BIZOMAT MAKEOS · Hydraulics
Hydraulic Line Sizing (Velocity, Pressure Drop and Tube Rating)
Is this tube big enough for the flow, how much pressure does the line lose, and is the tube rated for the system pressure?
Sizing check of a hydraulic pressure, return or suction line: the least bore for the flow at your velocity limit (as the pipe size by velocity tool), the actual velocity, Reynolds number, friction factor and pressure drop through the line and fittings (as the pipe friction tool), the power lost, and the tube burst pressure by Barlow’s formula on the outside diameter with the working pressure rating at your design factor.
Open the calculatorRuns in your browser. No account needed.
How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Flow | L/min | Flow through the line (for a return line from a cylinder, include the area ratio). |
| Velocity limit for this line | m/s | Velocity limit you design to (commonly about 0.6–1.2 m/s suction, 2–4 return, 3–6 pressure lines). |
| Tube bore | mm | Inside diameter of the tube. |
| Tube outside diameter | mm | Outside diameter of the tube. |
| Working pressure | bar | Highest working pressure in the line. |
| Tube tensile strength | MPa | Tensile strength of the tube material. |
| Design factor (burst ÷ working) | Ratio of burst to working pressure (often 4 for hydraulic tube; follow your standard). | |
| Line length | m | Length of the line. |
| Sum of fitting loss coefficients | Sum of loss coefficients of bends, fittings and valves in the line. | |
| Oil viscosity at working temperature | cSt | Kinematic viscosity at operating temperature. |
| Oil density | kg/m³ | Density of the oil. |
| Tube roughness | mm | Absolute roughness of the tube bore (about 0.0015 mm drawn tube). |
| Allowed pressure drop in the line | bar | Pressure drop you accept along this line. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Flow
- What it is
- The volume of oil passing through this line each minute at the highest flow condition.
- Units and format
- Enter a number in L/min: litres per minute. The form accepts 0.000001 L/min to 100000 L/min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60 L/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Velocity rises in proportion; pressure drop rises faster than flow in turbulent flow.
- Get it right
- A return line from a cylinder’s cap end carries the pump flow times the area ratio, so it can exceed the pump flow.
What this means: Velocity limit for this line
- What it is
- The highest oil velocity you design this line for. Suction, return and pressure lines each have their own limit.
- Units and format
- Enter a number in m/s: metres per second. The form accepts 0.000001 m/s to 15 m/s. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 m/s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Sets the least bore and the velocity check only; pressure drop is unaffected.
- Get it right
- Use the limit for this line’s duty; a suction line needs a much lower velocity than a pressure line.
What this means: Tube bore
- What it is
- The inside diameter of the tube, through which the oil flows.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 500 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 16 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Velocity falls with the square of the bore, so pressure drop falls steeply as it grows.
- Get it right
- Tube is sold by outside diameter and wall; bore = outside diameter − 2 × wall.
What this means: Tube outside diameter
- What it is
- The outside diameter of the tube, used with the wall thickness in Barlow’s formula for the burst pressure.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 600 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 22 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- For a fixed bore, a larger outside diameter means a thicker wall and a higher pressure rating.
- Get it right
- It must be larger than the bore. Use the nominal tube size, not a fitting size.
What this means: Working pressure
- What it is
- The highest pressure the line sees in normal work, compared with the tube’s working pressure rating.
- Units and format
- Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0 bar to 1000 bar. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 160 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- It does not change velocity or drop, only the pressure rating check.
- Get it right
- Use gauge pressure at the relief valve setting, not the average operating pressure.
What this means: Tube tensile strength
- What it is
- The ultimate tensile strength of the tube material, from the tube standard or mill certificate.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 2000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 410 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Burst pressure and working rating are directly proportional to it.
- Get it right
- Use tensile strength, not yield strength; Barlow’s formula here is written for burst.
What this means: Design factor (burst ÷ working)
- What it is
- The ratio of burst pressure to allowed working pressure that your standard or company practice requires for hydraulic tube.
