Inadvertent chemical reactions

This past week I was cleaning some Tupperware containers that had had moldy food in them. I have discovered that one of the best way of cleaning stained/smelly plastics is household hydrogen peroxide and sunshine. I had put the containers on roof of my car in the sun and then filled them with hydrogen peroxide. After leaving them outside for an hour or so I brought them in.

On the way inside through the garage I spilled some of the hydrogen peroxide onto a stain on the garage floor. On the way back out to the car to get a few more of the containers I saw the garage floor was steaming where the stain and the spilled hydrogen peroxide coincided.

Being an engineer I was reflected on this and it made perfect sense. The stain was some hydrocarbon and the hydrogen peroxide a strong oxidizer (though diluted for household use) when they came in contact they reacted. fortunately not very energetically.

Here is someone else's experience with an unplanned experiment. Unfortunately his was a bit more energetic.

I have reproduced the account below:

A safety coordinator at the Esso Oil Company plant in Longford, Australia, was using a belt grinder in his home workshop to smooth the edge of a hacksaw cut on a 2" length of 1.5" angle iron. He had been grinding for about 1.5 to 2 minutes when there was a loud "THUMP" accompanied by an approximately 2-foot diameter brilliant yellow orange fireball. The fireball lasted no more than 1 second and then completely extinguished itself. It completely enveloped the machine, his hands to half way up his forearms, and the front of his torso.

Injuries included deep second-degree burns to about 60% of the victim's left hand and 50% of his right hand and first degree burns to his neck, chin, cheeks, lips, and the end of his nose. The right cuff of his shirt was smoldering, his face felt a burning sensation, and he could hear the front of his hair sizzling. Nothing on the bench was burning. A few streaks of white powder were deposited on the bench top and on a few items lying on the bench. The workshop was filled with dense white smoke with very little odor. His fingers and the ends of his thumbs escaped relatively unscathed as they were protected from the heat flash. He was wearing glasses, which protected his eyes. He also lost half his moustache, one of his eyebrows, and about 1 inch off the front of his hair. His eyelashes were curled by the heat but not singed. The burns to his face were caused solely by radiant heat, as the fireball did not come that high.

Analysis:
A few days before the event, the man's son had ground the heads off about twelve aluminum pop rivets. Finely divided aluminum mixed with finely divided ferrous oxide (the black powder residue from grinding steel) produced a compound called thermite. Thermite is used to fill incendiary bombs and is used commercially to weld large steel items. It burns at approximately 3500C (6300F), hence the extensive burns from such a short exposure time.


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chemistry
accident analysis
garage safety

Think before you act

This morning I was fixing breakfast for my son. The flat griddle we use for cooking bacon and pancakes had been left on the stove and not put away after being washed the day before. I decided to use the griddle instead of getting out a frying pan to scramble some eggs. Being in a hurry I didn't wait for the griddle to get completely hot before pouring on the eggs.

The stovetop is not quite level. Because I didn't wait for the griddle to get completely hot the eggs didn't set quickly. There was time enough for the eggs to slowly flow to the edge of the griddle and then onto the stovetop making a big mess.

If I had taken the time to get the griddle hot so the eggs would set quickly, or if I had gotten out the frying pan, or if the stove had been level a big mess would have been avoided.

I recall from a class long ago that most catastrophes are not the result of a single failure but the consequence of a chain of minor events and decisions which culminate in a tragedy.

There is a story in a recent Design News column The Case of the Languid Latch of how an assembly error combined with some wear and careless use led to a fatality.


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mechanical design
failure
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Using statistics to design a holder

I recently designed a bracket to hold some scintillation detectors. The detectors were not all uniformly sized. To determine the opening size to make for the detectors I followed the procedure outlined below.

I measured the detector widths from a sample and collected the data listed in the table 1. The data has a mean of 1.192 inches (3.0277 cm) and a standard deviation of 0.008071 inches (0.0205 cm).

Table 1. Detector sizes.
data (in) data (cm)
1.18 2.9972
1.184 3.0074
1.185 3.0099
1.193 3.0302
1.195 3.0353
1.197 3.0404
1.200 3.048
1.202 3.054

At this point I assumed the data to follow a normal distribution. I determined to make the opening fit the 99th percentile detector. From a z-score table I determined that the 99th percentile is at 2.4 standard deviations above the mean (2.4 std devs above mean is actually 99.2 percentile). The z-score is a dimensionless number obtained by subtracting the population mean from an individual (raw) score and then dividing the difference by the population standard deviation. The z score reveals how many units of the standard deviation a case is above or below the mean. Figure 1 shows a normal distribution with cumulative percentiles and z-scores.



Figure 1. Z-Scores and normal distribution (source: Wikipedia)
This gives the size for the 99th percentile detector of

1.192”+2.4X.008071”=1.211” (3.076 cm)

The number above is the width of a 99th percentile detector. To this number I added 0.005”. This addition was to allow for any inaccuracies in locating the holes for the dowel pins. This addition would allow the dowel pin on each side of a detector to move in by 0.0025”.

The mount was designed with nominal opening for a detector of 1.216” (3.0886 cm).

Unfortunately, designing for 99th percentile detector to ensure that almost all detectors will fit inside the provided opening means that the average detector with a width of 1.192” (3.028 cm) will have 0.024” of side to side travel if the pins are in their correct nominal position.

It turns out that due to the excessive play in the small detectors I will need to redesign the holder.


At this point I created a normal plot to see if the assumption that the data was normally distributed. The normal plot is shown in figure 2.The straight line on the plot indicates normally distributed data. With the limited sample size of the data deviation from a straight line is to be expected. The data is approximately normal. Given the small sample size there is no point to conducting a more formal test for normality.




Figure 2. Normal plot.


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mechanical design
statistics