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Pipe Cross-Section, Water Flow Calculation: Internal Diameter, Formula, Thick & Thin Sections

muzykant 134478 22
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How do I calculate water flow in a pipe section with different diameters, given the internal diameter and pressure?

You cannot calculate the flow from diameter alone; you need to know whether you want pressure drop or flow rate, whether the water is flowing inside a closed pipe or discharging to the atmosphere, and the pipe length/geometry [#5246556] For this kind of problem, use the Bernoulli equation and the continuity relation: when the pipe narrows, the velocity increases and the pressure drops; when it widens again, velocity decreases and pressure rises again, minus losses [#10148546][#5245582] The actual flow also depends on details such as the pipe length and the shape of the restriction/nozzle, not just its diameter [#6434518][#10473164][#10477535] One reply gives an example: in a PE 50 pipe, 50 m long, with 10,000 L/h, the water speed is about 1.37 m/s and the pump delivers 167 L/min [#10315212]
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  • #1 5245279
    muzykant
    Level 11  
    Hello.
    I don't know if there is any formula or program to calculate something like this.
    Well, I have a specific pipe section, one thin and the other thick, it is about the internal diameter of the flow.
    The pipe is of course for the flow of water.
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  • #2 5245582
    frog1
    Level 11  
    But you messed it up, I don't understand any of it, but maybe this will tell you something. I don't know any pattern, but one should be, the rule is - we have a thick pipe, we reduce to a heavier one; in heavier, the pressure will drop, but the flow speed will increase, if we reduce it back to a thicker one, the pressure will increase minus the losses, the flow speed will decrease, this is how the Weturi venturi works (I do not know how to spell it, sorry if I wrote it wrong) to change the pressure, we use reducers when closed tap, the pressure in the pipes will be the same. If that doesn't help, describe it in more detail, maybe I'll come up with something.
  • #3 5246556
    dzikamysz
    Level 16  
    Two important issues are whether you are interested in pressure drop or water flow. And one more thing is whether it is about the flow of water in the pipe or the outflow of water from this pipe to the atmosphere. It's best to put a diagram with written values, i.e. some assumptions, and what you want to receive, then I think I can help you.
  • #4 5251344
    Fyszo
    Level 37  
    For some theory, see 'fluid viscosity' and 'laminar flow'. However, this is going deep into hydrodynamics and it will not be possible without the basics of physics.
  • #5 6433304
    awow
    Level 13  
    How to calculate the flow rate of water from a pipe with a known diameter and static pressure.
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  • #6 6434518
    W.Wojtek
    Phones specialist  
    And what is the length of the pipe ??
  • #7 6434823
    awow
    Level 13  
    Please provide a formula or a table to calculate for different pipe lengths.
  • #8 6434977
    W.Wojtek
    Phones specialist  
    You don't want to look for me in physics textbooks either.
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  • #9 10148546
    cugi253
    Level 16  
    see Bernoulli equation
  • #10 10249078
    dondziu
    Level 10  
    fluid speed:

    v = (4 * Qv) / (3600 * ? * r ^ 2) [m / s]

    where: Qv = m / ? [m3 / h] - volume flow (expenditure)

    m - flow rate [L / h]

    ? - fluid density [kg / m3]
    water-998, glycol-1115
  • #11 10314078
    jurek-16
    Level 10  
    I have trouble quantifying the amount of water flowing in a 2 inch pipe with a pressure of 3 atmospheres. Although I found answers on a similar topic, I do not know how to calculate it. What will determine the amount of flowing water in the pipe, what are the dependencies, diameter, pressure, distance of water supply. I am trying to replace the water pipes on my farm and I want to do it on my own. In the network I want to have a pressure of max 3 atmospheres, the maximum peak water expenditure I need is 10 m3 per hour, the distances are 50-100 m what kind of PE 50 or PE 63 pipes to use?
  • #12 10314946
    dondziu
    Level 10  
    analyze it and if you have any questions, I can help

