Gas Detection Inside Laboratory Hoods: Prudent Safety Measure or Wasted Money?

Clients often ask me if they should provide continuous gas detection in their laboratory hoods. I always start by asking them why they believe it is necessary and usually get some version of the following. (These are numbered for reference not any priority.)

1.      To warn them they have a leak or similar loss of containment

2.      To warn them their scrubbing, neutralization, or treatment systems are not working properly

3.      To allow them to lower the area electrical classification inside the hood to unclassified (AKA general purpose or ordinary)

4.      To warn them “something” happened inside and alert the operators that they may be at risk

The issue with any gas detection in a hood is dilution. Assume a fairly typical 4 ft long benchtop hood with a vertical sash designed for a maximum 24” opening and an average hood face velocity of 100 ft/min. This hood would require at least 800 CFM. That means you are going to need a lot of leakage to get to a detectable level. Look at these examples:

For a toxic gas with a very low TLV, PEL, or STEL that size leak may be realistic. For a flammable gas it is much more problematic that you could get that large a leak.  Hence it is no clear that you will detect even a large leak with a gas detection system inside the hood unless it is a toxic material and one that has a sensor that can alarm at low levels.

Trying to place the sensor in a location away from the plenum so that the dilution is less is difficult. You cannot be sure how much exhaust is flowing over the sensor so determining the dilution may be difficult. Worse, it is very possible that a leak a short distance away may be exhausted and never be detected. So you almost always have to realistically place the sensor in the hood exhaust duct which means you need a leak as shown above. Trying to set the sensor limit lower is often impractical. For the above example you would need to set the flammable limits at 1/800th their value to get them to trip at an equivalent of 1 CFM.

How about setting the alarm and warning levels lower based on the dilution? For very toxic gases this may be feasible; for flammables it is rare one can set the sensor to reliably trip at such low levels. Just as importantly, routine hood operations often produce brief leaks during filling, emptying, transfers, maintenance and similar operations. Most are trivial and the reason for placing the equipment in the hood. However these often lead to numerous false trips which create needless additional work.

Given this problem one has to ask, does providing gas detection inside a hood really add any significant safety to most installations? Let’s examine the cases above.

#4, alerting the operators that they are at risk, is the easiest to address. If a laboratory hood is working properly then any release should be captured inside the hood and not escape to pose any hazard to the operators. The hood alarm, required by NFPA-45 Fire Protection for Laboratories Using Chemicals section 7.7.7, should be adequate if installed properly and tested annually as required to alert the operating personnel if the hood has stopped working[i]. Hence, any release inside a hood should not escape and pose an issue to the operating personnel as long as they use the hood properly and keep their head outside at all times.

A more valid concern is a pressurized leak which can easily jet out from the hood and into the laboratory even when the hood is functioning perfectly. This is a realistic and not unknown issue. Gas detection inside the hood is unlikely to identify the problem reliably. A better approach is to install the gas detection outside the hood to alert the operating personnel that, for whatever reason, there is a potentially hazardous environment outside the hood and allow them to initiate their emergency plans.

#3, lowering the area electrical classification inside the hood, is not allowed by code. NFPA 70 The National Electric Code in Article 500.7(K)(1) that only 3 specific applications allow a gas detection system to lower the area electrical classification.

·        A location, enclosed space, or building that is classified as a Class I, Division 1 location due to inadequate ventilation.

·        A building or enclosed space that does not contain a source of flammable gas or vapors that is located in, or with an opening into, a Class I, Division 2 hazardous (classified) location.

·        Inside the interior of a control panel containing instrumentation or other equipment utilizing or measuring flammable liquids, gases, or vapors.

None of these can apply to a laboratory hood so you cannot lower an area’s area electrical classification by adding a gas detection system[ii].

#2, to indicate a failure of a treatment system, may be valid but has issues. First, any vent inside a hood that can be piped should be piped to behind the hood plenum. This helps to minimize the potential for a higher pressure or higher velocity leak from escaping out into the laboratory itself and minimizes the chance that a flammable stream my contact an ignition point inside the hood before being adequately diluted. Hence, the sensor will usually end up being inside the plenum as well with all the problems of dilution. It may be possible to place the sensor in the vent piping itself although often sensors have short lives when continuously exposed to even low levels of the gases they are designed to detect. Maintenance can then become an issue. Additional problems may arise if the vented gas is too hot or cold, too high a moisture content, or too low an oxygen content. These are issues that can be addressed but usually add significant complexity and cost if measures are required. And, even if they are not required, my experience with these type installations is that they are always subject to significant false trips due to frequent but trivial process upsets. So this has some validity but a lot of potential problems.

