One of the greatest risks workers face when working in a permit-required or alternate-entry confined space job is a hazardous atmosphere. If the space has a hazardous atmosphere, or there is a potential for one to develop, ventilation after testing is always required.

Knowing proper setup and understanding how to ventilate a confined space correctly significantly reduces the risk of injuries and fatalities.

To protect the safety of your workers, follow these steps.

1. Test, ventilate and retest before initial entry

OSHA and Canadian provincial authorities require the testing of the air in a confined space before anyone enters. By testing first, a baseline is set for what is known to be in the space, and provides a clue as to what might return to the space. Ventilation prior to testing may cause the loss of information about a potential returning atmospheric hazard.

Test at the spacing intervals recommended by the manufacturer of the gas detector. This may require vertical and horizontal testing, depending on the configuration of the space since different gases may be present at different levels. The duration of the initial ventilation, or purge, of the space will depend on the results of the original test. Retesting of the atmosphere is always required after the original purge when a toxic gas was encountered.

2. Use a nomograph to calculate purge time

The generally accepted industry standard for purging a confined space is seven changes of air. If a toxic gas has been detected, the purge time increases by 50%.

Ventilation equipment manufacturers can provide a graphical calculating chart called a nomograph to help you determine how long pre-entry purging should last. To calculate that time, you’ll need to know the volume of the confined space and the net cubic feet per minute (CFM) of the confined space ventilation blowers you will be using.

3. Maintain air quality with continuous ventilation

After the space has been properly purged, the space may be considered safe for entry. But don’t assume the atmosphere won’t change. Activities such as welding or cleaning with chemicals can create an atmospheric hazard in a hurry. Remember, it is not just what might have been in the space, but may include potential hazards that are brought in to the space by entrants.

The industry-accepted standard for confined space ventilation is 20 air changes per hour. The net CFM of your confined space ventilation fan must meet that minimum flow requirement. You may need multiple fans to ventilate effectively.

4. Draw air from an untainted source

OSHA and Canadian provincial authorities require that air used for ventilation come from a known fresh air source. The supplied ventilation must be continuously-fed forced fresh air. Choose this site carefully. An open spot in the middle of a field might look like a good choice, but if the area is also used for staging idling vehicles or using fuel-powered equipment that emits fumes, it may not be.

5. Set up ductwork correctly

Set up your ventilation ductwork or hoses to ensure fresh air is discharged in the area where crews are working. Ductwork should be set up to direct the ventilation to the area where the employees are, or will be, present.

Work in a manhole may mean that employees and the ventilation duct will have to share the entry point. To eliminate blocking the manhole entry with duct, consider using a Manhole Ventilation Passthru. The Passthru allows air to continue to be fed into the space while allowing entrants to exit without affecting air flow.

6. Add booster blowers as needed

Because of friction, the amount of air flowing through a duct decreases as the length of the ductwork increases. As a rule, an inline booster is needed to for every 3 additional 25’ duct added.

Always follow the manufacturer’s instruction as to the impact of friction loss due to long spans or duct turns, and adjust the size of the ventilator to compensate.

7. Take additional precautions for dangerous chemicals

Certain chemicals create a toxic atmosphere when combined. While planning your confined space ventilation procedures, consult the safety data sheets for the chemicals that may be used in the space so you can understand whether a potential atmospheric hazard might result.

In some cases, confined space ventilation may not be enough. You may also be required use local exhaust ventilation (LEV), or workers may need self-contained breathing apparatus (SCBA).

8. Monitor continuously

Work activities inside the confined space can quickly change the quality of the air inside it. Continuous gas monitoring is the only way to ensure your workers remain safe from any atmospheric hazard while inside the space.

9. In the event of interrupted ventilation

In the event of loss of power to mechanical ventilation equipment, ensure that all employees immediately evacuate the space until proper ventilation is restored. The space should then be retested to ensure acceptable entry conditions exist.

With project schedules tighter than ever, skipping steps when it comes to confined space ventilation is tempting. But taking chances with the air quality in a confined space — and ultimately the lives of your workers — is not worth the risk.

Confined space planning and execution is a large undertaking and you should always consult with an expert regarding the specific circumstances, applicable rules and regulations related to your site and to your situation.

For help determining your confined space ventilation needs or to rent ventilation equipment, call us at 833.482.3761 or visit your local United Rentals branch.