Understanding And Selecting Wheeled Fire Extinguishers

   
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.

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

The primary fire protection value of wheeled fire extinguishers
is in their ability to:

1. Contain greater extinguishing agent capacities

2. Deliver higher agent flow rates

3. Provide additional fire fighting ranges

4. 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
   

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

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

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

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.