Ventilation is necessary in confined space entry to ensure safe breathing conditions for personnel working inside tanks, vessels, and other enclosed spaces. In this article, we will discuss what are the types of ventilation in confined space entry and share my experience with the ventilation selection on the shop floor.
What is Ventilation in a confined space entry permit?
Ventilation is the use of natural or mechanical means to maintain safe atmospheric conditions by supplying fresh air and removing hazardous contaminants, ensuring oxygen levels and gas concentrations remain within acceptable limits before and during entry, as required by OSHA.
Effective ventilation serves three critical purposes:
- Life Support: It maintains adequate oxygen levels.
- Contaminant Removal: It removes hazardous gases, fumes, or dust generated during work.
- Fire Prevention: It prevents the buildup of flammable atmospheres (keeping the air below the Lower Explosive Limit).
Without effective ventilation, even a cleaned and isolated confined space can quickly become unsafe.
Types of Ventilation Used in Confined Space Entry
Different confined space scenarios require specific airflow strategies to ensure worker safety. These are the primary ventilation methods used in the industry.
- Natural Ventilation
- Positive (Supply) Ventilation
- Negative (Exhaust) Ventilation
- Combined (Push–Pull) Ventilation
- Local Exhaust Ventilation (LEV)
Let’s start discussing one by one
1. Natural Ventilation
Natural ventilation relies on wind or open manholes to allow air movement into the confined space.
This type of ventilation may be acceptable only when:
- The space is open or shallow
- No toxic or flammable gases are present
- Atmospheric testing confirms safe conditions
Natural ventilation is generally not reliable and should not be used for permit-required confined space entry.
Example
A shallow, open-topped excavation pit or a newly constructed trench (less than 4 feet deep) in an open plant area where there is a strong cross-breeze and atmospheric testing confirms 20.9% oxygen with no contaminants.
2. Positive (Supply) Ventilation
In positive ventilation, fresh air is forced into the confined space using a blower and flexible ducting. This is the most commonly used ventilation method in industries.
Positive ventilation is used when:
- There is a risk of oxygen deficiency
- The confined space is deep or fully enclosed
- Natural airflow is not sufficient
Engineer’s Note – Fresh air must be taken from a clean location, and the duct should be extended close to the working area to avoid stagnant zones. I have seen cases where the blower accidentally sucked in a vent from a nearby pump, pumping chemical gases leaked from the pump seal (HCL) directly into the confined space.
Example
During maintenance work inside a deep process pit, there is a risk of oxygen deficiency due to limited natural airflow. To ensure safe breathing conditions, positive (supply) ventilation is provided. A portable air blower is placed outside the pit, and flexible ducting is extended near the working area. Fresh air is continuously forced into the confined space, improving oxygen levels and diluting any hazardous gases present. Atmospheric testing is carried out before entry and during the work to confirm that conditions remain safe.
3. Negative (Exhaust) Ventilation
Negative ventilation, also called exhaust or negative-pressure ventilation, works by extracting air from the confined space. This creates a slight negative pressure inside the space, which causes fresh “make-up” air to be drawn in through the entry openings.
Negative ventilation is preferred when:
- Toxic or flammable gases are heavier than air or concentrated in a specific area.
- Fumes are generated during specific work such as welding, organic solvent cleaning, or painting.
- There is a risk of pushing hazardous vapors out into the surrounding plant or work areas (negative pressure keeps the contaminants contained within the ducting).
Example
During internal cleaning of a solvent storage tank, residual solvent vapors are present even after draining and isolation. To remove these vapors, negative (exhaust) ventilation is provided. A portable exhaust blower is connected to the tank manway, and flexible ducting is routed inside the tank. The blower continuously extracts contaminated air from the tank, creating slight negative pressure inside the vessel. Fresh air enters naturally through the open manway, helping dilute and remove solvent vapors. Continuous gas monitoring is carried out to ensure safe oxygen levels and vapor concentration before and during entry.
Care must be taken to ensure the ‘make-up air’ drawn into the tank is clean and not coming from nearby generator exhausts or chemical vents. Also, ensure the airflow doesn’t ‘short-circuit’ by traveling straight from the inlet to the exhaust without circulating through the tank.”
4. Combined (Push–Pull) Ventilation
Push–pull ventilation uses both supply and exhaust fans to control airflow inside the confined space.
This type of ventilation is recommended for:
- Large tanks, vessels, and reactors
- High-risk confined space entry
- Long-duration maintenance work
It ensures better air circulation and effective removal of contaminants compared to single-mode ventilation.
Example
For decommissioning and internal cleaning of a Horton Sphere in LPG service, push–pull ventilation is used to ensure proper air circulation. Due to the large volume and curved geometry, air pockets can form along the walls. Fresh air is supplied through the top manway using a blower, while hydrocarbon vapors are removed from the bottom nozzle using an exhaust fan.
This arrangement prevents gas accumulation and dead zones inside the sphere and helps maintain safe atmospheric conditions during confined space entry.
5. Local Exhaust Ventilation (LEV)
Local exhaust ventilation captures hazardous fumes at the source of generation, such as welding fumes or solvent vapors. It is used as an additional control measure and does not replace general ventilation.
Example
During welding inside a small tank, fumes are generated continuously and can quickly make the air unsafe. To control this, local exhaust ventilation (LEV) is used. A portable exhaust unit with a flexible duct is placed near the welding area, with the duct opening positioned close to the welding point. This setup captures welding fumes at the source, preventing them from spreading inside the tank and reducing the welder’s exposure to hazardous fumes.
LEV controls fumes at the source, but the need for an airline BA set depends on oxygen levels, gas concentration, and permit conditions.
Important: LEV protects the worker from fumes, but general ventilation is still required to maintain overall oxygen levels and temperature control.
Note – Ventilation should not be stopped once entry has started and must continue throughout the job.
Ventilation Details to Be Mentioned in the Entry Permit
The confined space entry permit should clearly mention:
- Type of ventilation provided
- Blower or exhaust fan capacity
- Air inlet and outlet points
- Duration of ventilation (pre-entry and continuous)
- Gas testing frequency
Providing proper ventilation, along with gas testing and work permit compliance, greatly reduces the risk associated with confined space entry.
FAQ
1. Why is ventilation required in confined space entry?
Ventilation is required to maintain safe oxygen levels and to remove hazardous gases, vapors, fumes, or dust from confined spaces. It helps prevent oxygen deficiency, toxic exposure, and the buildup of flammable atmospheres.
2. What is positive ventilation in a confined space?
Positive ventilation supplies fresh air into a confined space using a blower and ducting to improve oxygen levels and dilute hazardous gases.
3. Does ventilation eliminate the need for gas testing?
No, ventilation does not replace gas testing.
4. Is breathing air required when ventilation is provided?
Breathing air is required when ventilation cannot maintain safe oxygen levels or control toxic gases, based on gas test results and permit conditions.
Wrapping Up
This was a detailed article on what is ventilation in confined space entry, its importance and types. If you have any doubt regarding this then feel free to use the comment section.
