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Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)

by Larry Zitko, ChromeTech, Inc.
June, 2002

Meshpads and Pressure Drop Calculations

Q. Do you have any guidance on how to properly calculate and comply with the pressure drop parameter when a company uses a composite mesh pad to control the chromium mist acid? (In compliance with the federal regulations 40 CFR 63.340)

A. It can sometimes be difficult to interpret the published regulations. The writing is often complex, and typically contains numerous references to other sections and paragraphs. However, there are good sources of information to augment the published final rule for the chromium MACT standards, that are easier to read and understand.

One of my favorite sources is EPA document number EPA-453/B-95-001 entitled "A Guidebook on How to Comply with the Chromium Electroplating and Anodizing National Emission Standards for Hazardous Air Pollutants". It is usually available from your State Small Business Assistance Program , State Small Business Ombudsman, or your local air agency. If you are a subscriber at the National Metal Finishing Resource Center, you can find a list of these Ombudsmen and Technical Assistance Directors under the "subscriber" area at www.nmfrc.org . In language that is easily understood, the Guidebook spells out all of the responsibilities that must be met to comply with this regulation, including those addressed in your question: pressure drop readings. I would highly recommend that you procure a copy of this book and use it as needed.

Unlike some other pollutants, EPA did not require continuous emission monitoring (instruments for measuring and recording exhaust stack discharges on a continuous basis) for chromium emissions from electroplating and anodizing sources. I suspect this was due to the lack of proven instruments for this application. Instead, EPA settled on an alternate strategy that would provide confidence that:

1) The air pollution control equipment installed at a facility is capable of meeting the strict discharge limits for chromium. The mandatory Initial Performance Test demonstrates that, at least on stack test day, the control equipment can produce emissions low enough to meet the limits.

2) The plating, exhaust ventilation and air pollution control equipment will be operated properly and maintained in good working order. This is accomplished by the Work Practice Standards, Ongoing Compliance Monitoring, and Operation and Maintenance Plan provisions in the regulation. Your pressure drop question applies to the Ongoing Compliance Monitoring section.

For facilities that use a composite meshpad mist eliminator (CMP) system for controlling chromium emissions, the air pressure differential across the control device, measured in units of "inches of water", must be recorded daily. This is easily done by installing commercially available air pressure instrumentation to the meshpad control device. I have often used the "magnehelic" and "photohelic" air pressure differential gages is made by Dwyer Instruments Inc. It is also possible to use a "U-tube" or "inclined" manometer as the pressure instrument. The idea is to plumb one port on the instrument to a pressure tap located upstream of the first meshpad, and the other port to a tap situated downstream of the last meshpad. In this way, the gage or manometer reads the pressure differential across the entire control device, which will typically have multiple pads. It is important to select an instrument which has a full-scale range that agrees with your control device. For example, if the actual pressure drop is around 3.5" H2O, an instrument with a 0-5" H2O range would be a better choice than one with a 0-20" H2O range, because it will offer better precision.

Facilities using a CMP control device must operate the device within a relatively narrow range of +/- 1.0 in. H2O from a baseline value representing the average value measured during the Initial Performance Test. There are exceptions to this default rule, but companies rarely are willing to conduct multiple performance tests to establish a wider operating pressure differential range. If a facility operates outside of this allowable pressure range, this is considered an "exceedance", and the periods of exceedance must be recorded in the "exceedance log", which is part of the Operations and Maintenance Plan. If periods of exceedance are high enough, the required "Ongoing Compliance Status Reports" must be submitted more frequently.

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