Halar/ECTFE Minimum Explosible Concentration 1

[DonyWane]

I posted the following message on the plastics forum - but it doesn't seem to get much mileage these days. Perhaps one of you could help....

A general web search didn't turn up any results and I do not have any plastics literature - so I wanted to run this up the flag pole with you guys/gals. Does Halar/ECTFE have an MEC and if so, what is it?

Also, what are acceptable methods for determining source terms that would feed into calculations determining plastic concentration? The process that I am interested in is electrostatic coating using Halar. The room has a defined ventilation rate, and I need to determine wheter or not the flow rate is acceptable. However, I am missing two key inputs: MEC and plastics concentration. Would you perhaps use a flow rate of plastics from the electrostatic coater and assume none "sticks" to the object being coated?

Thank you, in advance, for any help you offer.

 

[25362]

Although it is not of my personal experience I've been accustomed to the idea that teflons have outstanding flame retardancies. When Googleing around you may find sites containing MSDS on HALAR/ECTFE telling that it is not combustible, it doesn't have a self-ignition temperature, and it is not explosible.

However, plastics may accumulate static electricity and become a risk in the presence of combustible materials. Internal or external antistatics are frequently in use to reduce static charging.

 

[Fizzhead]

DonyWane:

I think that 25362 gives you a pretty good lead there concerning the combustibility of Halar/ECTFE. I would expect that if you have a potential problem here, it is not the combustibility of the product, but the potential for dust explosions that you are worried about here.

From an explosion avoidance point of view, there are two static conditions that you want to assess and then the assessment of the "transition zone" within the spray booth.

Because electrostatic applications are normally very efficient, I would expect the spray zone to be the greatest concern. Yes, the condition here is 100% of the application rate dispersed across the spray region in a three dimensional shape and concentration distribution determined by the specific type (including the specific manufacturer and model) of the application equipment and the geometry of the part being coated. You will have to either estimate, or actually measure, the spray zone ventilation rate to develop an accurate picture of the conditions. Given the conditions you are most likely working under, I would recommend that you make the best engineering estimate of the cross ventilation rate and proceed. While in situ measurements are feasible, they rarely improve the undertanding of the process condition sufficiently to justify the cost. The rest is simple straightforward process calculation to arrive at a concentration distribution for comparison to explosivity limits.

In the exhaust stream, if you are interested, the normal approach is to assume ideal mixing of the ventilation stream and overspray at the exhaust inlet and then extrapolate a distribution back to the application zone.

Don't forget to also investigate the health effects along with the possible explosion potential, for both the specific chemical and for dust concentrations, respirator type and use, etc. You also might look into the potential generation of fumes or gases other than ECTFE from the process to be on the sure side.

 

[DonyWane]

Fizzhead/25362,

Thank you for your posts! The explosion risk I was interested in evaluating was a dust explosion. In regard to dust explosion, any suggestions on how to find a MEC?

Also, to clarify a statemetn you made Fizz, is the spray "plume" something that the electrostatic precipitator manufacturer can offer suggestions on modeling/calculating? I am not very familiar with this sort of operation or dust explosions in general.

Thanks!

 

[Fizzhead]

DonyWane:

Finding the MEC could be difficult. Dust explosions are an inexact science. I will do some looking and let you what I can find. In the mean time, yes the manufacturer should be able to give you the spray pattern characteristics. After that you will have to calculate the exact distribution/concentration based upon your exact application rate. The manufacturer may be able to help with the modeling overall, depends upon the amount of interest they have found in this question. Can't hurt to ask. Just be sure you get the question to the manufacturer, local agents are often not too driven to pursue this question and may drop the ball. Good luck! Let you know when I find something.

 

[DonyWane]

Fizzhead,

NFPA 33 outlines a simple method for determining what percentage of the MEC is being reached in a spray booth based on MEC, maximum powder release rate, and air flow rate. It is pretty much the most straightforward calculation that can be imagined.

In the calculations you described above, you detailed some 3D plume analysis. Is the purpose of this to evaluate what the concentration profile looks like in the booth before being completely mixed and swept into the exhaust duct (I can see how the NFPA method is aimed at duct concentrations)? In what case would you use the NFPA approach of ((Application Rate/Exhaust Rate)/MEC)*100 vs. the analysis you detailed?

FYI: The system I am to address is an open-faced spray booth with filter bank at opposite end from the open end. (3 walls, floor, and roof) - no curtains.

Thanks, again, for your responses.

 

[Fizzhead]

DonyWane:

Sorry for the delay. Had to relocate my office and have been out of touch lately. Anyway, to answer your question. Yes, the idea of the 3D type modeling was to look at the concentration profile so that once you had an MEC identified, you could identify when you reached it and when you moved out of the that envelope (if indeed you do). Yes, the NFPA method looks at 100% dilution and that means it's point of best applicability is in the ductwork. Problem with only doing this check is that many such spraying operations pass through the explosive range and then in the ductwork, after taking into account transfer efficiency and exhaust filtration, are not in the explosive range anymore and some people get mislead by this and think their operation is never in the explosive range. In many cases, even if you assume zero transfer to target, you can get a result indicating nonexplosive conditions.

Modeling of the spray pattern is a common exercise and, as I believe I said before, the vendor should be able to provide you with some support here. This information allows you to calculate explosive conditions in the spray plume prior to the target. Once you get near the target, the modeling is more up to you. However, knowing transfer efficiency etc. you can model the dilution from that point on with relative simplicity, based on the assumption that you have two diverging streams, the Halar being transferred to the target, and the sweep air/overspray mixture. Again, you can keep the approach simple and just assume that you have a steady transition from the spray plume conditions to the conditions upstream of your exhaust filters. Remember, what you are trying to do is determine the potential for explosive conditions, not calculate everything down to the gnats eyebrow!

Ok, now you have to look at your process characteristics and determine what the worst case conditions are. Is there a human manning the spray gun? Or a machine? Especially if there is a human, there will be periods of time when the gun will be spraying, but not directed at a 'target'. Thus giving you a worse case condition than production because now all the Halar gets swept into the exhaust stream. Now, after you have reviewed that condition, then look at the cleaning and maintenance procedures for the spray apparatus. Are the guns solvent cleaned in the booth? If so, this is commonly considered environmentally unfriendly (exhausting solvents instead of using a closed gun cleaner) and may create worse conditions than production activties depending upon the solvent.