I am pumping various chemicals (oils, resins and solvents) over a distance of up to 12 metres. The inlet and outlet points are at the same height. For the majority of the distance, the pipework will be 1.5". At the final 500mm of pipework, the simplest and cheapest option for my design is to step the pipes down to 1.25".

Can I expect much reduction in flow rate due to this diameter reduction right at the end? Or will the losses over the rest of the pipework make this step-down a negligible drop?

Edit: to narrow down the answer, assume 12m total pipe length, 6x 90° bends, no valves or other obstructions, liquid 960 kg/m3 and 240 mPas. Flow rate up to the diameter reduction is about 55L per minute. Some clarification of the pipework:

  1. material is mostly stainless steel with maybe 25% poly hose.
  2. I am not expecting the pipe to degrade, rust, or otherwise change internal properties over time.
  • $\begingroup$ Yes, it will reduce flow. How significant the reduction is would be subject to your specs. Are you trying to engineer a solution or just ask if fluid will still flow? Yes it will still flow, but there isn't enough information here to determine the actual pressure drops. $\endgroup$
    – Tiger Guy
    Nov 15, 2021 at 3:41
  • 2
    $\begingroup$ scaling (at laminar = low-medium flow rates, unverified assumption): p_drop/length proportional to 1/D^4 .... so the last 1/24th of your total length, with 1.25" pipe, will contribute about twice the pressure drop compared to an equivalent length of the 1.5" pipe. Overall should be barely noticeable. $\endgroup$
    – Pete W
    Nov 15, 2021 at 4:42
  • 2
    $\begingroup$ @PeteW I think your comment with just a little effort would make a fine answer. Please post it as such. $\endgroup$
    – Eric S
    Nov 15, 2021 at 5:10


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