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How does pressure relief valve work?

Two Main Functions:

System protection

  • Limits pressure in a system or circuit

Pressure control

  • Relieves excess pump pressure

Two Main Styles:

Direct acting

  • Simple, fast acting, robust
  • Preferred style for system protection

Pilot operated

  • Efficient, more easily adjustable

Direct Acting Relief Valves

Direct Acting Relief Valve Types

Pilot Operated Relief Valves

Pilot operated relief valves use a pressure balanced spool that is controlled by a pilot seat. They provide much greater flow capability for a given cavity size.

Pilot Operated Relieve Valve

Direct vs Pilot Operated Valves

Direct vs Pilot Operated

Direct: Simple, robust but flow limited. Can see benefits of poppet geometry (secondary lift area) at higher pressure settings.

  • Fast response
  • Simple
  • Robust
  • Dirt tolerant

Pilot Operated: Main spool is normally pressure balanced via sensing orifice. Once valve setting is reached, ball unseats and pressure is relieved from back of spool, allowing valve to open. Allows compact design due to small pressure area on pilot seat.

  • Compact
  • High flow capability
  • Easy to adjust

How does pilot operated relief works?

Pilot operated valve working principle

Relief Valve Typical Applications

  • Main System Relief
  • Sub System Relief
  • Pump and Motor Protection
  • Thermal Relief
  • Over Load Protection
  • Over Torque Protection

Mechanical vs Hydraulic Damping

Damping Types

Kickdown Relief Valve Structure

Operating mode

  • Initial operation as pilot operated relief valve
  • Valve opens completely when extra spool drillings open to extra head drilling
  • Reseats only when inlet pressure < return spring load (~2 bar)
Kidckdown Relief Valve Structure

In situation where pressure must be dumped completely once valve relieves, a kickdown valve can be used. E.g. where achieving the relief setting indicates that an unsafe situation has occurred, machine jam, overload, etc.

Kickdown relief valve working principle is as below:

How kickdown valve works

Ventable Relief Valves

Additional port 3 giving great flexibility.

  • External control of spool balancing chamber pressure
  • Permits great flexibility of performance
Ventable Valve

Ventable Relief Valve Usage

Option 1

  • Port 3 connected to Proportional Relief Valve
  • Main setting controlled by Proportional Valve
  • Retains high flow capacity of pilot operated spool
Ventable Valve Usage

Option 2

  • Port 3 connected to normally closed solenoid valve
  • Solenoid de-energised, valve setting normal
  • Solenoid energised, valve unloads pump
  • Could use normally open solenoid valve to protect pump on start up.
Ventable Valve Usage 2

Pilot Operated Relief Valve

Excellent flow / pressure drop performance coupled with extremely low leakage poppet design, and also incorporating reverse check. It can be used in load holding applications.

How to adjust hydraulic pressure relief valves?

Pilot Operated Relief Valve

PED Cat IV Compliant Valves
A04B2Y & A04J2Y (CE marked)

Pressure Equipment Directive Compliant Valves

Pressure Equipment Directive 97/23/EC

Where does it apply?

  • Vessels above specified pressure / volume levels
  • Accumulators >1 liter and >50 bar-liter
  • Applies to any safety equipment associated with qualifying systems

What does it mandate?

  • Fully documented design verification and validation
  • Fully traceable manufacture
  • Regular external audits to maintain approval

Key Features / Benefits

  • Hardened working parts for maximum durability.  (with differential hardness between mating parts)
  • 4:1 minimum Ultimate Tensile Strength Factor of Safety
  • Springs rated to 10,000,000 cycles (Infinite life)
  • Valves rated to 1,000,000 impulse & cyclic duty cycles
  • Wide adjustment range in single valve
  • Wide range of functions available
  • Flows up to 400 l/min. Pressures up to 450 bar.
  • Commonality of parts 
  • CE Marked (Complies Pressure Equipment Directive (PED) 97/23/EC) 

Relief Valve Typical Applications:


  • Use direct acting valves for main system relief
  • When relieving, kickdown valves need the inlet pressure to be removed (~2bar) to reset

How does pressure relief valve work?

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