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Technical guide · method and practical tips

Hydraulic pressure, flow and power: what is the difference?

Pressure creates force, flow rate determines actuator speed, and power combines pressure and flow.

Reviewed

pumpcontrolactuatorQΔpPₕ = Δp · Q
The pump supplies flow. Pressure develops when the circuit resists motion.

Pressure does not directly create speed

F = p · A

Pressure develops when fluid meets resistance. For a given area, it determines theoretical available force.

With little load, a circuit can have flow while pressure remains low.

Flow rate determines speed

Q = v · A

Flow rate is fluid volume moved per unit time. In a cylinder, speed equals flow rate divided by active area.

Doubling flow approximately doubles speed if area remains unchanged.

Power links pressure and flow

Pₕ = Δp · Q

A system can operate at high pressure with low flow, or low pressure with high flow. Hydraulic power accounts for both quantities.

Electric motor input power will exceed useful hydraulic power because of pump, valve and actuator efficiencies.

Practical units and shortcut formula

In a coherent calculation, pressure is in Pa and flow rate in m³/s. Bar and L/min remain convenient in workshops.

  • 1 bar = 100,000 Pa.
  • 1 L/min = 1/60,000 m³/s.
  • Pₕ(kW) = p(bar)·Q(L/min)/600.
  • With overall efficiency η: Puseful = η·Pinput.

Numerical application

Worked example: 100 bar and 20 L/min

A pump supplies 20 L/min at a useful pressure difference of 100 bar.

  1. Use the practical formula Pₕ = p·Q/600.
  2. Pₕ = 100 × 20 / 600.
  3. Pₕ = 3.33 kW of hydraulic power.
  4. At 80% overall efficiency, useful power is about 2.67 kW.
  5. The drive motor must be sized above absorbed power, not only useful power.

Result: The circuit carries 3.33 kW of hydraulic power before losses. Part of this power will become heat.

Example based on the energy relation Δp·Q and SI conventions. Open exact source ↗

For checking and further study

Direct references

Each link points to the exact course, standard or publication page used, rather than a generic homepage.

  1. ISO 4413:2010 — Hydraulic fluid powerInternational Organization for StandardizationGeneral requirements for design, safety, efficiency and maintenance.
  2. The International System of Units, 9th editionInternational Bureau of Weights and Measures (BIPM)Official reference for coherent units, symbols and conversions.
  3. NIST Special Publication 811 — Guide to the SINational Institute of Standards and TechnologyPractical rules for writing and converting physical quantities correctly.