The term Pred is thrown around quite a bit in the dust explosion world but what is it exactly and why is it so important? Let’s start at the beginning.
You have a material and it is combustible. You send if off to a test lab and get it tested. They send you back a report that gives you the two magic values that everyone keeps asking for: Kst and Pmax. You take this coveted information to your explosion equipment supplier thinking you’ve got this problem all figured out and they come back by asking you what’s the Pred of your vessel?
Should this not have been something the lab gave you in all those test results? Why can’t you just use Pmax? What is Pred again? How are Pred and Pmax different?
Pmax is a parameter of the tested material, under the tested conditions. It is the maximum pressure the tested material will create in a contained deflagration, at optimal concentration. The only time you would use Pmax for explosion protection is if you decided you are not going to put any sort of explosion protection on your vessel. The vessel must then be built for containment; it must contain the Pmax with a safety factor.
Pred, on the other hand is the maximum pressure developed that your vessel must withstand when it has been vented. Pred is the shortened term for “pressure reduced” or “reduced pressure”, and technically only applies to explosion venting applications. If you are dealing with active explosion protection (chemical suppression system), the term used is PTSP (total suppressed pressure), but Pred is often still referred to when talking about suppression.
The differences between Pmax and Pred are shown below in Figure 1.
Calculating Pred Value
Now that you have an understanding of what Pred is, how exactly do you figure it out what the value is?
The easiest way for an end user of the equipment to get a vessel Pred is to ask the supplier/manufacturer of the vessel. If you are dealing with new equipment, this information should be on the shop or approval drawings. If the Pred is not on the drawings, you should ask the manufacturer to add it or supply additional documentation with the necessary information.
If you are trying to obtain the Pred of an existing vessel, where do you start? Start in the same place, ask the equipment supplier. This is always a great place to start, because in a lot of cases the equipment manufacturers have the information but only supply it on request. Keep in mind the manufacturer Pred is based on a brand new vessel. Wear and tear have to be factored in if the equipment is not in like new condition.
If the equipment supplier does not have the Pred of the equipment or the equipment is very old, then a finite element analysis can be completed on the unit to reverse engineer the vessel strength. This is usually performed by a third-party engineering company. To complete the finite element analysis, you will need to supply detailed drawings and information, or the consultant will have to come and visit the vessel in person to collect the relevant data. The information you would have to supply would include-but not be limited to--the type and thickness of all materials used to construct the vessel, size measurements of the unit, location and size of all reinforcing on the vessel, all inlets/outlets, how the vessel is constructed, and any damage or deterioration. With this, a finite element analysis can be completed and a Pred can be determined.
The person conducting the finite element analysis may come back and ask to what strength you would like the vessel to withstand. What more questions? You want it to withstand the Pred, right? Wrong.
If you design the vessel to the Pred and you have a full-blown deflagration at maximum concentration the vessel will likely rupture.
NFPA 68 (2018) sections 220.127.116.11. and 18.104.22.168 talk about the parameters for Pred. There are two options for Pred selection: the vessel can be designed to two-thirds of the ultimate strength or two-thirds of the yield strength for the vented enclosure (see Figure 3).
Ideally all vessels should be designed to two-thirds yield strength, but it’s not always possible with older vessels. Two-thirds yield strength means that in the event of a deflagration--at full concentration--the vessel will show no signs of physical deformation. In most cases the vessel can be inspected, cleaned, and put back into service with minimal effort. If the vessel is designed to two-thirds ultimate strength, the vessel may deform, but not rupture. The vessel may or may not be reusable. This is only acceptable if the end user is aware that the vessel may deform. I personally think this should only apply to pre-existing or retrofitted vessels. Why would you want to install a brand new unit, have an explosion in it with explosion protection, and then have it all deformed beyond use?
During the finite element analysis process, it is often determined that the existing vessel is too weak to meet the necessary Pred for the explosion protection system. Reinforcing can be designed to increase the strength of the vessel. It is worth noting that sometime there is a point where the reinforcing efforts become excessive or not cost effective. At this point it must be decided which path to move forward with: additional reinforcing, additional explosion protection, or even perhaps a new vessel. The installation of more explosion vents often can reduce the required Pred, which can be more cost effective than installing excessive reinforcing. If you must reinforce a vessel, it is often worthwhile to discuss with the explosion protection equipment supplier what is the lowest obtainable Pred.
When the finite element process is complete you should be supplied with a document that tells you the vessel Pred and if necessary, how to reinforce it.
Why Is Pred so Important?
Determining the pressure a vessel can withstand is often something that is overlooked or forgotten about completely. Project timelines get shortened; process changes can happen that change the Kst/Pmax that then affects the sizing of the explosion venting or any other number of things. The vessel strength verification often just gets overlooked.
Let’s use an example to explain why Pred is important. You install explosion venting. The Pred of the vessel was not known and someone sizing the vents assumes the Pred of your square vessel is 0.2 bar, because that sounds like a reasonable number. Time goes by and then one day, the unthinkable happens, an explosion occurs in that vessel. The explosion protection system works as designed but there are three things that can occur to the vessel:
1. No effect to the vessel. It can be cleaned, inspected, and put back into use (you lucked out).
2. It will be deformed but can still be used (you’re still lucky).
3. The vessel does not contain the explosion, ruptures, and allows the explosion to propagate to other parts of the facility.
Not knowing the vessel Pred, and designing to an unknown pressure value, could result in any one of these options, with the third option obviously being the worst. When the explosion protection system is designed around an unknown or assumed Pred you are just playing the lottery, and the dust lottery is not something you want to win.
The final thoughts on Pred are to make sure that it is not overlooked. If you are considering a new vessel that will be purchased with explosion venting, ask the equipment supplier to provide the documentation. Make it part of the equipment supply documents. If it is an existing vessel, contact the equipment supplier or complete a finite element analysis. Regardless, you must make sure you have the information. Don’t use assumed or estimated values because they are often an overestimate of the strengthen of the unit, which could end in failure.