National
Institute for Occupational Safety and Health
NIOSH
ALERT: July 1986
DHHS (NIOSH) Publication No. 86-118
Fire departments responding to incidents involving oxygen-limiting silos are cautioned that directing water or foam onto the fire through the top openings of an oxygen-limiting silo may result in the silo exploding. |
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This Alert
requests the assistance of fire department personnel, farm owners
and workers, and silo manufacturers in the prevention of fatalities
due to fires and explosions occurring in oxygen-limiting silos.
Several
recent incidents occurred while fighting oxygen-limiting silo
fires which resulted in the death of fire fighters. Other
fire fighters lost their lives as a result of similar explosions
in the late 1960s. The problems associated with burning silos
appeared to have abated during recent years, but these incidents
demonstrate the need to renew efforts to minimize their recurrence.
A concerted effort should be made to prevent silo fires from
occurring and to provide training programs on controlling
this type of fire.
Oxygen-limiting
silos by design have all their openings sealed to prevent
oxygen from entering the silo. Generally, these silos are
of steel or concrete construction of varying heights and diameters.
The openings (bottom and top) are normally sealed with rubber-gasketed
hatches. When these hatches are tightly closed and the silo
is filled, the oxygen concentration should be insufficient
to support a fire. If the hatches are left open or the oxygen-limiting
features are not properly maintained, spontaneous heating
can occur with subsequent ignition of the silage [1].
If improperly
sealed or otherwise not operating as designed, the amount
of oxygen entering the silo may be sufficient to allow a fire
to smolder, causing an accumulation of combustible gases due
to incomplete combustion. Any additional increase in oxygen
content in such an environment can create an explosive atmosphere.
Thus, merely opening the top hatches of such silos, or applying
water or foam by hose stream from the top of the silo, could
allow sufficient oxygen to enter the silo and create an explosive
atmosphere [1-4]. Dust explosions may also occur if dust inside
the silo becomes suspended as a result of the hose stream,
and is ignited by the heat of the smoldering fire [3,5].
The
following case report resulted from a NIOSH investigation
of the circumstances of the incident as part of the NIOSH
Fatal Accident Circumstances and Epidemiology Program.
On August
27, 1985, three fire fighters were killed when a burning oxygen-limiting
silo exploded. The fire fighters were spraying water onto
the fire from the top of the silo at the time of the explosion.
The explosion lifted the concrete roof of the silo approximately
four feet in the air and the fire fighters were thrown from
the silo.
This
explosion was due either to a build up of combustible gases
from incomplete combustion or a dust explosion, or a combination
of the two. Regardless of the ultimate cause of the explosion,
directing water into the top of the silo appears to have been
an improper method for fighting this silo fire.
In this
incident nothing should have been done to increase the level
of oxygen inside the silo. Opening the top hatches to apply
water to the fire could have increased the level of oxygen
and created an explosive atmosphere. Air entrained in the
water stream may have also contributed. Additionally, the
water spray could have suspended the dust and increased the
risk of explosion.
NIOSH
is aware of three other explosions that occurred in oxygen-limiting
silos at about the same time as the incident described in
the case report. Two of the incidences occurred in the same
geographical area as the incident described above. No fire
fighters were applying water to these silos at the time, and
there were no injuries. The third fire which occurred in another
geographical area resulted in the fatal injury of one fire
fighter [4].
There
are no specific OSHA regulations covering fire hazards of
oxygen-limiting silos. Also, since most farms employ less
than ten workers, other general OSHA regulations that might
apply are not used. Therefore, OSHA estimates that over 90%
of all farms in the U.S. are not covered by OSHA regulations.
A. Basis
for Needed Actions The
following collected in this case study suggests that the following
factors may have contributed to the fatal accident as reported:
- Improper
fire fighting methods; and
- Lack
of proper operating and maintenance procedures on the silo.
B. Recommended
Measures
Acknowledging
concern for the above factors, NIOSH recommends the following
steps for both the prevention of fires and explosions in oxygen-limiting
silos, and for fire control procedures once a fire has developed:
- Prevention
- When
not being filled or emptied, oxygen-limiting silo hatches
should be kept closed. If an oxygen-limiting silo is
properly sealed, there is very little likelihood of
a fire occurring by spontaneous heating, since the amount
of oxygen trapped in the silo is usually insufficient
to support a fire.
- Proper
maintenance of the silo should be performed to ensure
the integrity of the oxygen-limiting features. The manufacturer
of the silo should be contacted for proper operating
and maintenance procedures for the silo.
