Corrosion Considerations Filter Design, Acid Resistant Filters

Many industrial operations generate acid or caustic fumes that can damage or destroy filters, system components, and construction materials. The air cleaning system designer must select components and materials of construction suitable for the corrosive environment to ensure high levels of system performance and reliability.

Acid resistant pre-filters and HEPA filters are available. These filters utilize media constructed with Nomex or Kevlar fibers mixed with glass fibers during manufacturing, epoxy-coated separators to extend the life of the aluminum separators, and stainless steel frames.

Metal filters with a demonstrated suitability for a corrosive atmosphere, in accordance with the ASME AG-Code 4 , are recommended for hydrogen fluoride or other highly acidic applications. Hydrogen fluoride is a concern because it will attack the glass media. Wood-case filters are vulnerable to attack by nitric acid that will form nitrocellulose.

Stainless steel is recommended for ductwork and housings when corrosion can be expected. Even this material may be insufficient in some cases, and coated (e.g., vinyl, epoxy) stainless steel or fiber-reinforced plastics may be necessary. The system designer can either: (1) use existing databases containing information about the performance of materials (including the filter media) exposed to various concentrations of corrosive contaminants, or (2) perform actual testing to validate the air cleaning system design.

Scrubbers or air washers may be employed to pre-treat the air or gas before it enters the air cleaning system or to scrub the air stream of caustic agents, but consideration must also be given to moisture carryover if the scrubbers or air washers are not designed and operated properly. Stainless steel moisture separators are recommended ahead of the filters. Corrosion is always a danger, but is not always obvious.

Electrical and electronic components are particularly susceptible to corrosive atmospheres. Plastics become brittle over time, contacts corrode, etc. For this reason, all electronic components must be environmentally qualified for the intended application.

Care must be exercised in selecting and using gaskets, as some gasket material reacts with the moisture in the airstream and releases chlorides that can corrode steels (including stainless steel). Gasket material selection should also include consideration of the effects of the materials use in acidic, radioactive, or other harsh environments.