Pressure Relief Valve (PRV) vs. Pressure Reducing Valve (PRV): Functions, Operation & Industrial Importance

Pressure Relief Valve (PRV) vs. Pressure Reducing Valve (PRV): Functions, Operation & Industrial Importance

Pressure management is one of the most critical aspects of any hydraulic, pneumatic, steam, or process-fluid system. Whether the application involves boilers, pumps, pipelines, or industrial automation equipment, controlling pressure ensures not just operational efficiency—but more importantly, safety.

Among the most frequently discussed and often misunderstood pressure-control devices are the Pressure Relief Valve (PRV) and the Pressure Reducing Valve (PRV). Though their abbreviations appear similar, their functions, mechanisms, and applications differ dramatically. One protects a system from catastrophic failure, while the other ensures stable downstream pressure for proper functioning of equipment.

This article provides a comprehensive deep dive into the working principles, mechanical construction, flow behaviour, failure modes, and industrial relevance of both valves—supported by the illustration you generated.

 

1. Introduction: Why Pressure Control Matters

Fluid systems, whether hydraulic oil circuits, steam distribution networks, compressed air systems, or water supply pipelines, inherently face pressure variations due to:

• Pump start/stop cycles
• Load fluctuations
• Temperature changes
• Blockages or line restrictions
• Rapid demand changes

If these pressure variations are left unmanaged, the system can suffer from:

• Burst pipes
• Seal failures
• Equipment damage
• Inefficient operation
• Complete system downtime
• Dangerous explosions (especially in boilers and high-pressure steam applications)

This is why two foundational devices—Pressure Relief Valves and Pressure Reducing Valves—are used to maintain safe and stable operating conditions.

 

2. Pressure Relief Valve (PRV): Safety Through Over-Pressure Protection

2.1 What is a Pressure Relief Valve?

A Pressure Relief Valve (also called Safety Valve, Safety Relief Valve, or PSV) is a normally closed protective device designed to open automatically when system pressure exceeds a set limit, allowing excess fluid to vent to atmosphere or return to a tank.

It is a safety component, often mandated by industrial standards, certifications, and legal requirements.

2.2 How It Works

As shown in the illustration, the PRV consists of:

• A spring-loaded poppet or disc
• A sealed valve body
• An inlet connected to the high-pressure line
• An outlet connected to a tank or atmosphere

Under normal operating pressure:

• The spring force keeps the valve fully closed.
• Fluid cannot pass through the valve.

When pressure rises above the preset relief pressure:

• The upward force of the fluid overcomes spring tension.
• The valve lifts, opening a path for the excess fluid to escape.
• Pressure in the system immediately drops.

Once pressure returns to normal, the spring pushes the valve closed again.

2.3 Key Characteristics

Attribute	Description
Function	Safety, protection from overpressure
Normal State	Closed
Action Mode	Opens fully when pressure exceeds setpoint
Fail Mode	Fails open (preferred for safety)
Flow Direction	To tank, drain, or atmosphere
Control Type	ON–OFF, not modulating

 

 

2.4 Applications

PRVs are indispensable in:

• Hydraulic power units
• Boiler systems
• Compressed air receivers
• Steam lines
• Water pipelines
• Chemical processing tanks
• Pressure vessels

Any system capable of generating excessive pressure must include a PRV to avoid damage or explosion.

2.5 Why PRVs Are Critical

A PRV is the last line of defense.
While sensors, transmitters, and PLC-based logic can help regulate pressure, none are foolproof. Electronic systems can fail; a mechanical PRV is required to prevent catastrophic failure.

 

3. Pressure Reducing Valve (PRV): Stable Output Pressure Through Regulation

3.1 What is a Pressure Reducing Valve?

A Pressure Reducing Valve (also abbreviated PRV but better called Pressure Regulating Valve or Pressure Reducing Regulator) is a normally open, modulating valve that reduces high inlet pressure to a consistent, lower outlet pressure.

Its purpose is control, not safety.

3.2 How It Works

From the illustration, the reducing valve operates using:

• A spring and diaphragm/piston assembly
• An inlet for high-pressure fluid
• An adjustable screw or set-spring to set downstream pressure
• An outlet providing reduced pressure

When downstream demand increases:

• Pressure falls → the diaphragm moves → the valve opens more, allowing more fluid.

When downstream pressure rises:

• Pressure pushes the diaphragm → compresses the spring → the valve throttles or closes slightly.

This constant adjustment ensures the downstream pressure remains within a narrow, stable range.