- Units and format
- Enter a plain number with no unit. The form accepts 1.5 to 10. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The working pressure rating is the burst pressure divided by this factor.
- Get it right
- Follow the standard that applies to your machine; it is a ratio, not a percent.
What this means: Line length
- What it is
- The length of straight tube in the line, from where the flow enters to where it leaves.
- Units and format
- Enter a number in m: metres. The form accepts 0.000001 m to 1000 m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The pipe friction part of the pressure drop is proportional to length.
- Get it right
- Measure along the route of the tube, not straight-line distance; bends are counted in the fitting coefficients.
What this means: Sum of fitting loss coefficients
- What it is
- The total of the loss coefficients for every bend, fitting, valve and entry or exit in the line, each counted once.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 200. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The fitting part of the pressure drop is proportional to it.
- Get it right
- Add one coefficient per fitting; a valve’s loss may be given as a pressure drop instead, which should be added separately.
What this means: Oil viscosity at working temperature
- What it is
- The kinematic viscosity of the oil at the temperature it actually runs at in this line.
- Units and format
- Enter a number in cSt: centistokes (kinematic viscosity, 1 cSt = 1 mm²/s). The form accepts 0.000001 cSt to 5000 cSt. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 46 cSt as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Higher viscosity lowers the Reynolds number; in laminar flow pressure drop rises in proportion.
- Get it right
- One cSt is one mm²/s. The ISO grade is the viscosity at 40 °C; cold oil is far thicker.
What this means: Oil density
- What it is
- The mass of one cubic metre of the hydraulic oil, from its data sheet.
- Units and format
- Enter a number in kg/m³: kilograms per cubic metre. The form accepts 0.000001 kg/m³ to 1500 kg/m³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 870 kg/m³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Pressure drop in turbulent flow is roughly proportional to density.
- Get it right
- Enter kg/m³, not kg/L or g/cm³.
What this means: Tube roughness
- What it is
- The average height of the roughness on the inside wall of the tube, used in the friction factor for turbulent flow.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 5 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.0015 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Rougher bores raise the turbulent friction factor and pressure drop.
- Get it right
- It only matters in turbulent flow; the laminar friction factor ignores it.
What this means: Allowed pressure drop in the line
- What it is
- The pressure loss you are willing to accept along this line, from your pressure budget for the circuit.
- Units and format
- Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0.000001 bar to 100 bar. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- It does not change the drop, only the pressure drop check.
- Get it right
- Base it on what the actuator or return filter can tolerate, not on the pump’s full pressure.
Method and formulas
- Least bore d = √(4Q ÷ πv_max); velocity v = Q ÷ (π d² ÷ 4).
- Re = v d ÷ ν; f = 64 ÷ Re laminar, Colebrook turbulent; Δp = (f L ÷ d + ΣK) ρ v² ÷ 2 (as the pipe friction tool).
- Barlow burst pressure p_b = 2 S_u t ÷ D_o with t = (D_o − d) ÷ 2; working rating = p_b ÷ design factor.
- Example: 60 L/min in a 16 mm bore runs at 4.9736 m/s, loses 3.0261 bar and the tube is rated about 279.55 bar.
Limitations
- Straight-tube rating; fittings, hoses and welded joints have their own ratings.
- Viscosity must be taken at the actual oil temperature.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Choose a tube size for a new hydraulic circuit
- Find why a return line runs hot or noisy
- Check a tube’s pressure rating against a pressure increase
Who it is for
- Hydraulic engineers
- Maintenance engineers
- Machine designers
What you get
- Least bore and actual velocity
- Pressure drop and power lost
- Tube burst and working pressure rating
Interpreting the result
- Pressure drop rises about as velocity squared in turbulent flow: one tube size up often halves the loss.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3024–3027, 281–286. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Flow in lines, hoses and filters
What it covers: Calculate laminar and transitional pressure drop in oil lines with the Hagen–Poiseuille law and the BIZOMAT line sizing tool, and see how a cold start multiplies losses in lines, strainers, filters.
BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
Open the Hydraulic Line Sizing (Velocity, Pressure Drop and Tube Rating) calculator