A cylinder chamber held between closed valves is a trapped hydraulic volume. If its temperature changes, pressure can rise even though the pump is off. This is especially relevant to equipment exposed to sun, nearby heat sources or large ambient swings during storage and service.
The circuit can become a sealed container
A cap-end chamber blocked by a pilot-operated check valve has no easy volume change if the piston is restrained by the machine. When the oil warms, its volume tends to expand. If it cannot move, pressure may rise until something else deflects or a relief path opens. The exact rise depends on oil properties, trapped volume, hose elasticity, cylinder stiffness and the load; it should not be guessed from a single ambient-temperature number. Enerpac’s manual identifies temperature change as a cause of trapped pressure in disconnected cylinders.
Enerpac’s cylinder manual explicitly notes trapped pressure in disconnected cylinders after changing ambient temperature. The response is a circuit-level design review, not an automatic extra valve: determine which chamber can be isolated, the pressure ratings of the cylinder and hoses, how a load affects that pressure, and where controlled relief can discharge. A relief path must not compromise load holding or allow unintended movement. Record expected temperature extremes and verify the selected solution in the approved schematic.
Do not solve it by defeating load holding
A small thermal-relief function can be appropriate in some circuits, but it must discharge to a safe location and must not allow an unsupported gate or platform to move. The engineer needs to distinguish pressure caused by thermal expansion from pressure caused by the external load. The chamber may require both load control and overpressure protection, coordinated in the approved schematic. A main pump relief valve does nothing for oil isolated beyond a closed check valve.
An outdoor-gate example
A floodgate cylinder may be parked with both ports blocked overnight, then exposed to direct sun. The pump is off, but the trapped chamber warms. At the same time the gate and seals may hold a varying water load. These effects should be evaluated separately: one asks how trapped fluid pressure is protected; the other asks how the load is held without unintended movement. A relief route that simply dumps oil to tank might address pressure while allowing the gate to shift. Review the actual valve network and structural support before choosing hardware. The commissioning record should include a safe service-state pressure-release test.
Design and test the service state
Mark every position in which both cylinder ports are blocked, including power-off and maintenance modes. State the maximum and minimum temperature expected at the actual installation, not just in the workshop. Review the lowest-rated hose, fitting, sensor and cylinder component exposed to the trapped pressure. Finally, write an approved depressurization procedure with local measurement and mechanical support. Commissioning should verify both load holding and controlled pressure release; one test does not prove the other.
Checks to put on the drawing or test plan
- Mark all valve positions that can block a cylinder chamber.
- Review component pressure ratings and mechanical load cases at both temperature extremes.
- Test the intended pressure-release procedure while the load is safely supported.
Failure mode to avoid: A relief valve located only at the pump may be separated from the trapped chamber by a closed valve.
Information to request before approval
- Hydraulic schematic and valve fail positions.
- Temperature range and storage arrangement.
- Cylinder and hose pressure ratings.
For a related explanation, see trapped-pressure service guide.
Технические ссылки: Enerpac cylinder operating manual. The cited manuals and product data describe their makers’ equipment; this article is independent engineering guidance and does not assign their ratings to Yongjian products.