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Pressure Relief Valve in Hydraulics – Working Principle, Types and Selection Guide

Pressure Relief Valve in Hydraulics – Working Principle, Types and Selection Guide


Pressure Relief Valve in Hydraulic Systems: Working Principle, Types and Selection Guide

In hydraulic systems, the pressure relief valve is the primary safety component that protects pumps, hoses, cylinders and other critical equipment from overload. A properly selected and adjusted relief valve ensures system stability, improves energy efficiency and prevents catastrophic failures.

This comprehensive guide explains the working principle, direct vs. pilot operated designs, damping methods, unloading function and correct sizing criteria for industrial applications.

What Does a Pressure Relief Valve Do?

A pressure relief valve:

  • Allows the system to build pressure only up to the required working level
  • Limits pressure when resistance increases (end of stroke conditions)
  • Diverts excess flow to tank when the set pressure is reached
  • Protects the hydraulic circuit from overload

In simple terms, it is the safety fuse of the hydraulic system.

Working Principle

The operation is based on force equilibrium:

Fhyd = Fspring = P × A

  • Hydraulic pressure acts on the poppet or spool
  • Spring force keeps the valve closed
  • The adjustment screw sets the opening pressure

When system pressure exceeds the spring force, the valve opens and flow is redirected to tank, limiting pressure.

Main Components

A typical pressure relief valve consists of:

  • Body
  • Poppet or spool
  • Spring
  • Adjustment mechanism
  • Pilot stage (in pilot-operated designs)

This structure enables accurate and repeatable pressure control.

Direct Acting Pressure Relief Valves

In direct acting valves, system pressure directly acts on the poppet.

Advantages

  • Simple and compact design
  • Fast response time
  • Suitable up to 330 L/min
  • Ideal for low to medium flow applications

Limitations

Higher flow requires a larger spring and larger poppet area, which leads to:

  • Bulky construction
  • Installation constraints
  • Reduced adjustment sensitivity

Pilot Operated Pressure Relief Valves

For high-flow systems, pilot operated relief valves are preferred.

Advantages

  • High flow capacity
  • Stable pressure control
  • Lower pressure override
  • High accuracy

A small pilot valve controls the main stage, eliminating the need for large springs while maintaining precise regulation.

Pressure Oscillation and Vibration

The spring–mass system can generate oscillations during operation, causing:

  • Pressure fluctuations
  • Noise
  • Reduced valve life

Solution: Damping

Oscillations can be minimized by:

  • Damping spools
  • Orifice restriction
  • Cushioning techniques

These methods improve system stability and extend component life.

Unloading Function (Pressure-Free Operation)

When combined with a check valve, a relief valve can switch the system from:

  • Pressurized mode → Unloaded (pressure-free) mode

using a simple control signal.

Benefits

  • Energy savings
  • Reduced heat generation
  • Longer pump life

This function is widely used in power units and accumulator circuits.

Characteristic Curves: Direct vs Pilot

Direct acting valves:

  • Steeper pressure rise
  • Higher pressure override
  • More sensitive to flow changes

Pilot operated valves:

  • Flatter characteristic curve
  • Stable pressure control
  • Lower pressure peaks

For precision systems, pilot operated valves are the industry standard.

Typical Operating Example

Example application parameters:

  • Flow: 60 L/min
  • Pressure: 170 bar
  • Oil temperature: 50–62 °C

These values highlight the importance of considering:

  • Fluid viscosity
  • Thermal effects
  • Flow capacity

during valve selection.

How to Select the Right Pressure Relief Valve

  1. Maximum System Pressure

The adjustment range must exceed the working pressure.

  1. Flow Capacity

The valve must handle the pump’s maximum flow.

  1. Valve Type
  • Low flow → Direct acting
  • High flow → Pilot operated
  1. Mounting Style
  • Screw-in cartridge
  • Subplate mounted
  • Inline
  1. Stability Requirements

Use damped designs for vibration-sensitive systems.

Why Relief Valves Are Critical

Operating without a properly set relief valve can cause:

  • Pump failure
  • Cylinder rupture
  • Hose burst
  • Excessive heat generation

A correctly selected valve:

  • Extends system life
  • Improves energy efficiency
  • Ensures safe operation

Conclusion

Pressure relief valves are not just components—they are the core safety and stability elements of hydraulic systems. Correct sizing, proper type selection (direct vs. pilot), and damping implementation directly affect performance, efficiency and reliability.

Hidroman engineering approach:
Selecting the right relief valve is an investment in system safety, uptime and energy efficiency.