Chapter 09

© 2008 Delmar, Cengage Learning

Wet and Dry Chemical Extinguishing Systems

Chapter 9


Learning Objectives

• Describe the characteristics and types of wet and dry chemical systems

• Discuss the most likely hazards wet and dry chemical systems protect

• Define pre-engineered system• Discuss the events that lead to the

standardization of UL300


Learning Objectives (continued)

• List the major components that make up wet and dry chemical systems

• Describe how wet and dry chemical agents control and extinguish fire

• Discuss the various inspection, testing, and maintenance requirements for fixed and dry chemical systems


Introduction• Wet and dry chemical systems provide an

alternative to water– Used when water would have no effect, or make the

situation worse

• Wet chemical systems most commonly used to extinguish fires in commercial kitchens

• Dry chemical systems extinguish flammable and combustible liquid fires

• Both systems use gas to expel the agent and have a finite amount of agent available


Wet Chemical Extinguishing Systems

• Protect appliances in commercial kitchens– Deep fat fryers, ranges, griddles, grills etc.

• Chemicals used are proprietary for the manufacturer

• Water-based; forms a medium for the agent to flow to the hazard

• Saponification decreases or eliminates the fuel vapors by smothering and cooling the fuel


Figure 9-1 Wet chemical extinguishing system protecting a commercial kitchen


Dry Chemical Extinguishing Systems

• Protect equipment and processes against flammable and combustible liquid fires

• Fine powders that use pressurized gas as the transport medium– Particles suspend in the gas to facilitate distribution

• Knock down the flame and coat the surface area, cutting off oxygen

• Effective on three-dimensional surface fires


Standards for Wet and Dry Chemical Systems

• Many wet and dry chemical systems are pre-engineered and adaptable to many situations

• Some hazards require full engineering– Amount of agent needed is the basis for system

design

• NFPA 17 and 17 A are the standards• UL 300 and UL 1254 provide specific

requirements


System Component and Requirements

• Wet and dry chemical systems use similar types of components

• Main components:– Agent storage containers– Expelling gas cartridges– Piping and fittings– Nozzles – Activation devices– System alarms and indicators


Agent Storage Containers and Gas Cartridges

• Containers that store wet and dry chemicals are made of non-reactive metal– Must handle high pressures

• Size depends on amount of agent needed to protect the hazard

• Many manufacturers do not combine wet chemical agent and expelling gas

• Dry chemical containers may be larger


Piping and Fittings

• Piping and fittings must be made of non-combustible material– Must be compatible with the agent– Must be able to handle anticipated pressure– Usually a metal specifically approved for use with the

specific agent and extinguishing system

• Arrangement and size of the pipe and fittings depends on the hazard and the agent

• Fluidization: mixing dry agent and expelling gas


Nozzles

• Different types made for wet and dry chemical systems– Require listing for the intended application

• Distribute the extinguishing agent by delivering a specific application pattern

• NFPA 17 and 17 A require the nozzles to be noncombustible– Should not deform when exposed to fire

• Nozzles on wet systems require in-line strainer


Figure 9-6 Wet chemical system nozzles with blow-off caps


Activation Devices

• Wet and dry chemical extinguishing systems operate by automatic and manual activation– Failure of one does not inhibit the other

• Automatic activation by mechanical or electrical sensing device located in the hazard area– Mechanical device example: fusible link– Electrical device example: heat detector

• Pull station must be 42- 48 inches above the floor in path of egress


System Alarms and Indicators• Upon activation, wet and dry chemical systems

must initiate an alarm– Must tie into building fire alarm system– If no alarm system is in building, local audible alarm or

visual indicator provides notification

• When flowing, dry chemical discharge creates potentially dangerous cloud of powder– Inhibits breathing and visibility

• Indicators notify when recharging or investigation is needed


Wet and Dry Chemical Agents

• Wet chemicals are liquid-based• Dry chemicals are solid particle-based • Each agent is appropriate for particular

applications and conditions• Design professional must consider:

– Environment– Type of system– Amount of agent needed


Wet Chemical Agents

• Proprietary water-based solutions• Mixed with:

