Where perchloric or other acids will be heated above ambient temperature, a dedicated acid hood shall be installed or provisions made to trap and scrub vapors at the point of emission, before they enter the laboratory ventilation system.
If perchloric acid is heated above ambient temperature, it will give off vapors that can condense and form explosive organometallic perchlorates. Limited quantities of perchloric acid vapor can be kept from condensing in laboratory exhaust systems by trapping or scrubbing the vapors at the point of origin. Nitric, hydrochloric, sulfuric, and other mineral acids are often used in digestion procedures at high temperatures.
NFPA 45, Chapter 6-11.1
Acid hoods and exhaust ductwork shall be constructed of materials that are acid-resistant, nonreactive, and impervious to the acid being used. No organic materials, including gaskets, shall be used in the hood construction unless they are known not to react with perchloric or other hot acids and/or their by-products;
Perchloric acid digestion may over time result in the condensation and consequential formation of metal perchlorate crystals, which can pose an explosion hazard, especially if combined with organic chemical condensate. Typically, 316 stainless steel or unplasticized PVC has been used. 316 stainless steel may not be appropriate for other hot acid applications, particularly hydrochloric acid. Consult with EH&S for guidance on the appropriate materials of construction of perchloric/hot acid digestion exhaust systems.
ANSI Z9.5 3.2.4
NFPA 45, Chapter 6-11.2 and 6-11.5
A water spray system shall be provided for washing down the hood interior behind the baffle and the entire exhaust system, including the stack and exhaust fan. It is not necessary to wash down the interior of the fume hood work area. The hood work surface shall be watertight with a minimum depression of 13 mm (1/2 in) at the front and sides. An integral trough shall be provided at the rear of the hood to collect wash-down water.
Perchloric/hot acid digestion hoods should be washed down after each use to minimize accumulations of potentially explosive perchlorate salts and other highly corrosive by-products.
NFPA 45, Chapter 6-11.6
Wash-down spray nozzles shall be installed in the ducts no more than 5 ft apart. The ductwork shall provide a positive drainage slope back into the hood. Ductwork shall consist of sealed sections, and no flexible connectors shall be used.
NFPA, Chapter 6-11.4
The hood baffle shall be removable for inspection and cleaning.
NFPA 45, Chapter 6-11.7
Ductwork for perchloric/hot acid hoods and exhaust systems shall take the shortest and straightest path to the outside of the building and shall not be manifolded with other exhaust systems. Preference shall be given to a routing that is as vertical as possible. Horizontal runs shall be a short as possible, with no sharp turns or bends.
Sealants, gaskets, and lubricants used in perchloric acid hoods, ductwork, and exhaust systems shall be acid resistant and nonreactive with perchloric acid.
NFPA, Chapter 6-11.4
The exhaust fan shall be acid-resistant and spark-resistant. The exhaust fan motor shall not be located within the ductwork. Drive belts shall be conductive.
NFPA 45, Chapter 6-11.3