hydraulic-networks

name	hydraulic-networks
description	Hydraulic networks — series/parallel circuits, accumulator sizing, pressure compensation, variable displacement pumps, load-sensing systems, flow sharing, mobile hydraulics, ISO 4413.
metadata	{"priority":7,"promptSignals":{"phrases":["hydraulic network","hydraulic circuit","hydraulic system design","accumulator hydraulic","load sensing hydraulic","variable displacement pump","parallel hydraulic"],"minScore":3}}

Hydraulic Networks — Complete Skill

Fundamental Relationships

Flow continuity: Q_pump = Q_actuator1 + Q_actuator2 + ... + Q_leakage [L/min]

Pressure drops (series): P_supply - P_tank = Σ ΔP_component [bar] ΔP_components: valve ΔP, pipe ΔP, filter ΔP, cylinder back-pressure

Hydraulic power: P = Q × ΔP / 600 [kW; Q in L/min; ΔP in bar]

Pump efficiency: η_pump = Q_actual × ΔP / P_motor = η_vol × η_mech η_vol (volumetric): 90–97%; η_mech: 88–95%; combined: 80–90%

Series vs. Parallel Hydraulic Circuits

Series

Same flow through all components; pressures add Q_total = Q (one flow rate) P_supply = P_cyl1 + P_valve1 + P_cyl2 + P_valve2 + ΔP_lines

Use for: synchronized motion (both actuators travel at same speed) Problem: if one cylinder loads more → all pressure drop there; others starve

Parallel

Same pressure at inlet junction; flows divide Q_total = Q₁ + Q₂ + ... Each branch: Q_i = Cv_i × √(ΔP_i) for valve-controlled branches

Unequal loading: higher load takes less flow; lower load steals flow → velocity runaway Solution: flow-sharing valves (LUDV) or pressure-compensated flow controls

Pressure Compensation

Individual Pressure Compensators

Each directional control valve section has own pressure compensator Maintains constant ΔP across metering orifice regardless of load Flow ∝ spool position only; independent of load

CEMB (Closed Center Metering Block): full pressure compensation; every section independent

LUDV (Load-Independent Flow Distribution)

German: "Lastdruckunabhängige Durchflussverteilung" How: all sections get same max load pressure as signal; each compensator adjusts to maintain equal flow priority If pump output exceeded: all functions reduce speed proportionally (priority preserved) Used in: mobile cranes, excavators, multiple-function mobile equipment

Open Center (Fixed Pump, Uncompensated)

Pump always runs; unneeded flow returns over-center to tank at low pressure Simple; low cost; energy inefficient (pump always running at full flow) Common in low-cost agricultural/construction equipment

Load-Sensing (LS) Systems

Concept: Pump displacement adjusts to maintain pressure margin ΔP_LS = P_pump - P_load_max Typical margin: 10–30 bar

Variable displacement pump regulator: If P_pump < P_load + ΔP_LS → increase displacement (more flow) If P_pump > P_load + ΔP_LS → decrease displacement (less flow)

Advantages: Energy efficient: pump output tracks actual demand Quiet: variable speed/displacement Fast response: load changes → pump adjusts in < 50 ms

LS signal line: Signal shuttle valve selects highest pressure section → feed to pump regulator Anti-void control: pump minimum displacement = small (maintains pressure even at zero demand)

Negative LS Control

Pressure differential across metering restriction → regulates pump displacement Hydraulic signal without separate signal line

Accumulator Types and Sizing

Types

Bladder accumulator: rubber bladder separates gas and oil; fast response; most common Piston accumulator: floating piston; larger volumes; slower response; better for energy storage Diaphragm accumulator: small volume (< 1 L); compact; fast; simple

Gas Pre-Charge (Nitrogen)

P₀ = pre-charge pressure at ambient T (before system pressurizes) Typical: P₀ = 0.9 × P_min [bar; P_min = minimum working pressure]

Sizing for energy storage (pulsation damping, emergency):

Volume calculation (gas law — polytropic): For adiabatic (fast cycle): P₁V₁^1.4 = P₂V₂^1.4 For isothermal (slow cycle): P₁V₁ = P₂V₂

Usable oil volume ΔV: ΔV = V₁ - V₂ = V_gas0 × [(P₀/P₁)^(1/1.4) - (P₀/P₂)^(1/1.4)]

Required accumulator size V₀: V₀ = ΔV / [(P₀/P_min)^(1/1.4) - (P₀/P_max)^(1/1.4)]

Emergency function (fail-safe): Size to provide n full actuator strokes at reduced pressure after pump failure ΔV = n × A_cyl × L_stroke; P₂ = minimum functional pressure

Pulsation Damping

Size accumulator to attenuate pump pressure ripple Attenuation ratio: A = ΔP_attenuated / ΔP_pump; A = 0.1–0.3 typical

Pipe and Hose Sizing for Hydraulic Systems

Flow velocity guidelines: Suction lines: V ≤ 1.0 m/s (prevent cavitation) Return lines: V ≤ 3.0–4.0 m/s Pressure lines: V ≤ 4.0–6.0 m/s (use higher end for short sections) Case drain: V ≤ 2.0 m/s

Reynolds number: Re = ρ V D / μ = 2100 V D / ν [ν = kinematic viscosity in cSt; D in mm; V in m/s] Laminar if Re < 2300 (common in hydraulics at high viscosity)

Laminar pressure drop: ΔP = 128 μ L Q / (π D⁴) = 384 ν ρ L Q / (π D⁴) [Pa; ν in m²/s] = 64/Re × L/D × ρV²/2 [Hagen-Poiseuille in Darcy form]

Hydraulic Fluids

Fluid	Viscosity at 40°C [cSt]	T_range [°C]	Application
Mineral oil (ISO VG 46)	46	-20 to +90	General industrial
Mineral oil (ISO VG 68)	68	0 to +100	High-T systems
HFC (water-glycol)	40–60	-20 to +55	Fire-resistant
HFAE (oil-in-water)	≈ 1	5 to +50	Fire-resistant; low cost
HFD (phosphate ester)	30–100	-20 to +80	Aviation; fire-resistant
Biodegradable (HEES, VG 46)	46	-20 to +80	Environmental

Viscosity at operating T (Walther's equation): log log(ν + 0.7) = a - b log T [T in K; ν in cSt; a, b from 2-point test]

ISO 4413:2011 — Hydraulic Fluid Power Safety

Cleanliness: ISO 4406 contamination class: ISO-17/15/12 for precision servo systems; 18/16/13 for general Filter rating: β₁₀ ≥ 75 for servo valve systems; β₃ ≥ 75 for general directional valves

Pressure setting: relief valve ≤ 1.25 × system MAWP Thermal management: oil T ≤ 60–70°C (check reservoir size; heat exchanger if needed) Reservoir sizing: V_reservoir ≥ 3–5 × Q_pump [L; Q in L/min]

Output

Provide: pump displacement and pressure [cc/rev and bar], flow at each actuator [L/min], velocity of actuators [mm/s], pressure drop at each component [bar], accumulator size V₀ [L] with pre-charge P₀ [bar] and working range P₁-P₂ [bar], pipe diameters at each segment [mm NB], fluid type and cleanliness class (ISO 4406), reservoir size [L], heat generation estimate Q_heat [kW], applicable standard (ISO 4413, ISO 4406).