By Pulp & Paper Canada
By Pulp & Paper Canada
While troubleshooting at a mill, I encountered a series of apparently insoluble problems. Flow rates could not be set correctly, chemical dosages did not match settings, and results bore no apparent r…
While troubleshooting at a mill, I encountered a series of apparently insoluble problems. Flow rates could not be set correctly, chemical dosages did not match settings, and results bore no apparent relation to expectations. We were puzzled and so I went back to basics, using a thermometer (my favourite ‘temperature sensor’) to check temperatures, as well as a bucket and a stop watch (my favourite type of ‘flow meter’) to survey flows of water and chemicals throughout the mill. The results were amazing, as they did not match the DCS settings or readings! Using my very basic ‘instrumentation,’ we were able to bring the process under control quickly. Later, in reviewing the logic in the DCS, we found that most of the expected calculations and control loops were in place, but many did not control anything. A large number of processes were controlled by set figures, and were unaffected by the operators’ settings. After fixing the problems, we tracked down one of the people originally involved in setting up the control logic. He told us that at the time, everyone was under great pressure to get the mill running, and the programmers could not get answers to their questions about the chemicals and processes. So they made a few assumptions: all chemicals have a density of 1.0; temperature was unimportant; if nobody noticed immediately, it could not be important. They also set several of the control parameters to constants, as the required calculations were too complex to work out in the time available. Up to a certain point they were correct, as nobody tracked the problem back to the DCS for over a year!
At another location, I had worked with the mill for many years pursuing various changes to the process. We ran trials, performed lab work and considered the results. About ten years later, now with a completely different set of personnel, we started a different project, though with some similarities to the previous work. One of the process engineers sent me a spreadsheet that he thought might be useful in monitoring and controlling a planned trial. It was a fairly complex spreadsheet, allowing the calculation and print out of tables for dosage control, easy recording of results with automated generation of graphs. I complimented the engineer on a fine piece of work! He did not seem too impressed — he just told me to look at the author’s name in the file properties. It was my own spreadsheet from the similar work ten years before!
I must say that I was very pleased, particularly as this sort of good news does not come to me all that often. More often, I end up reworking old presentations or other documents to make them suitable for re-use, wondering what I was thinking when I wrote it originally! However, I save every document I create, in the hopes that it will be of some use in the future, saving me from repeating effort. I re-use documents so often that I now look at every one I create, with an eye to making it suitable for future work.
The above are two opposite examples: one of a legacy of hasty work coming back to haunt those who inherit it a year or more later; the other of good work producing a document with lasting value. Either result could have occurred in either situation. Prior to our current level of technology, even 20 years ago, these situations would not occur, as such legacies were much less likely. Now, the logic installed with a DCS will, to a large degree, remain in place until the next control system is installed. Even upgrades will often inherit the same logic — possibly carrying the original errors even further.
However, it does not have to be that way. A good piece of work is also a legacy, carried into the future even further as it will be preserved by the deliberate efforts of those who found it useful. Each of us has the choice with every project we do — work on each one as if preparing something that you will be able to use ten years from now and who knows? Maybe you will.
If you have anything to add or would like to suggest another topic, please contact the author. Dan Davies is a freelance writer. He can be reached at firstname.lastname@example.org