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Craftworktools.com Tips
Confined Space Ventilation
Among the many safety precautions that must be observed prior to entering a confined workspace, such as
a manhole vault, is the monitoring, purging and ventilation of the space. Never enter a manhole, even
momentarily, until it has been tested for combustible and noxious gases and then purged with the blower
operating at a maximum rated speed for a length of time sufficient to make seven (7) complete changes of
air in the space. See the example below.
(workspace volume ÷ blower capacity) × 7 = minutes of purge time
Consider this typical example:
vault dimensions = 10' x 8' x 6' high = 480 cubic ft.
blower capacity = 640 cubic ft. / min. with 1- 90° bend
480 ÷ 640 = .75
.75 × 7 = 5.25 minutes of purge time
5 minutes is considered to be the minimum purge time.
Once the workspace has been tested with a certified gas indicator confirming a satisfactory atmosphere
and the space has been effectively purged of hazardous gases, only then can the workspace be entered
while gas monitoring and fresh air ventilation continue for the duration of the time the space is occupied.
The fresh air ventilator (blower) must be adequately sized to achieve twenty (20) complete air exchanges
per hour. A rule of thumb calculation for ventilator sizing is:
workspace volume ÷ 3 = ventilator CFM with 2 - 90° bends. For example, a manhole vault of 480 cubic ft. ÷
3 = ventilator CFM of 160 with 2 - 90° bends.
Important Considerations:
Craftworktools.com has products for confined space safety available here.
Always remember the safety creed that states, "No job is so important and no service is so urgent that we
cannot take time to perform our work safely."
Among the many safety precautions that must be observed prior to entering a confined workspace, such as
a manhole vault, is the monitoring, purging and ventilation of the space. Never enter a manhole, even
momentarily, until it has been tested for combustible and noxious gases and then purged with the blower
operating at a maximum rated speed for a length of time sufficient to make seven (7) complete changes of
air in the space. See the example below.
(workspace volume ÷ blower capacity) × 7 = minutes of purge time
Consider this typical example:
vault dimensions = 10' x 8' x 6' high = 480 cubic ft.
blower capacity = 640 cubic ft. / min. with 1- 90° bend
480 ÷ 640 = .75
.75 × 7 = 5.25 minutes of purge time
5 minutes is considered to be the minimum purge time.
Once the workspace has been tested with a certified gas indicator confirming a satisfactory atmosphere
and the space has been effectively purged of hazardous gases, only then can the workspace be entered
while gas monitoring and fresh air ventilation continue for the duration of the time the space is occupied.
The fresh air ventilator (blower) must be adequately sized to achieve twenty (20) complete air exchanges
per hour. A rule of thumb calculation for ventilator sizing is:
workspace volume ÷ 3 = ventilator CFM with 2 - 90° bends. For example, a manhole vault of 480 cubic ft. ÷
3 = ventilator CFM of 160 with 2 - 90° bends.
Important Considerations:
- Effective ventilator (blower) capacity, as stated by the manufacturer must have been determined by
precision airflow chamber test performed by an independent testing lab. - The ventilator inlet must be positioned upwind.
- Ensure that the blower hose is not damaged and is securely connected to the ventilator outlet.
- Position the blower hose with one bend for purging and two bends for ventilating as shown in the
illustration below.
Craftworktools.com has products for confined space safety available here.
Always remember the safety creed that states, "No job is so important and no service is so urgent that we
cannot take time to perform our work safely."
