Ventilation of Confined Spaces
Confined Space Ventilation
 Confined spaces are not normally designed
for convenient ventilation
 Must take steps to:
• ensure air is breathable before entering
confined space
• maintain acceptable air quality in the confined
space during entry
Hotwork in Confined Spaces
 Presents additional ventilation challenges
in confined spaces
 Includes torch cutting, welding, brazing
and soldering, arch gouging
Hotwork in Confined Spaces
 Remember… confined spaces concentrate
hazards!
 Hotwork can create atmospheric hazards in
confined spaces from fumes, gasses and
vapors
 Effective ventilation sometimes may only
be accomplished by mechanical
ventilation
Natural Ventilation in
Confined Spaces
 “Chimney Effect”
• convection process created by temperature
changes
 Occurs by sunlight heating vessel walls
and air within
 If there is an opening in bottom and top of
vessel, upward draft created
Natural Ventilation Problems
 Confined spaces rarely experience uniform
thermal conditions
 Various sources of heat in confined spaces
• human bodies
• lighting
• hotwork processes
Natural Ventilation Problems
 Factors such as sunlight, body heat,
lighting and hotwork are usually not
sufficient to move enough air to provide an
acceptable atmosphere
Overcoming Natural Ventilation
Problems
 Must establish techniques and procedures
to provide adequate ventilation
 It is easier to work with than against
natural convection.
Use of Mechanical Ventilation
 Properly installed, can reduce or eliminate
respiratory protection requirements
 Effective engineering controls usually less
dependent on worker attention than
respiratory protection
29 CFR 1910.134
When feasible, effective engineering
controls must be used rather than
respirators
When is ventilation necessary?
 If atmosphere:
• contains insufficient oxygen or is oxygen rich
• contains flammable dusts or vapors
• contains hazardous or toxic vapors, mists,
fumes, gases, or fibers
When is ventilation necessary?
 If atmosphere:
• is subject to activities that may generate
hazardous mists, vapors, fumes or gases, or
may create either an oxygen deficiency or
oxygen excess, and
• increases heat stress on workers to
unacceptable levels
CAUTIONS
 Many people resist wearing PPE, including
respiratory protection
 Tangle of supplied air hoses in vessel with
many welders can present a hazard
Ventilation- 29 CFR 1910.252(c)
 Spaces that require ventilation
• work space less than 16 feet high
• volume less than 10,000 cubic feet per welder
• work areas with partitions, structural barriers,
or other barriers that significantly obstruct
airflow
Ventilation- 29 CFR 1910.252
 Ventilation options:
• provide at least 2000 cfm of airflow for each
active welder; or
• provide each welder with a local exhaust
device
– local exhaust devices must be capable of
maintaining a velocity of 100 fpm toward the air
intake
Ventilation Requirements
 29 CFR 1910.252 and 29 CFR 1926.353
require use of local exhaust ventilation or
supplied air respiratory protection when
performing hotwork using certain
substances
Ventilation Requirements
 Fluorine compounds
 Zinc
 Lead
 Cadmium
 Mercury
 Beryllium (local exhaust and supplied air
respirators are required)
Ventilation Requirements
 Construction :
• chromium
• stainless steels (if using MIG processes)
Air Moving Devices
 Two types:
• Fans
• Venturi-type eductors
 Can be air, steam or electrically driven
Eductors
 Sometimes called “air horns”
 Air powered and rely on venturi effects to
move air
Criteria for Rating Air Movers
 Free Air Delivery
 Effective Blower Capacity
 Quantity of air and air pressure required
(air-driven devices)
 Electrical power requirements (electrically
driven devices)
Factors Reducing Performance
 Equipment components in confined space
 Maintenance/construction materials
erected in space
 Obstructions in make-up air manway
 Insufficient number of make-up air
manways
Factors Reducing Performance
 More restrictions to overcome = less air
moved
 Air moving into space equals amount of air
leaving the space
Air Driven DevicesPoor Performance
 Reduced pressure and volume to air driven
devices from multiple users
 Excessively long air hoses
Increasing Performance
 Supplemental air compressors dedicated
to air mover use (air driven devices)
 Supplemental air compressors connected
directly to plant air system
 Eliminate “short-circuiting” of airflow
Designing Ventilation Systems
 Configuration, contents and tasks
determine type of ventilation
• opening configuration
• properties of expected atmospheric hazards
• type of work being performed
Electrically Driven Centrifugal Fans
 Designed to overcome higher static
pressures
 Usually heavier than air-driven equipment
 Can be used remotely to reduce noise
 Due to power, can suck up debris
Local Exhaust
 Single manway vessels
 Interior obstructions that create “dead
spots”
 Lack of feasible way to attach air-moving
device
 Work with toxic metals
Local Exhaust
 Effective only when it captures and
removes welding fumes and gasses at the
source as they are emitted
 100 fpm capture velocity at the source of
fumes or gasses
Local Exhaust- Considerations
 Long runs reduce airflow
 Airflow loss minimized by use of smooth
ducting with large radius bends
 Run flexible ducting as straight as possible
 Consider using plenums for multiple
welders
 Field test flow/personal air monitors
Make-up air quality
 Mechanical ventilation uses surrounding
air
 Make sure the make-up air is not a source
of airborne contaminants