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Statistical Process Control: Process and Quality Views

Jul 14 2012 06:50 PM | Chris Haslego in Process and Reactions *****

SPC: Danger Signs and Where to Start Looking

As a process or quality engineer, you'll eventually come across some charts that make your eyes pop out and spell "O-V-E-R-T-I-M-E". Imagine going to work one morning and find the chart below:

Depending on where the results came from, the problem could be several things.
If the results are from online measuring devices:
1. Check the operator's log for any abnormal behavior during the time that the results were out of standard.
2. Check the calibration schedule for the measuring device.

If the results are from a laboratory:
1. Check laboratory equipment for correct calibration.
2. Review laboratory notes on the tests for any errors that may have been made in the testing procedure.
3. Ask the laboratory technician if he/she remembers anything strange about the tests. For example, sample collection container abnormalities that may have led to contamination.

Any investigating beyond these ideas may begin to be counterproductive. However, the process should be monitored closely to see if this result is repeated later. The process has returned to normal operation and the out of standard results were not a serious compromise of quality.

Now suppose you find a chart resembling Figure 6:

You may initially think that since no results are out of the quality control standards that you don't have a problem. To the contrary, unless there has been an intentional process change, you have a very serious problem. It's not a matter of where the process has been or where it is now, but where it is going. This type of trend cannot be attributed to simple error, there is something seriously wrong! Depending on many factors, you must find a place to start investigating. You may want to start with equipment that can immediately affect the SG. Look closely at the process data over the past few days. It can be helpful to compare data over a comparable period of time when the process was in control versus this new trend that you're seeing. I might suggest starting with the separation equipment.

Until this point we've considered only one characteristic of n-hexane, specific gravity. Process engineers must simultaneously monitor all important characteristics of a product. Suppose your manager presents you with the following two charts:

You notice that the SG seems to be fine while the concentration has dropped off dramatically. While SG is generally a good indicator of concentration, it doesn't appear to be so in this situation. Since you have no reason to doubt the accuracy of these results at this time, where should you start? The first question you should answer is: "What else is in the stream that is lowering the concentration?" A quick look at the gas chromatograph show that on 3/4/99 at 1500 (when the n-hexane concentration was at 92%) there was also significant amounts of two other chemicals: 2-methyl-butene-3 (SG=0.63) and isoprene (SG=0.68)[hypothetical components]. Since the 2-methyl-butene-3 is lighter than n-hexane and isoprene is heavier than n-hexane, the contaminant mixture did not force the specific gravity out of standard, but the concentration is being seriously affected (this is why you monitor both SG and concentration). Now all you have to do is find out how it got there. I might start with heat exchangers that may be leaking.

Processes and Reactions Articles


Sep 01 2012 02:23 AM
thanks,that's really good stuff.I have been training for a Six Sigma Green Belt,use these charts a lot!

Hi Thanks very much

Could you please provide me with example of  daily,monthly  performance report of equipment,production report, specifically the activities of process engineer with examples

I'll be so great full for you  , i have solid experiences in oil and gas operation, however  i am chemical engineer ,but i didn't experience engineering issue 

Aug 25 2014 03:08 PM

excelente pagina, los temas son bien específicos y de muy buena calidad.


gracias a ustedes por tan maravillosa pagina y del tiempo que le dedican a cada articulo.