Dry and Wet Chem Systems

• Ch. 8, pages 148-162

• Dry Chemical Systems

• Dry Chem. System Components

• Dry Chem. Sequence of Operation

• Applications for Dry Chem Systems

• Types of Dry Chem. Systems

• Wet Chemical Systems

Dry Chemical Systems

• either pre-engineered

or

• designed by supplier

• design details are proprietary

• commercial cooking

• dry chem is different from dry powder

Dry Chemical Examples

• Commercial cooking

• heavy-duty vehicles– graders, buses

• portable units – (up to 300 lb)– flamm. Liquids

• combined with foam (twin agent)

Dry Chem. Agents

• Small solid particles

• suspended or fluidized in air

• more complicated than gaseous or liquid agents

Dry Chem. Agents

• 3 broad categories

• sodium carbonate based

• potassium based

• multipurpose

Sodiium Bicarbonate based

• NaHCO3

• baking soda

• Flamm. Liquids (class B)

• Electrical (class C)

• ideal for grease fires

• saponify (soap-like layer)

Potassium based

• Flamm. Liquids (class B)

• Electrical (class C)

• better than sodium bicarbonate except on grease fires

• KHCO3, potassium bicarbonate, Purple K

• KCl, potassium chloride, Super K

• KC2N2H3O3, Monnex

Multi-purpose

• Monoammonium phosphate

• A B or C

• Form molten residue

• Not effective on deep fat fires

• Not as effective as narrow purpose agent

Dry Chem. advantages

• Rapid knockdown– minimizes damage

• 3D surface coating– minimizes reignition

• sprays, leaks etc.

Dry Chem. disadvantages

• Surface coating– messy residue– expensive cleanup

• caking– from moisture– vibration– particles commonly coated

Personnel hazard

• Material is non-toxic

• particles too large to penetrate into deep lung

• products of combustion harmful

• usual precautions to prevent exposure

Testing of systems

• Testing – messy– expensive– requires clean-up

• most are not tested• reliability issue• least risk with popular pre-engineered

systems

Extinguishing mechanism

• Coating

• smothering

• heat absorption

• block chain reaction

Container

• From 1lb to 3,000 lb

• small are pressurized

• large have separate expellant gas cylinder

• minimize piping runs

• rupture disk

• see fig 8-1

Expellant gas cylinder

• For large systems

• 100’s of lb

• N2 or CO2

• to fluidize dry chem

Piping and nozzles

• Minimize runs

• remix at each tee

• withstand pressure

• corrosion resistant

• not cast iron

• variety of nozzles available

Sequence of operation

• See 8-4

1. Detector senses fire

2. Signal to panel

3. Panel interprets signal, if interpreted as fire

4. Alarm sounded

5. Equipment, HVAC shutdown

6. Could be manual activation

Sequence of operation

7. Could have remote activation

8. Signal to actuator on propellant tank

9. Valve opened

10. Propellant flows to dry chem tank

11. Powder fluidized

12. pressure builds in tank

Sequence of operation

13. Rupture disk bursts

14. Fluidized particles/propellant gas flow

15. Discharge from nozzle

Applications

• Commercial cooking

• petrochemical

• paint spray booths

• dip tanks

• tranformers

• generators

• conveyors

Types of systems

• Total flood

• local application

• hand hose line

• pre-engineered

Total Flood

• Fill enclosure

• paint spray booth

• enclosure must be sealed

• ventilation shut down

• additional dry chem for unclosable openings

Local Application

• Hazard must be isolated– drum filling stations

• if liquid, consider splashing

• if outdoors, consider wind dispersion

Hand lines

• As a supplement to fixed system

• minimum 30 sec capacity/line

Pre-engineered

• Commercial restaurants

• vehicle fueling

• mobile equipment

• packaged units

• highest reliability

Wet Chemical

• Water and extinguishing chemical

• usually potassium based chemical

• pre-engineered restaurant systems