Pressure reducing valves and relief valves are both hydraulic pressure control components, but their functions are not the same.

Relief valves are mainly used to limit the maximum upstream pressure of a hydraulic system, protecting hydraulic pumps, cylinders, hoses, and other components from overpressure damage.

The function of a pressure reducing valve is to establish and maintain a downstream pressure in a certain branch of a hydraulic system that is lower than the main system pressure.

The simplest way to understand this is:

Relief Valve → Limits maximum system pressure

Pressure Reducing Valve → Maintains a lower downstream pressure

Although the two types of valves sometimes look similar in appearance, they usually cannot be used interchangeably. The key to correct selection is to first determine which pressure position in the system actually needs to be controlled.

What Does a Hydraulic Relief Valve Do?

A hydraulic relief valve is mainly used to prevent system pressure from becoming too high.

Under normal working conditions, the relief valve usually remains closed.

When system pressure reaches the set value, the relief valve begins to open, releasing excess hydraulic oil back to the tank.

The basic working logic can be understood as:

Hydraulic pump → hydraulic system

When pressure is below the set value:

Relief valve closed

When pressure reaches the set value:

Relief valve opens → excess oil returns to tank

This prevents system pressure from continuing to rise.

The relief valve mainly protects:

• Hydraulic pumps
• Hydraulic cylinders
• Hydraulic hoses
• Seals
• Valve blocks
• Directional valves
• Other hydraulic components

Common relief valves can be divided into direct-acting and pilot-operated structures.

What Does a Hydraulic Pressure Reducing Valve Do?

A pressure reducing valve is mainly used in situations where different branches of the same hydraulic system require different working pressures.

For example:

The main system pressure is:

200 bar

But the clamping circuit only requires:

80 bar

In this case, a pressure reducing valve can be installed in the clamping branch:

200 bar main system pressure → pressure reducing valve → 80 bar branch pressure

Unlike a relief valve, a pressure reducing valve usually allows oil to pass through normally.

When downstream pressure approaches the set value, the pressure reducing valve begins to restrict flow, stabilizing the outlet pressure at a lower level.

Therefore, the core function of a pressure reducing valve is:

To reduce and stabilize downstream pressure.

The Biggest Difference Between Pressure Reducing Valves and Relief Valves

The most core difference is that the two valves control pressure at different positions.

Comparison Item	Pressure Reducing Valve	Relief Valve
Main function	Reduce and stabilize downstream pressure	Limit maximum system pressure
Pressure sensed	Outlet / downstream pressure	Inlet / upstream pressure
Normal state	Normally open	Normally closed
When set pressure is reached	Restricts or throttles oil flow	Opens and relieves to tank
Common installation position	Branch circuit	Main pressure line
Main purpose	Establish low-pressure branch	Prevent system overpressure
Oil flow direction	Continues into downstream circuit	Excess oil returns to tank

Therefore, although both belong to pressure valves, their working logic is completely different.

Why Is a Pressure Reducing Valve Usually Normally Open?

The task of a pressure reducing valve is to let oil enter the downstream branch while controlling outlet pressure.

When downstream pressure is low:

The valve is open

As downstream pressure rises:

The valve begins to restrict flow

When pressure approaches the set value:

The valve opening gradually decreases

Therefore, its working logic is:

Low downstream pressure → valve open

↓

Pressure rises

↓

Reaches set value

↓

Valve restricts flow

So a pressure reducing valve can usually be understood as:

Normally Open Pressure Control Valve

That is, a normally open pressure control valve.

Why Is a Relief Valve Usually Normally Closed?

During normal system operation, a relief valve should not allow oil output by the hydraulic pump to return directly to the tank.

Therefore, when system pressure is below the set value, it usually remains closed.

The working logic is:

Normal pressure → relief valve closed

↓

Pressure continues to rise

↓

Reaches set pressure

↓

Relief valve opens and returns oil

Therefore, a relief valve generally belongs to:

Normally Closed Pressure Control Valve

That is, a normally closed pressure control valve.

The Difference Between Upstream Pressure and Downstream Pressure

This is a very practical way to distinguish the two valves.

Relief Valve

Mainly senses:

Inlet / upstream pressure

It focuses on:

“Is system pressure too high?”

If pressure is too high, it opens and relieves.

Pressure Reducing Valve

Mainly senses:

Outlet / downstream pressure

It focuses on:

“Has the branch pressure reached the pressure reducing set value?”

If it has, it restricts the flow entering that branch.

Simple understanding:

Relief Valve → Controls upstream pressure

Reducing Valve → Controls downstream pressure

Using Both Valves in One Hydraulic System

In many hydraulic systems, pressure reducing valves and relief valves are used at the same time.

For example:

Main system working pressure:

210 bar

The main cylinder requires the full:

210 bar

But the clamping cylinder only requires:

70 bar

It can be designed as:

Hydraulic pump → main system: 210 bar

At the same time:

Main pressure line → pressure reducing valve → clamping circuit: 70 bar

At this point:

The relief valve is responsible for protecting the entire main system

And:

The pressure reducing valve is responsible for controlling the low-pressure clamping branch

So the two valves do not replace each other; instead, they each perform different functions.