- The
moisture content of stored silage should be controlled,
as should the type of cut of the silage. Filling rates
recommended by the manufacturer should also be followed
to reduce the possibility of spontaneous heating of
stored silage. "Elements of good silage" can
be obtained from the bulletin, "Extinguishing Silo
Fires," NRAES-18, published by the Northeast Regional
Agricultural Engineering Service, Cornell University,
Riley Robb Hall, Ithaca, New York 14853.
- Fire
Control
- During
fire fighting operations on oxygen-limiting silos, water
or foam should not be directed onto the fire through
the top hatches, since this may allow oxygen to enter
the silo and cause the suspension of explosive dust.
- Placards
should be placed on the oxygen-limiting silos warning
fire fighters that the silo is in fact an oxygen-limiting
silo, and should include information concerning the
proper extinguishing techniques.
- If
the roof hatches of oxygen-limiting silos are open,
no attempt should be made to close them if there is
smoke or steam coming from the open hatches or if the
silo is vibrating.
- The
roof hatches should be safe to close if the silo is
quiet and there has been no smoke or steam coming from
the hatches for several hours. Do not secure the hatch.
This will permit the relief of any subsequent pressure
that may build up.
- Large
quantities(*) of carbon dioxide
or liquid nitrogen should be injected into the silo
to extinguish the fire. Some silos have valves specifically
designed for this. If it is necessary to drill a small
hole in the side of the silo for insertion of the gas
tube, care should be taken not to allow additional oxygen
to be pulled into the silo. All precautions normally
associated with either nitrogen or carbon dioxide should
be taken when handling these gases.
- Manufacturers,
in conjunction with local fire departments, should establish
a program to provide valves designed for injection of
gases for fire control on all new and existing oxygen-limiting
silos.
- Certain
manufacturers have step-by-step instructions on how
to extinguish fires in their silos. Therefore, farm
owners are encouraged to contact the silo manufacturer
to obtain these instructions.
NIOSH
has published the following documents which contain further
information.
NIOSH
Alert: Request for Assistance in Preventing Hazards in the
Use of Water Spray (Fog) Streams to Prevent or Control Ignition
of Flammable Atmospheres, DHHS (NIOSH) Publication No.
85-112.
Occupational
Safety in Grain Elevators and Feed Mills, DHHS (NIOSH)
Publication No. 83-126.
NIOSH
requests that the technical information and warning contained
in this Alert be disseminated to personnel of fire departments,
fire training academies, other emergency response organizations,
farm extension associations, farm workers and owners, and
manufacturers of silos.
Requests
for additional information or questions related to this announcement
should be directed to Mr. John Moran, Director, Division of
Safety Research, National Institute for Occupational Safety
and Health, 944 Chestnut Ridge Road, Morgantown, West Virginia
26505, Telephone (304) 291-4595.
We greatly
appreciate your assistance.
J. Donald
Millar, M.D., D.T.P.H. (Lond.)
Assistant Surgeon General
Director, National Institute for Occupational Safety and Health
Centers for Disease Control
* As
an example, for a 20-foot diameter by 60-foot-high silo, the
estimated amount of carbon dioxide or liquid nitrogen would
be: 20 standard cylinders of carbon dioxide or 40 standard
cylinders of liquid nitrogen. Reference #1 provides estimated
amounts of CO2 or liquid nitrogen for other silo
sizes.
- Murphy,
DJ, Arble WC: Extinguishing Silo Fires. NRAES-18.
Ithaca, NY: Northeast Regional Agricultural Engineering
Service (1982).
- NIOSH
Alert: Request for Assistance in Preventing Hazards in the
Use of Water Spray (Fog) Streams to Prevent or Control Ignition
of Flammable Atmospheres. DHHS (NIOSH) Publication No.
85-112. National Institute for Occupational Safety and Health,
4 pages (1985).
- Bahme
CW: Fire Officer's Guide to Emergency Action. NFPA
No. FSP-38. Boston, MA: National Fire Protection Association,
185 pp. (1974).
- Upgrade
Training Programs Aimed at Controlling Silo Explosions,
Say National Volunteers. Fire Control Digest, 12(2):6
(1986).
- Occupational
Safety in Grain Elevators and Feed Mills, DHHS (NIOSH)
Publication No. 83-126. National Institute for Occupational
Safety and Health, 85 pp. (1983).
Disclaimer
and Reproduction Information: Information in NASD does not represent
NIOSH policy. Information included in NASD appears by permission
of the author and/or copyright holder. More
NASD Review: 04/2002
This NIOSH
Update is DHHS(NIOSH) Publication number
86-118
.
This page was last updated: 2/11/97
For
more information about these or other occupational safety
and health concerns, call toll free: 1-800-35-NIOSH
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