Hydraulic hoses are the lifeblood of every machine. Unfortunately, in many DIY projects and workshop upgrades, they are selected entirely by eye or by choosing the cheapest fittings. This is a huge mistake.
Using too thin a hose in a high-capacity pump guarantees choked flow, rapid oil boiling, and loss of machine power. Oversizing the hoses, on the other hand, unnecessarily increases costs and complicates installation in tight spaces.
How to find the perfect balance? BRhydraulic engineers have prepared a comprehensive guide that explains the principles of flow rate and shows how to easily calculate the ideal inner diameter (DN) for your system.
1. Three zones of the system - different flow rates
The hose size is selected based on where in the system it will be used. Fluid behaves differently when pushed under extreme pressure than when flowing freely into a tank.
In power hydraulics, the following optimal oil flow rates (so-called speed tolerances v ) are assumed:
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Suction Line (From Tank to Pump): This is where the hose must be the thickest! Oil is being sucked in, not pushed out. A hose that's too thin will cause negative pressure and damaging cavitation in the pump (pieces of metal being torn from the teeth/pistons).
👉 Recommended oil speed: 0.5 to 1.2 m/s .
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Return/Return Line (From Manifold to Tank): Oil returns to the tank at very low pressure. If the hose is too thin, it will create artificial resistance (back pressure) that will damage the distributor seals.
👉 Recommended oil speed: 2.0 to 3.0 m/s .
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Pressure/Discharge Line (Between Pump, Manifold and Cylinder): Here the oil is pumped under pressure (e.g. 160-250 bar), so the hose can be the thinnest.
👉 Recommended oil speed: 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 the diameter of a hose? (Mathematical formula)
If you know the capacity of your pump (in liters per minute) and what type of line you are building, you can calculate the exact inner diameter of the hose (in millimeters) using the formula:
d = √ (21.22 * Q / v)
Where:
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d = inner diameter of the hose 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're building a system with a pump with a capacity of 60 l/min. We're looking for a hose for the pressure line (we assume an optimal speed of v = 5 m/s).
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Calculation:
d = √ (21.22 * 60 / 5)=√ 254.64≈15.9mm. -
Conclusion: You need a hose with an internal diameter of at least 16 mm (i.e. a standard DN16 or 5/8" hose).
3. Method for practitioners: Hydraulic Nomogram
If math isn't your thing, engineering uses what's known as a nomogram. It's a simple graph 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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Center Axis: Inside diameter of the hose.
How to use it? Simply place a ruler connecting your pump's capacity point (on the left axis) to your target flow rate (on the right axis). Where the ruler intersects the center axis will indicate the finished hose size (DN) in millimeters and inches.
Summary
Don't skimp on return and suction hose diameters – that's the quickest way to boil the oil and ruin the new pump. Always select diameters (DN) with an appropriate margin, using fluid velocity converters.