– Potassium acetate– Potassium carbonate– Potassium citrate– Mixture of the above

• Wet agents are most effective for Class K fires• May corrode, stain equipment if not cleaned up• Generally harmless to humans


Dry Chemicals• Small solid particles propelled by pressurized

gas• When discharged, chemical covers and

smothers the burning material• Three categories: sodium bicarbonate-based,

potassium-based, multipurpose• Sodium bicarbonate and potassium-based work

well on Class B and C fires• Ammonium phosphate works well on Class A, B

and C fires


Types of Fixed Pipe Wet and Dry Extinguishing Systems

• Two types of fire extinguishing systems:– Local application systems– Total flooding systems

• Local application systems necessary when the hazard is exposed and not surrounded – Can be wet or dry

• Total flooding system protects enclosed hazards or hazards in confined areas


Local Application Systems• Protects a specific area, piece of equipment,

process or operation• Use wet or dry extinguishing agents

– Discharge the agent directly onto the hazard– Smother, cool, extinguish the fire

• Must protect entire hazard– Hazard must be isolated from other hazards to avoid

spreading the fire

• Nozzle placement critical to ensure proper application of the agent


Total Flooding Systems

• Protect enclosed hazards and areas within a structure

• When discharged, the agent fills entire protected area to disrupt the fire’s chain reaction

• Amount of agent and nozzles needed depend on total volume of area– Based on manufacturer’s design calculations

• Successful suppression requires adequate sealing or minimal openings in protected area


Other Wet and Dry Chemical Application Methods

• Hand hose lines can stand alone, supplement other systems

• Installation of hand hose line permissible if approved by governing authority

• When supplementing dry chemical system, hand hose line must have separate chemical supply

• Minimum 30-second dry chemical supply• Must be accessible and reach the hazard


Wet and Dry Chemical Extinguishing System Operation

• May be manual or automatic• Automatic activation: fire must build to a

sufficient level to activate the sensing device• Manual activation: person must make physical

connection with releasing device• Manual or electronic release starts operational

sequence: – Agent flows; associated utilities are controlled; alarm

sounds


Figure 9-11 Once the system activates, the cable attached to the gas shut-off valve releases to close the valve.


Inspection, Testing, and Maintenance

• Wet and dry chemical systems must undergo acceptance inspections and tests

• Relatively simple for both types of systems– May be many interconnected systems

• As systems age, periodic inspections, tests, maintenance are required

• Some inspections and tests must be done by specially trained personnel


Visual Inspections

• Visual inspection confirms equipment is located and installed according to documentation

• Includes verification that:– Correct nozzle types are installed– Nozzles are properly oriented and located– Pipe material, sizes, and length are correct– Agent type and amount are correct– Auxiliary equipment is correctly installed


Discharge Test

• Discharge tests verifies that system nozzles and associated equipment operate correctly

• Balloons or bags capture the agent when discharged

• Capturing and weighing the product more critical to the dry systems than the wet systems

• Gas propellant is usually nitrogen or compressed air– Avoids corrosion by wet agent


Figure 9-12 Bags tied to the nozzles to collect the wet chemical agent during a discharge test to ensure proper flow to each nozzle


Acceptance Test

• Confirms proper operation by manual or automatic release

• Activating manual release, automatic detector or fusible link minimizes spread of fire– Fuel shut off equipment that produces heat– Electrical supply shut off – Supply air shut down – Alarm activation– Exhaust fan shut down and damper closes


Periodic Inspection, Testing, and Maintenance

• Many tests performed when commissioning a system are the same tests for inspection– More frequent and the owner’s responsibility

• Inspection: on a monthly basis, visually inspect for conditions that could cause failure

• Periodic testing determines if system is operating as designed

• Routine maintenance follows manufacturer’s guidelines


Summary

• Wet and dry chemical systems provide an alternative to water-based systems

• Wet chemical agents are a mixture of water and other chemicals, and are proprietary

• Dry chemical agents coat the surface of the hazard, preventing oxygen from reaching fuel

• Many systems are pre-engineered• Two types of systems: local application and total

flooding systems

Business

Chapter 09