5. The sketch shows a tank which contained a highly flammable volatile liquid at

Question

5. The sketch shows a tank which contained a highly flammable volatile liquid at a slightly higher than Stirrer Scraper rod Flammable vapour and nitrogen Solid residues had formed on the stirrer shaft and required removal. The vessel was pumped out and charged with nitrogen to minimise the risk of air entering to form a flammable atmosphere of hazardous vapour and air. off with a scraper rod. The operator, however, went for his lunch before completing the job but left the manhole cover off. Upon his return, a flash fire was caused when he generated a spark by striking the stirrer shaft with the rod Write a short report suggesting reasons why this accident occurred

Explanation / Answer

Now if you examine the above case there are certain aspect why the actual spark took place. Now let just analyse the basic requirement for flash to happen :

1.oxygen

2.Flammable Material

3. Ignition source.

we will be going to analyse the report byy trying to cover the above points.

Now when intially vessels was pumped off and nitrogen was pumped in for cleaning at that point of time there was no supply of oxygen hence there was no any flash took place. But when the workman clean the tank halfway and gone for the lunch at that point of time vessel do contain some of the flammable material which is recently scrached off from the stirrer using the scrapper.

Now when he goes for lunch he left open the tank hence which means the oxygen present in the atmoshphere enter in the vessel and as soon as the lunch gets over and he starts cleaning the shaft again .He once strike the Scrapper rod with the shaft and now due to impaction of two metal rod causes a small spark.  

Although the intensity of sparked caused is low but the enough to cause flash with the highly flammable material vapors in the vessel.

As the all the things required to cause a spark was present flash took place.

Summary :

1.Ignition Source : Spark caused due to impaction of Two Metal Rods.

2.Oxygen : enters in while manhole was left open.

3. Flammable material : Material that has been scrapped of from the shaft and is in the vessel.

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