Pressure Loss in Hydraulic Tubes

Pressure loss in hydraulic tubes includes friction loss, local pressure loss, and total pressure loss. Friction loss is caused by friction and resistance, while local loss is caused by pipe bends, joints, etc. To reduce pressure loss, measures such as increasing pipe diameter, reducing the number of bends and joints, and increasing fluid temperature can be taken.

Pressure loss in hydraulic tubes is a common problem in hydraulic systems, which can lead to reduced system efficiency and even affect the normal operation of equipment. So, what are the different types of pressure loss in hydraulic tubes?

Types of Pressure Loss in Hydraulic Tubes

1. Friction Loss

Friction loss refers to the pressure reduction caused by friction and resistance as the fluid flows in the hydraulic tube. This loss is related to the pipe length, inner diameter, material, and the properties and flow rate of the fluid. This is generally expressed as a pressure drop, called the friction loss ΔP_λ, and is calculated using the following formula:

Where, λ—friction coefficient, a function of Reynolds number Re and relative roughness Δ/d;

l—length of the pipe along the friction path, m;

d—inner diameter of the pipe, m;

v—average velocity inside the pipe, m/s;

ρ—fluid density, kg/m³.

2. Local pressure loss

Local pressure loss refers to the pressure reduction caused by changes in fluid direction, velocity, and collisions as the fluid flows through elbows, joints, valves, and other local resistance components in a hydraulic tube. This loss is related to the type, number, and installation location of the resistance components. Overcoming local resistance requires energy, generally manifested as a pressure drop.

In the formula, ξ—local resistance coefficient, which is related to the shape of the pipe fitting and the Reynolds number;

v—average flow velocity, m/s, generally refers to the average velocity on the flow cross-section after the local pipe fitting unless otherwise specified.

3. Total pressure loss

Total pressure loss is the sum of friction loss and local pressure loss. It is one of the important indicators for evaluating the performance of a hydraulic system.

Causes of Pressure Loss in Hydraulic Piping

Pressure loss in hydraulic piping is mainly caused by factors such as fluid viscosity, friction of the pipeline inner wall, fluid velocity changes, and fluid turbulence. In addition, local resistance components such as bends, joints, and valves in the pipeline can also impede fluid flow, leading to pressure loss. The following is an analysis of key factors:

1. Fluid Properties

Viscosity: High-viscosity fluids (such as heavy-duty hydraulic oil) have greater frictional resistance and higher pressure loss. For example, ISO VG 46 hydraulic oil has a viscosity of 46 cSt at 40°C, and its pressure loss is about 15%-20% higher than that of low-viscosity oils.

Temperature: Increased temperature reduces viscosity, thus reducing pressure loss. However, excessively high temperatures may lead to oil oxidation, requiring balanced control.

2. Pipeline Design

Pipe Diameter: The smaller the diameter of the hydraulic tube, the higher the flow velocity and the greater the pressure loss. According to the Darcy-Weisbach equation, pressure loss is inversely proportional to the fifth power of the pipe diameter.

Pipeline Length: Long hydraulic tubes increase friction loss; the design should aim to shorten the pipeline as much as possible or use segmented pressure reduction.

3. Flow Velocity and Flow Rate

Excessively high flow velocities exacerbate turbulence, significantly increasing pressure loss. A flow rate of 2-6 m/s is generally recommended, with lower values used for high-pressure systems.

Sudden changes in flow rate (such as pump start-up and shutdown) can cause instantaneous pressure fluctuations, which need to be mitigated by accumulators or buffer valves.

4. Hydraulic Component Types and Layout

Valves: Components such as throttle valves and directional valves introduce localized losses. For example, the pressure loss of a typical check valve is approximately 0.1-0.3 MPa.

Layout Optimization: Reducing the number of bends and using large-radius bends (R/D ≥ 1.5) can reduce losses.

Common Values of Hydraulic System Pressure Loss

Based on practical experience and theoretical research, common values for hydraulic system pressure loss are typically between 10% and 30%. This range is based on the combined effects of multiple factors.

Measures to Reduce Hydraulic Line Pressure Loss

To reduce hydraulic line pressure loss, the following measures can be taken:

1. Increase line diameter. Increasing the line diameter reduces fluid velocity and frictional resistance, thereby reducing pressure loss along the flow path.

2. Reduce the number of elbows and joints. Reducing the number of elbows and joints can decrease fluid turbulence and collision effects, thereby reducing local pressure loss.

3. Increase the fluid temperature. Increasing the fluid temperature can reduce fluid viscosity and frictional resistance, thereby reducing pressure loss along the flow path. However, it should be noted that excessively high temperatures may affect fluid stability and sealing performance.

Summary

Hydraulic tube pressure loss is a core factor affecting the efficiency, energy consumption, and reliability of hydraulic systems. It essentially stems from frictional losses along the flow path and local resistance losses. In engineering practice, by appropriately selecting pipe diameters, controlling flow rates, optimizing pipeline layout, and using low-roughness, high-quality hydraulic tubes, system pressure loss can typically be controlled within 10%, significantly improving system performance.