Fluid Metering with Defined Precision

01.06.2012

Metering systems in process technology have the task of supplying a fluid, under specified process conditions (pressure, temperature), either in a defined amount or in a time-related flow rate, or of mix-ing two fluids according to specifications.

This is no trivial task. To solve it, several system components must be perfectly coordinated (meter-ing pump, flow meter, controller, and actuators) and the entire system must be integrated into the application (whether in the lab, in the testing facility, or in production).

For optimum monitoring of metering systems, Lewa offers a controller, the Lewa smart control. It compares the actuating variable of the metering pump (motor speed or stroke length) and the meas-urement signal from the flow meter and checks both values permanently for plausibility. This allows the reliable identification of deviations, ensuring high system availability. Even wear in the metering pump is detected. The smart control regulates deviations very quickly, and the controller can even access saved pump characteristic curves. The learning function, or adaptive control, ensures fast adjustment of the actual value, without the significant transient fluctuations produced by conventional PID controllers. At process start-up, with smart control, the actual value adjustment already reaches a maximum deviation of ± 1 % after only about 30 seconds. That lowers the scrap and keeps product quality uniformly high, even with pressure fluctuations in the system.

Examples where the operator gets maximum benefit from Lewa smart control:

  • Controlling the metering flow: If the process requires to maintain a continuous mass or volumetric flow, this flow has to be controlled flexibly and reliably, for example by injecting blowing agents during extrusion processes. Here, the speed of the extruder is the guide value for the diaphragm metering pump. Using smart control, the blowing agent can be added proportionally to the extruder speed into the plastic melt.
  • Batch metering: This case occurs in all filling or refilling processes, for example if an adjustable metering volume in a tank must be tracked by level. This regulation type is also used, if a reactor needs a specific quantity of a chemical agent metered according to a recipe.
  • Mixing processes (such as dilution of caustic soda) are the most frequent use of metering systems in processing industry. In these applications, smart control handles the adjustment of two metering flows in the correct ratio.

By means of up to four actuators the controller regulates one or even two closed control loops. This allows the operator to control a redundant system or two completely independent metering systems with a single controller.

The controller operates with almost every type of metering pump and measuring system.

Picture: Lewa smart control offers operators in process technology intelligent metering flow controls, for example when main-taining a continuous mass or volumetric flow (Image: Lewa)

Source: LEWA GmbH

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