empty chemical hoods are turned off when the laboratory is not in use.
9.C.2.8 Testing and Verification
The OSHA lab standard includes a provision regarding laboratory chemical hoods, including a requirement for some type of continuous monitoring device on each chemical hood to allow the user to verify performance and routine testing of the hood. It does not specify a test protocol.
Laboratory chemical hoods should be tested at least as follows:
• containment test by manufacturer;
• containment test after installation and prior to initial use (commissioning);
• annual or more frequent face velocity and airflow visualization;
• performance test any time a potential problem is reported; and
• containment test after significant changes to the ventilation system, including rebalancing or recommissioning.
9.C.2.8.1 Initial Testing
All laboratory chemical hoods should be tested before they leave the manufacturer according to ANSI/ASHRAE Standard 110-1995 or equivalent, Methods of Testing Performance of Laboratory Fume Hoods (ANSI, 1995). They should pass the low- and high-volume smoke challenges with no leakage or flow reversals and have a control level of 0.05 ppm or less on the tracer gas test. It is highly recommended that chemical hoods be retested by trained personnel after installation in their final location, using ANSI/ASHRAE 110-1995 or equivalent testing. The control level of tracer gas for an “as installed” or “as used” test via the ANSI/ASHRAE 110-1995 method should not exceed 0.1 ppm.
The ANSI/ASHRAE 110-1995 test is the most practical way to determine chemical hood capture efficiency quantitatively. The test includes several components, which may be used together or separately, including face velocity testing, flow visualization, face velocity controller response testing, and tracer gas containment testing. These tests are much more accurate than face velocity and smoke testing alone. Respectively, ASHRAE and ANSI found that 28% or 38% of chemical hoods tested using this method did not meet the pass criteria, even though face velocity testing alone found them to be in an acceptable face velocity range.
Performance should be evaluated against the design Specifications for uniform airflow across the chemical hood face as well as for the total exhaust air volume. Equally important is the evaluation of operator exposure. The first step in the evaluation of hood performance is the use of a smoke tube or similar device to determine that the laboratory chemical hood is on and exhausting air. The second step is to measure the velocity of the airflow at the face of the hood. The third step is to determine the uniformity of air delivery to the hood face by making a series of face velocity measurements taken in a grid pattern.
Leak testing is normally conducted using a mannequin equipped with sensors for the test gas. As an alternative, a person wearing the sensors or collectors may follow a sequence of movements to simulate common activities, such as transferring chemicals. It is most accurate to perform the in-place tests with the chemical hood at least partially loaded with common materials (e.g., chemical containers filled with water, equipment normally used in the chemical hood), in order to be more representative of operating conditions.
For the ASHRAE 110-1995 leak testing, the method calls for a release rate for the test gas of 4 liters per minute (Lpm), but suggests that higher rates may be used. One-liter per minute release rate approximates pouring a volatile solvent from one beaker to another. Eight liters per minute approximates boiling water on a 500-W hot plate. The 4-Lpm rate is an intermediate of these two conditions. If there is a possibility that the chemical hood will be used for volatile materials under heating conditions, consider a higher release rate of up to 8 Lpm for worst-case conditions.
The total volume of air exhausted by a laboratory chemical hood is the sum of the face volume (average face velocity times face area of the hood) plus air leakage, which averages about 5 to 15% of the face volume. If the laboratory chemical hood and the general ventilating system are properly designed, face velocities in the range of the design criteria will provide a laminar flow of air over the work surface and sides of the hood. Higher face velocities (150 fpm or more), which exhaust the general laboratory air at a greater rate, waste energy and are likely to degrade hood performance by creating air turbulence at the face and within the chemical hood, causing vapors to spill out into the laboratory (Figure 9.3).
An additional method for containment testing is the EN 14175, which is the standard adopted by the European Union and replaces several other procedures that were in place for individual countries. Parts 3 (Type tests) and 4 (On-site tests) of this standard address methods for “as manufactured” and “as installed/used” systems, respectively.
9.C.2.8.2 Routine Testing
At least annually, the following test procedures should be conducted for all chemical hoods: