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Flow rates

  • As a matter of principle, there are hardly any limitations to the maximum permissible flow rate. On the other hand, when the flow rates drop below a minimum value, problems frequently arise in practice
    • With water or aqueous liquids, noise is developed at flow rates beginning at approx. 3.5 m/s; for this reason, this is usually considered to be the upper limit. Liquids with higher density or higher viscosity should flow more slowly. With gases, the flow rates are considerably higher. The usual maximum flow rate for carbon dioxide is approx. 30 m/s, for compressed air approx. 50 m/s and for steam approx. 100 m/s.
    • At increasing flow rates, the pressure loss also increases. However, this hardly has any practical significance with short pipelines.
    • Small nominal sizes and high flow rates are preferable, because this
      • reduces the purchasing and installation costs
      • reduces losses in product, rinsing water and cleaning agent
      • makes CIP cleaning more effective and
        • the mixing zones,
        • cleaning agent quantities and
        • heat radiation losses are reduced.
    • Usually, products similar to water are pumped at flow rates  ensuring turbulent flow (in actual processes frequently above approx. 0.3 m/s). 
    • If the flow rates are too low, any solids can settle down in the line.
      • Lines for untreated water sludge up easily at low flow rates.
      • Flow rates greater than 1.6 m/s are therefore usually recommendable for these reasons.
    • The influence of the flow rate on product damage which results from shear forces is negligible for most applications. Many, usually older literature specifications no longer correspond to the current state of the art.
      • When a butterfly valve is closed quickly, the flow separation causes a vacuum on the flap and in the area of the butterfly valve gasket. At flow rates of approx. 2.5 m/s, the valve should be closed only at significantly reduced speed to reduce pressure impacts and prevent a damage of the valve.

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