Cleaning of porous metal filter elements can be accomplished in two ways:
Cleaning or removal of elements from their housings for cleaning.
Cleaning in liquid/solids systems can be accomplished
Do not mechanically clean porous metal elements.
Wire brushing, scraping, sand or glass bead blasting can smear the pores and close off the porous media.
Chemical cleaning of porous elements: Chemical cleaning follows a logical sequence: First, determine what contaminants are present in the filter. Next, determine the chemical agents that will dissolve these contaminants (without dissolving or attacking the porous metal).
The chemical agents compatible with 316L stainless steel porous media include, but are not limited to:
• Water at any temperature
• 15% nitric acid at up to 150°F
• 20% caustic up to 212°F
• Acetic acid
• Organic solvents
• Methylene chloride
• Industrial cleaners such as Oakite 31 or Sonicor #103 (to remove grease)
• Solvents and detergents
Soak the elements in the chemical agent as required, flush with clean, filtered water or other compatible fluid, blow out with clean air or steam. Two chemical agents may be required, such as 15% nitric acid to remove iron oxide and 20% caustic to remove aluminum particulate,with a water flush between soaks.
Back to porous filter cartridges.
Evaluation of cleaning effectiveness:To evaluate the effectiveness of cleaning and the integrity of the filter elements, it is necessary to air flow test, and leak and/or bubble point test the elements after cleaning. For post-cleaning evaluation to be meaningful, newelements should be serialized and “base lined”, with air flow and bubble point data recorded before they are put into service. This provides a basis for comparison and evaluation.
Air flow testing determines the differential pressure through the media at a given air flow, usually set at 2scfm/ft2. After cleaning, the differential pressure at the same air flow may be only slightly higher than the “base line” value; a 10-15% increase is generally accepted as clean. A significantly higher differential pressure indicates ineffective cleaning. A second cleaning may be required. Bubble point testing, or leak testing, is a measure of integrity of a filter element or cartridge. Bubble point testing is the determination of the actual bubble point or “first bubble” of an element. Leak testing determineswhether there are any “leaks” in an element at a pressure below the minimum bubble point for the particularporous media being tested. Each filter grade has a specific bubble point range which is used as the basis for bubble point or leak tests.
Another factor for evaluating filter cleaning is weight, “before and after”. Record the weight of the new element on a precision scale and the weight of the element after cleaning and drying to determine the effectiveness of thecleaning. A higher weight indicates contaminants have remained in the filter media. Lower weight could indicateweight loss due to corrosion. The same weight indicates that the filter is free of contaminants.