3.3 Key Characteristics

Attribute	Description
Function	Control, maintaining constant downstream pressure
Normal State	Open
Action Mode	Modulating (throttling)
Fail Mode	Fails closed (to avoid overpressure on downstream side)
Flow Direction	Inline (inlet → outlet)
Control Type	Continuous, proportional

 

 

3.4 Applications

Pressure Reducing Valves are widely used in:

• Pneumatic control circuits
• Water distribution systems
• Steam applications
• Hydraulic circuits requiring stable pilot pressure
• Domestic plumbing
• Industrial gas distribution
• HVAC systems

They enable the system to deliver consistent pressure even when the inlet pressure fluctuates.

 

 

 

 

 

 

4. PRV vs Pressure Reducing Valve: A Clear Technical Comparison

Parameter	Pressure Relief Valve (Relief PRV)	Pressure Reducing Valve (Regulating PRV)
Purpose	Safety	Control
Normal Position	Closed	Open
Operates When	Pressure exceeds set limit	Pressure varies downstream
Flow Direction	To tank, drain, or atmosphere	Inline, from inlet to controlled outlet
Operation Type	Snap open (ON/OFF)	Modulating (variable opening)
Fail Mode	Fails open (safe)	Fails closed (protects downstream equipment)
Key Application	Prevents burst/explosion	Ensures constant controlled pressure
Adjustment	Set at one value	Adjusts continuously during operation

This fundamental contrast clarifies that they cannot replace each other. A pressure reducing valve cannot protect against dangerous overpressure. A relief valve cannot stabilize downstream pressure.

 

5. Internal Mechanics and Flow Behavior

5.1 Relief Valve Flow Behavior

When pressure exceeds the setpoint:

• The valve snaps open instantly.
• A large volume of fluid is discharged.
• The system pressure drops rapidly.
• When pressure normalizes, it returns to the closed position.

This rapid response is crucial to safety.

5.2 Reducing Valve Flow Behavior

Reducing valves work in a continual equilibrium between:

• Spring force
• Downstream pressure
• Diaphragm/piston movement

The valve opening continuously adjusts to deliver consistent downstream pressure irrespective of:

• Fluctuating upstream pressure
• Changing demand on the downstream side

This "continuous throttling" is characteristic of regulating devices.

 

6. Failure Modes and Safety Considerations

6.1 Relief Valve (Fails Open)

If the relief valve fails, the safest condition is for it to remain:

• Open

This ensures pressure does not rise uncontrollably. Though it may cause loss of fluid or system shutdown, it avoids catastrophic failure.

6.2 Reducing Valve (Fails Closed)

If the reducing valve fails, the safe condition is for it to remain:

• Closed

This prevents excess pressure from reaching downstream instruments, actuators, and equipment.

 

7. Industry Standards and Codes Related to Valves

Relief Valve Standards

• ASME Boiler and Pressure Vessel Code
• API Standards (API 520, API 521, API 526)
• OSHA pressure safety guidelines

Reducing Valve Standards

• ISO 5208
• ASSE 1003 (water pressure reducing valves)
• Industrial equipment manufacturer specifications

These standards consistently reinforce that relief valves are mandatory safety components.

 

8. Real-World Example Scenarios

8.1 Hydraulic Press System

• Pressure Relief Valve prevents dangerously high pressure if the pump or hydraulic cylinder malfunctions.
• Pressure Reducing Valve is used to supply a lower pressure to a pilot-operated valve or actuator.

8.2 Water Supply System

• Reducing Valve ensures every outlet receives a consistent flow pressure.
• Relief Valve protects the system from sudden peaks, such as pump overrun or line blockage.

8.3 Steam Boiler System

• Relief Valve prevents explosion.
• Reducing Valve supplies lower pressure steam to equipment requiring controlled conditions.

 

9. Choosing the Right Valve

A common engineering mistake is selecting a pressure reducing valve thinking it can also relieve pressure—this is incorrect. When selecting valves:

Choose a Pressure Relief Valve when:

• Overpressure may cause damage or pose danger
• Legal or safety compliance is required
• Protecting pumps, pipelines, pressure vessels

Choose a Pressure Reducing Valve when:

• Constant downstream pressure is required
• Flow fluctuates based on demand
• You need stable process control

Many systems actually require both valves to ensure safe and smooth operation.

 

10. Conclusion

Pressure Relief Valves and Pressure Reducing Valves play fundamentally different roles in fluid systems. While one ensures protection, the other ensures precision control. The relief valve prevents disasters by opening during overpressure, while the reducing valve maintains constant downstream conditions essential for equipment performance.

Understanding their differences is not just beneficial—it is essential for engineers, technicians, and professionals working in hydraulics, pneumatics, water management, steam systems, and industrial automation. The illustration clearly highlights how each valve functions mechanically and how its role fits into an overall system.

A Pressure Relief Valve saves the system;
a Pressure Reducing Valve stabilizes the system.

Both are vital, but neither substitutes the other.