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.
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:
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:
- Recommended design pressure that equals or exceeds maximum operating pressure.
- 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:
Table 2: Pressure Ratings
Table 3: Design and Derating Factors
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.
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.
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, Research & Development - Engineering Manager, Parker Tube Fittings Division
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