1. What is residual pressure?

Generally, residual pressure in a hydraulic system is defined as the pressure that “remains” in the actuators (cylinders, motors) or local oil circuits after the system stops operating (e.g., pump shutdown, control valve de-energization), due to circuit design flaws, component malfunctions, or other reasons. It is not the system’s designed “working pressure,” but rather “pressure that shouldn’t exist but hasn’t been released.” Data shows that the residual pressure in a typical hydraulic power unit should be between 0.05-0.1 MPa.

2. How is residual pressure generated?

The generation of residual pressure is essentially due to the inability of pressurized oil to flow smoothly back to the oil tank. Common causes can be categorized into four main types:

(1) Sticking of check valves/one-way valves

Check valves in hydraulic systems (such as cylinder pressure-holding check valves and pump outlet check valves) rely on valve core seals to prevent oil backflow. However, if impurities are mixed into the oil, they can stick to the valve core, preventing the check valve from opening—the pressurized oil is trapped in the cavity, thus creating residual pressure.

(2) Accumulator not depressurized

Accumulators are a major source of residual pressure. Many high-pressure systems (such as hydraulic presses and mining machinery) are equipped with accumulators to buffer pressure fluctuations or provide emergency pressure maintenance. However, if the accumulator is not depressurized after the system is shut down, the nitrogen gas inside the accumulator will continuously compress the oil, maintaining pressure in the circuit. During maintenance of a hydraulic crusher at a cement plant, the accumulator was not depressurized. The moment the technician disconnected the oil pipes, high-pressure oil sprayed out, wetting nearby electrical cabinets. Subsequent testing revealed that the accumulator pressure was still 12 MPa (the system operating pressure was only 16 MPa).

(3) Malfunction or Improper Design of Directional Control Valves

If core control valves such as electromagnetic directional control valves and electro-hydraulic directional control valves are stuck in the non-neutral position, or if the neutral position function is improperly designed (such as closed circuits in O-type or M-type neutral positions), although the directional control valve can quickly return to the neutral position after shutdown, the oil in the cylinder chamber cannot be completely released, and the pressure is “stifled,” resulting in residual pressure.

(4) Influence of Oil Temperature

In low-temperature environments, the viscosity of the oil increases significantly, and the flow resistance increases, causing the oil in the circuit to be unable to be drained quickly; the residual oil, due to its own compressibility, eventually forms residual pressure. Some people have observed an increase in hydraulic pressure when a closed hose filled with hydraulic oil is placed under the sun, causing the temperature to rise.

3. How serious are the hazards of residual pressure?

(1) Damage to equipment

① Accelerated component aging:

Long-term operation of the hydraulic cylinder with residual pressure will cause continuous compression of the seals, shortening their lifespan by more than 50%;

② Pump failure:

When the hydraulic pump restarts, if there is residual pressure in the circuit, the pump will “start under pressure,” causing excessive stress on the pump bearings, increased noise, and even “cavitation”;

③ Affecting accuracy:

If a precision hydraulic valve (such as a proportional valve) is impacted by residual pressure, the valve core is prone to displacement, leading to a decrease in the accuracy of equipment operation.

(2) Threat to maintenance personnel

This is the most fatal hazard of residual pressure—if residual pressure is not checked during maintenance, the pressure oil will push the actuator to move suddenly when the oil circuit or component is disassembled, causing the hydraulic cylinder to lurch, the motor to rotate unexpectedly, etc., resulting in pinching or crushing injuries. In 2023, when a car parts factory was repairing a hydraulic press machine, the mechanic disassembled the hydraulic cylinder joint without checking the residual pressure. The hydraulic cylinder suddenly moved downward, deforming the tooling on the workbench. Fortunately, no one was below at the time. Subsequent testing revealed that the residual pressure in the rodless chamber of the hydraulic cylinder reached 8 MPa, sufficient to push a slider weighing several tons.

5. A Misconception About Residual Pressure

Some hydraulic systems, even if they generate residual pressure, usually don’t maintain it for long. The pressure will spontaneously seek a way to release it, and even if the pressure relief circuit is not smooth, the pressure will automatically find any leaking channels to gradually dissipate. Therefore, after a period of shutdown, the residual pressure in the system will basically disappear on its own. Circuits with accumulators may retain residual pressure for a slightly longer time, but it will eventually release naturally. After all, without energy input, pressure cannot be created out of thin air; it can only be gradually consumed. Therefore, there is generally no need to worry about this issue when disassembling the machine after it has been idle for a period of time.

6. Final Reminder: Residual Pressure Detection Should Be “Proactive,” Not “Luck”

Many people check residual pressure by “feel” (e.g., shaking the cylinder to see if it moves). This method is unreliable. After the system is shut down, connect a pressure gauge (the range must cover the system’s working pressure) to the oil port of the actuator; wait 5-10 minutes for the pressure to stabilize, then read the pressure gauge reading—if it exceeds 0.5 MPa, it is considered “abnormal residual pressure,” and the cause must be investigated. Before maintenance, regardless of whether the pressure gauge is displaying anything, you must release the pressure through the pressure relief valve and confirm that the pressure is 0 before proceeding.