What you need to know about DHAs and NFPA 652

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DHS and NFPA 652: What you need to know about DHAS and NFPA 652. Every material is different, and therefore every valve has to be designed to handle each different combustible dust in order to properly prevent deflagration. Watch this video to find out what you need to know.

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"P-red" or reduced pressure is the maximum pressure developed in a vented enclosure during a vented deflagration. This is important as your rotary airlock housing, endplates, and rotor shall be of adequate strength to withstand the maximum projected explosion pressure, also known as p-red.

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Every material is different and has a different potential explosion value. You must determine the value of your material being handled. As soon as you can, you should obtain a copy of NFPA 652: Standard on the Fundamentals of Combustible Dust, Edition 2016. This is super important because it is your starting point to evaluate your particulate being handled and the safety in your whole system and facility. 652 outlines the mandatory requirement for owners/operators in the facility to have DHA (dust hazard analysis) completed on each different dust their facility handles. A DHA is a dust hazard analysis.

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NFPA states that this is a systematic review to identify and evaluate the potential fire, flash, or explosion hazards associated with the presence of one or more combustible particulate solid in a process or facility. It will also help determine how combustible your particulate dust is. This will determine which preventative and protective measures you will need in your systems and facility. It is important to start the process of your DHAs now, as owners/operators must schedule and complete DHAs for existing processes and facilities within three years of the date of this standard. This clock started ticking in October 2015.

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Note the importance of starting this now is that you must be able to demonstrate reasonable progress in each year of the three years you have to comply. There are many reputable companies out there that can help you with your analysis and risk assessment. Don't look at it as an overwhelming task. As I said, there are great companies out there that can help you pinpoint and help you organize and prioritize which items should be addressed first for overall safety compliance. Failure to conduct a hazard analysis is a huge oversight. Don't put it off any longer.

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As I said, your DHA will help you determine how combustible your particulate is. Your dust will be given a KST value. For example, there are four classes for KST values. The first has zero explosion value. An example of this would be rock dust. Class One is if your KST value is anywhere from 1 to 200. An example of this type of dust would be grain and sugar dust.

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Class Two is in a range between 200 and 300. Wood dust is an example of this class, ST Class 2, i.e., wood dust. Class Three, 300 to 400, and that would be an aluminum dust. The higher the KST value, the higher the explosion risk.

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Getting back to the design of the rotary valve as outlined in NFPA 69: Specific Explosion Characteristics, KST and the p-max, or maximum pressure of the powder being handled, is needed to make certain the design of the valve can handle the maximum anticipated explosion pressure. KST value plus the p-red is what determines the pressure rating required of your rotary valve.