In this episode of the DustSafetyScience Podcast, we talk to Burke Desautels, Vice President of Aftermarket Engineering with IEP Technologies based out of Marlborough, Massachusetts, about choosing the right reduced pressure (Pred) for your explosion protection system design.
Burke has over 25 years’ experience in the design of explosion protection systems. He also sits on the technical committees for NFPA 654, 664, and 91. It takes a certain level of experience to make an accurate Pred estimate for a system design, so he’s sharing his knowledge by answering the following questions:
Burke explained that reduced pressure, or Pred, is the resulting or reduced pressure when a combustible deflagration event inside a vessel is either vented or suppressed. When explosion venting is provided for a particular application, the vent area is sized based upon the volume and combustibility of the dust.
“The NFPA 68 calculations tell you how much effective vent area is required for a given Pred,” he said. “It needs to be lower than the actual strength of the vessel itself. NFPA 68 is actually very, very detailed and explicit in that the Pred should be two thirds of the yield strength if no deformation is desired, or it can be two thirds of the ultimate strength of the vessel, if some deformation is permissible.”
When Pred is inaccurately sized or selected, your protection equipment could be greatly over- or under-sized. A Pred that is too high can provide a false sense of security, and if it’s too conservative, a lot of extra protection has to be put in place to achieve that Pred. Sometimes it is not even possible, depending on the application.
The most simplistic approach is that round vessels or ducts are stronger than flat or rectangular ducts. DS 7-76: Prevention and Mitigation of Combustible Dust- FM Global recommends that when you’re sizing the venting for a vessel of unknown strength, you use 0.2 bar or about 3 psi for a rectangular vessel and 0.3 bar or 4-5 psi for a round or circular vessel.
“Those are the parameters that Factory Mutual has established over the years of testing and investigating these types of combustion events,” Burke explained.
He added that these parameters could often be exceeded. “In North America particularly, we see about 0.3 to 0.4 bar as a typical range for most of the rectangular type vessels and dust collectors that we come across. They might be slightly higher with cyclones, round vessels. In the case of mills, which tend to be really strong, they can be very easily a 1 to 2 bar range, which is 15 to 20 to 30 psi.”
Pred is a function of vessel strength, but Burke conceded that estimations can be more conservative for sizing venting.
“It’s easier to take a lower Pred value, meaning you need more vent area, assuming you don’t go too low or you’re not that confined by surface area. If you have enough surface area, it’s a lot easier to use a lower Pred with venting.”
Sizing for suppression is different because you’re governed by the application characteristics of the dust, KST, Pmax, volume size, and L/D characteristics. The fireball and result in reduced pressure in a vessel with an L/D greater than 2 is much more severe than if it were not considered a compact vessel.
“So when you’re dealing with suppression, you put in your inputs, you determine how many extinguishers are required. The software modelling that we use takes into account the KST pressure time curve, so we know what the fireball growth is and how fast our detectors detect and how fast our agent is discharged. After performing heat extraction calculations, we compare that to the pressure time curve that the dust would produce in that size vessel. Then we have the resulting Pred for the application.”
Burke recommended that materials be tested for the application in question because the parameters could be different than those in published guidelines. Factors like particle size, moisture content, chemical composition of the dust, and its propensity to oxidize could impact the KST value.
“NFPA also recommends that the DHA be repeated periodically, and part of that DHA should be retesting the material to make sure that the dust characteristics haven’t changed as well.”
He once worked with a system designed for a certain KST. It consisted of a mill and a product receiver. After 25 years of use, the mill was replaced with a newer version that produced a higher quantity of smaller particle sizes. When the system was called into action, deformation occurred on the sidewall of the vessel.
“We were asked to investigate and try to understand what the situation was. So we sent the material out and had it tested. The original KST was around 100, and the new material with the much finer particle size practically doubled the KST. Our model predicted that the existing suppression system would fail, so extra hardware had to be added for that application. It’s a very real scenario: you may have a protection system in place that is now inadequate because you’re changing processes.”
For venting, Burke follows the NFPA 68 calculations, but still needs to interpret the data and confirm the proper sizing for each of the vents. With regards to the suppression, IEP has a proprietary software that was designed and developed by Dr. Peter Moore from the UK.
With systems that are especially complex, additional steps are taken. Burke often sees this with dust collectors.
“When you’re in the lower hopper and that volume (is) below the others, that’s essentially an open area. But when you get into that volume in between the bags – what we call an included volume- it’s an area where detection could be delayed. Another case is when you have a truly complicated volume with baffles separating volumes. We then treat each volume individually.”
A lot of the data on explosion protection systems are based on organic dusts and their characteristics, but metal dusts behave differently. They tend to burn two to three times hotter than organic dust and don’t follow the same scalability.
“Since they burn a lot hotter, they tend to produce higher reduced pressures,” Burke said. “So metal dusts are a unique application that we come across in explosion protection. The original design approach with metal dusts was to put it outside, put a vent on it, isolate it, and don’t let returning air back in. And that’s OK for some applications, but it’s certainly not a fix for all the applications that are out there. So industry does need to come up with these types of solutions for the folks who don’t have that sort of latitude with their process.”
Burke recommended that those with questions about Pred and explosion protection system design follow the NFPA codes.
“They’re out there to assist you. They are really good guidance on how to deal with extremely challenging hazards that are out there, as well as a good starting point for you to become familiar with the different types of applications and protection techniques. But at the end of the day, consult experts when it comes to life, safety and client and personnel protection. Contact an expert to get the right protection system for your application.”
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