken, the fire goes out. Most agents have a lasting effect upon the fuel to reduce the possibility of rekindling. Generally, the agents applied are water, chemical foam, dry powder, halon, or carbon dioxide (CO2). Unfortunately, no one agent is effective in fighting all types (classes) of fires. The type and environment of the combustible material determines the type of extinguisher to be kept nearby.
Fire extinguishers, in one form or another, have probably postdated fire by only a short time. The more practical and unitized extinguisher now commonplace began as a pressurized vessel that spewed forth water, and later, a combination of liquid elements. The older extinguishers comprised cylinders containing a solution of baking soda (sodium bicarbonate) and water. Inside, a vessel of sulfuric acid was positioned at the top of the body. This design had to be turned upside down to be activated, so that the acid spilled into the sodium bicarbonate solution and reacted chemically to form enough carbon dioxide to pressurize the body cylinder and drive out the water through a delivery pipe. This volatile device was improved by placing the acid in a glass bottle, designed to be broken by a plunger set on the top of the cylinder body or by a hammer striking a ring contraption on the side to release the acid. Cumbersome and sometimes ineffective, this design also required improvement.
Aside from using different agents, manufacturers of extinguishers generally use some type of pressurized vessel to store and discharge the extinguishing agent. The means by which each agent is discharged varies. Water fire extinguishers are pressurized with air to approximately 150 pounds per square inch (psi)—five times a car tire pressure—from a compressor. A squeeze-grip handle operates a spring-loaded valve threaded into the pressure cylinder. Inside, a pipe or "dip tube" extends to the bottom of the tank so that in the upright position, the opening of the tube is submerged. The water is released as a steady stream through a hose or nozzle, pushed out by the stored pressure above it.
Water extinguishers of the "gas cartridge" type operate in much the same manner, but the pressure source is a small cartridge of carbon dioxide gas (CO2) at 2,000 psi, rather than air. To operate a gas cartridge unit, the end of the extinguisher is struck against the floor, causing a pointed spike to pierce the cartridge, releasing the gas into the pressure vessel. The released CO2 expands several hundred times its original volume, filling the gas space above the water. This pressurizes the cylinder and forces the water up through a dip-pipe and out through a hose or nozzle to be directed upon the fire. This design proved to be less prone to leakdown (loss of pressure over time) than simply pressurizing the entire cylinder.
In foam extinguishers, the chemical agent is generally held under stored pressure. In dry powder extinguishers, the chemicals can either be put under stored pressure, or a gas cartridge expeller can be used; the stored-pressure type is more widely used. In carbon dioxide extinguishers, the CO2 is retained in liquid form under 800 to 900 psi and is "self-expelling," meaning that no other element is needed to force the CO2 out of the extinguisher. In halon units, the chemical is also retained in liquid form under pressure, but a gas booster (usually nitrogen) is generally added to the vessel.
Fire extinguishers can be divided into four classifications: Class A, Class B, Class C, and Class D. Each class corresponds to the type of fire the extinguisher is designed for, and, thus, the type of extinguishing agents used. Class A extinguishers are designed to fight wood and paper fires; Class B units fight contained flammable liquid fires; Class C extinguishers are designed to fight live electrical fires; and Class D units fight burning metal fires.
Water has proven effective in extinguishers used against wood or paper fires (Class A). Water, however, is an electrical conductor. Naturally, for this reason, it is not safe as an agent to fight electrical fires where live circuits are present (Class C). In addition, Class A extinguishers should not to be used in the event of flammable liquid fires (Class B), especially in tanks or vessels. Water can cause an explosion due to flammable liquids floating on the water and continuing to burn. Also, the forceful water stream can further splatter the burning liquid to other combustibles. One disadvantage of water extinguishers is that the water often freezes inside the extinguisher at lower temperatures. For these reasons, foam, dry chemical, CO2, and halon types were developed.
Foam, although water based, is effective against fires involving contained flammable liquids (Class B). A two-gallon (7.5 liters) extinguisher will produce about 16 gallons (60 liters) of thick, clinging foam that cools and smothers the fire. The agent itself is a proprietary compound developed by the various manufacturers and contains a small amount of propylene glycol to prevent freezing. It is contained as a mixture in a pressurized cylinder similar to the water type. Most aircraft carry this type of extinguisher. Foam can also be used on Class A fires.
The dry powder agent was developed to reduce the electrical hazard of water, and thus is effective against Class C fires. (It can also be used against Class B fires.) The powder is finely divided sodium bicarbonate that is extremely free-flowing. This extinguisher, also equipped with a dip-tube and containing a pressurizing gas, can be either cartridge-operated or of the stored pressure type as discussed above. Many specialized dry chemical extinguishers are also suitable for burning metal fires, or Class D.
Carbon dioxide (CO2) extinguishers, effective against many flammable liquid and electrical fires (Class B and C), use CO2 as both the agent and the pressurizing gas. The liquified carbon dioxide, at a pressure that may exceed 800 psi depending on size and use, is expelled through a flared horn. Activating the squeeze-grip handle releases the CO2 into the air, where it immediately forms a white, fluffy "snow." The snow, along with the gas, substantially reduces the amount of oxygen in a small area around the fire. This suffocates the fire, while the snow clings to the fuel, cooling it below the combustion point. The greatest advantage to the CO2 extinguisher is the lack of permanent residue. The electrical apparatus that was on fire is then more likely to be able to be repaired. Unlike CO2 "snow," water, foam, and dry chemicals can ruin otherwise undamaged components.
