Relief Valve Function

A relief valve is a safety device that protects hydraulic systems from over-pressurization. It opens when system pressure exceeds a set point (cracking pressure) and diverts excess flow back to the reservoir.

Relief valves are classified by their operating characteristics:

• Direct-acting: Spring-loaded, simple construction, responds directly to pressure
• Pilot-operated: Uses pilot pressure to control main valve, better pressure regulation
• Proportional: Electronic control of pressure setting

Key Pressure Terms

• Cracking Pressure: Pressure at which the valve begins to open (first oil flow)
• Full Flow Pressure: Pressure at which valve is fully open and passes rated flow
• Reseat Pressure: Pressure at which valve fully closes after opening
• Override: Pressure rise from cracking to full flow (Full Flow - Cracking)

Sizing Criteria

Relief valves must be sized to handle the maximum flow that could be delivered to them. This is typically:

• Full pump flow for main system relief
• Cylinder regeneration flow for circuit protection
• Motor bypass flow for deceleration control

The valve capacity rating should exceed the maximum anticipated relief flow by at least 10-25%.

Heat Generation

When a relief valve bypasses flow, the hydraulic energy is converted to heat:

Power (kW) = Q (LPM) x dP (bar) / 600

Power (HP) = Q (GPM) x dP (psi) / 1714

Where dP is the pressure drop across the valve (approximately equal to system pressure when relieving to tank).

Continuous Bypass Warning

Relief valves are designed for intermittent operation. Continuous bypass causes:

• Rapid fluid heating: 1 HP generates ~2,545 BTU/hr
• Accelerated fluid degradation: Each 18F (10C) rise doubles oxidation rate
• Valve wear: Erosion of seat and poppet
• Energy waste: All bypassed flow represents lost efficiency

For systems requiring continuous pressure limiting, consider a pressure-compensated pump instead.