How to properly size hydraulic hoses and fittings

 Introduction

Hydraulic systems are an essential part of modern machinery, and their performance and safety largely depend on the proper design of the piping system. Correctly selecting hydraulic hoses and fittings ensures efficient system operation, extends service life, and reduces maintenance costs.

If hoses and fittings are not properly selected, the following issues may occur:

Pressure drop: If the hose diameter is too small or the routing is improper, system pressure may be insufficient, reducing the responsiveness of actuators.

Leakage: Mismatched fittings or inadequate pressure rating can lead to leaks, compromising system safety.

Reduced service life: Long-term operation under overpressure or high temperatures can accelerate hose aging, cracking, or bursting.

Reduced system efficiency: Pressure loss and insufficient flow can lead to energy waste and increased operational costs.

Therefore, properly sizing hydraulic hoses and fittings is fundamental to ensuring a safe, reliable, and efficient hydraulic system.

Understanding Hydraulic System Requirements

Before selecting hoses and fittings, it is essential to understand the basic operating parameters of the hydraulic system, including:

System Operating Pressure

Normal and maximum pressure: Identify the system’s continuous operating pressure and peak pressure to ensure that hoses and fittings have adequate pressure ratings.

It is recommended to include a safety factor of 1.25–1.5 in the design.

Flow Rate Requirements

Flow rate (L/min or GPM) determines the hose diameter. Too small a diameter increases pressure loss, while too large increases cost.

Flow rate and hose length should be evaluated together for optimal sizing.

Hydraulic Fluid Temperature Range

High or low temperature environments affect hose material performance.

Ensure hoses are rated for the required temperature range and can accommodate thermal expansion of the fluid.

Hydraulic Fluid Type and Material Compatibility

Different fluids (mineral oil, synthetic oil, water-glycol, etc.) have varying effects on rubber or plastic hoses.

Select materials compatible with the hydraulic fluid to prevent swelling or softening of the hose.

Determining Hose Inner Diameter

Hose inner diameter directly affects flow rate, pressure loss, and system efficiency. When selecting the diameter, consider:

Flow Rate Impact on Hose Size

Higher flow rates require larger diameters to reduce flow velocity and pressure loss.

Too small a diameter reduces system efficiency, while too large increases cost and bending radius requirements.

Minimizing Pressure Loss

Long hoses or frequent bends increase pressure loss. Optimize routing or increase diameter when necessary.

Using Manufacturer Flow Charts or Formulas

Most manufacturers provide pressure-flow reference charts to help select the correct diameter.

Pressure loss formula:

$$\Delta P = \frac{8 \cdot L \cdot \mu \cdot Q}{\pi \cdot r^4}$$

Where ΔP = pressure loss, L = hose length, μ = fluid viscosity, Q = flow rate, r = inner radius of the hose.

1. Considering Hose Length and Bend Angles

• Larger bend angles increase flow resistance; avoid sharp bends and increase diameter if necessary.

• Ensure the routing respects the hose’s minimum bend radius.

Selecting Hose Type

When choosing a hose, consider pressure, temperature, fluid type, and environmental conditions:

Material Selection

Common materials: synthetic rubber, thermoplastics, PTFE (Teflon)

Different materials suit different fluids and temperature ranges.

Reinforcement and Pressure Rating

Single or multi-layer steel wire reinforcement increases pressure capacity.

Ensure hose pressure rating ≥ system maximum pressure.

Temperature and Environmental Suitability

Outdoor or high-temperature applications require UV resistance and heat aging resistance.

Also consider wear resistance, oil resistance, and chemical resistance.

Selecting Fittings

Fittings are critical components of a hose system. Key considerations:

Fitting Type and Hose Compatibility

Ensure the fitting size and material fully match the hose.

Thread Type and Sealing Method

Common types: O-ring seal, flare, NPT, JIC, BSP, etc.

The sealing method must be reliable to prevent leakage.

Fitting Pressure Rating

Fitting pressure rating must ≥ system maximum operating pressure to ensure long-term safe operation.

Safety and Performance Considerations

Pressure Safety Factor

Hoses and fittings should have a pressure rating 25–50% higher than the system’s maximum pressure for reliability.

Minimum Bend Radius

Each hose has a minimum bend radius; exceeding it can cause kinking or damage.

Routing Guidelines

Avoid abrasion, vibration, and over-tight fastening.

Allow for thermal expansion so hoses can flex with temperature changes.

Testing and Verification

Post-Installation Pressure Testing

Ensure the system is leak-free and meets pressure requirements.

Check for Leaks and Performance Under Load

Recheck hoses and fittings after running the system for a period.

Regular Inspection and Maintenance

Clean hose surfaces and inspect for cracks, aging, or oil leaks.

Common Mistakes During Sizing

Hose Oversized or Undersized

Too small: high pressure loss

Too large: increased cost, larger bending radius required

Ignoring Minimum Bend Radius and Flexibility

Can lead to hose kinking or premature aging

Using Incompatible Fittings or Materials

May cause leaks, insufficient pressure rating, or chemical corrosion

(FK9016)