Relief Valve vs Pressure Reducing Valve vs Sequence Valve
Core Differences in Hydraulic Pressure Control Valves
Article Overview
I. Fundamental Concepts and Common Principles
Hydraulic valves are essential components in hydraulic transmission systems, used to control fluid pressure, flow rate, and direction. Among pressure control valves, the primary types include relief valves, pressure reducing valves, sequence valves, and unloading valves.
Core Common Principle
These three pressure control valves operate on the same fundamental principle: using controlled pressure oil to open and close valve ports. Understanding this is the foundation for distinguishing their differences.
II. Relief Valve
2.1 Working Principle
A relief valve is a hydraulic pressure control valve primarily serving four functions: pressure setting relief, pressure stabilization, system unloading, and safety protection.
Detailed Operating Process
- Low Pressure State: When inlet pressure is low, the main valve spool remains closed under spring force, valve port closed
- Pressure Increase: As inlet pressure rises, oil flows through the damping orifice into the upper chamber of the main valve spool, simultaneously acting on the pilot valve spool
- Pilot Valve Opens: When pressure reaches the set value, the pilot valve opens, allowing oil to flow back to the tank through the pilot valve
- Main Valve Opens: A pressure differential is created across the main valve spool. When the force generated by this differential exceeds the spring force, the main valve spool lifts, allowing large volumes of oil to overflow back to the tank
2.2 Technical Parameter Characteristics
| パラメータ | Relief Valve Characteristic |
|---|---|
| 制御圧力 | Inlet pressure control, maintains constant inlet pressure |
| Normal State | Valve port normally closed |
| Operating State | Valve port opens, oil overflows to tank |
| Drain Method | Internal drain – spring chamber oil drains internally to outlet |
| Working Mode | Passive operation |
III. Pressure Reducing Valve
3.1 Working Principle
A pressure reducing valve is used to reduce pressure in a branch circuit, maintaining outlet pressure lower and more stable than the main circuit pressure.
Detailed Operating Process
- Initial State: Valve port at maximum opening position, valve port normally open
- Pressure Regulation: Outlet pressure oil enters the lower chamber of the main valve spool through the damping orifice, simultaneously acting on the pilot valve
- Pressure Balance: When outlet pressure is below the set value, the pilot valve remains closed, main valve spool maintains maximum opening
- Pressure Reducing Action: When outlet pressure reaches the set value, the pilot valve opens, the main valve spool moves upward, valve port narrows, restricting flow to maintain stable outlet pressure
Key Difference: The pressure reducing valve operates in active mode. Within the set pressure range, the reducing valve is also closed like a relief valve. However, as system pressure increases and reaches the reducing valve set pressure, the reducing valve opens, allowing some oil to return to the tank, and the pressure in this branch circuit will not increase further. The reducing valve provides pressure reduction and stabilization for this branch circuit.
3.2 Technical Parameter Characteristics
| パラメータ | Pressure Reducing Valve Characteristic |
|---|---|
| 制御圧力 | Outlet pressure control, maintains constant outlet pressure |
| Normal State | Valve port normally open |
| Operating State | Valve port narrows, restricting flow |
| Drain Method | External drain – has separate drain port |
| Working Mode | Active operation |
IV. Sequence Valve
4.1 Working Principle
A sequence valve is used in systems with two or more branch circuits to control the operating sequence of multiple actuators based on circuit pressure.
Detailed Operating Process
- Initial State: Valve port closed, Actuator A operates, Actuator B does not operate
- Pressure Build-up: When Actuator A completes its stroke, system pressure increases
- Sequential Opening: When pressure reaches the sequence valve set value, valve port opens
- Actuator B Operation: Pressure oil enters Actuator B, achieving sequential motion control
4.2 Multi-Function Applications
Depending on configuration, sequence valves can perform different circuit functions:
1. Relief Valve Function
Internal control, internal drain
2. Sequence Control
Controls sequential action
3. Backpressure Valve
Internal control, external drain
4. Unloading Valve
External control, internal drain
5. Balance Valve
Prevents uncontrolled descent
4.3 Technical Parameter Characteristics
| パラメータ | Sequence Valve Characteristic |
|---|---|
| 制御圧力 | Inlet or external pressure control |
| Normal State | Valve port normally closed |
| Operating State | Valve port opens, oil flows through |
| Drain Method | External drain |
| Working Mode | Passive operation |
V. Core Differences Comparison
| Comparison | 安全弁 | Pressure Reducing | Sequence Valve |
|---|---|---|---|
| 制御圧力 | Inlet pressure | Outlet pressure | Inlet or external |
| Normal State | Normally closed | Normally open | Normally closed |
| Operating State | Opens (overflow) | Narrows | Opens |
| Drain Method | Internal drain | External drain | External drain |
| 主要機能 | Safety protection | Pressure reduction | Sequential control |
| Working Mode | Passive | Active | Passive |
VI. About Fisher
フィッシャー is a leading brand in industrial process control, providing comprehensive solutions for pressure control, flow control, and process safety applications.
For industrial process control valves and instrumentation, YUNRUI offers a complete range of Fisher products including control valves, regulators, positioners, and digital valve controllers. Contact us for technical support and product selection assistance.
VII. Summary
主なポイント
- Relief valve operates in passive mode, pressure reducing valve operates in active mode
- Relief valve controls inlet pressure, pressure reducing valve controls outlet pressure
- Relief valve has normally closed valve port, pressure reducing valve has normally open valve port
- Sequence valve can be used for multi-circuit sequential control
- The three valves have different drain methods
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Last Updated: May 2026 | Categories: Hydraulic Valves, Pressure Control Valves, Technical Guides