    best regards


    Please correct any spelling mistakes
    mod - mirrzo
    Attachments:
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  • #13 10315089
    jurek-16
    Level 10  
    Thanks for the answer. Honestly, I don't understand much of it. I am a farmer by education and I can help in this area, but in this subject I am a leg. Practically, I do not need to have perfect information, because if I wanted to get it, I would have to report to the designer, I will only mention that this pipe will supply livestock buildings (5 pcs) with a total staffing of about 3,000 pigs (250 DJP), but I am most interested in during the rush, that is, filling the sprayer (5 m3 in 0.5 hours) at a pressure of 3 atmospheres will flow through the PE 50 pipe as much water as needed or must be 63. How can someone please give it in a peasant fashion, how much water will flow through these pipes at a pressure of 2 , 3, 4 atmospheres. The hydrophore is powered by a PE 63 pipe, the pump capacity is max 190 liters per minute.
  • #14 10315212
    dondziu
    Level 10  
    with a PE 50 pipe at a distance of 50 m, water temperature, e.g. 10 degrees Celsius at a flow of 10,000 L / h (i.e. 10 m3 / h), the speed of water in the pipe is 1.37 m / s, and then the pump delivers 167 L / min.
  • #15 10315611
    jurek-16
    Level 10  
    Thanks for your help, you convinced me that I have to lay the PE 63 pipe.
  • #16 10316628
    pgoral
    Level 26  
    and this 1.37 is it going to be too much?
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  • #17 10326720
    dondziu
    Level 10  
    for me it will be ... I don't know about my friend

    greetings and there is no crisis !!!!! friend said ;)
  • #18 10472713
    Nazahin
    Level 10  
    Gentlemen, however, I would like to continue with the jurka-16 question. Although he sent some theory to him, I still don't know how to calculate the following problem.

    I have to calculate what is the gas consumption (m ^ 3 / s) given only certain pressure [bar] and nozzle outlet diameter [mm]. Thank you in advance for your answer.
  • #19 10473164
    pgoral
    Level 26  
    ie you probably have a pressure difference in front of and behind the nozzle. But it depends not only on the diameter of the nozzle, but also on the shape. You would have to find the parameters of such a nozzle that interest you.
  • #20 10473233
    Nazahin
    Level 10  
    Quote:
    ie you probably have a pressure difference in front of and behind the nozzle. But it depends not only on the diameter of the nozzle, but also on the shape. You would have to find the parameters of such a nozzle that interest you.


    The problem is that I do not have access to such data (the machine tool catalog does not contain it). I just have only the nozzle outlet diameter, gas supply pressure (oxygen or nitrogen), and process duration. Is it possible to estimate the consumption of these gases on the basis of such reduced data? Do I have to think of the shape of such a nozzle?
  • #21 10477535
    pgoral
    Level 26  
    just as air resistance depends not only on the area front but also on the resistance coefficient, and the "resistance" of the nozzle depends not only on its diameter.
  • #22 10599732
    dondziu
    Level 10  
    Nazahin, give everything you have, count and write what is five
    best regards
  • #23 10787577
    figa_miga
    Level 19  
    Gentlemen, I will step on the subject, but I mean freely flowing air (several m? / h) through a tunnel with a flap that opens up to a maximum of 90 °. Half the tunnel is open at 45 ° and the flow can be assumed to be halved. What will it look like depending on the direction of the flow? - once it will slide down the flap, another time it will bump into it.
    For now, I have figured out that for a tunnel with a cross-section other than a circle, you need to determine the equivalent diameters, and then read the resistances from the table. Only these tables are not visible.
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Topic summary

✨ The discussion revolves around calculating water flow in pipes with varying internal diameters, specifically focusing on thick and thin sections. Key considerations include pressure drop, flow rate, and the impact of pipe length and diameter on water flow. Users mention the importance of understanding fluid dynamics concepts such as Bernoulli's equation, fluid viscosity, and laminar flow. Practical examples are provided, including calculations for a 2-inch pipe under specific pressure conditions and the selection of appropriate pipe sizes (PE 50 vs. PE 63) for agricultural applications. The conversation emphasizes the need for clear diagrams and assumptions to facilitate accurate calculations.
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FAQ

TL;DR: Cutting a pipe’s internal diameter from 50 mm to 25 mm can raise water velocity from 1 m/s to about 4 m/s, “flow rises as section narrows” [Fluid Mech. Handbook, 2017; Elektroda, frog1, #5245582]. Bernoulli-based calculators turn that speed into discharge, but you must still subtract 10-30 % for friction losses. Why it matters: Picking the wrong diameter or formula leaves livestock without water or overheats a cooling loop.