A more pertinent question is would the failure of the treatment system create a hazard at the exhaust? In almost all cases I have encountered the answer is no it would not. If it would, then normally the entire hood exhaust needs to be scrubbed rendering the issue somewhat mote. Sometimes, the issue is not the hazard if the treatment system fails to work properly but the fact that if it does then the emissions may create a reportable release, something most research organizations avoid like the plague. Again, I usually find that this requires scrubbing the entire hood exhaust since leakage from before the treatment is usually as important and just as credible a concern. In that case a gas detection system in the outlet of the scrubber may be required but it is more likely to be an instrument than a standard detection system as usually these cases require reporting (or at least monitoring) the output emissions.

#1, indicating a leak or loss of containment, is very dependent on the size of the leak. Most research equipment needs to be leak tested very regularly to exacting standards of tightness to operate properly[iii]. This routine leak test usually is much more effective than relying on a gas detection system. If the gas detection system indicates a problem the research equipment has usually completely vented itself. At that point I am not sure how much mitigation a gas detection system provides. Some organizations prefer this approach to alert the operating personnel to this type of rare failure but it can generally be indicated much more reliably, easily, and cheaply by a low pressure alarm or switch.

Given all the above, I suspect you now recognize my most common answer to the question posed at the beginning of the article is no, adding a gas detection system to a hood is usually not a very good mitigative measure. I caution that rarely is not never. I have encountered some situations where the gas detection system is either necessary or very prudent. These are usually where there are regulatory issues that require the organization to take a belt and suspenders approach to ensuring no reportable release occurs. All too often, however, I find that even then the gas detection system does not eliminate the potential for a release. So think through the issues carefully before deciding to spend all the effort to install one to ensure it does what you want it to do.

For more information on this and other issues using Hoods you may want to consider the University of Wisconsin course Laboratory Hood Operation, Selection, and Use for Safe Installations  be given virtually on June 9, 2021. See https://epd.wisc.edu/courses/laboratory-hood-operation-selection-and-use-for-safe-installations/ for more information.

[i] See "Just Ignore That Noise, It's Just The Hood Alarm Again": Hood Alarms Issues at https://www.dhirubhai.net/pulse/just-ignore-noise-its-hood-alarm-again-alarms-issues-richard-palluzi/ for more information on issues with proper hood alarm functioning.

[ii] See R P Palluzi, Area Electrical Classification in Research Applications: Is It the Only Mitigative Choice? (Chemical Engineering, Feb, 2021) and R P Palluzi, Install Ventilation to Reduce Area Electrical Classifications, (Chemical Engineering Progress, August, 2020) for a more detailed discussion of the code’s requirements. Some other aspects of this issue are addressed in Laboratory Area Electrical Classification at https://www.dhirubhai.net/pulse/laboratory-area-electrical-classification-richard-palluzi/ and Ventilated Enclosures for Lowering Area Electrical Classification at https://www.dhirubhai.net/pulse/ventilated-enclosures-lowering-area-electrical-richard-palluzi/.

[iii] See R P Palluzi, Leak Prevention In Small-Bore Piping And Tubing (Chemical Engineering, Oct 1, 2020) and R P Palluzi, Leak Testing Procedures, (Chemical Engineering, Feb 1988). Other aspects of the prevalence of leakage and the need for routine leak testing are discussed in “Not Another Leak!”: Leakage in Pilot Plants, Laboratory Unit, and Laboratory Equipment at https://www.dhirubhai.net/pulse/another-leak-leakage-pilot-plants-laboratory-units-richard-palluzi/, Locating Leaks in Pilot Plants and Laboratory Units and Equipment at https://www.dhirubhai.net/pulse/locating-leaks-pilot-plants-laboratory-units-richard-palluzi/, and Testing For Leaks In Pilot Plants at https://www.dhirubhai.net/pulse/testing-leaks-pilot-plants-richard-palluzi/.