Significant New Alternatives Policy Program

Fire Extinguishing and Explosion Prevention Sector

Risk Screen on Substitutes for Halon 1301

Total Flooding Systems in Normally Occupied or Unoccupied Spaces

Substitute: N2 Towers Inert Gas Generator Fire Suppression System

This risk screen does not contain Clean Air Act (CAA) Confidential
Business Information (CBI) and, therefore, may be disclosed to the
public.



1. 	Introduction

Ozone-depleting substances (ODS) are being phased out of production in
response to a series of diplomatic and legislative efforts that have
taken place over the past decade, including the Montreal Protocol and
the Clean Air Act Amendments of 1990 (CAAA).  The U.S. Environmental
Protection Agency (EPA), as authorized by Section 612 of the CAAA, runs
the Significant New Alternatives Policy (SNAP) Program, which identifies
acceptable and unacceptable substitutes for ODS in specific end-uses
based on assessment of their health and environmental impacts.

EPA’s decision on the acceptability of a substitute is based largely
on the findings of a screening assessment of potential human health and
environmental risks posed by the substitute in specific applications. 
EPA has already screened a large number of substitutes in many end–use
applications within all of the major ODS-using sectors, including
refrigeration and air conditioning, solvent cleaning, foam blowing,
aerosols, fire suppression, adhesives, coatings and inks, and
sterilization.  The results of these risk screens are presented in a
series of Background Documents that are available in EPA's docket.

The purpose of this risk screen is to supplement EPA’s Background
Document on the fire suppression and explosion protection sector (EPA
1994) (hereinafter referred to as the Background Document). This risk
screen discusses potential human health and environmental risks posed by
the N2 Towers Inert Gas Generator Fire Suppression System (N2 Towers
System) when used as a total flooding agent in normally occupied or
unoccupied spaces as a replacement for Halon 1301.

The N2 Towers System is an inert gas system designed for total flooding
applications for fires in normally occupied or unoccupied spaces.  The
nitrogen gas generating propellant grains are formed into purple-colored
discs, each with a diameter of 39 mm and a height of 15 mm.  Each disc
contains 19 perforations, with an inside diameter of 2.5 mm. Each
ten-inch N2 generator unit contains 96 propellant grain discs in a total
of 14 layers: seven discs per layer for the 12 middle layers and six
discs per layer for the top and bottom layers. Depending on the fire
suppression requirement, several generators may be stacked inside an N2
Tower in a room, or a single generator may be bracketed inside an
armored vehicle.

Upon activation, the substitute undergoes the following chemical
reaction: 

→ 2Na2O.FeO + 6N2

The nitrogen is discharged into the space and the remainder of the
activation products are filtered and contained inside the N2 generator,
where moisture is absorbed from the ambient air, neutralizing the sodium
hydroxide until all that is left inside the generator is a burnt
propellant sinter. The sinter is contained within the filter inside the
sealed N2 generator until the entire unit is disposed of at an
appropriate disposal facility.  The largest N2 generator, 24” long by
7” in diameter, can discharge up to 6.9 pounds of nitrogen gas in
approximately 100 milliseconds. The submitter will use this size for the
existing stockpile of grains. Each 24” long by 6” diameter generator
can protect up to 400 cubic feet of volume, and multiple generators may
be combined to protect a single space (N2 Towers Inc., 2009, 2010a,
2010b).

Table 1 provides the composition of propellant grains contained in N2
Tower Systems, prior to activation.

Table 1.  Composition of N2 Towers

Component	Chemical Formula	CAS No.	Weight Percent

Sodium Azide (s)	NaN3	26628-22-8	50-70

Ferric Oxide	Fe2O3	1309-37-1	20-35

Graphite	C	7782-42-5	2-7

Bentonite	Al2O3.4(SiO2).H2O	1302-78-9	0-5

Sodium Nitrate	NaNO3	7631-9-4	0-5

Potassium Perchlorate	KClO4	7778-74-7	0-3

Amorphous Silica	SiO2	112926-00-8	0-2

Source: N2 Towers Inc. (2009)

Section 2 of this risk screen summarizes the results of the risk screen
for the proposed substitutes listed in Table 1.  The remainder of the
risk screen is organized into the following sections:

Section 3: Atmospheric Assessment; 

Section 4: Occupational Exposure Assessment; 

Section 5: End-Use Exposure Assessment; 

Section 6: General Population Exposure Assessment; 

Section 7: Volatile Organic Compound Assessment; and

Section 8: References.

