May 23, 2020

Parameters that affect Fire Severity of Group A Plastics

For the protection of Group A Plastics Storage, Designers face with some items such as Expanded or Non-expanded, Stable or Unstable, Exposed or Cartoned that effect the demand of fire sprinkler systems. In this text, we want to compare the items with this question:

Which one of these terms will create more challenging fire?

Expanded or Unexpanded? Expanded
Expanded plastics (Like Polystyrene Foam) ignite faster because of lesser thermal inertia.
The thermal inertia of a material is the direct product of three physical properties:

thermal conductivity (k), density (ρ), and heat capacity (C).

Thermal inertia characterizes the rate of surface temperature rise of the material when exposed to heat.

Low values of thermal inertia lead to elevated surface temperatures for a given applied heat flux scenario, and hence, to more rapid ignition and a greater flammability hazard if the material is combustible.[Fire Protection Handbook- Section 2, Chapter 3- 20th Edition].
If we assume that the thermal conductivity and heat capacity of expanded and unexpanded plastics are the same,

but we know that the density of expanded plastics are lower than unexpanded.

So the thermal inertia of expanded plastics are smaller and the flammability hazards will be greater.

Stable or Unstable? Stable
Fires established on vertical combustible surfaces are especially dangerous because of the potential for rapid upward fire spread, which will be discussed in detail below.

Flames on vertical burning surfaces transition from a steady laminar flow to a turbulent flow near the flame base.

With increasing height on the vertical surface, flame thickness increases proportionally, which results in increased thermal radiation both to the material surface and outward to potential targets. [Fire Protection Handbook- Section 2, Chapter 2- 20th Edition].
According to the definition of Stable pile in NFPA13 (Those arrays where collapse, spillage of content, or leaning of stacks across flue spaces is not likely to occur soon after initial fire development),

the stable pile creates more challenging fires because the loads stay longer in vertical situation.

Although finding “Soon after” definition is not an easy task. For this condition, designing according to the Stable pile is more conservative.

Exposed or Cartoned? Exposed
When Group A Plastics are stored in a carton, water from sprinklers will be absorbed by the carton and retard burning hazard. Wetting surrounding commodities by discharging water of sprinklers can limit the fire size and delayed fire spread.

Example: Compare the minimum flow rate of Solid Piled storage, Expanded Group A Plastics, Storage height:15 ft., Ceiling height: 20 ft., in two conditions:

A) Exposed, Stable
B) Cartoned, Unstable

Answer of A) For Expanded, Exposed, stable Group A plastics:
Minimum Required Flow rate for Sprinklers: 0.6 gpm/ft.² x 2500 ft.²= 1500 gpm
Total Require Flow rate (Sprinklers and Hose):1500+500= 2000 gpm

Answer of B) For Expanded, Cartoned and Unstable Group A plastics:
Minimum Required Flow rate for Sprinklers: 0.4 gpm/ft.² x 2500 ft.²= 1000 gpm
Total Require Flow rate (Sprinklers and Hose):1000+500= 1500 gpm

For commodities that are Encapsulated (Plastic sheet completely covers the sides and top of pallet load),

Plastic sheets behave as a shield and doesn’t let the water reach to the load,

so fire control is harder in comparison with Non-encapsulated commodities.

Although we saw in this text the differences between Expanded or Unexpanded, Stable or Unstable, Exposed or Cartoned, Encapsulated or Non-encapsulated,

but we should remember that in some parts of NFPA13, design requirements of two terms may be the same.

For example, in accordance with 17.2.1.2.1(NFPA13, 2016 Edition),

there is no difference between Encapsulated or Non-encapsulated requirements where they are in cartons.

To see more content, please click here nsvsoft/blog