10 replies to this topic

[Mahesh@A&M]

Operators open up tank hatch to charge catalyst in an acetone filled tank. We got a blower on top of the hatch and the vapors go to a scrubber. Last week when I analysed this vapor sample, I found O2 conc - 19% and Acetone conc is 2%.

Acetone LEL is 3% - 13%. Maximun o2 conc to prevent explosion is 13%-21% = 8%.

By chanig blower suction position I am not able to prevent air being pulled by the blower from tank hatch sorroundings. How should I resolve this problem??

The other thought I have is since acetone conc is less than LEL , it will not catch fire.

Thanks for your advice


Roopa

[latexman]

The tank should be inerted and vapor tight to not asphyxiate anyone. Nitrogen might be a good choice. The catalyst should be transferred into the tank by some other means than humans.

An engineer should be willing to operate the equipment/process they are responsible for. With the limited information you gave us, I would not operate that process, and since you are asking about it, I suspect you may have the same feeling.

[Mahesh@A&M]

Latexman,

you are right. I am looking to automate catalyst charging. We have a 20" valve that opens up for charing big bales of rubber in to the same tank. 90% of this area is covered whne when the valve opens for raw material charging. Here also I get the same O2 conc.
We do have N2 blanketing on this tank. But when the valve opens and blower runs, we get 19% O2 and 2 - 2.5% acetona and the rest being N2.
Is this composition safe as Acetone LEL and UEL is 3-13%.

[gvdlans]

In principle you're right, if the concentration is below LFL you can not get a fire or an explosion. However, you state that the measured concentration is 2 - 2.5% while LFL is 3%, so the margin is very (too) small. I also get the impression that the concentration is normally not measured at all.

For example, normally flammable gas detectors are activated at concentrations above 25% of LFL, so this would be 0.75 % (I assume this is vol%).

In the European Union, companies are required to prepare an Explosion Safety Document as per ATEX directives, where they show that fire and explosion risks have been assessed and that sufficient measures are in place to ensure the employees are not exposed to an untolerable risk. Measures may include use of equipment (e.g. the blower) that is suitable for use in flammable atmospheres (as per the hazardous area classification) and/or continuous measurement of flammable concentrations and instructions to stop operations when 0.25% is reached.

Relevant documents related to this are: API RP 500, API RP 505, NFPA 69, NFPA 497, IP 15, IEC 60079

[latexman]

Chemroopa,

As gvdlans indicated, the concentrations are much too close to the explosive region to be considered safe. It isn't hard to imagine a slightly abnormal condition occurs at the right time and there could be an explosion. Or, would that be the wrong time?

Anyway, you are on the right track in automating the catalyst. Strive to do it in a way such that containment of the tank/process remains intact at all times, i.e. the tank is not open to atmosphere. Then, it will remain inerted. Now I'd run a process like that!

[Mahesh@A&M]

Thanks for your advice.

Until we get the automated system in place, I will dilute the gas using nitrogen to get Acetone conc below 1.5% with a gas detector in place. Oxygen conc will drop a bit but not below Limited oxidant concentration(11%). any comments will be appreciated

Thanks

[gvdlans]

I checked a couple of sources and found that LFL for acetone is often quoted as 2.5 vol%, not 3% !

See for example: http://www.bu.edu/es.../MSAcetone.html
or
http://www.jtbaker.c...hhtml/A0446.htm

Flammability limits are shown on these MSDSs below "fire fighting measures"

[Mahesh@A&M]

Yes Gvdlans.Acetone LEL is 2.5%. Thank you

[latexman]

25% of 2.5% = 0.625%. I only believe one significant figure on those hand held meters that get thrown around day after day anyway. So, why not say 0.5% or less just to be safe.

[Mahesh@A&M]

Sample analysis report says
Acetone : 2.37%
Oxygen: 18.65%
Nitrogen: 78.98%

Mole percent. Acetone is close to its LEL. Limiting oxidant conc is 11%(NFPA 69) . Total flow rate is 16.86 lb mole/hr.

If I increase nitrogen by 300lb/hr ( 4000SCFH), Acetone conc will be 1.44% which is close to 60% of acetone LEL. A monitoring device in place will work. Blower seems to have enough capacity to handle this flow rate.

[gvdlans]

Limiting oxidant conc is 11%(NFPA 69) .

For the record: NFPA 69 Annex C indicates for Acetone a LOC of 11.5 vol% and an adjusted LOC of 9.5 vol%. As per section 7.7.2 of NFPA 69, a safety margin shall be maintained between the LOC and the normal working concentration in the system.