Define the operating duty first
Begin with the fluid, normal working pressure, maximum pressure including peaks, and minimum and maximum operating temperatures. Include start-up conditions and cleaning fluids where relevant. A fitting in a mobile machine may experience a different combination of vibration, pressure cycling and environmental exposure from a fitting on a stationary hydraulic power unit.
Record the tube outside diameter, wall thickness, material and condition. The fitting body is only one part of the pressure boundary. The tube, equipment port, seals and other connected components must all be suitable for the duty. Select using the permissible rating of the complete combination rather than the highest value shown for one component.
Compare carbon steel and stainless steel
Carbon steel, 316L stainless steel and 316Ti stainless steel may be specified in an F-LINE enquiry, subject to confirmation of the supplied configuration and material. Carbon steel can suit many hydraulic oil installations when its surface protection and the surrounding environment are appropriate. Include exposure to moisture, washdown or coastal conditions when requesting a recommendation.
Stainless steel can be considered where corrosion resistance is an important selection factor. Identify the required grade explicitly: 316L and 316Ti are different material specifications and should not be treated as identical order descriptions. Suitability still depends on the fluid, concentration, temperature and exposure conditions. Stainless steel does not automatically give a fitting a higher pressure rating than its carbon-steel equivalent.
Treat pressure ratings as item-specific data
LL, L and S identify fitting series, not one universal working pressure. Within a series, published ratings can change with tube size, material, connection design and fitting configuration. A straight union, banjo connection and adjustable fitting should each be checked against the data for that exact item.
Temperature can reduce the permissible pressure of a material or assembly. Pressure peaks, repeated cycling, external loads and vibration also need consideration. Request the applicable pressure and temperature data for the proposed part and tube combination. A nominal pressure designation, test pressure or burst pressure must not be used interchangeably as the system working pressure.
Specify seals and tubing together
NBR, FKM/FPM and other seal materials have different compatibility and temperature characteristics. Choose a seal for the actual medium and conditions, including any additives or cleaning agents. An O-ring and an ED profile ring are different sealing components; dimensions, profile, material and hardness may all matter. A similar-looking spare seal is not enough to establish suitability.
Tube selection requires more than matching the outside diameter. Confirm the wall thickness, dimensional tolerances, hardness and preparation requirements for the fitting system. Thin-wall or softer tubing may need an approved support arrangement. Obtain confirmation before making substitutions, and account for the most restrictive temperature and pressure limits across the assembly.
- Provide the fluid name, composition where relevant, and operating temperature range.
- State working pressure, maximum peaks, cycling and vibration conditions.
- Include tube dimensions, material grade, installation environment and required documentation.
