Short answer
A hydraulic return line should not be treated as equivalent to an ordinary liquid hose because of its seemingly low pressure. The fluid, temperature, flow rate, back pressures caused by filters and coolers, and transient loads are assessed. Suitability for return service must be verified from the product code.
Source: S017
Technical explanation
The conditions of the flow returning to the hydraulic system's tank differ from the pump outlet line; however, it is not assumed to be zero pressure. System components and operating states can affect the pressure on the return side. In particular, there may be different flow resistance when the fluid is cold, or transient changes at certain valve positions. The hose's inside diameter, temperature class and fluid compatibility are addressed together with the connections. The pressure requirement should be determined from machine data or suitable measurement rather than estimation.
Source: S017
Topic-specific checkpoints
| Item | Description |
|---|---|
| Flow path | Sequence of valve, filter, cooler and tank connections. |
| Pressure | Normal and transient back pressure; data source. |
| Temperature | Start-up and steady operating conditions. |
| Geometry | Passage through fittings and reducers in addition to the hose inside diameter. |
Source: S017
How is it assessed in practice?
In the recommended request, show the filter, cooler and valves connected to the return line in a simple diagram. Record normal operation and start-up as separate rows. If there is a reducing connection at the end of the existing hose, only enlarging the inside diameter of the new hose does not make the whole line equivalent. Also check the candidate product's minimum radius, whether it has a suction function, and the external environmental conditions. Do not write the phrase 'tank line' in place of technical data.
Original editorial recommendation; not a clause of a standard.
Frequently asked questions
Is it enough to look only at temperature for a return hose?
No. There are also fluid, pressure, flow rate, connection and installation requirements. Base it on the defined limits for the return side of the system.
Source: S017
Source coverage
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S017
Parker Safety Guide, Publication 4400-B.1 Parker Hannifin · accessed 2026-10-20 PDF page 2; 2.2 PressurePDF page 3; 3.1 Component Inspection, 3.2 Hose and Fitting Assembly and 3.3 Related AccessoriesPDF page 2; 2.4 Temperature | PDF page 3; 2.17 Radiant Heat | PDF page 2; 2.5 Fluid CompatibilityPDF page 2; 2.4 Temperature | PDF page 3; 2.17 Radiant Heat | PDF page 2; 2.5 Fluid Compatibility | PDF page 2; 2.7 Size