Can a Pressure Reducing Valve Replace a Relief Valve?

Usually not.

A pressure reducing valve is mainly used to control downstream pressure in a certain branch. It is not the main overpressure protection component for the entire hydraulic system.

If the hydraulic pump or main system experiences abnormal pressure rise, a relief valve is still required for safety protection.

Therefore:

Pressure reducing valve ≠ main system safety relief valve

Can a Relief Valve Replace a Pressure Reducing Valve?

Usually not.

Assume the main system pressure is 200 bar, while one branch can only use 80 bar.

A relief valve cannot stably maintain this branch at 80 bar like a pressure reducing valve.

A relief valve mainly deals with:

Relieving when pressure exceeds the set value

A pressure reducing valve mainly deals with:

Continuously maintaining a lower branch pressure

Therefore:

Need to reduce branch pressure → use a pressure reducing valve

Need to limit maximum system pressure → use a relief valve

Direct-Acting and Pilot-Operated Structures

Both pressure reducing valves and relief valves may use direct-acting or pilot-operated structures.

Direct-Acting

Pressure acts directly on the spool or poppet.

Common features:

• Simple structure
• Relatively fast response
• Compact size
• More suitable for smaller flow

Pilot-Operated

The pilot stage controls the main spool.

More suitable for:

• Larger flow
• Higher pressure
• Industrial hydraulic systems
• Applications with higher pressure stability requirements

Therefore, after determining whether it is a pressure reducing valve or a relief valve, it is also necessary to further confirm:

Direct Acting or Pilot Operated

Common Applications of Relief Valves

Relief valves are used in almost all types of hydraulic power systems.

Common equipment includes:

• Hydraulic power units
• Hydraulic presses
• CNC machine tools
• Agricultural machinery
• Construction machinery
• Steel equipment
• Injection molding machinery
• Automated production equipment

Their main function is always:

To limit maximum pressure and protect the system.

Common Applications of Pressure Reducing Valves

Pressure reducing valves are suitable for situations where different pressure requirements exist in one hydraulic system.

Common applications include:

• Hydraulic clamping systems
• Machine tool fixtures
• Auxiliary hydraulic circuits
• Lubrication circuits
• Multi-actuator systems
• Industrial automation equipment
• Hydraulic station branches

This allows one hydraulic pump to supply oil to both a high-pressure main circuit and a low-pressure branch at the same time.

Common Selection Mistakes

A common mistake is seeing:

Pressure Valve

and assuming that all pressure valves have similar functions.

In fact, hydraulic pressure control valves also include:

• Relief Valve
• Pressure Reducing Valve
• Sequence Valve
• Unloading Valve
• Counterbalance Valve
• Pressure Cut-Off Valve

Although all are related to pressure, their control logic is completely different.

Another common mistake is selecting a replacement model based only on:

• Product appearance
• Mounting dimensions
• Pressure range

Even if two valves have similar appearance and pressure range, they cannot be directly replaced if their hydraulic symbols are different.

What Needs to Be Confirmed During Selection?

Whether selecting a pressure reducing valve or a relief valve, it is recommended to confirm:

• Complete model
• Hydraulic symbol
• System working pressure
• Maximum pressure
• System flow
• Mounting type
• Port specifications
• Adjustment range
• Equipment used

If it is a pressure reducing valve, also focus on confirming:

Inlet pressure + required outlet pressure

If it is a relief valve, focus on confirming:

Normal working pressure + maximum allowable pressure + required relief flow

During repair and replacement, it is best to provide:

Nameplate + product photos + hydraulic symbol

This allows more accurate judgment of product function.

How Does XYIYEYA Hydraulics Support Pressure Valve Selection?

XYIYEYA Hydraulics provides a variety of industrial hydraulic pressure control products, including:

• Hydraulic relief valves
• Pressure reducing valves
• Pilot-operated relief valves
• Direct-acting pressure valves
• Solenoid-controlled relief valves
• Proportional pressure valves
• Pressure cut-off and unloading valves

For repair and replacement projects, customers can provide:

Original model + product photos + nameplate + hydraulic symbol + pressure + flow + equipment used

The first step is usually to confirm:

Which pressure position actually needs to be controlled?

Then further confirm:

Pressure range + flow + hydraulic function + mounting interface

This can reduce selection problems caused by incorrect function confirmation.

Conclusion

Although pressure reducing valves and relief valves both belong to pressure control valves, their functions are completely different.

The simplest way to remember is:

Relief Valve

Limits maximum system pressure

System pressure too high → valve opens → excess oil returns to tank

Pressure Reducing Valve

Maintains a lower downstream pressure

High inlet pressure → pressure reducing valve adjusts → lower outlet pressure

Therefore:

System needs overpressure prevention → choose a relief valve

A branch needs lower pressure than the main system → choose a pressure reducing valve

Correct selection cannot be based only on pressure grade. It should also confirm at the same time:

Pressure control position + hydraulic function + flow + pressure range + mounting method

Understanding the difference between the two valves can help avoid incorrect replacement and help hydraulic systems achieve more stable and reliable pressure control.