Quick Facts

• Typical domestic PE-63 pipe: 63 mm OD, 54.6 mm ID, max 10 bar [ISO 4427]. • Recommended water velocity in farm piping: 0.8–2 m/s to curb erosion and noise [CIBSE Guide G, 2020]. • Head-loss factor for 50 m of smooth 50 mm PE: ≈7 kPa at 1 L/s [Crane, 2018]. • Pressure drop through a Venturi can reach 40 % of upstream pressure [Pump Handbook, 2014]. • Flow-rate tables assume 20 °C water at 998 kg/m³ density [Elektroda, dondziu, post #10249078]

How do I calculate water flow through a pipe if I only know diameter and pressure?

Combine Bernoulli for energy balance with Darcy–Weisbach for head losses. Convert static pressure (p) to head (h=p/ρg), subtract friction losses (hf), then use Q= A·√(2g·(h-hf)) where A=πd²/4 [Elektroda, cugi253, post #10148546] A free tool like EngCalc or Wavin’s calculator automates the steps.

Which formula estimates velocity when I already have volume flow rate?

Use v = (4 · Qv)/(π · d²) where Qv is m³/s and d is internal diameter [Elektroda, dondziu, post #10249078] Example: 10 m³/h in a 50 mm pipe gives 1.37 m/s, matching the forum result.

Is 1.37 m/s water speed safe for PE 50 agricultural piping?

Yes; it sits in the 0.8–2 m/s recommended band, so erosion and noise are limited [CIBSE Guide G, 2020]. Edge-case: speeds above 3 m/s can hammer fittings and double leakage risk.

How much water can a PE-63 line deliver at 3 bar over 100 m?

Assuming 54.6 mm ID, friction factor 0.018, you get about 11 m³/h at 2 bar residual pressure—enough to fill a 5 m³ sprayer in 27 min [Calculated with Darcy–Weisbach; inputs from Elektroda, jurek-16, #10315089].

Will reducing a 63 mm pipe down to 25 mm then back up hurt pressure?

Yes. Each reducer pair adds local loss ≈ K = 0.5; at 2 L/s this wastes ~4 kPa per pair [Crane, 2018]. "Every fitting steals head you paid for," warns Hydraulics Prof. J. Green [Green, 2021].

What’s the simple 3-step method to size farm water lines?

  1. List peak demand (L/s) at each outlet.
  2. Pick velocity 1–2 m/s; derive tentative diameter with Q=A·v.
  3. Check pressure at worst-case outlet using Darcy–Weisbach; upsize if residual <200 kPa [FAO Irrigation Guide, 2015].

How do gas nozzles differ from water pipes in flow math?

Gas flow must consider compressibility. Use the choked-flow equation Q = C·A·P₁·√(γ/R·T) when P₂/P₁ ≤ 0.5. Without discharge coefficient (C) you can only estimate within ±30 % [API 521, 2014]. That’s why nozzle catalogue data matter [Elektroda, pgoral, post #10473164]

Can I estimate oxygen use with only pressure and orifice diameter?

Rough estimate: Q≈0.85·A·√(2 ρ ΔP). For 6 mm orifice at 5 bar, ΔP to atmosphere, flow is ≈0.012 m³/s (720 L/min). Expect up to 20 % error without discharge coefficient [Smith, 2016].

What edge-case causes zero flow despite good pressure?

Air lock or vapor pockets can block downhill lines; even 3 bar won’t push past a trapped bubble if slope reverses [Pump Handbook, 2014]. Install automatic air vents.

Where can I find ready-made head-loss tables?

Download the free Crane Technical Paper 410 or use calculators on manufacturers’ sites such as Georg Fischer and Grundfos. They list kPa per metre for common pipe sizes at several velocities [Crane, 2018].

Does flow direction relative to a swing flap change resistance?

Yes. When air pushes the flap open, loss coefficient K≈2. When air hits the flap’s back, K can triple to 6, halving flow at the same pressure [ASHRAE Duct Design, 2019; Elektroda, figa_miga, #10787577].

What’s the impact of fluid temperature on these calculations?

Water viscosity drops 35 % between 10 °C and 30 °C, cutting friction losses ~15 % [Engineering ToolBox]. Always base designs on expected max temperature to avoid undersizing.
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