Overview
ISO 6358-3:2014 - Pneumatic fluid power - Part 3 - specifies a numerical method to calculate steady-state flow-rate characteristics of systems made from components and piping whose individual flow characteristics are known. The standard lets engineers estimate overall system behavior for compressible fluids (air) without performing full-system measurements. It covers both subsonic and choked (sonic) flow regimes and provides practical formulae and calculation procedures for systems with components in series or parallel.
Key technical topics and requirements
- Purpose: Use a simple numerical technique to estimate overall flow-rate characteristics of an assembly from component C, b, m and Δp parameters.
- Characteristic parameters:
- Sonic conductance (C) - defines maximum (choked) flow.
- Critical back‑pressure ratio (b) - boundary between choked and subsonic flow.
- Subsonic index (m) - refines subsonic flow behaviour.
- Cracking pressure (Δp₍c₎) - for valves that open with upstream pressure (check valves, one‑way controls).
- Flow regimes covered: Explicit formulae for choked and subsonic flows; zero-flow condition when upstream pressure below cracking pressure.
- Calculation organization: Step‑by‑step procedures for components connected in series (Clause 6) and parallel (Clause 7), including determination of sonic conductance, critical back‑pressure ratio, subsonic index, and optional pressure‑dependence coefficient.
- Assumptions and hypotheses: Adiabatic flow, matching stagnation pressures at measurement points, same inlet/outlet pressure conventions; conductance values are valid for the upstream pressure at which they are determined.
- Conversion guidance: Methods to obtain equivalent ISO 6358 parameters for components expressed with other flow coefficients (see Annex D).
- Supporting material: Worked examples, flow charts, visualization guidance and bibliography (Annexes A–E).
Practical applications and users
ISO 6358-3:2014 is valuable for:
- Pneumatic system designers estimating system flow capacity and pressure drops without prototype tests.
- Component manufacturers reporting how valves, silencers, filters and piping combine in assemblies.
- System integrators and OEMs performing early-stage performance modelling and sizing.
- Test laboratories and maintenance engineers comparing calculated system behavior with measured results.
- Researchers analyzing pneumatics where compressible-flow behavior (choked vs subsonic) matters.
Practical uses include sizing supply lines, selecting valves and silencers, predicting blow-off/nozzle performance, and rapid "what‑if" analysis during system optimization.
Related standards
- ISO 6358-1:2013 - General rules and test methods for steady‑state flow (normative reference).
- ISO 6358-2 - Alternative test methods (discharge/charge tests).
- ISO 5598 - Fluid power vocabulary (terms and definitions).
Keywords: ISO 6358-3:2014, pneumatic fluid power, flow-rate characteristics, compressible fluids, sonic conductance, critical back-pressure ratio, subsonic index, cracking pressure, system flow calculation.