Flow Simulation Finds Failures, Tunes Systems

Several flow simulation packages are available to solve problems in everything from injection molding to designing hydraulic and pneumatic controls

By Rich Merritt

Engineers were stumped. A massive pump station at Monroe County's wastewater treatment plant in Hilton, Pa., on the shore of Lake Ontario, had problems with vibration. They installed 4-20 mA vibration sensors from IMI Sensors on the variable frequency drives. The sensors transmitted vibration data to the SCADA system, so they could see the problem. However, they still couldn't determine why vibration was going off scale at certain motor speeds.

Suspecting impeller buildup, the engineers back-flushed the pumps, but found no debris. Using bump tests and run-up and coast-down tests, they discovered a case of resonance. To cure resonance, the classic solution is to stiffen or "unstiffen" the mounts. "We didn't want to arbitrarily place braces just anywhere," says Jeff Helfer, supervisor at Monroe County Water, "because of the possibility of shifting the problem to another area, or exciting another natural frequency."

Suppose you supply motor-driven pumps with your equipment, or maybe a system integrator installs them. What do you do when your customer calls to complain about resonance problems in the motors and drives you chose? Would you know where to start?

The answer is to use Operational Deflection Shape (ODS) simulation to determine why the pumps shake at various flow conditions. Helfer built a model of the motor-driven pumps in the simulator, and used it to test various ways to brace the system and eliminate resonance. Without simulation, they would've had to use trial-and-error methods.

Machines built for filling and dispensing, thermoforming, injection molding, extrusion, blow molding, and steel casting also have various flow-related problems. For example, a flow problem occurs when cooling slabs in continuous steel casting, according to Richard Hardin, assistant research engineer at the University of Iowa, Iowa City.

"The practice of controlling water flow based on casting speed results in transient variations in the slab temperature," he says. "Sprayed cooling water must be controlled dynamically, and computer-based control models represent the best, if only, method for reliable dynamic control." In this case, he says, trial and error doesn't work.

Machine builders are familiar with trial-and-error when designing a new machine, or when they're trying to control the material flow rate, heat transfer relationships, or cooling needed to make the process run efficiently. Essentially, they build a prototype part or system, try to make it work, and go back to the drawing board to try something else when it doesn't. This is an expensive, time-consuming way to solve problems.

Today, instead of trial and error, you can simulate the flow and heat transfer conditions in a computer, run various models at assorted control settings, and see how the machine or part will react, before you commit yourself to the design. Fluent's website describes and demonstrates several flow-related solutions:

  • Filtering and Dispensing simulates liquid flow, providing fill time, predictions of entrapped air and bubbles, and splashing. It helps users design nozzles and dispensers.
  • Thermoforming simulates thickness distribution of a molded part, providing information for temperature control and mold motion control.
  • Runner Balancing simulates flow of material around bends in runners used in injection molding. Results help set temperature control parameters to ensure correct material flow.
  • Blow Molding simulates all phases of blow molding, including extrusion, pinch-off, blowing and cooling.

Meanwhile, Moldflow specializes in injection molding simulation. Its flow-simulation tools help machine builders design injection molding equipment, and its simulation software helps set up, optimize, control, and monitor the machine. "The more complex the part, material or mold, the narrower the processing window, and the higher the need for continuous monitoring and optimization during production," says Ken Welch, business manager at Moldflow. 

Welch adds, "Production capacity and yield ultimately are determined by the speed, reliability and quality by which the melt flows from the machine into the mold." In other words and with different parameters, this is probably true of many processes. Consequently, any simulation software that helps determine the limits and setting control parameters contributes to optimizing machine operations.

Several flow simulation packages are available to solve problems in everything from injection molding to designing hydraulic and pneumatic controls. So, don't build and break prototypes. Instead, simulate the problem.

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