It tough to avoid use of hazardous gases in fume hoods in sizes larger than lecture bottle. I have always been concerned though that when one does so, you do not not toss out the safety features you would typically employ for gases in a gas cabinet.

For high hazard gases one of the first things to consider is use of a flow restricting orifice in the cylinder valve, along with normally closed valves which shut down in the event of power and ventilation failure. State and local code may be more prescriptive (Section 3704 of the International Fire Code provides more detail and is adopted in almost all states).

The following is a snippet from our safety manual on this topic. Some is prescriptive and there are various ways to skin the cat, but provide this in case it is helpful to some degree.

Fume Hood Use of Compressed Gases

Hazardous gas use in fume hoods is appropriate under the following conditions:

1. The experimental apparatus fed by the hazardous gas is located inside the same hood.
2. The experimental apparatus is appropriate to be stored in the hood.
3. The experiment involves low gas pressure and flow rates
3. The experiment will be attended.
4. The engineering controls used for the hazardous gas in the fume hood provide equivalent safety to a gas cabinet installation.

Since fume hood face velocities provide insufficient protection against pressurized gas leaks, special care must be taken when hazardous gases are used in fume hoods.

The following is required for fume hood applications:

1. The smallest possible cylinder should be used for the experiment (a six-month bottle supply for routine use gases is appropriate, while smaller cylinder supplies are suggested for short term use). Make an effort to obtain gas cylinders in returnable bottles. Order bottles with lowest cylinder pressure possible.

2. Use a flow restricting orifice or needle valve to restrict flow to only that needed for the experiment.

3. Toxic and corrosive gases must be used with a normally-closed pneumatic shutoff valve, located immediately downstream of the cylinder regulator, which closes with exhaust loss or power failure. This valve should be fed from a pneumatic air (or nitrogen) supply valve, located at the entrance to the lab. Activation of this quarter turn valve, causes the pneumatic cylinder valve to close in the event of an emergency. The air supply to the pneumatic shutoff valve should also be connected to a three-way electric solenoid that fails open on power loss, bleeding air from the pneumatic supply line, and closing the valve. A static pressure alarm, monitoring the hood exhaust duct, will be interlocked to the pneumatic shutoff valve. Contact 513-1422 for parts, design details, and installation assistance.

4. Where determined necessary, run reactive or toxic gases through a suitable scrubbing media, then directly into the exhaust hood plenum (scrubber output hose should be placed into the exhaust slot).

5. Place the cylinder in rear of the hood. High pressure leaks can readily escape the hood and capture is best in the rear of the hood.

6. Assure all components in experiment can withstand full bottle pressure or incorporate pressure relief (run relief line into a hood slot)

7. All gas lines connected to the hazardous gas source, including purge lines and gas supply lines, must be completely contained inside of the hood.

8. See gas monitoring section of safety manual for gas detection requirements.

Ken