Hydraulic hoses are the lifeblood of any machine. Unfortunately, in many DIY projects and workshop modernizations, they are chosen entirely "by eye" or dictated by the cheapest crimp fittings. This is a huge mistake.
Using a hose that is too thin for a high-capacity pump guarantees flow restriction, rapid oil overheating, and loss of machine power. Conversely, oversizing hoses unnecessarily increases costs and complicates installation in tight spaces.
How do you find the golden mean? BRhydraulic engineers have prepared a comprehensive guide explaining the principles of flow velocity and showing how to easily calculate the ideal internal diameter (DN) for your system.
1. Three System Zones – Different Flow Velocities
Hose size is selected depending on where in the system it will operate. Liquid behaves differently when it is pushed under enormous pressure compared to when it flows freely into the tank.
In power hydraulics, the following optimal oil flow velocities (so-called velocity tolerances v) are assumed:
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Suction Line (From tank to pump): This is where the hose must be thickest! Oil is sucked, not pushed. A hose that is too thin will cause vacuum and the destructive phenomenon of cavitation in the pump (tearing off metal pieces from gears/pistons).
👉 Recommended oil velocity: 0.5 to 1.2 m/s.
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Return Line / Drain Line (From distributor to tank): Oil returns to the tank under very low pressure. If the hose is too thin, it will create artificial resistance (backpressure), which will damage the distributor seals.
👉 Recommended oil velocity: 2.0 to 3.0 m/s.
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Pressure Line / Delivery Line (Between pump, distributor, and cylinder): Here, oil is pumped under pressure (e.g., 160-250 bar), so the hose can be the thinnest.
👉 Recommended oil velocity: 4.0 to 6.0 m/s (in continuous systems) or up to 8.0 m/s (for short-term cycles).
2. How to Calculate Hose Diameter? (Mathematical Formula)
If you know your pump's flow rate (in liters per minute) and know which line you are building, you can calculate the exact internal diameter of the hose (in millimeters) using the formula:
d = √ (21.22 * Q / v)
Where:
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d = internal hose diameter in millimeters (mm).
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Q = oil flow (pump capacity) in liters per minute (l/min).
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v = recommended flow velocity in meters per second (m/s).
Example: We are building a system with a pump capacity of 60 l/min. We are looking for a hose for the pressure line (assuming an optimal velocity v = 5 m/s).
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Calculation:
d = √ (21.22 * 60 / 5)=√ 254.64≈15.9mm. -
Conclusion: You need a hose with a minimum internal diameter of 16 mm (i.e., a standard DN16 or 5/8" hose).
3. Method for Practitioners: Hydraulic Nomogram
If you don't like math, engineers use a Nomogram. This is a simple chart with three vertical axes:
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Left axis: Oil flow (l/min).
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Right axis: Flow velocity (m/s).
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Middle axis: Internal hose diameter.
How to use it? Simply place a ruler connecting your pump's flow rate (on the left axis) to the target flow velocity (on the right axis). The point where the ruler intersects the middle axis will indicate the ready hose size (DN) in millimeters and inches.
Summary
Do not skimp on the cross-sections of return and suction hoses – this is the fastest way to overheat the oil and destroy a new pump. Always select diameters (DN) with an appropriate margin, using liquid velocity converters.
