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Calculate Pressure Drop through Expansion/Reduction for Gas

  1. Jul 31, 2017 #1
    Something that routinely comes up as a problem at work is when we size flow projects for piping systems. We calculate pressure drop across a length of fixed diameter for compressible gasses, but we do not calculate pressure changes through an expansion or contraction with gas (for example, pressure drop of air flowing at 3,500 scfm through 2" SCH 40 piping, expands to 4" SCH 40 piping). I can recall from my time in school that we would have used CFD to assist with a problem like this.

    Am I overlooking a way to be calculating this? I had envisioned something that could be incorporated into Excel, but it may be more difficult than that for compressible flow.
  2. jcsd
  3. Jul 31, 2017 #2


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    From a theoretical standpoint for there will be a reduction in velocity in the enlarged section and therefore an increase in the static pressure component and a resulting reduction in flow friction loss due to the lower flow velocity in the larger pipe..
    For sudden expansion, there is a formula for estimating the pressure loss based upon the ratio of the small to large diameters but for a well formed pipe expansion fitting the pressure loss is considered to be negligible.
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