A Guide To Direct Acting Vs. Pilot Operated Relief Valves

A Guide to Pilot-Operated vs. Direct-Acting Relief Valves

Relief valves are an important part of hydraulic and pneumatic systems, safeguarding them from excessive pressure that may damage or destroy them. There are two main forms of pressure relief valves: direct-acting and pilot-operated. Knowing the differences, the benefits, and ideal applications for each is imperative to choosing the appropriate valve for your system.

1. Direct-Acting Relief Valves

How They Work:

Direct-acting relief valves use a spring-loaded action. As system pressure overcomes the preset force of the spring, the valve directly opens to relieve excessive pressure. The spring forces the valve to the closed position once the pressure has been returned to normal.

Characteristics:

Basic Design: Includes a valve seat, a spring, and an adjustable screw.
Quick Response: Opens rapidly as pressure goes over the setpoint.
Limited Capacity to Flow: Suits small to medium flow rates best.
Pressure Override: Pressure is slightly increased before full opening because of the force required to compress the spring.

Best Applications:

Small hydraulic circuits
Emergency pressure relief
Mobile equipment
Low-flow systems

2. Pilot-Operated Relief Valves

How They Work:

Pilot-operated relief valves employ a two-stage system:

A pilot valve (small, direct-acting valve) feels pressure and opens when the system approaches the set pressure.
This enables fluid to drain from the main valve, lowering system pressure. The bigger main valve opens more slowly, giving smoother pressure regulation.

Characteristics:

Increased Flow Capacity: Can handle larger system pressures and flows.
More Accurate Pressure Control: Less pressure override than direct-acting valves.
Slower Response: Opens more slowly but gives greater system stability.
More Complicated Design: Includes extra passages and elements.

Best Applications:

High-pressure hydraulic systems
Industrial machinery
Large-scale power units
Systems needing precise pressure control

Selecting the Proper Relief Valve

For rapid, small-volume relief: Employ a direct-acting relief valve.
For steady, high-flow conditions: Employ a pilot-operated relief valve.
For emergency relief systems where timing is everything: Direct-acting is the way to go.
For systems that require very close pressure control: Pilot-operated valves are the best choice.
By knowing these distinctions, you can choose the proper relief valve for your system, ensuring efficiency and safety in your hydraulic or pneumatic system.