“Ideas nearly always seem brilliant when they’re hatched, so we never act on a new idea for at least twenty-four hours.”  — Steven D. Levitt

Batch processes often rely on additions through open hatches for ingredients that don’t lend themselves to being pumped in through a pipe. These include liquid ingredients used in small quantities and solid ingredients, usually powders or granular ingredients, in any quantity.

It’s an understandable choice, but there are hazards associated with hatch additions. Process operators should be aware of those hazards, which depend on the material being added through the hatch. They also depend on what’s already in the vessel.

Hazards that Depend on the Contents of the Vessel

Whenever a volume of something—anything—goes into a vessel that is open to the atmosphere, an equal volume must come out of the vessel. Most likely, the volume displaced from the vessel will be from the headspace in the vessel. Even more likely, the displaced volume will come out through the open hatch. Right where an operator is standing, making the addition.

What will be in that displaced headspace volume? If the vessel contains a liquid, it will be the vapor of the liquid, at whatever concentration its vapor-liquid equilibrium dictates. A toxic liquid will result in toxic vapors in the headspace. A flammable liquid will result in flammable vapors in the headspace.

Because most toxic vapors have permissible exposure limits in the parts-per-million range, the vapor pressure of a toxic liquid doesn’t have to be very high to pose a hazard.

Flammable liquids are a different story. When a flammable liquid is in the vessel at a temperature below its flash point, it won’t generate a vapor pressure high enough to go above its lower flammable limit (LFL) and form a flammable mixture with air.

When a flammable liquid is hot enough to create a mixture above its upper flammable limit (UFL) in the headspace, the headspace gases cannot ignite. Once the mixture leaves the hatch, however, air will dilute it. So, somewhere between the hatch and the open atmosphere, the mixture is going to drop into the flammable range. An operator making additions through a hatch from which a flammable mixture is exiting is vulnerable to being engulfed in a flash fire.

What About Inerting the Headspace?

Inerting the headspace—replacing the air in the headspace with an inert gas like nitrogen—dramatically reduces the potential for ignition of the mixture in the headspace. However, there is still the problem of air diluting the headspace gas as it passes out of the hatch and into the atmosphere. It is possible for air to dilute a mixture outside of the flammable range and form a flammable mixture. Inerting the headspace makes the vessel safer, but a good understanding of the flammability curve is important before assuming that an unignitable atmosphere will remain unignitable when mixed with air.

There is also the problem of the inert gas itself. While inert gases are non-toxic—that is the very nature of being inert—they aren’t air. When the hatch is open, the inerting gas will dump into the vessel in an effort to maintain the slight positive pressure it’s set for. That gas will flow out the hatch, developing an oxygen-deficient atmosphere where the operator is working. OSHA considers any atmosphere at less than 19.5% oxygen hazardous. Again, it is important to understand this hazard when designing the inerting system.

Hazards of Liquids Addition

A concern associated with liquids addition into a potentially flammable gas mixture is static electricity. Non-polar liquids—hence, most flammable liquids—can carry a static charge. When allowed to freefall through the headspace, it is possible for the static charge to jump from falling liquid to wall of the vessel. This spark then becomes an ignition source for any flammable mixture in the headspace of the vessel.

To reduce the likelihood of a static discharge in the vessel headspace, large volume additions are typically subsurface, through a dip pipe. Using a dip pipe for small liquid additions through an open hatch is generally not practical. There is still a static discharge problem, however. To address static, it is important that the operator and the container of liquid be grounded and bonded to the vessel, bringing them to same electrical potential.

Hazards of Solids Additions

Solid materials, whether powder, granules, or pellets, contain fines. These small particles easily disperse as a dust cloud. Respirable dust poses a health burden on anyone exposed. Silica dust can lead to silicosis, but any respirable dust is a hazard.  When an operator adds a solid through a hatch, the displaced headspace vapor will sweep the fines out with it, exposing the operator to respirable dust.

When the dust is a combustible dust, the dust cloud becomes a greater hazard. Not only is there the exposure to respirable dust, but there is the potential for a dust cloud fire—a flash fire—or when confined, a dust cloud explosion. The operator is likely to be standing in the midst of it.


Finally, no discussion of manual hatch additions would be complete without a discussion of ergonomics. A common implement for hatch additions is a five-gallon pail. A five-gallon pail will typically weigh around 40 pounds, which is heavy lift of something that is also awkward to manage. This can lead to strains and musculoskeletal injuries. And because the container is awkward, pouring it into a hatch can also lead to spills, which have their own environmental and personnel exposure issues. It is important to ensure that every hatch addition is a manageable amount, and that buckets are not full to the top, but left with adequate freeboard.

Bigger Than a Stove-Top Cook Pot

Although hatch additions resemble adding ingredients to a stove-top cook pot or a witch’s cauldron, they’re even bigger and have bigger hazards. However, there are also measures available to address each hazard associated with hatch additions. These include proper PPE, e.g., respirators and flame-retardant clothing, and may also include bonding and grounding, vapor exhaust and dust collection systems, and appropriately sized transfer containers.

A Second Thought

In some facilities, operators make hatch additions without a second thought. Hatch additions may be hazardous, however, and deserve a second thought, maybe a third. If your facility uses hatch additions as part of any of its processes, make sure you have considered the hazards and addressed them.


  • Mike Schmidt

    With a career in the CPI that began in 1977 with Union Carbide, Mike was profoundly impacted by the 1984 tragedy in Bhopal and has been working on process safety ever since.