2.	Summary of Results

The N2 Towers System is recommended for SNAP approval as a total
flooding device in normally occupied or unoccupied spaces for fire
suppression.  EPA recommends that Section VIII of the Occupational
Safety & Health Administration (OSHA) Technical Manual be consulted for
information on selecting the appropriate types of Personal Protective
Equipment (PPE) recommended (OSHA 1999).  The risk screen indicates that
the use of the proposed substitute in normally occupied or unoccupied
spaces will be less harmful to the atmosphere than the continued use of
Halon 1301.  Additionally, the risk to the general population is
expected to be below levels of concern for non-cancer risks.  For
applications of this and all other water mist/inert gas fire suppression
equipment, EPA recommends that the latest editions of National Fire
Protection Agency (NFPA) Standards 750 and 2001 be followed.

3. 	Atmospheric Assessment

This section presents an assessment of the potential risks to
atmospheric integrity posed by the use of the N2 Towers System as a
total flooding system in normally occupied or unoccupied areas. The
constituents of N2 Towers Systems are solids before use and therefore
have zero ozone depleting potential (ODP) and global warming potential
(GWP) Further, as indicated in Table 2, the ODP of each of the
post-activation constituents of N2 Towers is zero and most of the
post-activation constituents also have zero GWP. The GWP and ODP of the
compounds in N2 Towers are well below the GWPs of previously
SNAP-approved fire-suppression agents (e.g. HFCs and other high-GWP
fluids).  Accordingly, use of the N2 Towers System is not expected to
pose any significant adverse atmospheric impacts.

Table 2.  ODP and 100-Year GWP of the N2 Towers System Constituents

Constituent	ODP	100-Year GWP

N2	0	0

CO	0	3a

CO2	0	1

aBased on indirect radiative effects, Fuglestvedt et al. (1996)

4.	Occupational Exposure Assessment



The potential for personnel exposure during manufacture, installation,
and maintenance are examined in this section.  According to information
provided in the N2 Towers SNAP application, all N2 Tower Systems will be
fabricated using an existing stockpile of TRW sodium azide propellant
grains. The stockpile of grains is stored in an airtight moisture proof
bag made of aluminum foil and are not opened until they are placed
inside the N2 generator. Thus, any exposure during manufacture will be
limited to propellant grain loading into the N2 generator. According to
the submitter, all loading of the grains will be performed using a down
draft loading table. Due to the inhalation, oral and dermal toxicity of
sodium azide, all employees involved in propellant loading should wear
appropriate PPE, such as protective clothing, gloves, goggles, and
respiratory protection, as described in the compound’s MSDS.  It is
expected that level B of the levels of protection presented in the OSHA
Technical Manual (OSHA 1999) will afford the appropriate protection. In
the case of an accidental spill, workers should immediately vacate the
premises and follow the first aid guidelines listed in the MSDS.  

Because of the minimal chance of worker exposure to the chemicals during
the manufacturing process, the use of these exposure controls and
adherence to the appropriate occupational safety guidelines as mentioned
above, are sufficient to ensure that the manufacture of N2 Towers
Systems does not pose a risk to human health. 

During installation or maintenance of an N2 Towers System, inadvertent
discharge of the substitute may occur.  According to the submitter,
during installation and maintenance, no exposures to the propellant are
expected to occur as the generator body is a sealed steel casing with
propellant grains contained therein.  However, in the event of
inadvertent discharge, oxygen concentration would be expected to decline
over time.  To ensure that exposures are minimized, it is recommended
that N2 Towers Systems provide safety training for technicians to
minimize the occurrence of an inadvertent discharge.  Training should
also be provided regarding egress from the space should such a discharge
occur.

5.	End-Use Exposure Assessment

The primary health concern for agents containing or releasing inert
gases, including nitrogen gas, is asphyxiation due to reduced oxygen
concentrations. To determine whether asphyxiation could occur at the
end-use, the N2 Towers System design parameters were analyzed.  N2
Towers Inc. conducted fire suppression tests to measure the oxygen,
carbon monoxide, carbon dioxide and particulates concentrations at the
time of flame suppression Table 3 presents the calculated design
concentrations based on the results of N2 Towers’ fire testing and a
conservative estimate provided by the submitter. To determine the
minimum design concentration for the N2 Towers System, a 30 percent
safety factor was applied to the extinguishing concentrations, in
accordance with NFPA 2001 Guidelines (NFPA, 2007).

