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I know that different geometries of piping, orifices etc... have different critical Reynolds values. If the value is exceeded laminar stream will turn to a turbulent form which causes a variety of problems. But why shouldn´t the value be too low?

I.e. for pressure piping up to around 300 bars I assume that I have to be inbetween of 3 to 12 m/s.

Do too low velocities cause problems like cavitation? What are the reasons?

FYI: To have a practical example. I am asking because I want to design a hydraulic system but I only have a NG25 valve available but NG6 is probably already enough.

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    $\begingroup$ The critical value is not fixed - it is a range and can be 1700 to 2200, but has experimentally, with very carefull preparation gone much higher 10000 or more. $\endgroup$ – Solar Mike Aug 1 '17 at 10:29
  • $\begingroup$ Thanks for your comment. Yes I already know that this value can even be as low as 200 for notches/slits. This value depends on the geometry/profile, viscosity and so much more. But the question is why I should not go below a certain point with the velocity. $\endgroup$ – André Rodriguez Aug 1 '17 at 11:54
  • $\begingroup$ A rule of thumb where I was once was up to 7m/s for delivery and less than 2m/s for the return as return was much lower pressure - think of the losses... $\endgroup$ – Solar Mike Aug 1 '17 at 12:17
  • $\begingroup$ I use: 0.5 to 0.8 m/s for Suction # up to 1,5 m/s for suction with intake # 2 to 4 m/s for return lines # 2 to 4 m/s @ 100bar for Pressure lines # 3 to 12 m/s @ max 315bar for Pressure lines $\endgroup$ – André Rodriguez Aug 1 '17 at 12:45
  • $\begingroup$ The lower your velocity, the bigger your pipe needs to be for a given flowrate, which means more money. So you run into a funding problem with very low velocities. $\endgroup$ – J. Ari Aug 1 '17 at 15:30
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There are lots of reliable and precisely controlled hydraulic systems that have very low or zero velocities. Some examples include:

  • Slow Hydraulic Positioners (rate controlled band saw arm)
  • Hydraulic Vises
  • Hydraulic Belt Tensoioners
  • Truck Scales

There is no base reason to imply a minimum velocity on a hydraulic system. Systems can be designed to work with zero flow. In some circumstances however it does make sense to have a minimum velocity:

  1. When line sizing for power transfer to a hydraulic motor, you wouldn't want to design for too low of velocity or lines become unnecessarily large, costing more money. This also increases the systems total volume requiring more oil to fill the system.
  2. In equipment for cold weather operation, we would drill small holes in the hydraulic cylinder heads to provide a small amount of flow to an intermittent or zero flow device (like a vise cylinder) in order to keep the hydraulic fluid and cylinder warm for responsive operation.
  3. Metering at very low velocities can be difficult with low quality proportional valves. High quality valves with positional encoder or pressure feedback can servo control at as low of flow-rate as you would like.
  4. Some low cost centrifugal hydraulic power units do not have pressure bypass or electric bypass valves. These units require some amount of continuous flow to keep from burning up.
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Most of these "guidelines" in engineering math is there purely for economic reasons. If the value goes very low it just means that your fluid is creeping along the pipe. Normally in a plant you would not want that, hence the guidelines of this must be above 3000 or for this pipe size you have this velocity. If you can work it out and get an answer then it is possible. You must personally decide what your requirements are.

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In a place where I worked they used as a rule of thumb was up to 7m/s for delivery (the pipes have to be a higher grade to support higher pressures and become expensive...) and less than 2m/s for the return as return was much lower pressure (with larger diameter pipes of a lower grade) - think of the losses and also think of the noise generated by the moving fluid, it is not always the economic reasons that come first but pumping power is also considered.

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  • $\begingroup$ Thanks for your comment. If I read the comments here i think my question has not been understood. I am constructing Hydraulic Power Units for years now and i know how to dimension a pipe. And i also know why higher velocities cause trouble. The bigger the pipe the lower the flow velocity if the volume flow stays the same. This is just a theoretical question. I absolutely know how to economically dimension the pipe diameter to be in a reasonable range. But still my question has not been answered. To say it in different words: "Why do very LOW flow velocities cause trouble".Thanks for your help! $\endgroup$ – André Rodriguez Aug 18 '17 at 10:54
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  • Low velocities can be problematic because they are difficult to meter (control) precisely.
  • High velocities can cause erosion in valves, and generate more turbulence due to the high momentum in geometry changes.
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