Hydraulic Component Selection: Line Size, Hose Routing, Fittings, and Connectors
Dec 13th 2023
Hydraulic component selection starts with the conductors that move fluid through the system. Steel tubing, pipe, and hose must be sized to control pressure drop, heat, turbulence, routing stress, and leakage risk.
After line size is established, hoses, fittings, and connectors need to match the pressure, movement, seal style, and installation requirements of the machine.
Line Size and Fluid Velocity
Fluid lines should be sized to avoid unnecessary pressure drop. Excessive velocity creates friction between the fluid and the line wall, which wastes energy and raises fluid temperature.
Maintaining practical velocity also protects pump inlet conditions and reduces the chance of excessive heat in the hydraulic circuit.
Velocity Guidelines
Common maximum velocity guidelines are 2 to 4 feet per second for inlet or suction lines, 10 to 15 feet per second for return lines, 15 to 20 feet per second for pressure lines operating from 500 to 2,000 PSI, and about 25 feet per second for systems around 3,000 PSI.
These values are starting points. The final size should reflect the equipment design, duty cycle, fluid viscosity, temperature, and allowable pressure drop.
Using Flow and Area
Fluid velocity can be estimated with V = 0.3208 x Q / A, where V is velocity in feet per second, Q is flow in gallons per minute, and A is conductor area in square inches.
The area of a round conductor can be estimated with A = 0.7854 x D squared, where D is inside diameter in inches. Rearranging the relationship helps estimate the inside diameter required for a given flow and velocity.
Example of Pressure Line Sizing
For a 10 gallon per minute system with a maximum velocity of 20 feet per second, the calculated inside diameter is about 0.452 inches, which points toward a one-half inch line size in many practical selections.
The calculation should still be checked against the actual tube, pipe, or hose rating and the machine layout.
Hydraulic Hose Selection
Hoses are used where rigid lines would crack or fail because of movement, vibration, or component motion. They are common on suction, discharge, and case drain lines as well as many mobile equipment connections.
The hose layline can help confirm the hose type, size, routing, and twist. A visible twist after installation is a warning sign. Even a small twist can sharply reduce hose service life.
Fittings and Connectors
Connectors and fittings must be selected to create a leak-free pressure boundary. Hydraulic systems may use metal-to-metal fitting seals, O-ring fittings, pipe-thread connections, flange connections, or other designs.
The fitting should be tightened to the specified method for that connection. Over-tightening can crack the fitting, damage the port, create leaks, or cause a dangerous rupture.
Final Selection Checks
Before the system is placed in service, inspect hose routing, layline orientation, fittings, threads, sealing surfaces, and visible damage. New equipment should still be visually checked before pressure is applied.
A correct hydraulic component selection keeps velocity controlled, hose movement managed, and fitting seals matched to the system pressure.