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I have come across various values for maximum flow velocity of water in pipes. At university we were told that 2 m/s is the upper limit where flow becomes turbulent.

In various planning reports I have read recently, 1 m/s is stated as the upper limit. I could calculate the exact velocity at which water in a specific size and type of pipeline will no longer flow in a laminar way, but for infrastructure planning purposes, this exercise is not required. What would a reasonable value for potable water be? Alternatively, what would the value for raw water (river) be?

SlydeRule
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    Do you know of the Reynolds number? Above a certain critical value (2000 or so in a pipe), the flow transitions towards a turbulent regime. The water speed at which this happens then depends on the size and fluid properties. – nluigi Jul 18 '16 at 16:33
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    "but for infrastructure planning purposes, this exercise is not required." Why not? – Algo Jul 18 '16 at 16:43
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    Are you sure the limit is to prevent the flow becoming turbulent? As for a velocity of 1m/s the pipe would have to have a diameter approx 2mm for it to be laminar. More likely the velocity restriction is due to keeping pressure drop low. – CleptoMarcus Jul 19 '16 at 14:33
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    Most flow restrictions are to prevent cavitation. – Usagi Jul 19 '16 at 15:47
  • Thank you all for the replies. For "cigarette box" or rough estimates during pre-feasibility stage for large projects one often works with rule of thumb values to accelerate the process and save on time. I also think the 1-2 m/s rule of thumb has to do with factors other than laminar/turbulent flow. – SlydeRule Jul 20 '16 at 11:57
  • Rules of thumb are dependent on context. For flow of water in a pipe generally, you are balancing the trade off of pressure loss and line size. Pressure loss = electricity usage where water is being pumped. Line size = capital costs and foot print of the plumbing. In this respect it's a tradeoff of operating vs. capital costs. To optimize these it depends on context: location, project goals, etc. If you want a rough estimate of what velocities are reasonable, a set of tables like this is instructive – Byron Wall Jul 21 '16 at 03:55
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    @Usagi, I don't think the concern when moving water around is cavitation. Vapor pressure of water at ambient temperature is around 0.02 atm. If the system is discharging to 1 atm, it's very unlikely to reach that sort of vacuum while still discharging the water. Also, I assume OP is talking about water velocity in a run of pipe (downstream of any pumps) so cavitation can't happen. – Byron Wall Jul 21 '16 at 04:03
  • @ByronWall When there is upstream control on a discharge pipe in a dam, cavitation is not uncommon, especially if there are no air vents supplied to relieve the lack of pressure. ( https://www.fema.gov/media-library-data/20130726-1516-20490-4203/fema484part2.pdf ) This occurs at much higher velocities than those that the OP is describing. – Adam Mar 27 '17 at 19:28

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Pipe diameter and material of construction is also a factor. One must take the volume of water into account. With smaller diameters, such as factory reticulation, 1 meter per second is usually the upper limit to prevent hydraulic shock (water hammer), which can destroy some valves, such as butterfly valves.
With larger diameter pipes, such as for wide area reticulation, even 1 meter per second can damage or destroy concrete pipes, so a bypass should be installed to protect valves and piping.

Barry
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Among mechanical engineers, such as me, a very general rule is about 10 feet per second. https://www.engineeringtoolbox.com/flow-velocity-water-pipes-d_385.html That's when water erosion of metal surfaces starts being significant, particularly when the flow turns, such as in elbows, Tees, and curves in general. It's also when friction loss becomes noticeable and for some elaborate systems of large extent, pumping costs go up. Whether the flow is turbulent isn't of consequence in itself, and it would be nearly impossible to avoid it in most practical systems. Yes, it may be surprising, but water erosion can be a problem, and in general that's different from cavitation, which occurs when the pressure in a liquid becomes lower than the liquid's vapor pressure. Most of that is avoided by keeping the inlet of a pump, for instance, above the Net Positive Suction Pressure (NPSH) specification for that pump. Cavitation can also occur in other applications, such as propellers with liquids. And again, the design parameter there is fluid pressure, not velocity.

ttonon
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  • Welcome on the Engineering SE! I suggest to put some reference into your post, it helps a lot to collect upvotes. :-) – peterh Mar 06 '20 at 17:42
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The velocity of water varies with pipe diameter, flow rate and nature of materials . Most range is 0.6-1.5m/s .

Petro
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  • Could you add more detail to this answer? Is this something that you have already heard, is it from experience, or is it from a reference of some sort? – hazzey Jun 29 '20 at 19:17
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Generally we want to limit the amount of velocity in a pipeline. One reason is that as velocity increases, the friction increases which in turn decreases the total energy we are delivering to the end user. This also means that all the money we spent adding energy in the pipeline is simply burned off with the friction.

Another reason is that sudden changes in velocity in a water system can have potentially catastrophic effects but if we limit the velocity then any sudden change in velocity will have a diminished impact on the system infrastructure.

Mbongeni Younsmus Bhebhe
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When modelling district heating network, I came across this figure showing recommended pipe diameters with respect to the flow velocity (of hot water). I hope it illustrates how maximum flow velocity should be considered w.r.t. pipe parameters.

To evaluate the effect of the chosen design guideline, it has been differentiated between four design guidelines. According to Nussbaum et al. [14] different design approaches exist, which are shown in Figure 1. [best2018impact]

As far as I understand, the reason behind the figure is how Reynolds number is calculated and related to turbulent flow.

  • [best2018impact] Best, I., Orozaliev, J., & Vajen, K. (2018). Impact of different design guidelines on the total distribution costs of 4th generation district heating networks. Energy Procedia, 149, 151-160.
Edward
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In South Africa we use the Guidelines for Human Settlement Planning and Design, or what we just call the (Civil) Red Book. This is a really good resource for smaller projects or quick feasibility estimates.

Chapter 9 in Volume 2 is about water supply and and it states on page 24: "Velocities in pipes should be approximately 0,6 m/s and should not exceed 1,2 m/s." This is in line with the 1 m/s you've picked up from reports.

ChP
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  • Hi Charl, thank you. We also use the Red Book in Namibia, for some reason I must have skipped that paragraph. – SlydeRule Jan 24 '18 at 19:08
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Higher velocities can lead to increased pumping costs and potential damage to the piping due to erosion or water hammer. On the other hand, lower velocities result in larger (and more expensive) pipes and may cause issues like fouling or solids deposition due to insufficient turbulence.

Cecil Harris
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Sometimes existing infrastructure limitations, such as chilled water pressure or pump capabilities, play a role in pipe sizing. In such cases, the available pressure drop may dictate the pipe size, regardless of general rules of thumb.

Cecil Harris
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