Let's say I have a pipe followed by a reduction and then another smaller diameter pipe continues. Using conservation of mass, the flow through the smaller diameter pipe will have higher velocity. If I apply Bernoulli to this system, while higher velocity, the smaller diameter pipe's flow has lower static pressure.

So this means if I have pressure gauges installed both before and after this reduction they would measure different pressures? And the pipe wall design should be different (eg. each side should take into account different pressure values)?

  • $\begingroup$ And the length of each pipe section will cause a pressure drop. You can derive the expression for pressure drop using dimensional analysis or it is stated in many texts. $\endgroup$
    – Solar Mike
    Jan 22, 2022 at 6:47
  • $\begingroup$ Yes, you will get the different pressure values at the intake and outlet, but the pressure differential is mainly caused by the velocity head, which I doubt has any significance in pipe wall design. I could be wrong though. $\endgroup$
    – r13
    Jan 22, 2022 at 20:13

1 Answer 1


You can't design for a lower pressure in higher velocity sections of piping because you have to design for the zero flow pressure as well.


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