BIZOMAT MAKEOS · Hydraulics
Hydraulic Orifice Restrictor Design (Drill Size, Flow, Heat)
What orifice size gives the flow I want at this pressure drop, what flow does the nearest drill size actually give, and how much heat does the restrictor put into the oil?
Design of a fixed orifice to limit flow in a hydraulic or lubrication circuit: exact orifice diameter for the target flow at the pressure drop, the diameter rounded to your drill steps or one you choose, the flow it gives and the pressure drop it needs for the target flow (as the orifice flow tool), jet velocity, the power turned to heat and the temperature rise of the oil passing through, checked against your flow tolerance, least orifice size for clogging and temperature rise limit.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Flow wanted | L/min | Flow you want through the restrictor. |
| Pressure drop available | bar | Pressure difference across the restrictor. |
| Discharge coefficient | Discharge coefficient of the orifice (about 0.6–0.7 for a sharp-edged orifice). | |
| Fluid density | kg/m³ | Density of the fluid. |
| Fluid specific heat | kJ/kg·K | Specific heat of the fluid (mineral oil about 1.9 kJ/kg·K). |
| Drill size step | mm | Steps between available drill sizes. |
| Orifice diameter chosen (0 = rounded) | mm | Orifice diameter you intend to use; 0 rounds the exact size to the drill step. |
| Limits | ||
| Flow tolerance | % | How far the flow may differ from the target. |
| Least orifice diameter (clogging) | mm | Smallest orifice you accept with your filtration. |
| Largest oil temperature rise | K | Largest temperature rise of the fluid passing through. |
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 wanted
- What it is
- The flow you want to pass through the restrictor at the available pressure drop, for example to feed a lubrication point or slow an actuator.
- 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 4 L/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The exact orifice diameter rises with the square root of flow.
- Get it right
- Type litres per minute.
What this means: Pressure drop available
- What it is
- The pressure difference between the upstream and downstream sides of the restrictor when the target flow passes.
- Units and format
- Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0.000001 bar to 5000 bar. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 70 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The temperature rise is proportional to the pressure drop.
- Get it right
- It is the difference across the orifice, not the system pressure.
What this means: Discharge coefficient
- What it is
- The discharge coefficient of the orifice: the ratio of the actual flow to the ideal flow through the same hole, which depends on the edge shape and length.
- Units and format
- Enter a plain number with no unit. The form accepts 0.1 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.62 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- For a given hole, flow is proportional to it.
- Get it right
- Take it from orifice data or a flow test for this geometry.
What this means: Fluid density
- What it is
- The mass per unit volume of the hydraulic or lubricating fluid at its working temperature, from the fluid data sheet.
- Units and format
- Enter a number in kg/m³: kilograms per cubic metre. The form accepts 0.000001 kg/m³ to 2000 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.
What this means: Fluid specific heat
- What it is
- The heat needed to warm one kilogram of the fluid by one degree, from the fluid data sheet. It sets the temperature rise through the orifice.
- Units and format
- Enter a number in kJ/kg·K: kilojoules per kilogram per kelvin. The form accepts 0.000001 kJ/kg·K to 10 kJ/kg·K. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.88 kJ/kg·K as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher specific heat lowers the temperature rise in proportion.
- Get it right
- Type kJ/kg·K; data in J/kg·K must be divided by 1000.
What this means: Drill size step
- What it is
- The size steps between the drills you have available. The exact diameter is rounded to the nearest multiple of this step.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.01 mm to 5 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.1 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- It is only used when no orifice diameter is chosen.
What this means: Orifice diameter chosen (0 = rounded)
- What it is
- The orifice diameter you intend to drill. Type 0 to round the exact diameter to the nearest drill step instead.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 200 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Flow at the pressure drop rises with the square of the diameter.
Limits
What this means: Flow tolerance
- What it is
- How far, either way, the flow through the drilled orifice may differ from the target flow, as a percentage of the target.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0.5% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Least orifice diameter (clogging)
- What it is
- The smallest orifice diameter you accept, so that it does not clog with the particle size your filtration lets through.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 50 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.8 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Take it from your filtration rating and design rules.
What this means: Largest oil temperature rise
- What it is
- The largest temperature rise of the fluid passing through the orifice that you accept, set by the fluid, seals or nearby components.
- Units and format
- Enter a number in K: kelvin (a difference of 1 K equals 1 °C). The form accepts 0.000001 K to 200 K. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 K as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- A rise in kelvin equals the same rise in °C.
Method and formulas
- Q = C_d A √(2Δp ÷ ρ), so d = √[4Q ÷ (π C_d √(2Δp ÷ ρ))] (as the orifice flow tool).
- Chosen d rounded to the drill step; flow at the chosen d and Δp needed for the target flow.
- Heat = Δp × Q; temperature rise = Δp ÷ (ρ c_p).
- Example: 4 L/min at 70 bar needs 1.0389 mm; a 1 mm hole gives 3.7063 L/min and heats the oil 4.2798 K.
Limitations
- Turbulent flow through a sharp-edged orifice; at low Reynolds numbers C_d falls.
- Cavitation and flow forces are not checked.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size a restrictor for a lubrication point or pilot line
- Limit cylinder speed with a fixed orifice
- Check heat from throttling
Who it is for
- Hydraulic engineers
- Machine designers
- Maintenance technicians
What you get
- Exact and chosen orifice diameter
- Flow at the chosen size and pressure drop for the target
- Jet velocity, heat and temperature rise
Interpreting the result
- Flow changes with the square of the diameter: a 0.1 mm step on a 1 mm hole is about 20% in flow.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3024–3027, 352–368. Topic reference; no source tables or recommended settings are reproduced.
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