Table 3.  Summary of N2 Towers Inc.’s Fire Tests and Calculated
Minimum Design Concentrations

Test	Extinguishing Concentrationa	

Design Concentration (% N2/% air (v/v))c,d 

	% O2 /% air (v/v)	% N2 /% air (v/v)c 	% CO/% air (v/v)b

	1	16.2 b	22.7	215.6	29.5

3	16.0 b	23.6	288	30.7

4	15.8 b	24.6	299	32.0

5	15.8 b	24.6	339	32.0

NAe	14.5e	30.8	NA	40.0

a Source: N2 Towers Inc. (2009 and 2010b).

b O2 and CO levels are experimentally measured.  

c N2 levels are calculated based on experimentally measured O2 levels. 
These values are exclusive of atmospheric nitrogen, i.e. represent the
concentration of nitrogen from the N2 Towers System only in the air.	

d Design concentrations were estimated by applying a 30 percent safety
factor to the extinguishing concentrations

e O2 level provided by submitter as a conservative estimate. 

According to the submitter, the N2 Towers System creates wind vorticity
and lowers the oxygen levels of a space down to approximately 14.5 %
oxygen per volume. Based on the conservative estimate of oxygen levels,
the nitrogen extinguishing concentration was calculated to be 30.8
percent and the design concentration to be 40.0 percent. 

Because N2 Towers Systems are designed to ensure that the oxygen
concentration in any protected space will not fall below 12 percent over
the discharge period, N2 Towers is considered safe for fires in normally
occupied or normally unoccupied spaces for inert gases.  

 

6. 	General Population Exposure Assessment

Given the large natural quantities of nitrogen and water in the
atmosphere, it is unlikely that an accidental release of N2 Towers will
affect the average atmospheric concentrations of these gases.  As such,
N2 Towers is not believed to pose a toxicity threat to the general
population.  

7.        Volatile Organic Compound Assessment

The constituents of the N2 Towers are not considered VOCs for purposes
of local air quality.    

8.	References

EPA.  1994. Risk Screen on the Use of Substitutes for Class I
Ozone-Depleting Substances: Fire Suppression and Explosion Protection
(Halon Substitutes).  Stratospheric Protection Division.  March 1994. 

Fuglestvedt, J.S., I.S.A. Isaksen and W.-C. Wang, 1996: Estimates of
indirect global warming potentials for CH4, CO, and NOx. Clim. Change,
34, 405-437.

N2 Towers, Inc. 2009.  SNAP Submission to EPA for N2 Towers.  December
22, 2009.	

N2 Towers, Inc. 2010a.   HYPERLINK "http://n2towers.com/" 
http://n2towers.com/  Accessed March 22, 2010.

N2 Towers, Inc. 2010b. Response to Incomplete SNAP Submission for N2
Towers. March 10, 2010.

NFPA, 2007. NFPA 2001: Standard on Clean Agent Fire Extinguishing
Systems.  2008 Edition.  Available at:   HYPERLINK
"http://www.nfpa.org/freecodes/free_access_document.asp" 
http://www.nfpa.org/freecodes/free_access_document.asp 

OSHA.  1999.  OSHA Technical Manual.  Department of Labor.  Occupational
Safety & Health Administration.  January 20, 1999.  Available online at:
  HYPERLINK "http://www.osha.gov/dts/osta/otm/otm_toc.html" 
http://www.osha.gov/dts/osta/otm/otm_toc.html   

 In evaluating the occupation exposures during manufacture, this risk
screen is limited to the existing stockpile of propellant grains that
have already been manufactured. According to the submitter, the quantity
of propellant grains in the existing stockpile can be used to
manufacture approximately 20,000 N2 generators. Any future manufacture
of N2 Towers Systems which requires production/manufacture of propellant
grains will require additional EPA screening for approval under the SNAP
program. 



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enerators by UTeC technicians. Once loaded, end caps are sealed and
screwed into place on the N2 generators. After sealing, the generator is
safe for storage and shipping (N2 Towers, Inc. 2010b)

September 20, 2011

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