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Understanding And Selecting Wheeled Fire Extinguishersby J.R. Nerat |
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Over the past 40 years, the demand for transportation, storage, and handling of flammable fuels that fall under the fire protection
classification of "High Hazard" has increased drastically. This has often occurred at a faster rate than most fire protection
requirements or coverages. Recent trends toward automation and work force reductions have placed additional demands upon
fast response types of fire fighting equipment. With fewer people available to respond to emergency situations where higher fire
loads exist, portable wheeled fire extinguishers that provide a quicker, more effective extinguishment solution than water, should
be utilized.
Portable wheeled fire extinguishers have been around for well over 50 years and were, in those early years, referred to as
"Wheeled Engines". The first true market niche for this equipment was established during the early 1940s when it was used to
protect military airport ramps.
Wheeled extinguishers can be easily operated by one person and are considered to be portable extinguishers falling under The
National Fire Protection Association's Standard NFPA-10. The portable "wheeled" units are differentiated from "hand portable"
types often simply on the basis of their capacity or total weight. For example, the portable extinguisher design standard for dry
chemical extinguishers ANSI/UL-299 specifies that hand portable models not have a total charged gross weight exceeding 60
pounds.
Wheeled units are available in a variety of model sizes that feature extinguishing agents such as carbon dioxide, dry chemical,
halons, dry powder, and AFFF-type foam pre-mix solutions. Agent capacities of 30 to 350 pounds are offered, with many of the
smaller-sized units sometimes being glorified hand portables on wheels marketed to meet minimum high hazard requirements or for
added ease of mobility.
Basic Extinguisher Operation Principles - NAFED
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The primary fire protection value of wheeled fire extinguishers
is in their ability to:
- Contain greater extinguishing agent capacities
- Deliver higher agent flow rates
- Provide additional fire fighting ranges
- Extend discharge times.
These unique fire fighting characteristics are also extremely valuable during emergency situations in providing assistance to
various rescue and evacuation efforts.
 Basic Extinguisher Operation Principles
There are basically four operation principles in which wheeled extinguishers are designed and
manufactured.
(Buckeye) 50 lb.
Carbon Dioxide
"Self Expelling" Model |
Self-Expelling: These models typically contain the extinguishing agent within a
single high-pressure cylinder. When the cylinder valve is opened, the
extinguishing agent's own vapor pressure forces the agent to travel up a pick-up
tube, through the operating valve and out the discharge hose nozzle valve onto the
fire. This principle is primarily limited to carbon dioxide wheeled models which are
not equipped with pressure gauges to indicate charge status.
Stored Pressure: With this principle, the extinguishing agent and expellant gas
source (normally super pressurized nitrogen) is contained within the same cylinder.
Upon actuation of an operating valve, internal pressure forces the extinguishing
agent up the pick-up tube, through the operation valve and out the discharge hose
nozzle valve on to the fire. This configuration of hardware is normally the least
expensive.
Pressure Transfer: This concept consists of separate extinguishing agent and expellant gas cylinders. The expellant (normally
compressed nitrogen) is contained within a specifically engineered volume and sized high-pressure cylinder designed to properly
pressurize and discharge the total contents of the agent cylinder. Upon opening of the expellant gas cylinder operating valve,
expellant gas is transferred through a special gas distribution tube assembly that fluidizes and pressurizes the agent cylinder to a
pre-determined head pressure. This pressure permits the extinguishing agent to travel up the agent tank pick-up tube and out the
discharge nozzle valve on to the fire. This concept is less susceptible to vibration pressure losses in rugged and abusive
environments. The separate unpressurized agent cylinder allows simple agent examination and quick recharges in the field.
Regulated Supply and Demand: While similar to the pressure transfer principle in most ways, this concept utilizes a much larger
volume expellant gas cylinder source and a pressure regulator. As the extinguishing agent is discharged, the regulator will flow
and meter additional expellant gas pressure to the agent tank, providing a more consistent operating pressure. This ensures stable
agent flow rates and ranges throughout the fire fighting evolution. While usually higher priced, many fire hazards justify the
additional cost associated with the regulator and larger gas cylinder necessary on these models.
Extinguisher Options
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There are a large variety of options available to wheeled extinguishers, with the nozzle design and wheel configuration being two
of the most often misunderstood and incorrectly applied.
Wheel diameters of 16 to 44 inches are commonly offered in various widths and finished rolling surface bases. The large diameter
wheel types will provide superior mobility and ease of operation over uneven surfaces like potholes,
sidewalks or small steps. Wide 6-inch wheel bases provide additional versatility on soft
surfaces such as loose sand, gravel or sod. When spark reduction or the marking of floor
finishes is a consideration, the pneumatic, semi-pneumatic and rubber-banded wheels should be
utilized rather than the steel base types. Because pneumatic wheels will absorb some surface
shock, they generally deliver the best unit ride and are often selected for large airport ramp areas
where towing operations exist. The semi-pneumatic and rubber-banded wheels eliminate
potential flat problems in and around areas where sharp objects may be present.
Before purchasing any unit, always check to ensure the overall width of the unit, under its
specific wheel option configuration, will fit through any necessary aisles or door passageways.
The nozzle design of the wheeled extinguisher directly contributes to various operation characteristics such as the discharge
range, pattern, velocity, flow rate and fire rating.
Range information published by manufacturers can often be somewhat misleading. The focus here should be on the unit's
"effective range" established through actual fire-fighting evolutions, subject to thermal up- draft factors. Different internal nozzle
bore designs provide various agent stream patterns, flow rates and velocities. Discharge characteristics should be properly
matched to the particular fire hazard. For example, the venturi designs generally provide higher velocities and ranges at lower
agent flow rates, which work well on large class "A" fire applications. On the other hand, cone-type nozzle designs normally
reduce agent velocity for a softer discharge, making them ideal around fuel-in-depth situations where splashing of fuels in, and
around, confined areas could present problems to firefighters.
Equipment specifiers should also keep in mind that the portable extinguisher class "B" rating system is based on minimum
discharge times and only one type of fire configuration. For the manufacturers to obtain higher and more impressive ratings, they
must increase discharge times by cutting back agent flow rates, which is detrimental to the extinguishment of many other common
industry class "B" fire configurations. Agent flow rates become critical in obstacle, gravity, 3-dimensional and pressure fire
configurations. The NFPA 10 standard recognizes this and specifically states under several such hazard areas that "The system
used to rate extinguishers on class "B" fires is not directly applicable to these types of hazards." For most wheel unit applications,
higher attention to the agent flow rates over extinguisher ratings is in order. Most manufacturers can provide recommendations
and furnish support data.
Maintenance
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Wheeled fire extinguishers, like hand portable models, are mechanical devices that require periodic service for optimum
performance and reliability. Both OSHA and NFPA requirements specify maximum extinguisher inspection intervals of 30 days and
for maintenance to be performed at least annually.
Training
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When properly selected and maintained, wheeled fire extinguishers provide an exceptional fire fighting resource for the high risk
industries of today. For companies to benefit and reap the full potential of these extremely effective units, training programs
properly designed to demonstrate and familiarize operators with their impressive capabilities should also be regularly conducted.
Written by J.R. Nerat, president of a Fire Protection Specialist, a
Michigan-based consulting and training company. He holds an
appointment on the NFPA-10 Portable Extinguisher Committee and is a
specialist instructor for Texas A&M Industrial Fire School.
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Aviation Fire Protection