Sizing Tube to Maximize Hydraulic System Efficiency

Learn how to size tube to maximize hydraulic system efficiency, from Parker Hannifin TFD techConnect
Hydraulic systems using properly sized tube will perform more efficiently and cost-effectively. Tube that is too small can increase fluid velocity causing pressure drops and heat generation, and in suction lines, pump cavitation damage. Tube that is too large unnecessarily increases costs and is more difficult to fit in tight spaces.

Outlined below is a simple procedure for optimizing tube size to maximize system performance.

Flow Diameter

The first step is to determine required flow diameter. The accompanying “Recommended flow diameter” table (Table 1)  gives guidelines for specific flow rates and types of line.

Table 1 is based on the following recommended flow velocities:

  • Pressure lines – 25 ft/sec or 7.62 meters/sec.
  • Return lines – 10 ft/sec or 3.05 meters/sec.
  • Suction lines – 4 ft/sec or 1.22 meters/sec.

If flow velocities differ from these, calculate the required flow diameter based on:

Calculate the required flow diameter of hydraulic systems with this formula from Parker Hannifin techConnect

Recommended flow diameter charts for hydrualic systems, learn more from Parker Hannifin TFD techConnect

                                                      Table 1: Recommended Flow Diameters

Diameter and thickness

Next determine tube OD and wall thickness. Using the “Pressure ratings” table (Table 2), find the diameter and thickness combination that satisfies the following two conditions:

  1. Recommended design pressure that equals or exceeds maximum operating pressure.
  2. Tube ID that equals or exceeds the required flow diameter determined earlier.

Another consideration is choosing the right wall thickness for bent tube. If bending without a mandrel, then wall thickness of less than 7% of tube OD is not recommended.

Design pressures in the table 2 are based on a severity of service rating “A” (design factor of 4) as listed in the “Design and derating factors” table (Table 3).

In more-severe operating conditions, multiply values in the pressure-ratings table 2 by the appropriate derating factors before determining the tube OD and wall thickness combination. You can contact a Parker expert when in doubt.

Allowable stress levels and the underlying specifications used to arrive at the pressure ratings are given in the “Design stress ratings” chart (Table 4). Values are for fully annealed tubing.

Design pressure can also be formulated based on Lame’s equation:

Lame's equation for calculating design pressure to ensure you size tube to maximize hydraulic system efficiency. Learn more from Parker Hannifin TFD techConnect

Pressure ratings for hydraulic tube based on size and material, from Parker Hannifin TFD techConnect
                                                            Table 2: Pressure Ratings

Design and derating factors for hydraulic tube systems, from Parker Hannifin TFD techConnect

                                                  Table 3: Design and Derating Factors

Design stress ratings for hydraulic tube systems, from Parker Hannifin TFD techConnect

                                                     Table 4: Design Stress Ratings

Maximum Working Pressures

The design factor is generally applied to the material’s ultimate strength (or tubing burst pressure) to provide a margin of safety against unknowns in material and operating conditions. Apply the derating factors listed in table 3 directly to values in the pressure ratings table (Table 2) to determine maximum recommended working pressures. That is, multiply values in table 2 by the derating factors.

Temperature Considerations

Besides severity of service, high operating temperatures also reduce allowable working pressure in tubing. Temperature derating factors for various tube materials are given in table 5. Where applicable, apply derating factors for severity of service and temperature to the design pressure values (from the table) to calculate the maximum recommended working pressure. For example, the combined derating factor for 316SS tubing for B (severe) service and 500°F operation is 0.67 × 0.9 = 0.60.

Temperature derating factors for hydrualic tube systems, from Parker Hannifin, TFD techConnect

                                                  Table 5: Temperature Derating Factors

If you have questions or comments, please post them and I’ll respond if warranted. If you want to talk to me directly, I can be reached at Parker Tube Fittings Division, 614.279.7070 or via email. Download a print-friendly version of Sizing Tube to Maximize Hydraulic System Efficiency.

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Burleigh Bailey Engineer at Parker Tube Fittings Division

Burleigh Bailey, Research & Development - Engineering Manager, Parker Tube Fittings Division




Additional helpful content:

The Truth About Pressure Ratings for Hydraulic Fittings and Adapters

Turn vs. Torque? How Making the Right Choice Keeps Your Hydraulic Fitting Connections Leak-Free

Four Easy Steps to Identify Hydraulic Threads

A Dollars and Sense Approach to Preventing Hydraulic Oil Leaks

How Many Times Can I Reassemble a Hydraulic Fitting?

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Comments for Sizing Tube to Maximize Hydraulic System Efficiency

Craig Wojanis
Great info Burleigh!
Hydraulic Cylinders
Hydraulic cylinders also recognized as a linear hydraulic motor, is a machine that is used to make a unidirectional power by apply a unidirectional stroke. It has many applications particularly in the industrialized division as well as in the area of civil application and manufacturing vehicles.
Adem Ozalp
Your metric equation above table one is wrong. First I calculate in inches and multiply it by 25.4 and then result is wrong for metric equation when same values in metrics and litre/min form.
Adem Ozalp
Sorry the equation is true.

I appreciate
João Luiz Amorim
Muito útil

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