We have 4 perchloric hoods at the CASC facilities, these hoods were not installed as per any regulating code such as NFPA, ANSI or other.
The existing perchloric hoods installation has the following characteristics that go against code:
- The hoods’ ducting is manifolded together
- See attached PDF. Highlighted system handles perchloric.
- The manifold includes other hoods that do not handle perchloric.
- There isn’t a dedicated fan for each hood.
- The hoods have damper valves where we’ve previously encountered perchloric crystals.
- Not exactly sealed sections.
- The hoods ducting includes over 100 feet of horizontal runs.
- The ducting includes about five 90 degree elbows.
- Please see attached a drawing with the highlighted perchloric ventilation hoods
- The system has several sprayers installed about every 8 to 10 feet that constantly spray to prevent the formation of perchloric crystals.
- Depending on the atmospheric air humidity a significant amount of water accumulates and flows through the duct work (~ 1” or 1.5” deep).
- See attached picture taken in July (Arizona)
- Since the ducting is horizontal, I doubt the water touches the whole area.
- The fan is not shut-down for cleanup, the sprayers are turned on while the fan is running.
- Sprayers are activated automatically every 8 hrs. for 2 minutes at the time
- The ducting has some view ports that allow partial inspection of the ducting for perchloric crystals.
- An analysis of the water flowing through the ducting showed that it only has between 8 and 10 PPM of perchloric acid.
- periodic samples of the fluid flowing through the duct are not done.
- The sample was taken when the 2 of the 4 hoods was ventilating samples at partial capacity.
- The ventilation system has two scrubbers
- One in the hood and the main scrubber at the end of the ducting system.
- I've been told that the scrubber in the back of the hood is not a scrubber, but a sprayer system.
- Each perchloric hood has a scrubber (90% efficiency as per the supplier)
- And a main scrubber system at the end of the ducting.
I also would like to get some more information about the recommended system by NFPA 45 for perchloric ventilation. The system requires a perchloric hood, a dedicated fan and am exhaust ducting routed straight up with the required sprayers.
But the code doesn't say a scrubber is needed anywhere. Is this because NFPA doesn't deal environmental regulations? or is the required scrubber the one in the back of the perchloric hood? is that a scrubber or just a sprayer system? would this system represent a potential for environmental issues such as exhaust of acid mist to the environment and roof?
Also, the NFPA 45 code section 7.12.1 (attached), includes an exception that reads as follows:
18.104.22.168* Perchloric acid shall be permitted to be used in a
chemical fume hood that is not specifically designed for perchloric
acid operations where the vapors are trapped and
scrubbed before they are released into the hood.
Some of us interpret that as saying that if the hood has a scrubber than the code doesn’t apply to our installation. We are working on getting NFPA membership to get them to provide interpretation of this section. But I wanted to know what you input was on this interpretation as well.
Please let me know what you think. We are trying to determine whether or not to modify the system.
Carlos E. Castillo
Sr. Mechanical Engineer
3443 East Highway 70, Safford, AZ 85546-9143