A pressure relief valve in a hydraulic system protects the system from overpressure by opening and diverting excess fluid when pressure exceeds a pre-set limit. Here's a breakdown of how it works:
1. Normal Operation (Pressure Below Set Point):
- The pressure relief valve remains closed under normal operating conditions. A spring holds a poppet or ball against a seat, preventing fluid flow through the valve. The force exerted by the spring is greater than the hydraulic pressure acting on the poppet/ball.
2. Overpressure Situation (Pressure Exceeds Set Point):
- When the hydraulic pressure in the system rises above the valve's pre-set pressure (also known as the cracking pressure or set pressure), the force exerted by the hydraulic fluid overcomes the spring's force.
- This causes the poppet or ball to lift off its seat, opening a flow path.
3. Fluid Diversion:
- The opening of the valve allows the excess hydraulic fluid to flow through the valve and be diverted. The fluid is typically directed to the reservoir (tank) or a low-pressure return line. This diversion reduces the pressure in the system.
- The amount of fluid diverted is proportional to how much the pressure exceeds the set pressure. This is described by the pressure-flow characteristic of the valve.
4. Pressure Regulation:
- As fluid is diverted and the system pressure decreases, the spring begins to overcome the hydraulic pressure acting on the poppet/ball.
- Once the pressure drops below a certain level (often referred to as the reseat pressure, which is slightly below the set pressure), the spring force becomes dominant, and the poppet/ball returns to its seat, closing the valve and stopping the flow of fluid.
- The valve therefore, modulates to maintain a safe system pressure.
Components of a Typical Pressure Relief Valve:
| Component | Function |
|---|---|
| Valve Body | Provides the housing for the internal components and connections to the hydraulic system. |
| Poppet/Ball | The moving element that opens and closes the flow path, controlled by the balance between hydraulic pressure and spring force. |
| Spring | Provides a calibrated force that opposes the hydraulic pressure, determining the set pressure of the valve. Different springs are available for different pressure ranges. |
| Seat | The surface against which the poppet/ball seals to prevent flow when the valve is closed. |
| Adjustment Screw | Allows the operator to adjust the spring force, and thus the set pressure of the valve. |
| Return Port | The connection point where the diverted fluid is returned to the reservoir or a low-pressure line. |
Types of Pressure Relief Valves:
- Direct-acting: The simplest type, where the poppet/ball directly responds to the hydraulic pressure.
- Pilot-operated: Uses a small pilot valve to control a larger main valve, offering faster response and higher flow capacity.
In summary, a pressure relief valve works by acting as a safety mechanism, automatically opening a flow path when system pressure exceeds a pre-determined level, diverting excess fluid and preventing damage to hydraulic components. The valve then closes when the pressure drops back to a safe level.
