Chemical Processing - September 2007 - (Page 32)

With all the computer power and sophisticated software, what responsibility is the user taking? What role does the seasoned engineer play in this new age? Generally the engineer with 30-to-40-years of experience prefers to do complex calculations by hand. This leaves the young computer jockeys of today to sit at the helm of the many robust simulation packages. But, because the software is handling many of the constraints and boundary conditions that govern the outcome of a solution, how much confidence can we have in the results? Read almost any major engineering magazine and you’ll see advertisements that make all sorts of claims about software capability — e.g., non-linear capability when addressing the physics, and “automatic” and “userfriendly” when discussing the model design itself. Many of the non-linear solutions require all sorts of coefficients that normally come from experiments or well-known solutions. These coefficients may not be completely accurate for the problem you are working on. Achieving accuracy What process or method should we use to ensure accurate solutions? When performing numerical modeling, seven basic rules have withstood the test of time: 1. No result can be more accurate than the input conditions. Not too long ago I was in a technical review where the engineers were touting the complexity and accuracy of their solution. When asked about critical boundary conditions it became quite clear that important factors that would affect the solution were no more than estimates. Sometimes we become so proud of our calculation that we forget the error and uncertainty of input data. Many models require calculation of boundary conditions. These are When performing numerical modeling, seven basic rules have withstood the test of time. either calculated by hand or by some other software package. Sometimes these boundary conditions are provided by measurements. A model or simulation can’t be any more accurate than the input data. One of the worst mistakes on a simulation is evaluating the results to four significant digits when the input data are limited to only one significant digit. Always evaluate the “uncertainty” of all aspects of the problem you are working on. (See CP’s ongoing series by Dr. Gooddata; Part 1 at www.chemicalprocessing. com/articles/2005/419.html.) 2. Nothing beats experience. It’s important to define the model that best fits the physical situation. Many 32 • September 2007 young folks are computer wizards and can develop a model and quickly get results. That’s great but it’s a good idea to have as much gray hair around the problem as possible. The experienced folks might not know all the details about the computer modeling but they have “been there and done it” in the field. They have an intuitive feel for the behavior of a problem that others don’t have. This is important for success. A few years ago I was in a meeting where a group was presenting simulation results on a structural dynamics problem. A well-respected senior engineer told the group its results were wrong. When asked specifically what the problem was, he said, “I don’t know what the actual problem is but the numbers you calculated in the results I have never seen in that range before.” The young guns just passed the comments off, but the senior engineer asked them to solve a problem he had done by hand that was a known solution in the industry. As it turned out, their program left out a gravitational acceleration term, causing an error in all results by a factor of 32.2. That senior engineer saved the team an embarrassing mistake; it’s a lesson those young guns will never forget. 3. Take it easy on the problem size. Don’t try to model the world. With computers as powerful as they are today, the trend is to make the model as big as the computer can handle. This isn’t the best strategy. Take the problem in increments and strategic “bite sizes.” Similarly, stay away from non-linear analysis until things are “tuned up and debugged” with linear analysis. Then, introduce complexity in small steps. As a friend once said, “Just because you can eat more doesn’t mean you should.” This step also ties to Rule 2, because with all the computer power now we can do more testing effectually and faster to make sure our approach is correct. 4. Always check the model in detail. I remember discussing a structural dynamics problem with an engineer who had a doctorate in mechanical engineering. The results of the problem were coming up with an incredibly unrealistic deflection. The experienced plant folks questioned this. The PhD’s response was a long theoretical “mumbo-jumbo” on how this could occur. As it turned out, the only problem was a bug in the input data. You analytical types should keep your ego in check when questioned by experienced design engineers and operating plant personnel who have lived with the situation. Don’t always trust those beautiful color plots. Assume everything in the model is wrong until all is proven correct. It’s always wise to perform hand calculations to doublecheck certain aspects of the problem. 5. Define a model that best depicts the physical situwww.chemicalprocessing.com http://www.chemicalprocessing.com/articles/2005/419.html http://www.chemicalprocessing.com/articles/2005/419.html http://www.chemicalprocessing.com

Table of Contents Feed for the Digital Edition of Chemical Processing - September 2007

Contents From the Editor Field Notes In Process Energy Saver Compliance Advisor Succeed at Simulation Rethink Your Approach to Process Safety Avoid Blending Blunders Get the Right Cartridge or Bag Filter Wireless Proponents Take HART Membrane Boasts Material Benefits Process Puzzler Plant InSites Chem Show Product Preview ISA Product Preview Equipment & Services Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - September 2007

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