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Wet Chemical Extinguishing System || NFPA 17A

Wet Chemical Extinguishing System

Wet Chemical Extinguishing System
Wet Chemical Extinguishing System

What is Wet Chemical Extinguishing System, its applications, and how it works?!

It’s a fire suppression system designed to provide fire protection for :-
  • Restaurant, commercial, and institutional hoods.
  • Plenums , ducts and filters with their associated cooking appliances.

System suppress fire by spraying the plenum area, the filters , cooking surfaces and exhaust duct system with a pre-determined flow rate by :

  • Cooling the grease surface.
  • React with the hot grease and forming a layer of soap like foam on the surface of the fat, this layer act as an insulation between the hot grease and the atmosphere, which helping to prevent the escape of combustible vapor.

We can define this system as a pre-engineered system (NFPA 17A 3.3.13), which mean having predetermined:-

  • Flow rates.
  • Nozzle pressure.
  • Quantities of extinguishing agent.
  • Specific pipe size.
  • Max. and min. pipe length.
  • Flexible hose specification.
  • Numbers of fittings.
  • Number and type of nozzles.

So the system should listed by a testing laboratory, max. and min. pipe lengths and number of fittings should be permitted to be expressed in equivalent feet (meter) of pipe. The hazard protected by these systems should limited as to type and size based on actual fire test, limitations on hazards that are permitted to be protected by these systems are contained in the manufacturer’s design , installation, and maintenance manual, which is part of listing of the system.

Wet Chemical Extinguishing System Components

1- Detectors

Wet Chemical Extinguishing System
Wet Chemical Extinguishing System
Fusible hear detector to be suitable for vapor and grease field, it consist of three components :-
  • Fusible link.
  • Fusible link holder (Bracket).
  • linkage.

2- Discharge Nozzle

Wet Chemical Extinguishing System
Nozzle is a device used to deliver the liquid agent with a specific flow rate and stream pattern, it shall have the followings spec.:-
  • An internal strainer or a separate strainer located immediately upstream the nozzle.
  • Made of a corrosion-resistance material , non-combustible material (Brass or Stainless Steel) or to be protected inside and outside with deformation.
  • Have a protective cover cap, to prevent the entrance of grease, vapor, moisture, environmental contaminants and other foreign materials get in pipes.
Nozzle shall be marked and identification for each application and flow rate.

3- Agent Cylinder

It’s a stainless tank with valve assembly, which containing the extinguishing agent, it comes in different sizes according to each manufacturers.

4- Extinguishing agent

A potassium carbonite based extinguishing agent designed specially to extinguish grease laden fires.

5- System Releasing Module

Is a fully mechanical head that can be connected directly or remotely to the system valve and its required for system actuation, system can actuate by three methods:-
  • Fusible heat detector.
  • Direct from cylinder head.
  • Remote pull station.

6- Actuation Cartridge

Wet Chemical Extinguishing System
Actuation Cartridge
It’s gas cartridge contain nitrogen or carbon dioxide used to expels the liquid agent from the agent cylinders (tanks) through the discharge piping.

7- Remote Mechanical Pull Station

Its connected to Releasing module to actuate the system, its required at the point of egress from the hazard area are being protected.

8- Shut-off gas valve

It’s used to shut off the flow of gas to the appliances upon actuation of the releasing module , and it comes with different size.

9- Shielded Cable

Wet Chemical Extinguishing System
Shielded Cable
It used to connect the fusible link system, gas valve, and/or remote pull station to the Systems Releasing Module, 1/16” stainless steel cable encased in a protective, flexible conduit.

Wet Chemical Extinguishing System design

First of all we are deal with a pre-engineered system, so must follow the manufacturers limitation according to their testing report.
We have to know well the hood components which as the followings:-

  • Ducts.
  • Plenum.
  • Cooking Appliance ( Fat Fryer, Stove , boiler , grill,…… etc.)
Wet Chemical Extinguishing System
Hood Components

To make a correct design we must divide our design to sub-section as following:-
  • Nozzle Type and Placement.
  • Tanks Capacity.
  • Actuation cartridge selection.
  • Distribution of pipe network.
  • Detection System.

1- Nozzle Type, Placement and aim point.

Selection of nozzle based on:-
  • Coverage area for the nozzle and applications area.
  • Application (duct, plenum, cooking appliances) so nozzles used for protecting duct differ from that used for protecting plenum. After selecting nozzle type, then define nozzle placement and aim point according to manufacturer design manual.

2- Tanks Capacity.

Selection of tank based on term (Flow Point).
Flow Point is a term used in system design to describe the flow capacity of each nozzle according to manufactures design manual , which used in determine the tank capacity. 
Wet Chemical Extinguishing System
Wet Chemical Extinguishing System
Note that: Selection of nozzle type based on application area and nozzle coverage area, flow point for each nozzle according to nozzle type and manufacturer design manual.

Tank capacity category based on max. flow point for each cylinder, so tank selection will be based on the total flow point for the whole system.
Wet Chemical Extinguishing System
Wet Chemical Extinguishing System

3 - Actuation cartridge selection. 

Actuation cartridge have different sizes, selection of based on the number of cylinders will actuated.

4 - Distribution of pipe network.

The max. and min. pipe length for each size agent cylinder is based on total internal pipe volume, regardless of the diameter of the pipe used. All piping parameter are based on total system piping and do not make a distinction between branches and supply line piping. all pipe connections have a limitation according to cylinders size based on manufacturer test report and design manual.

5 - Detection System.

Conduit runs, pulley, elbows and number of detection per system must be within the approved system guide line.

Minimum requirement for detection system is :-

  • At least one fusible link shall be installed within each exhaust duct opening in accordance with the manufacturing listing NFPA 17A
  • Other than appliances that a downdraft ventilation system a fusible link shall be provided above each protected appliance or in accordance with the extinguishing system manufacturer’s design, installation and maintenance manual NFPA 17A 5.6.15
  • In case of electrostatic precipitator presence, must install detector after and before it.

Selection of rating of fusible heat detector based on the max. ambient temperature and according to Table A. NFPA 17A.
Wet Chemical Extinguishing System
Wet Chemical Extinguishing System

References of Article:

NFPA 17A - 2017 edition: Standard for Wet Chemical Extinguishing Systems

Fire Protection Specialists
Keeping You Up To Code


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