As extinguishing agents, halons are up to ten times more effective in putting out fires than other chemicals. Most halons are non-toxic and extremely fast and effective. Chemically inert, they are harmless to delicate equipment, including computer circuits, and leave no residue. The advantage of the halon over the CO2 extinguisher is that it is generally smaller and lighter. Halon is a liquid when under pressure, so it uses a dip-tube along with nitrogen as the pressurizing gas.
Halon, at least in fire extinguishers, may soon become a footnote to history. In 1992, 87 nations around the world agreed to halt the manufacture of halon fire extinguishers by January 1, 1994. This will eliminate a potential threat to the earth's protective ozone layer, which halon molecules—highly resistant to decomposition—interact with and destroy.
Most of the other elements of a fire extinguisher are made of metal. The pressure vessel is generally made of an aluminum alloy, while the valve can either be steel or plastic. Other components, such as the actuating handle, safety pins, and mounting bracket, are typically made of steel.
Manufacture of the tank-type or cylinder fire extinguisher requires several manufacturing operations to form the pressure vessel, load the chemical agent, machine the valve, and add the hardware, hose, or nozzle.
Creating the pressure vessel
- 1 Pressure vessels are formed from puck-shaped (disc) blocks of special aluminum alloy. The puck is first impact extruded on a large press under great pressure. In impact extrusion, the aluminum block is put into a die and rammed at very high velocity with a metal tool. This tremendous energy liquifies the aluminum and causes it to flow into a cavity around the tool. The aluminum thus takes the form of an open-ended cylinder with considerably more volume than the original puck.
Necking and spinning
- 2 The necking process puts a dome on the open end of the cylinder by constricting the open end with another operation called spinning. Spinning gently rolls the metal together, increasing the wall thickness and reducing the diameter. After spinning, the threads are added.
- 3 The vessel is hydrostatically tested, cleaned, and coated with a powdered paint. The vessel is then baked in an oven where the paint is cured.
Adding the extinguishing agent
- 4 Next, the extinguishing agent is added. If the vessel is a "stored-pressure" type, the vessel is then pressurized accordingly. If a gas-cartridge is necessary to help expel the extinguishing agent, it is also inserted at this time.
- 5 After the extinguishing element is added, the vessel is sealed and the valve is added. The valve consists of a machined body made of metal bar stock on a lathe, or a plastic injected molded part on the economy versions. It must be leak free, and it must have provisions for threading into the cylinder.
- 6 The final manufacturing operation is the assembly of the actuating handle, safety pins, and the mounting bracket. These parts are usually cold formed—formed at low temperatures—steel or sheet metal forms, purchased by the manufacturer from an outside vendor. Identification decals are also placed on the cylinder to identify the proper fire class rating as well as the suitability for recharging. Many of the economy versions are for one time use only and cannot be refilled.
All fire extinguishers in the United States fall under the jurisdiction of the National Fire Protection Association (NFPA), Under-writer's Laboratories, The Coast Guard, and other organizations such as the New York Fire Department. Manufacturers must register their design and submit samples for evaluation before marketing an approved fire extinguisher.
One of the most crucial checkpoints during the manufacturing process occurs after the extinguishing agent is added and the vessel sealed. It is extremely important that the cylinder not leak down the pressurizing gas, because that would render the extinguisher useless. To check for leaks, a boot is placed over the cylinder to serve as an accumulator. A trace gas is released inside, and within two minutes any unacceptable rate of leakage can be recorded by sophisticated pressure and gas-detecting equipment. All extinguishers are leak tested.
With the gradual elimination of halon, a new, non-damaging agent will most likely replace the hazardous chemical within the next few years. In addition, new applications of the old designs are being seen; most prevalent are automatic heat and fire sensors that discharge the extinguisher without the need for an operator.
Where To Learn More
Fire Prevention Handbook. Butterworths, London, 1986.
Mahoney, Gene. Introduction to Fire Apparatus & Equipment. 2nd ed., Fire Engineering Books & Videos, 1986.
Portable Fire Extinguishing Equipment in Family Dwellings & Living Units. National Fire Protection Association, 1992.
—Douglas E. Betts and
"Fire Extinguisher." How Products Are Made. . Encyclopedia.com. (October 20, 2017). http://www.encyclopedia.com/manufacturing/news-wires-white-papers-and-books/fire-extinguisher
"Fire Extinguisher." How Products Are Made. . Retrieved October 20, 2017 from Encyclopedia.com: http://www.encyclopedia.com/manufacturing/news-wires-white-papers-and-books/fire-extinguisher
fire ex·tin·guish·er • n. a portable device that discharges a jet of water, foam, gas, or other material to extinguish a fire.
"fire extinguisher." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. (October 20, 2017). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/fire-extinguisher
"fire extinguisher." The Oxford Pocket Dictionary of Current English. . Retrieved October 20, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/fire-extinguisher
fire extinguisher: see fire fighting.
"fire extinguisher." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (October 20, 2017). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/fire-extinguisher
"fire extinguisher." The Columbia Encyclopedia, 6th ed.. . Retrieved October 20, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/fire-extinguisher